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MANUAL

PHOTOGRAPHY.

ROBERT HUNT,

l-KOFESSOU OF PHYSICAL SCIENCi! IN THE METKOPOLITAN SCHOOL OF SCIENCE — KEEPER

OF MINING BECOBDS IN THE MUSEUM OF PRACTICAL GEOLOGY — AUTHOR OF

" RESEAKCHES ON LIGHT," " THE POETRY OF SCIENCE," &C.

FOURTH EDITION, REVISED

Illujftvatc'lJ bg Numerous lEnsvabinfiji,

LONDON AND GLASGOW:
RICHARD GRIFFIN AND COMPANY,

PUBLISHERS TO THE UNIVEKSITY OP GLASGOW.

1854.

GLASGOW :

W. G. BLACKIE AND CO., PELNTEKS,

VILLAFIELD.

PREFACE,

The opportimity oifered in preparing a fourth edition
of the Manual of Photography for the press, has been
embraced for the purpose of making some alterations in
the aiTangements of the several divisions of the subject,
and of placing thereby each particular phenomena in a
clearer view. By this the amateur will advance more
readily in his Photogi'aphic studies, and the experienced
artist will find the references more easy to any particular
mode of manipulation which he may desire to consult.

All the most recent improvements have been compre-
hended; the processes of Photogi'aphic etching have been
embraced ; and, it is believed, nothing omitted which
can serve to maintain the flattering position which the
" Manual" has secured for itself as a standard work on
Photogi'aphy.

ROBERT HUNT.

London, March, 1854. ,

PREFACE TO THE THIRD EDITION.

The rapidity with which the second edition of this work
has been exhausted, is a most satisfactory proof that a
collection within convenient compass of all the facts con-
nected with the progress and practice of Photography,
was required by the increasing number of amateurs and
artists who are interested in its very beautiful phenomena.

In the present edition a new system of arrangement has
been adopted, which will, it is thought, prove generally
convenient. The first division contains the history of the
progress of Photogi'aphic discovery, in which the gi-eatest
care has been taken to insm-e exactness, and to give to
each discoverer the full merit of his labours: the date of
publication being taken in all cases where it could possibly
be obtained.

The second division embraces the science of the art ;
and it is a section to which the attention of the inteUigent
student in Photography is particularly directed, as he
may, by the knowledge he will thus acquire, relieve him-
self from many of the annoyances attendant upon frequent
failures, and probably advance himself in the path of new
discovery. The manipulatory details, given with all
necessary minuteness, ai-e included in the third division.

A very large portion of the present volume consists of
new matter — the Waxed Paper and the Collodion Pro-
cesses, amongst other sections, may be refen-ed to ; and

VIU PREFACE.

the appended chapter, On the Production of Pictui'es for
the Stereoscope, will be found to afford much useful in-
formation. The additional wood -cut illustrations which
have been introduced will be found to render important
aid to the amateur. I must acknowledge the ready as-
sistance, in the way of correction and information, which
has been afforded me by several of the most practised
photographic artists of the metropolis. To Mr. Claudet
and Mr. Home, in particular, I am indebted for many
novel and most useful details in the practice of the
Daguerreotype and the Collodion processes ; and to Mr.
Sandford, who has furnished me with many valuable
facts relating to the preparation of paper for Photography,
I am under many obligations. Those gentlemen who
have kindly hastened to supply me with the results of
their practice, which is duly acknowledged in the text,
will accept my thanks. In conclusion, let me hope that
the arrangement of the present volume will be found an
improvement upon the former editions, and that this
Manual may deserve to maintain its place as as a Text-
Book on Photography.

ROBERT HUNT.

London, Decemler, 1852.

CONTENTS

P.YPtT I.
HISTOEY OF DISCOVERIES IN PHOTOGRAPHY.

CHAPTEE I.

Early Eesearches on the Chemical Action of the Solar
Eats ........ 3

CHAPTEE II.

Heliography — The Process of M. Niepce . . .12

CHAPTEE III.

Mr. H. Fox Talbot's Photogenic Drawings, Calotype, &c.
Section I. Photogemc Drawing
„ II. Calotj^e .
„ III. Improvements in Calotype
„ IV. Pictures on Porcelain
„ V. Instantaneous Process

CHAPTEE IV.

Daguerreotype — The Discovery of M. Daguerre

Section I. The Original Process of M. Daguerre
„ II. Improvements in Daguerreotj^e

CHAPTEE V.

The Photographic Processes on Paper of Sir John Herschel 48
Section I. Cyanot^^^e . . . .48

II. C'hrysotyiDe . . . .54

III. Photographic Properties of Mercuiy . 57

IV. Ferro-Tai-trate of Silver . . .58
V. The Amphitype . . . .59

VI. The Colom-iniT Matter of Flowers . 6 1

rPE, &c.

lerre

CONTENTS.

CHAPTEE VI.

Miscellaneous Processes .....

Section I. Mr. Pontoii's process. (Bicliromate of
Potash) .
„ II. The Chromatype .
,, III. The Ferrotj-pe
„ IV. The Catalysotype
„ V. Ferrocyanide of Potassium
„ VI. The Fkiorot>T3e .
„ VI.*Bromide of Silver and Mercurial Vapour
„ VI r. Positive Photogi'aphs by One Process .
„ VIII. On the Application of the Daguerreotype

to Paper ....

„ IX. Salts of Grold as Photographic Agents .
„ X. The Influence of Chlorine and Iodine in
rendei'ing Wood sensitive to Light

CHAPTER VII.

Photographs on Glass Plates and Recent Improvements
Section I. Precipitates of Silver Salts

II.
III.

Albumen
Collodion

CHAPTER VIII.

Portraiture by the Daguerreotype .

CHAPTER IX.
General Summary of the History of Photography

69
71
73
75
79
80
81
82

85
89

92
92
95
95

101

PAET 11.
SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

CHAPTER I.

General Remarks on the Solar Agency producing Chemi-
cal Change .......

CHAPTER II.

Chemical Changes on Sensitive Preparations
Section I. Nitrate of Silver
„ II. Chloride of Silver
„ III. Iodide of Silver .
„ IV. Bromide of Silver
„ V. Miscellaneous Salts of Silver

CHAPTER III.

The Theory or the Daguerreotype .

107

114
114
115
118
118
120

124

CONTENTS. XI

CHAPTER IV.

On THE Photographic Registration OF Philosophical Instru-
ments, AND THE MEANS OF DETERMINING THE VARIATIONS OF Ac-

TiNic Power, and for Experiments on the Chemical Focus 1.39
Section I. Photographic Registration . . 139
„ II. Instniments for Measuring Actinic Varia-
tions, &c. .... 143
The Photographometer . . 143
The Focimeter . . .145
The Dynactinometer . . 146

CHAPTER V.
Thermography ....... 151

CHAPTER VI.

On the Possibility of Producing Photographs in their Na-
tural Colours ...... 158

CHAPTER VII.
On Lenses for the Photographic Camera . . . 163

?xrt: III.

PRACTICE OF PHOTOGRAPHY.

CHAPTER I.

Selection of Paper for Photographic Purposes . . 176

CHAPTER II.

On the Apparatus necessary for the first Practice of Pho-
tography on Paper . . . . . .183

CHAPTER III.

On the modes of Manipulation adopted in the Preparation
of Sensitive Papers . . . . . .193

Section I. Nitrate of Silver . . . 193

n. Chloride of SUver . . .194

„ III. Iodide of Silver . . . .197

CHAPTER IV.

On Fixing the Photogpjo'hic Pictures . . . 199

CHAPTER V.

The Calotype as now practised, and its Modifications . 206

Section I. Calotype on Paper . . . 206

„ II. Calotype on Gelatine or Albumen . 222

„ III. Calotype Process on Waxed Paper . 227

Xll CONTENTS.

CHAPTEE VI.

The Dagtterreottpe ...... 231

Section I. Daguerre's improved Manipulation . 231
„ II. Polishing the Plate . . . 235

„ III. To give the Sensitive Surface to the Plate 236
„ IV. To Develope the Image formed on the

Plate . . . . .243

„ V. Fixing the Daguerreotype Image . 245

„ VI. Simplification of the Daguerreoty|De . 248

CHAPTEE VII.
The Collodion Process ..... 252

CHAPTEE VIII.

The use of Albumen on Glass Plates, &c. . . . 276

Section I. Albumen on Glass . . . 276

„ II. Mr. Malone's Process . . .278

„ III. M. Martin's Process . . .280

„ IV. Miscellaneous Processes . . . 282
„ V. Positive Photogra^jhs from Etchings on

Glass Plates . . . .283

CHAPTEE IX.

On the Production of Positive Photographs by the use of
THE Salts of Iodine ...... 285

CHAPTEE X.

General Eemarks on the Use of the Camera Obscura —
The Photographic Pentagraph .... 298
Section I. Buildings, Statues, Landscapes, and Fo-
liage . . . . .298
„ II. Portraits from the Life . . . 300
„ III. Photographic Pentagraph . . 302

CHAPTEE XL
The Stereoscope ...... 304

CHAPTEE XII.

Photographic Engraving ..... 314

Appendix ....... 321

Photographic Patent Eight . . . .321

Correspondence of English and French Weights and
Measures . ... 324

Index ...... 325

PAET I.

HISTORY

DISCOVERIES IN PHOTOGEAPHY.

CHAPTER I.

EARLY RESEARCHES OX THE CHEMICAL ACTIOX OF THE SOLAR

RAYS.

It is instructive to trace tlie progi-ess of a tliscoveiy, fi-om tlie
fii-st indication of a truth, to the period of its fiill development,
and its application to jiiu-poses of ornament or utility. The
progress of discoveiy is ordinarily slow, and it often happens that
a gi-eat fact is allowed to lie dormant for years, or for ages, which,
when eventually revived, is found to render a fine interpretation
of some of Nature's haimonious phenomena, and to minister to
the wants or the pleasm-es of existence. Of this position, Photo-
gi-aphy is peculiarly illustrative.

The philosophers of antiquity appear to have had their atten-
tion excited by many of the more striking characters of light :
yet we have no account of their having observed any of its che-
mical influences, although its action on colom-ed bodies — deepen-
ing their colour in some cases, and dischai-giug it in others —
mus-t have been of every-day occiu-rence. The only facts which
they have recorded are, that some precious stones, particularly
the amethyst and the opal, lost their sparkle by prolonged
exposiu-e to the rays of the sun.

It has been stated — ^but on doubtful authority — that the jug-

4 HISTORY OF PHOTOGRAPHY.

glers of India were for many ages in possession of a secret
process, by wkicli tliey were enabled in a lirief space to copy the
profile of any individual Ijy the agency of light. However this
may have been, it does not appear that they know anything of
such a process in the present day.

The alchemists, amidst the multiplicity of theii" processes — in
their vain search for i\\e pliUosoplipys stone, and the elixir vitce —
stimibled upon a jDeculiar combination of silver with chlorine,
which they called horn-silver — as, by fasion, the white powder
they obtained by precipitation was convei'ted into a hom-like
substance. They observed that this horn-silver was blackened
by Kght, and as they taught that "silver only diflfered fi'om gold
in being mercmy intei-penetrated by the siilphiu-eous principle
of the sun's rays," they concluded that this change was the
commencement of the process by which their di-eams were to be
realized. Failing, however, to produce gold from hom-silver,
the fact of its blackening was simply recorded, and no fiu-ther
investigations were made into tliis remarkable ^^henomenon.

Petit, in 1722, noticed that solutions of nitrate of potash and
miu-iate of ammonia ciystallized more readily in the light than
they did in darkness.

The illustrious Scheele (1777), in his excellent Traite de
VAir et clu Feu, gave the world the first philosophical examina-
tion of this pecviliar change in a salt of silver, and proved the
dissimilar powers of the different rays of light in eflecting this
change. He writes, " It is well known that the solution of
silver in acid of nitre poiu-ed on a piece of chalk, and exposed to
the beams of the sun, gi'ows Ijlack. The light of the sun reflected
from a white wall has the same effect, but more slowly. Heat
without light has no efiect on this mixtiu-e." Again, " Fix a
glass prism at the window, and let the refi-acted sim-beams fall
on the floor; in this coloiu'ed light put a paper strewed with
luna cornua, and you will observe that this horn-silver gTOws
sooner black in the \"iolet I'ay than in any of the other i"ays."

Senebier repeated these experiments, and he states that he
foimd chloride of silver darkened in the violet ray in fifteen
seconds to a shade which requii-ed the action of the red ray for
twenty miuutes. He also experimented on the influence of light
in bleaching wax.

In the PMlosopMcal Transactions for 1798 will be found a
memoii' by Count Eumford, entitled, "An Inquiiy concerning
the Chemical Properties that have been attributed to Light." In
tills paper a number of experiments are brought forward to prove
that all the effects produced upon metallic solutions by bright
sunshine, can be obtained by a prolonged expos\u-e to a tempe-

EARLY EESEAECHES ON THE SOLAS RAYS. O

rature of SIO'^ Fahrenheit. We ai'e now, liowever, in a position
to show that the chemical eflects produced by raj's of dark heat,
are of a Tery diffei-ent character £i-om those iisually attributed to
light. Mr. Eobert Harrap, in a eommimication to Xicholson's
Journal in 1802, refuted the exj)eriment of Count Rumford,
showing that sevei-al salts of mercury were reduced by light
alone, and not heat.

In 1801, Ritter proved the existence of rays a considerable
distance beyond the visible spectrum, which had the property of
speedily blackening cliloride of silver. These researches excited
the attention of the scientific world : M. Berard, Seebeck, Ber-
thollet, and others, directed theii- attention to the pecidiar con-
dition of the difierent rays in relation to their kiniinous and
chemical influences; whUe Sii" William Hei"schel and Sii* Heniy
Englefield investigated the calorific powers of the colomed rays,
and were followed in these investigations by Seebeck and Wimsch.
Dr. WoUaston piu-sued and published an interesting series of
experiments on the decomposition efi"ected by light on gum-
guaiacum. He found that paper washed with a solution of this
giun in spirits of wine, had its yellow colour rapidly changed to
green by the violet rays, while the red rays had the property of
restoring the yellow hue. Sir Himiphiy Davy observed that
the puce-colom-ed oxide of lead became, when moistened, red
by exposme to the red i-ay, and black when exposed to the violet
ray ; and that the gi'een oxide of merciirj^, although not changed
by the most refrangible rays, speedily became red in the least
refrangible.

Morichini and Configliachi, M. Berard, and more recently, Mrs.
SomerAdlle made some cmious experiments on the power of the
"violet rays to induce magnetism in steel needles. Seebeck and
Berzelius investigated this involved subject: it has again and
again engaged the attention of experimentalists ; but to the
present time it must be regarded as an imsettled point, whether
magnetism can be induced in steel by the solai" rays.

A statement has been made by the French, to the efiect that
M. Charles was in possession of a process by which portraits
could be obtained by the agency of sunlight, producing a dark
impression upon a prepared surface. This is, however, exceed-
ingly doubtful, and even the Abbe Moigno, in hLs Repertoire,
states, that M. Charles never disclosed any fact connected with
this discovery, and that he left no evidence behind him of being
in possession of sixch a process.

In addition to the interesting facts already mentioned, it ■v\'ill
be insti-uctive to add a few particulars of other inquiries pui-
sued about the same time on various phenomena connecting

b HISTORY OF PHOTOGRAPHY.

themselves wdth tlie solar radiations. Although these do not
bear directly on Photography, they stand in very close relation
with it, and will seiwe possibly to indicate lines of reseai'ch which
have not been fixlly followed out.

Desmortiers in 1801 published a paper in. Gilhevt's Annals,
entitled, " Eecherches siu' la Decoloration spontanea du Blen de
Prusse," subsequently translated into Nicholson's Journal, in
which he has mentioned the influences of the solar rays in
producing the change. Bockman aboiit the same time observed
that the two ends of the spectnun acted differently on phos-
phorus; and Dr. Wollaston, examining the chemical action of
the rays of the spectiiim, ai-rived at nearly the same results as
Eitter. He states, " This and other effects usually attributed
to light are not in fact owing to any of the rays usually per-
ceived."

Wedgwood was certainly the fii*st person who made any
attempts to use the siuibeam for deHneating the objects it illu-
minated: it is therefore necessary that some more particular
account should be given of his processes. In 1802, he published
a paper in the Joiu-nal of the Royal Institution, under the fol-
lo\\-ing title : "An Account of a Method of Copying Paintings
upon Glass, and of making Profiles by the Agency of Light upon
Nitrate of Silver ; with Obseiwations by H. Davy." From this
communication, the following extracts, containing the more
impoi-tant indications, are made.

" White paper, or white leather, moistened with solution of
nitrate of silvei*, undergoes no change when kept in a dark place,
but, on being exposed to the daylight, it speedily changes colour,
and after passing throiigh different shades of gTay and brown,
becomes at length nearly black. The alterations of colour take
place more speedily in propoilion as the Hght is more intense.
In the dii-ect beam of the sun, two or thi-ee minutes ai'e sufficient
to produce the full effect ; in the shade several houi's are required;
and light transmitted throxigh different coloured glasses acts
upon it with different degTces of intensity. Thus, it is found
that red rays, or the common simbeams, passed thi'ough red glass,
have very little action upon it ; yellow and green are more
efficacious ; but blue and -violet light produce the most decided
ajid powerfiil effects.

" When the shadow of any figiu'e is thi'own upon the prepared
siu-face, the part concealed by it remains white, and the other
parts speedily become dark. For copying paiutmgs on glass,
the solution should be applied on leather; and in this case it is
more readily acted on than when paper is used. After the
colour has been once fix:ed on the leather or paper, it cannot be

EAJILT RESEARCHES ON THE SOLAR RAYS. 7

removed by the application of water, or -water and soap, and it
is in a liigh degree permanent. The copy of a painting, or the
ja'ofile, immediately after being taken, must be kept in an
'ibscui-e place; it may, indeed, be examined in the shade, but in
this case the exposure shoiild be only for a few minutes : by the
light of candles or lamps, as commonly employed, it is not
sensibly affected. No attempts that have been made to prevent
the uncoloured parts of the copy or profile fi*om being acted
upon by Hght, have as yet been successful. They have been
covered by a thin coating of fine vamish, but this has not
destroyed theii' suscej^tibility of becoming coloiu-ed ; and even
after repeated washings, sufficient of the active part of the
saline matter will adhere to the white parts of the leather or
paper to cause them to become dark when exposed to the rays
of the Sim. Besides the applications of this method of copying
that have just been mentioned, there are many others ; and it
will be useful for making delineations of all such objects as are
possessed of a textui'e partly opaque and partly transparent.
The woody fibres of leaves, and the wings of insects, may be
pretty accurately represented by means of it ; and in this case it
is only necessaiy to cause the dii-ect solar light to pass thi-ough
them, and to receive the shadows upon leather.

" The images foi-med by means of a camera obscm-a have been
found to be too faint to produce, in any moderate time, an effect
upon the nitrate of silver. To copy these images was the first
object of Mr. "Wedgwood in his researches on the subject ; and
for this purpose he fii'st used nitrate of silver, which was
mentioned to him by a friend as a substance very sensible to
the influence of light ; but all his numerous experiments as to
their primary end proved unsuccessful. In following these
processes, I have found that the images of small objects, pro-
duced by means of the solar microscope, may be copied without
difficulty on prepared paper. This will probably be a useftd
application of the method ; that it may be employed successfully,
however, it is necessaiy that the paper be placed at but a small
distance from the lens. {Davy.)

" In comparing the effects produced by light upon muriate of
silver with those produced iipon the nitrate, it seemed evident
that the muriate was the most susceptible, and both were more
readily acted upon when moist than when dry — a fact long ago
known. Even in the twilight, the colou.r of the moist mimate
of silver, spread upon paper, slowly changed from white to faint
violet ; though, imder similar circumstances, no immediate
alteration was produced upon the nitrate.

" Nothing but a method of preventing the imshaded parts of

8 HISTORY OF PHOTOGRAPHY.

the delineations fi^om being coloured by exposure to the day, is
wanting to render this process as useful as it is elegant."

An expeiiment on the dark rays of Ritter, by Dr. Young,
included in his Bakerian Lecture," is a very important one. Dr.
Young, after referi-iug to the experiments of Eitter and Wol-
laston, goes on to say : " In order to complete the comparison of
their properties (the chemical rays) with those of visible light,
I was desirous of examining the effect of their reflection from a
thin plate of air capable of producing the well-known rings of
colom-s. For this purpose I formed an image of the rings, by
means of the solar microscope, with the appai-atus which I have
described in the Journals of the Royal Institution ; and I threw
this image on paper dipped in a solution of nitrate of silver,
placed at the distance of about nine inches from the microscope.
In the course of an horn-, portions of three dark rings were very
distinctly visible, much smaller than the brightest rings of the
coloiu'ed image, and coinciding very nearly, in their dimensions,
with the rings of violet light that appeared upon the interpo-
sition of violet glass. I thought the dark rings were a little
smaller than the violet rings, but the difference was not suffi-
ciently great to be accurately ascei-tained : it might be as much
as ^Q or ^Q of the diameters, but not greater. It is the less
surpiising that the difference should be so small, as the dunen-
sions of the coloui-ed rings do not by any means vary at the
violet end of the spectrum so rapidly as at the red end. The
experiment in its present state is sufficient to complete the
analogy of the invisible with the visible rays, and to show that
they are equally liable to the general law, which is the principal
subject of this paper :" that is the interference of light.

M. B. G. Sage, in the Journal de Physique, 1802, mentions
a fact observed by him, that " the realgar which is sublimated at
the Solfaterra under the form of octahech-al crystals, known
under the name of ruby of arsenic, effloresces by the light ; "
and that ordinaiy native realgar from Japan changes to orpi-
ment by exposure to sunshine.

In 1806, Vogel exposed fat, carefully protected from the
influence of the air, to light, and found that it became in a short
time of a yellow colour, and acquii-ed a high degree of rancidity.
Vogel subsequently discovered that j^hosphorus and ammonia
exposed to the sim's rays were rapidly converted into phosj^hu-
retted hydrogen, and a black powder, phosphuret of ammonia.
He also noticed that the red rays produced no change on a
solution of corrosive sublimate (bichloride of merciuy) in ether,

* PldlosopMcal Transactions, 1804.

CHEMICAL ACTION OF THE SOLAR RAYS. 9

but that tlie blue rays rapidly decomposed it. Dr. Davy, much
more recently, repeated a siuiilar set of experiments to those of
Yogel. He found that corrosive sublimate was not changed by
exposui'e ; but that the Liquor Hydrary. Oxymur. of the old
London Pharmacopoeia quickly vmdei-went decomposition in the
sunshine, depositing calomel (chloride of merciuy).

Seebeck, in and subsequently to 1810, made some important
additions to omt knowledge of the influences of the solar
radiations, the most striking of his statements being the 2>'>'o-
duction of colour on chloride of silver ; the violet rays rendering
it bro-wTi, the blue producing a shade of blue, the yellow pre-
serving it white, and the red constantly gi'v'ing a red colour to
that salt. Sii' Hemy Englefield, about the same time, was
enabled to show that the phosphorescence of Canton's phos-
• phoiois was gi'eatly exalted by the blue rays.

Gay-Lussac and Thenard, being engaged in some investigations
on chlorine, on which elementary body Da^y was at the same
time experimenting, observed that hycb'ogen and chlorine did
not combine in the dark, but that they combined with gTeat
rapidity, and often with explosion, in the sunshine, and slowly
in ditfused light. Seebeck collected chlorine over hot water,
and, combining it with hych-ogen, placed difierent portions of it
in a yellowish-red beU glass and in a blue one. In the blue
glass combination took place immediately the mixtiu'e was
exposed to daylight ; but without explosion. The mixture in
the red glass was exposed for twenty minutes without any
change ; but it was fomid that the chlorine had undergone
some alteration, probably a similar one to that subsequently
noticed by Dr. Draper, who found that chlorine having been
exposed to simshine would unite "with hych-ogen in the dai-k. If
the gases were placed in a white glass and exj)osed to sunshine,
they exjDloded ; but if the gas had been preidously exposed to
the action of the solar i-adiations in the yellow-red glass, it
combined with hycb'Ogen in the wliite glass in the brightest
sunsliine ■v\dthout any explosion.

BerzeHus noticed some peculiar conditions in the action of
the solar rays upon the salts of gold ; and Fischer pursued some
researches on the influence of the prismatic rays on horn-silver.

The most important series of researches, however, were those
of Bei-ard in 1812, which were examined and reported on by
BerthoUet, Chaptal, and Biot. These philosophers write : " He
(M. Berard) found that the chemical intensity was gi-eatest at
the violet end of the spectrmn, and that it extended, as Bitter
and WoUaston had observed, a Kttle beyond that extremity.
When he left substances exposed for a certain time to the action

10 HISTOEY OF PHOTOGRAPHY.

of each ray, he observed sensible effects, though with an inten-
sity continually decreasing in the indigo and blue rays. Hence
we must consider it as extremely probpble, that if he had been
able to employ reactions still more sensible, he would have
observed analogous eifects, but still more feeble, even in the
other rays. To show clearly the great disproportion which exists
in this respect between the energies of different rays, M. Berard
concentrated, by means of a lens, all that part of the spectrimi
which extends from the green to the extreme violet ; and he con-
centrated, by means of another lens, all that portion which ex-
tends from the green to the extremitij of the red. This last
pencil formed a ivhite jwint so brilliant that the eyes toere scarcely
ahle to endure it ; yet the muriate of silver remained exposed more
than two hours to this brilliant point of light iviihout tmdergoing
any sensible alteration. On the other hand, when exposed to
the other pencil, which was much less bright and less hot, it was
blackened in less than six minutes." Tliis is the earliest inti-
mation we have of any hypothesis that the luminous and chemical
powers may be due to dissimilar agencies. On this, the Com-
missioners remark : — " If we wish to consider solar light as
composed of three distinct substances, one of which occasions
light, another heat, and the third chemical combinations, it will
follow that each of these substances is separable by the prism
into an infinity of different modifications, like light itself; since
we find by experiment, that each of the three properties, chemical,
colorHic, and caloriRc, is spread, though unequally, over a cer-
tain extent of the spectrum. Hence we must suj)pose, on that
hypothesis, that there exist three spectrums, one above another ;
namely, a calorific, a colorific, and a chemical sj)ectrum. We
miist likewise admit that each of the substances which compose
the three spectrums, and even each molecule of imequal refran-
gibility which constitutes these substances, is endowed, like the
molecules of visible light, with the property of being polarized
by reflection, and of escaping from reflection in the same positions
as the Imninous molecules, &c."

From the time when the difficulty of fixing the photographs
which they obtained, stopped the progress of DavyandWedg•^vood,
no discoveries were made until 1814, when M. Niepce, of Chalons,
on the Soane, appears to have first dhected his attention to
the production of pictures by light.

It does not seem that his early attempts were very successful ;
and, after pursuing the subject alone for ten years, he, from an
accidental disclosure, became acquainted with M. Daguerre, who
had been for some time endeavouring, by varioiis chemical pro-
cesses, to fix the images obtained with the camera obscura. In

CHEiriCAL ACTION OF THE SOLAR EAYS. 11

December, 1829, a deed of copartneiy Tvas executed between M.
Xiepce and M. DagneiTe, for m\itually investigating the subject.

M. Niepce had named his discoA'eiy Heliogi-aphy.* In 1827,
he presented a paper to the Royal Society of London, on the
subject ; but as he kept his process a secret, it conld not, agi-ee-
ably with one of theii* laws, be received by that body. This
memoir was accompanied with several heliographs on metal
(plated copper and pewter) and on glass plates ; which were
aftei'wards distributed in the collections of the cxmons, some of
them still existing in the possession of Mr. Robert Brown, of
the British Museum. They pi-ove M. Xiepce to have been then
acquainted with a method of fonning pictiu-es, by which the
lights, semi-tints, and shadows, were represented as in nature ;
and he had also succeeded in rendei-ing his Heliographs, when
once formed, imjieiwious to the fiuiher effects of the solar rays.
Some of these specimens appear in a state of advanced etchings ;
but this was accomplished by a process similar to that piu-sued
in common etchings.

The ease with which nitric acid could be applied to etch
these Heliographic plates will be apparent when the process of
obtaining the pictures is undei-stood.

* Sun-drawing : a more appropriate name than Photography, since it remains
a problem (1853) of diflSeult solution, whether Light, or some agent associated
with Light, is active in producing the chemical changes we ai-e considering.

CHAPTER II.

HELIOGEAPHY. THE PROCESS OF M. NIEPCE.

M. NiEPCE "was tlie first inquirer who appears to have produced
permanent pictm-es by the influence of the sun's rays. This
process — Heliography — is in many resj^ects peculiax*, which
renders it necessary, although his preparation was only acted on
by an exposiu'e of many hoiu's to fiill sunshine, to give a parti-
cidar account of it ; the more so, as some points of considerable
interest requii-e fiu-ther elucidation.

The substance employed by M. Niepce was asphaltum, or
bitmnen of Judea, He thus directs its preparation : — " I about
half fill a wine-glass with this pulverized bitumen ; I pour upon
it, drop by ch-op, the essential oil of lavender,* imtil the bitumen
is completely satiu'ated. I afterwards add as much more of the
essential oil as causes the whole to stand about thi-ee lines above
the mixtiu'e, which is then covered and submitted to a gentle
heat until the essential oil is fully impregnated with the colouring
matter of the bitumen. If this varnish is not of the requii-ed
consistency, it is to be allowed to evaporate slowly, without
heat, in a shallow dish, care being taken to protect it from
moisture, by which it is injured, and at last decomposed. In
winter, or during rainy weather, the precaution is doubly neces-
sary. A tablet of plated silver, or well cleaned and wai-m glass,
is to be highly jiolished, on which a thin coating of the varnish
is to be appKed cold, with a light roll of very soft skin : this will
impart to it a fine vermilion coloiu-, and cover it with a veiy thin
and equal coating. The plate is then placed upon heated iron,
which is "\vrapped round "svith several folds of paper, fi-om which
by this method all moisture had been pre"viously expelled.
When the varnish has ceased to simmer, the plate is withdi'awn
fi'om the heat, and left to cool and dry in a gentle temperature,
and protected from a damp atmosphere. In this part of the
operation a light disk of metal, with a handle in the centre,

* The English oil of lavender is too expensive for this purpose. An article
sold as the French oil of lavender, redrawn, is very much cheaper, and answers
in every respect as well, if not better.

HELIOGRAPHY. THE PROCESS OF M. NIEPCE. 13

slioiild be held before tbe mouth, in order to condense the mois-
ture of the breath."

The plate thus prepared is now in a fit state for use, and may-
be immediately fixed in the correct focus of the camera. After
it has been exposed a sufficient length of time for receiving the
impression, a very faint outhne alone is visible. The next opera-
tion is to bring out the hidden pictvu'e, which is accomplished by
a solvent.

This solvent must be carefully adapted to the piu-jjoses for
"which it is designed : it is difficult to fix vdih cei-taiaty the pro-
portions of its components, but in all cases it is better that it
be too weak than too strong ; in the former case the imao-e
does not come out strongly ; in the latter it is comjjletely de-
stroyed. The solution is prepared of one part — not by weight, I '
but volimie — of the essential oil of lavender, poiu-ed ujwn ten ■
parts, by measm-e also, of oil of white petroleum. The mixture \
which is fii'st milky, becomes clear ra two or three days. This •■
compound will act until it becomes satiu-ated with the asphaltimi,
which state is readily distinguished by an opaque apjiearance, and
dark brown colour. A tin vessel somewhat larger than the pho-
tographic tablet, and one inch deep, must be provided. This is
to have as much of the solvent in it as will cover the plate. The
tablet is plimged into the solution, and the operator, obseiwing
it by reflected light, begins to see the images of the objects to
which it has been exposed slowly unfolding then.' forms, though
still veiled by the gi-adually darkening supernatant fluid. The
plate is then Hfted out, and held in a vertical position, till as
much as possible of the solvent has been allowed to drop away.
When the dropping has ceased, we proceed to the last, and not
the least important operation, of washing the plate.

This is performed by carefully placing the tablet upon a

board, b, fixed at a large
angle, in the trough A a, the
sujjports being joined to it
by hinges, to admit of the
necessary changes of inclina-
tion, under difiercnt circum-
stances : two small blocks,
not thicker than the tablet,
are fixed on the board, on
which the plate rests. Water
must now be slowly povu'ed
'" upon the upper pai-t of the

board, and allowed to flow evenly over the sui-face of the picture.
The descending stream clears away aU the solvent that may yet

14 HISTORY OF PHOTOGRAPHY.

adhere to the varnish. The plate is now to be di'ied with gi"eat
care by a gentle eTaporation : to preser^'e the j)ictm'e, it is
requisite to cover it up frcm the action of light, and protect it
from hmnidity.

The vai-nish may be applied indifferently to metals, stone, or
glass; but M. Nieijce prefers cojDper plated with silver. To
take copies of engTa'v'ings, a small quantity of wax is dissolved
in essential oU of lavender, and added to the vamish ah-eady
described : the engTa\TJig, first varnished over the back, is placed
on the siuface of the prepared tablet, face towards it, and then
exposed to the action of the light. In the camera obscui'a an
exposiu-e of from six to eight hours, varying with the intensity
of light, is requii'ed ; while fi'om foiu' to six hours is necessaiy
to produce a copy of an engi'aving. The pictiu-e, in the fij^t
instance, is represented by the conti"ast between the polished
sUver and the varnish coating. The discoverer aftei-wards
adopted a plan of darkening the sUver by iodine, which appears
to have led the way to Daguerre's beautiful process. To darken
the tablet, it was placed in a box in which some iodine was
strewed, and watched until the best effect was produced. The
varnish was afterwards removed by spiiit of wine.

Of the use of glass plates M. Kiepce thus speaks : — " Two
exjjeriments in landscape upon glass, by means of the camera,
gave me results wliich, althoi^gh imperfect, appear deserving of
notice, because this vai'iety of application may be brought more
easily to perfection, and in the end become a more interesting
department of heliogTaphy.

" In one of these trials the light acted in such a way that the
varnish was removed in proportion to the intensity with which
the light had acted, and the pictui-e exhibited a more marked
gradation of tone ; so that, viewed by transmitted light, the
landscape produced, to a certain extent, the well-known effects
of the diorama.

" In the second trial, on the contrary, the action of the
Imninous fluid having been more intense, the parts acted upon
by the strongest lights, not having been attacked by the solvent,
remained transparent ; the difference of tone resulted from the
relative thickness of the coatings of varnish.

" If this landscape is viewed by reflection in a miiTor, on the
varnished side, and at a certain angle, the effect is remarkably
striking ; while, seen by transmitted light, it is confused and
shapeless : but, what is equally surprising, in this position the
mimic tracery seems to affect the local colour of the objects."

A statement that M. I^iepce was enabled to engi-ave by light,
went the round of the press ; but this does not a^jpear to have

HELIOGRAPH Y. THE PROCESS OF M. NIEPCE. 15

been the case. All tliat the author of heHogi-aphy effected, was
the etching of the plate, after it had undergone its varioxxs
processes, and the diuwing was completed by the action of nitric
acid in the usual manner : the parts of the coj)perplate pro-
tected by the varnish remained, of course, unacted on, whilst
the other pai-ts were rapidly attacked by the acid. Niepce re-
marks that his process cannot be used dviring the winter season,
as the cold and moistiu-e render the varnish brittle, and detach
it from the glass or metal.

M. Niepce afterwards used a more unctuous varnish com-
posed of bitumen of Judea, dissolved in animal oil of Dijypel.
This composition is of much greater tenacity and higher colour
than the formei", and, after being apj^lied, it can immediately be
submitted to the action of light, which appears to render it solid
more quickly, from the greater volatility of the animal oil. M.
Daguerre remai^ks, that this very propei'ty diminishes still
fm-ther the resources of the process as respects the lights of the
drawings thus obtained. These processes of M. Niepce were
much improved by M. Daguerre, who makes the following
remarks on the subject : —

" The substance which should be used in preference to bitu-
men is the residuum obtained by evaporating the essential oil of
lavender, which is to be dissolved in alcohol, and applied in an
extremely thin wash. Although all bituminous and resinous
substances are, without any exception, endowed with the same
property — that of being affected by light — the i^refei-ence ought
to be given to those which are the most unctuous, because they
give greater finnness to the di-awings. Several essential oils
lose tliis character when they are exposed to too strong a
heat.

" It is not, however, from the ease with which it is decomposed,
that we are to prefer the essential oil of lavender. There ai-e,
for instance, the resins, which, being dissolved in alcohol, and
spread upon glass or metal, leave, by the evaporation of the
spiiit, a very wliite and infinitely sensitive coating. But this
greater sensibility to light, caused by a quicker oxidation,
renders also the images obtained miich more liable to injmy
from the agent by which they were created. They grow faint,
and disappear altogether, when exposed but for a few months
to the sun. The residuum of the essential oil of lavender is
more effectually fixed, but even this is not altogether unin-
fluenced by the eroding effects of a direct exposure to the
sun's light.

" The essence is evaporated in a shallow tlish by heat, till the

16 HISTORY OF PHOTOGRAPHY.

resinous residuum acquii-es such a consistency, tliat when cold,
it rings on being struck vntli the poiut of a knife, and flies ofi"
in pieces when separated from the dish. A small quantity of
this material is aftenvards to be dissolved in alcohol or ether ;
the solution formed should be transparent, and of a lemon-yellow
coloiu'. The clearer the solution, the more delicate will be the
coating on the plate : it must not, however, be too thin, because
it would not thicken or spread out into a white coat ; indispen-
sable requisites for obtaining good effects in photogi-aphic
designs. The use of the alcohol or ether is to facilitate the
application of the resin under a very attenuated form, the spmt
being entirely evaporated before the light effects its delineations
on the tablet. In order to obtain gTeater vigour, the metal
ought to have an exquisite polish. There is more charm about
sketches taken on glass plates, and, above all, much greater
delicacy.

"Before commencing operations, the experimenter must care-
fully clean his glass or metal plate. For this purpose, emeiy,
reduced to an impalpable powder, mixed wdth alcohol, may be
used ; applying it by means of cotton-wool : but this part of the
process must always be concluded by diy-poHshing, that no trace
of moistui'e may remain on the tablet. The plate of metal or
glass being thiis prepared, in order to supply the wash or coating,
it is held in one hand, and with the other, the solution is to be
poured over it from a flask or bottle having a wide mouth, so
that it may flow i-apidly, and cover the whole siu-face. It is at first
necessary to hold the plate a little inclined j but as soon as the
solution is poiu'ed on, and has ceased to flow freely, it is raised
perpendicrdarly. The finger is then passed behind and below
the plate, in order to di'aw ofi" a poi'tion of the Hquid, which, tend-
ing always to ascend, would double the thickness of the covering :
the finger must be Aviped each time, and be passed very rapidly
along the whole leng-th of the plate from below, and on the side
opposite the coating. "When the liquid has ceased to iim, the
plate is chied in the dark. The coating being well chied, it is
to be placed in the camera obscm-a. The time required to pro-
cui'e a photogi-aphic copy of a landscape is from seven to eight
hom's ; but single monuments strongly illuminated by the sun,
or very bright in themselves, are copied in about three hours.

" When operating on glass it is necessaiy, in order to increase
the light, to place the plate upon a piece of paper, with great
care that the connection is perfect over every part, as, other-
wise, confusion is produced in the design by imperfect reflection.

" It frequently happens that when the plate is removed from

HELIOGRAPHT. THE PROCESS OF M. ^^EPCE. 17

tlie camera, there is no trace of any image upon its surface : it
is therefore necessary to use another process to biing out the
hidden design.

" To do this, pro-\-ide a tin vessel, larger than the tablet,
having all round a ledge or border 50 millimeters (2 English
inches) in depth. Let this be three-quai-tei"s full of the oil of
petroleum ; fix your tablet by the back to a piece of wood which
completely covers the vessel, and place it so that the tablet, face
downwards, is over but not touching the oil. The vapoiu" of the
petroleum penetrates the coating of the plate in those parts on
which the light has acted feebly ; that is, in the portions which
corresjiond to the shadows, imparting to them a transparency
as if nothing were there. On the contrary the points of the
resinous coating, on which light has acted, having been rendered
impei-Aious to the vapoiu', remain imchanged.

"The design must be examined fi'om time to time, and with-
drawn as soon as a vigorous efiect is obtained. By lU'ging the
action too far, even the strongest lights wiU be attacked by the
vapoTU', and disappear, to the destmction of the piece. The pic-
ture, when finished, is to be protected from the dust by being
kept covered ^vith a glass, which also protects the silver plate
from tarnishing."

It may perhaps appear to some that I have needlessly given
the particulars of a process, now sxiperseded by others, possessing
the most infinite sensibility ; producing in a few minutes a better
efiect than was obtained by the heliographic process in several
hours. There are, however, so many cra-ious facts connected
■with the action of light on these resins, that no treatise on pho-
tography could be considered complete ^^'ithout some description
of them ; and this process is now revived ■with a view to the pro-
duction of etchings directly fi'om nature.

M. Daguerre remarks, that numerous experiments tried by
him "w-ith these resinous preparations of M. Niepce, prove that
light cannot fall upon a body without leaA^ing traces of decompo-
sition; and they also demonstrate that these bodies possess the
power of renewing in darkness, what has been lost by luminous
action, pro^dded total decomposition has not been eflected. This
heKographic process must be regarded as the earhest successful
attempt at fixing on solid tablets the images of the camera ob-
scura, and at developing a dormant image.

CHAPTER III.

MR. H. FOX TALBOT's PHOTOGENIC DRAWINGS, CALOTYPE, ETC.

Section I. — Photogenic Drawing,

On tlie 31st of Jamiaiy, 1839, six montlis prior to the publica-
tion of M. Dagiiei"re's process, Mr. Fox Talbot communicated to
the Royal Society his photographic discoveries, and in Febi-uary
he gave to the world an accoimt of the process he had devised
for preparing a sensitive paper for photographic ch'awings. In
the memoir read before the Royal Society, he states — " In the
spring of 1834, 1 began to put in practice a method which I had
devised some time pre\"iously, for employing to pm-poses of utility,
the very cui'ious property which has been long known to che-
mists to be possessed by the nitrate of silver, namely, its dis-
coloration when exposed to the violet rays of light." Prom
this it appears that the EngHsh philosopher had pursued his
researches ignorant of what had been done by others on the
Continent. It is not necessaiy to enlarge, in this place, on the
merits of the two discoveries of Talbot and Daguerre; but it
may be as well to show the kind of sensitiveness to which Mr.
Talbot had aiTived at this early period, in his pre^jarations ;
wliich will be best done by a brief extract from his own com-
mmiication.

"It is so natural," says this experimentalist, "to associate the
idea of labour with great complexity and elaborate detail of exe-
cution, that one is more struck at seeing the thousand florets of
an Ayrostis depicted with all its capillaiy branchlets, (and so
accvu-ately, that none of all this multitude shall want its little
bivalve calyx, requii'ing to be examined through a lens), than
one is by the pictm-e of the large and simple leaf of an oak or a
chestnut. But in trath the difficulty is in both cases the same.
The one of these takes no more time to execute than the other;
for the object which woidd take the most skilfid artist days or
weeks of labour to trace or to copy, is eflected by the boundless
powers of natural chemistry in the space of a few seconds." And
again, "to give some more definite idea of the rapidity of the
process, I will state, that after various trials, the nearest valua-

Talbot's photogenic drawing. 19"

tion wMch I could make of tlie time necessary for obtaining tlie
picture of an object, so as to have pretty distinct outlines, when
I employed the full sunshine, was half a second.'''' This is to be
imderstood of the paper then used by Mr. Talbot for taking ob-
jects by means of the solar microscope.

In the Philoso'phical Magazine, Mr. Fox- Talbot published the
first accoimt of his Photogenic exjieriments. This term was in-
troduced by this gentleman : and his experiments cannot be
better described than in liis own words. " In order to make
what may be called ordinary photogenic paper, I select, in the
first place, paper of a good fijrm quality and smooth sm-face. I do
not know that anything answers better than supei'fine wi'iting-
paper. I dip it iuto a weak solution of common salt and wipe it
dry, by which the salt is unifoi-mly distributed throughout its
substance. I then S2:)read a solution of nitrate of silver on one
surface only, and diy it at the fire. The solution should not
be satui'ated, but six or eight times diluted with watei*. When
dry, the paper is fit for use.

" I have found by experiment that there is a certain propor-
tion between the quantity of salt and that of the solution of
silver which answers best, and gives the maximum effect. If
the strength of the salt is augmented beyond this point, the
efiect diminishes, and, in cei-tain cases, becomes exceedingly
small.

" This paper, if properly made, is veiy useful for aU photoge-
nic pm-poses. For example, nothing can be more perfect than
the images it gives of leaves and flowei's, especially with a sum-
mer smi, — the light passing tlu'ough the leaves, delineates every
ramification of their nerves.

" Now, suppose we take a sheet thus pre})ared, and wash it with
a saturated solution of salt, and then diy it. We shall find (es-
pecially if the paper is kept some weeks befoi'e the trial is made)
that its sensibility is greatly diminished, and, in some cases,
seems quite extinct. But if it is again washed with a liberal
quantity of the solution of silver, it becomes again sensible to
light, and even more so than it was at first. In tliis way, by
alternately washing the paper with salt and silver, and (hying it
between times, I have succeeded in increasing its sensibility to
the degree that is requisite for receiving the images of the ca-
mera obscm'a.

"In conducting this operation, it wUl be fovmd that the re-
sults are sometimes more and sometimes less satisfactory in con-
sequence of small and accidental variations in the proportions
employed. It happens sometimes that the chloride of silver is
disposed to darken of itself without any exposm-e to light : this

20 HISTORY OF PHOTOGRAPHY.

shows that the attempt to give it sensibility has been canied too
far. The object is to approach to this condition as near as pos-
sible without reaching it, so that the substance may be in a state
ready to yield to the slightest extraneous force, such as the
feeble impact of the violet rays when much attenuated. Having,
therefore, prepared a number of sheets of paper with chemical
proportions sbghtly diiferent from one another, let a piece be
cut from each, and, liaving been duly mai'ked or nimibered, let
them be placed, side by side, in a very weak diffused light for a
quarter of an hour. Then, if any one of them, as fr-equently hap-
pens, exhibits a marked advantage over its competitoi-s, I select
the jmper which beai's the corresponding number to be placed in
the camera obsciu-a."

The increased sensitiveness given to paper by alternate ablu-
tions of saline and argentine washes, the striking diiferences of
effect produced by accidental variations of the proportions in
which the chemical ingi'edients are applied, and the spontaneous
change which takes place, even in the dark, on the more sensitive
varieties of the paper, are aU subjects of gi'eat intei'est, which
demand fru'ther investigation than they have ever yet received,
and which, if followed out, promise some most important expla-
nations of chemical phenomena at present involved in uncer-
tainty, particularly those which appear to show the influence of
time, an element not sufficiently taken into account, in ovei'-
coming the weaker affijiities. Few fields of research promise a
gi-eater measiu'e of reward than these ; already the art of making
sun-pictui-es has led to many very important physical discoveries,
but most of the phenomena are yet involved in obscurity.

Section II. — The Calotype.

Although, in order of date, the investigations of Sfr John
Hei-schel and others have a priority over those particular expe-
riments of Mr. Talbot's which resulted in the discoveiy of his
very beautiful process, the calotype, yet to avoid confusion, it is
thought advisable to group together the discoveries of each in-
vestigator, where thLs is practicable, in our historical division.

The earUest productions of Mr. Talbot were simply such pre-
parations as those already described, in which a chloride of silver
was formed on the siu-face of the paper, with some nitrate of
silver in excess. These need not be any fru'ther described than
they have afready been.

Eai-ly in 1810, di-awings on paper were handed about in the
scientific cfrcles of London and of Paris, which were a great ad-

THE CALOTYPE. 21

vance upon anytMng which had been previously done. These
were the residts of a new process discovered by Mr. Talbot, and
then attracted so much attention, that INI. Biot made them the
subject of a communication to the Academy of Sciences in Paris.
His remarks are printed in extenso in the Coviptes Rendus,
fi'om which the following passages ai'e translated, as they bear
particularly on many of the defects which still continue to prove an-
noyances in the photogi-ajihic process to which they have reference.

Many of the remarks have a peculiar value fi'om the sugges-
tions they contain, and they are worthy of record as marking
the period when the French were first made acquainted with
the i^i'ocesses on paper, as 23i'actised in England. Some dispo-
sition has been shown on the part of several continental photo-
graphers to claim originality for processes published in England
many years before their o\m were devised, and which singularly
resemble them. After remarking that many very important
physical facts were being developed by the study of photogi-aphy,
M. Biot continues : —

"It is not to be expected that photogenic drawings, made on
paper, can ever equal the clearness and fineness of those obtained
on level and polished metallic plates. The texture of jmper, its
superficial roughnesses, the dej)th of the imbibitions, and the
capillaiy communication established between the various un-
equally marked parts of its surface, are so many obstacles to
absolute strictness of delineation, as well as to the regular
gradation of tints in the camera obscura ; and the influence of
these obstacles is greater when the chemical operation is slowly
cari'ied on. But when there is no pretence or necessity for
submitting to the delicacies of ai-t — when it is requii-ed, for
example, to copy rare manuscripts faithfully — if we have jiapei-s
which are very susceptible of receiving impressions in the cameiu
obsciu-a, they will suffice perfectly ; particularly when they
present, Kke those of Mr Talbot, the facility of immediately
procuring copies of the primitive ch-awing. It will, therefore,
doubtless be found more commodiou.s, and often even more
practicable, to put four or five hundred dra^\-ings in a portfolio,
than to carry about a similar pro'vision of metallic plates with
tliose indispensable protectors, squares of glass, to cover them.
Attempts are being made, at this time, to fix the images pro-
duced by the Daguerreotype — perfect 2)rints, it is tme, but which
are as light as the vapoxu- fi.-om which they are produced ; and,
indeed, to bring a voluminous collection of these fragile products
through the accidents incident to long, and sometimes perilous
voyages, is a task requiring no ordinary care. But whoever has
attentively studied the combination of physical conditions whence

22 HISTORY OF PHOTOGRAPHY.

these admirable images result, will find it very difficidt — T am far
from saying impossible — to fix them, 'without destroying, or at
least without essentially altering, the causes which produce their
charm ; and then, for the purposes which 1 have mentioned,
jDapers very suscej^tible of impression would still have the advan-
tages of being less troiiblesome in removal from place to place,
as also of more easy presei'vation.

" The utility of sensitive papers for copying texts was a natural
consequence of the clearness of the copies of engi*avings which
Mr. Talbot had ah-eady obtained by application, and which were
presented to the Academy. He has inckided others among those
just sent : there are also added specimens of this especial appli-
cation, consisting of copies of a Hebrew psalm, of a Pei*sian
Gazette, and of an old Latin chart of the year 1279. Our
brethi'en of the AcoxUmie des £elles Lettres, to \^hom. I exhibited
these impressions, were pleased to remai'k the fidelity of the
charactei-s, and theii- clearness, by which they ai-e rendered as
legible as the original text. Doubtless an old manuscript may
be copied more C{uickly and more accui'ately by this means than
by hand, even when the language in which it is written is
undei*stood. However, we must stoj^ here. These copies are
obtained by application : we must be enabled to obtain them by
immediate radiation in the camera obsciu-a. It is the only
means of extending the process to papyrus and other ojijaque
manuscripts, or which are not snfficiently transparent for radia-
tion to traverse them. Moreover, the application of leaves is
very difiicult when they are bound up in a volume, and cannot
be detached from one another.

"But this important extension will require much physical per-
fecting, towards which experunenters should direct their efibrts.
The fii'st thing will be to augment the sensibility of the paper as
much as possible, in order that the capillaiy communication of
its various parts may not have sufiicient time to deteriorate the
effects of the local and immediate action of the radiation. I
should be led to believe that it is principally to this kind of
commimication shoidd be attributed the fact remarked by Mr.
Talbot, that, in experiments by application, it is more difficult
to copy clearly a tissue of black lace spread on a w-hite ground,
than w'hite lace on a black ground ; two cases of which he here
gives examples. But another more liidden and more general
difficiilty seems to me to proceed from the unequal faculty of
vaiious substances for reflecting the radiations which strike
them, and perhaps from then* aptitude for making them undergo
physical modifications. For example, you wish to copy by radia-
tion in the camera obscm-a a pictiu'e painted on canvas, wood,

THE CALOTYPE. 23

or porcelain : the different colonring substances employed by
the painter are placed and distributed in sucli a manner that
each of them absorbs certain portions of the total incidental light,
and reflects especially towai'ds your eye the complementaiy por-
tions, wherein predominate the rays proper to foi-m the tint
of which it would give you the sensation. ■ But the chemically
active reagent which the same pai-ts of the picture receive and
reflect, is distinct from the light which affects your retina. In
order that the chemical effect which it produces on the sensible
paper, or on M. Daguen-e's layer of iodine, may present, in light
or in shade, the equivalent of the colom-ed pai-ts, it is requisite
— 1st, that this reflected radiation be chemically active ; 2d,
that the energy of its action be proportional to the intensity of
illumination operated in the eye by the portion of himinous
radiation reflected fi-om the same point of the picture. Now this
latter concordance certainly should not be fulfilled in an equal
degree, by the varions coloiu'ing matters, which affect the eye in
the same manner, and which the painter may substitute for one
another in his work. Substances of the same tint may present,
in the quantity, or the nature of the invisible radiations which
they reflect, as many diversities, or diversities of the same order,
as sxibstances of a different tint present relative to light : in-
versely they may be similar in then- property of reflecting
chemical radiations, when they are dissimilar to the eye : so that
the differences of tint which they presented in the pictm-e made
for the eye, will disappeai- in the chemical pictui-e, and will be
confused in it in a shade, or of an imiibrm whiteness. These
are the difficulties generally inherent in the formation of
chemical pictm-es ; and they show, I think, evidently, the illu-
sion of the experimenters who hope to reconcile, not only the
intensity, but the tints of the chemical impressions produced
by radiations, with the coloiu-s of the objects li-om which these
radiations emanate. However, the distant or near relations of
these two species of phenomena are veiy cm-ious to study, not
only as regards the photogenic art, smce that name has, very
improijerly, been given it, but likewise as regards experimental
physics. I doubt not that examples of these peculiarities may be
remarked in the images of natm-al objects and coloui'ed pictures
executed by the Daguerreotype ; but veiy apparent ones may be
seen among Mr. Talbot's present impressions. Thus, some of them
represent white procelain vases, coloured shells, a candlestick (of
metal) with its taper, a stand of white hyacinths. The whole of
these objects ai-e felt and perceived veiy well in then- chemical
image ; but the pai-ts which reflect the purely white light, probably
also the radiations of eveiy kind, are, relatively to the othei-s, in

HISTOET OF PHOTOGRAPHY.

an exaggerated proportion of iUiiniination, which, it seems to me,
must result, paitially, from the capillaiy conmiunication diu-ing
the continuance of the action ; so that the inequaKty would he
less if the paper M-ere more sensitive or more rapidly acted on.
In the hyacinth, the stalk and the green leaves have produced
scarcely a faint trace of theii' configii ration ; hut they are strongly
defined, especially in the parts of the outline, where more or less
perfect speculai" reflection takes place. The points of the candle-
stick (metallic), where this reflection occui-red, are copied by white
stains locally applied, and which deteriorate the effect of the
whole hy theii* disproportion. But tliis is seen especially in a
pictiu-e by Correggio, the frame of which was veiy vividly copied,
whilst the figiu-e on the canvas was hai-dly pei'ceptible. This
disproportion of lustre in the reproduction of some white parts,
especially when they ai-e dull and consequently veiy i-adiating,
is sensible in certain pai-ts of views taken by Mr. Talbot, to the
point of rendering dilficult the interpretation of the object to
which they belong. However, these views are very satisfactory,
as being obtained on paper, in the present season. Moreover,
by an advantage pecidiar to the chemical preparation which ]\Ir,
Talbot uses, it appeal's that the operations once completed, the
di'awiugs ai-e no longer alterable by radiation, even acting with
much energy. Indeed, we have here, as an example, four proofs
of the same \^ew of INIr. Talbot's house, with an identical dispo-
sition of lights and shades ; so that some, at least, if not thi-ee
out of foiu', must have been procm-ed by superposition. Mr.
Talbot is right in representing this property of reproduction as
an especial advantage of his process, and it would indeed be very
useful in voyages. I have exposed one of these di-awings to the
action of the sun, not very powei-fid, it is tiiie, for several hours,
and I have not perceived the slightest alteration in the lights.
I think I understand that, in Mr. Talbot's opinion, the shades
alone ai-e streng-thened xmder this influence. According to what
I have just said, it should be expected that the trimnph of this
process, as of every other photogenic reproduction, would take
place with objects of white and dull plaster. Indeed, Mr.
Talbot's parcel contains eight copies of busts and statues ; six
of which chiefly, of various forms and sizes, present very remark-
able results, especially taking into consideration the unfavoui-able
season at which they were produced. Truly, there is not found
in them the strict perfection of trace, nor the admirable grada-
tions of lights and shades, which constitiite the charm of M.
Daguen-e s impressions ; and I again repeat it, that my expres-
sions may not be exaggerated. But I also repeat, that repre-
sentations on sensitive papers must be considered as principally

THE CALOTYPE. 25

applicable to a different object, wliicli does not impose sucli
strict conditions of art, reqiiii-iug only faithful images, siifficiently
clear in tbeii' details to be readily recognized, and which, more-
over, beiag obtained with rapidity, by an easy manipulation, may
be kept with veiy little care, comprised in great number in a
small compass, and moved from place to place with facility. ]\Ir.
Talbot's papers abeady present many of these essential qualities,
with the advantage of being able to fimiish niimerons copies
immediately. His efforts, and those of others occupied with the
same subject, will conclude by adding to them eveiy thing which
may be desirable, provided that expectation, or the pretension
of a perfection of art physically incompatible with operations on
paper, do not give a false direction to their endeavour's. How-
ever, not to appear to despair too much of the futm-e, I may add
that the height of success would consist in discovering a sub-
stance very susceptible of receiving impressions, which might be
apphed on a papp-aceous leaf without penetrating deep into it,
and which might, however, be fixed in it after the operation, as
in Mr. Talbot's impressions. It does not seem necessaiy even
that the fii'st impression thus rapidly obtained should copy the
lights and shades in their proper jjlaces, provided that its trans-
pai-ency and fixedness were such that we might deduce them
from the application of copies whereia the invei'sion would be
corrected. And perhaps, by this decomposition of the problem
into two successive operations, one of the best ways is opened
by which it may be resolved."

Numerous improvements have been introduced, but still physi-
cal difficulties, such as those which he has indicated, surroimd the
photogi-aphic processes, and even where INI. Biot has proved
"WTong in his conjectm^es, his remarks form a curious chapter in
the histoiy of the art.

Mr. Talbot's description of his process, the patent for which
is dated 1841, is as follows : — *

Take a sheet of the best writing-paper, having a smooth sur-
face, and a close and even texture. The water-mark, if any,
should be cut off, lest it should injure the appearance of the
picture. Dissolve 100 gi-ains of crystallized nitrate of silver in
six oimces of distilled water. "Wash the paper with this solution
with a soft bnish on one side, and put a mark on that side,
whereby to know it again. Diy the paper cautiously at a dis-
tance fi'om the fii-e, or else let it diy spontaneously in a dark

* Mr. Talbot, by a letter in the Times of Atigust 13, 1852, gives to the
public the right of using any of his patents for any purpose not involving the
production of portraits from the life. The letter is printed in the Appendix.

26 HISTORY OF PHOTOGRAPHY.

room. When dry, or nearly so, dip it into a solution of iodide
of potassium, containing 500 gi-aias of that salt dissolved in one
pint of water, and let it stay two or three minutes in the sohi-
tion. Then dip the paper into a vessel of water, diy it lightly
with blotting-paper, and finish diying it at a fire, which will not
injiu-e it even if held pretty near : or else it may be left to chy
spontaneously. All this is best done in the evening by candle-
light : the paper, so far prepared, is called iodized paper, because
it has a uniform pale-yellow coating of iodide of silver. It is
scai'cely sensitive to light, but nevertheless it ought to be kept
in a portfolio or drawer until wanted for use. It may be kept
for any length of time "without spoiling or undergoing any change,
if protected from sunshine. When the paper is requii-ed for use,
take a sheet of it, and wash it with a liquid pi-epai'ed in the
following manner : —

Dissolve 100 grains of crystallized nitrate of silver in two
oimces of distilled water; add to this solution one-sixth of its
volume of strong acetic acid. Let this be called mixtiu'e A.

Make a saturated solution of crystallized gallic acid in cold
distilled water. The quantity dissolved is veiy small Call this
solution B.

Mix together the liquids A and B in equal volumes, btit only
a small quantity of them at a time, because the mixtm-e does not
keep long without spoiling. This mixture ]Mr. Talbot calls the
gallo-nitrate of silver. This solution must be washed over the
iodized pajaer on the side marked, and being allowed to remain
upon it for half a minute, it must be dij^ped into water, and then
lightly dried with blotting-paper. This operation in particular
requires the total exclusion of daylight ; and although the paper
thus prepared, has been found to keep for two or tliree months,
it is advisable to use it within a fcAv hoiu's, as it is often rendered
useless by spontaneous change in the dark.

Paper thu^s prepai-ed is exquisitely sensitive to light; an ex-
posiu'e of less than a second to diffused daylight being quite
sufficient to set up the process of change. If a piece of this
paper is partly covered, and the other exposed to daylight for
the briefest p)Ossihh period of time, a veiy decided impression
will be made. This impression is latent and invisible. If, how-
ever, the paper be placed aside in the dark, it will gi-adually
devehpe itself; or it may be brought out immediately by being
washed over with the gallo-nitrate of silver, and held at a short
distance from the fii-e, by which the exposed portions become
brown, the covered j^arts remaining of theii- original colour. The
pictm-es being thus procured, are to be fixed by washing in clean
water, and lightly di}-ing between blotting-paper, after which

IJIPKOVEMEXTS IX CALOTYPE, 27

tliey ai'e to be washed over Tvdtli a solution of bromide of potas-
sium, containing 100 graias of that salt, dissolved in eight or ten
ounces of water ; after a minute or two, it is agaia to be cUpped
into water, and then finally dried.

Such was, ia aU its main features, the description given by
Mr. Talbot in his specification of his process for producing the
Calotype, or beautiful 2)icture (as the term sigiiifies) : he, in a
second patent, included the points stated in the next section.

Skctiox III. — Improvemknts IX Calotvpe.

Such is the tenn employed by Mr. Talbot, and these im-
provements consist of the follo^sdng particulai's, constituting that
gentleman's second claim.

1. Removing the yeUo^dsh tint which is occasioned by the
iodide of silver, from the paper, by plvmging it into a hot bath of
hyposulphite of soda dissolved in. ten times its weight of water,
and heated nearly to the boiling point. The picture shoiild
remain in the bath about ten minutes, and be then washed ia
warm water and di'ied.

Although this has been included by Mr. Talbot in his specifi-
cation, he has clearly no claim to it, since, in Febiiiary 1840, Sir
John Herschel published, in liis Memoir " On the Chemical
Action of the Rays of the Solar Spectrum," a process of fixing
with the hot hy[:)Osulpliite of soda.

After undei-going the operation of fixing, the picture is placed
upon a hot iron, and wax m.elted uito the pores of the paper to
increase its transparency.

2. The calotype paj)er is rendered more sensitive by placing
a warm iron behind in the camei-a whilst the light is acting
upon it.

3. The preparation of io-gallic pfiper, which is simply washing
a sheet of iodized paper with gaUic acid. In this state it will
keep in a portfolio, and is rendered sensitive to light by washing
it over with a solution of nitrate of silver.

4. Iodized paper is washed with a mixture of twenty-six parts
of a saturated solution of gallic acid to one pai-t of the solution
of nitrate of silver ordinarily used. It can then be di'ied without
fear of spoiling, may be kept a little time, and used without
foi-ther preparation.

5. The improvement of photogi-aphic dra^\'ings by exposing
them twice the tisual time to the action of sunlight. The shadows
are thus rendered too dark, and the lights are not sufliciently
white. The di'awing is then wa.shed, and plunged into a bath of

ZQ HISTORY OF PHOTOGRAPHY.

iodide of potassium, of tlie strengtli of 500 grains to each, piut
of water, and allowed to remain in it for one or two minutes,
wliicli makes tlie pictiu'es brigliter, and its lights assvune a pale-
yellow tint. After tliis, it is washed, and immersed in a liot batli
of liyposulphite of soda until tlie pale-yellow tint is removed,
and the lights remain quite white. The pictures thus finished
have a pleasing and peculiar effect.

6. The appeai'ance of photograjihic pictures is improved by
■waxing them, and placing white or coloiu'ed paper behind them,

7. Enlarged copies of dagueiTeotypes and calotypes can be
obtaiued by tlu'owing magnified images of them, by means of
lenses, upon calotyjie pajaer.

8. Photograjjhic printing. A few pages of letterpress are
printed on one side only of a sheet of paper, which is waxed if
thought necessary, and the letters are cut out and sorted ; then,
in order to compose a new page, a sheet of white paper is ruled
with straight lines, and the words are formed by cementing the
separate letters in theii' proper order along the lines. A nega-
tive photographic copy is then taken, having white letters on a
black ground; this is fixed, and any number of positive copies
can be obtained. Another method proposed by the patentee is
to take a copy by the camera obsciu-a from large letters painted
on a white board.

9. Photographic publication. This claim of the patentee con-
sists in making, first, good negative di-awings on papers prepared
with salt and anmionio-nitrate of silver ; secondly, fixing them
by the process above described ; thii-dly, the formation of positive
di-awings from the negative copy, and fixing.

These claims, taken from the specification as published in the
Repertory of Patent Inventions, are preserved, in their original
form, for the purpose of showing how much that is now fully
accomplished was foreseen by Mr. Talbot as the result of his
discoveries.

Section IY. — Pictures on Porcelain Tablets.

A third patent has been obtained by Mr. Talbot, mainly in-
volving the use of porcelain as a substitute for glass, and con-
tains some useful facts noticed by Mr. Malone.

The fii'st part of the patentee's invention consists in the use of
plates of unglazed porcelain, to receive the photogTaphic image.
A plate intended for photographic pm-poses should be made of the
finest materials emjiloyed by the manufactiu-ers of porcelain ; it
should also be flat, very thin, and semi-transparent ; if too thin, so

PICTUSES ON PORCELAIN TABLETS. 29

that there would be a chance of breaking, it may be attached by-
means of cement to a piece of glass, to give it strength. The sub-
stance of the plate shotild be sUghtly porous, so as to enable it to
imbibe and retaiu a sufficient quantity of the chemical solutions
employed. To prepare the plate for use, it is fii-st required to give
it a coating of albumen, or white of eggs, laid on veiy evenly,
and then gently di'ied at a foe. According as the plate is more
or less porous, it requires more or less of the albuminous coating ;
it is best to employ a veiy close-gi'ained porcelain, which requii-es
but little white of egg. The prepared plate may be made sensi-
tive to light in the same way in which a sheet of paper is
rendered sensitive ; .and we generally find the same methods ap-
plicable for photographic pictiu"es on paper, applicable to those
on porcelain plates, and one of the processes employed by the
patentee is nearly the same as that patented by jNIr. Talbot in
1841. The prepai'ed plate is dipped iato a solution of nitrate
of silver, made by dissolving twenty-five gi-ains of nitrate in one
ounce of water : or the sohition is spread over the plate uni-
formly with a bi-ush; the plate is then di-ied, afterwards dipped
into a solution of iodide of potassium, of the strength of about
twenty-five gi-ains of iodide to one ounce of water, again dried,
and the smface inibbed clean and smooth ^vith cotton. The
plate is now of a pale-yellow coloiu", owing to the formation on
its sm-face of iodide of silver. The plate, prepared as above
directed, may be kept in this state vmtil required, when it is to
be rendered sensitive to light by washing it over with a solution
of gallo-nitrate of silver, then placed in the camera; and the image
obtained is to be rendered visible, and sufficiently strengthened,
by another washing of the same liquid, aided by gentle wai-mth.
The negative pictui'e thus obtained is fixed by washing it with
water, then with bromide of potassium, or, what is still better,
h}qiosulphite of soda, and again several times in water. The
plate of porcelain being semi-transparent, positive pictures can
be obtained fi'om, the above-mentioned negative ones by copying
them in a copying-frame.

The pictm-e obtained on porcelain can be altered or modified
in appeai*ance by the application of a strong heat, a process not
applicable to pictiu*es taken on paper. "With respect to this part
of their invention, the patentees claim : — " The obtaining by
means of a camera, or copying-frame, photographic images or
rictures upon slabs or plates of porcelain." The second part
relates to the process which has been discovered and improved
upon by INIr. Malone, who is associated vrith. Mr. Fox Talbot in
the patent. The patentees' improvement is a method of ob-
taining more complete fixation of photographic pictures on paper.

30 HISTORY OF PHOTOGRAPHY.

For this purpose, tlie print, after vindergoing tlie iisual fixing
process, is dipped into a boiling solution of strong caustic
potash, which changes the colour of the print, and usually, after
a certain time, acquii'es something of a gi-eenish tint, which
indicates that the process is terminated.

The picture is then well washed and dried, and if the tint ac-
quii'ed by it ig not pleasing to the eye, a slight exposui'e to the
vapoiu's of sulphiu'etted hydi'ogen will restore to it an agi-eeable
brown or sepia tint. Under this treatment the pictiu'e dimi-
nishes in size, insomuch that if it were previously cut in two,
and one paiii submitted to the potash process, and the other
not, the two halves, when affcei'wards put together, would be
found not to correspond. The advantages of this process for re-
moving any iodine which, even after fixing with the hyposulphite,
remains in the paper, is gi'eat, and it will tend much to preserve
these beautifid transcripts of natiu-e.

The patentee also claims as an improvement the use of var-
nished, or other transparent paper, impendous to water, as a
substitute for glass, in certain circumstances, to support a film
of albumen for photogTaphic pm'poses. A sheet of -wiiting-paper
is brushed over with several coats of varnish on each side : it
thus becomes extremely ti-ansparent. It is then brushed over on
one side A\T.th albimien, or a mixture of albumen and gelatine,
and diied. This film of albumen is capable of being rendered
sensitive to light by exposing it to the vapour of iodine, and by
following the rest of the process indicated in the preceding
section of this specification. The advantages of using varnished
or oil paper do not consist in any superiority of the images over
those obtained upon glass, but in the gi-eater convenience of
using paper than glass in cases where a large number of ijictures
have to be made and can'ied about for considerable distances :
besides this, there is a well-knowTi kind of photographic pictiu'es
gi^-ing panoramic views of scenery, which are produced upon a
curved sm-face by a movement of the object-glass of the camera.
To the production of these images glass is hardly applicable,
since it cannot be readily bent to the requii-ed cmwe and again
sti-aightened ; but the case is met by emploj-ing talc, varnished
paper, oiled paper, &c., instead of glass. It will be seen that
the varnished paper acts as a support to the film of albiunen or
gelatine, which is the surface on which the light acts, and forms
the j^ictm-e. The next imi^rovement consists in forming photo-
gi'aphic pictui'es or images on the siu'faces of polished steel plates.
For this purpose, one pai-t (by measin-e), of a saturated solution
of iodide of potassium is mixed with 200 parts of albumen, and
spread as evenly as possible upon the sui-face of a steel plate

INSTANTANEOUS PROCESS. 31

and dried by the heat of a gentle fire. The plate is then taken,
and, whilst still wann, is washed oyer with an alcoholic solution
of gallo-nitrate of silver, of moderate strength. It then becomes
very sensitive, and easily receives a photogi-aphic image. If the
plate be cold, the sensibility is considerably lower. The image
obtained is fixed by washing with h^-posiilphite of soda, and
finally 's\'ith water.

Section Y. — Instantaneous Process.

The next invention and patent of Mr. Fox Talbot possesses
many peculiarities, and as the results are of a remai-kable character,
it is important that the px'ocess should be given uncurtailed in its
main particvdai-s. The following description must be regarded
as an absti-act of ]Mi'. Talbot's communication to the Atlienceum,
Dec. 6, 1851. An expei-iment was tried in June, at the Royal
Institution, in which an instantaneous image was produced; but
as the process was the subject of another patent, it was not pub-
lished \intil the above date. The expei-inient in question was
that of obtaining a photogi-aphic copy of a printed paper fa.stened
to a wheel, which was made to revolve as rapidly as possible, by
illuminating it for a moment by the light obtained fi-om the
discharge of a Leyden batteiy : the bill was faithfully printed,
not even a letter being indistinct.

A glass plate is employed, and Mr. Talbot thus directs that it
should be prejDared.

1. Take the most liquid portion of the white of an egg, yq-
jecting the rest. Mix it with an equal quantity of water. Spread
it veiy evenly upon a plate of glass, and diy it at the fire. A
strong heat may be used without injuring the plate. The film
of dried albumen ought to be unifonn and nearly invisible.

2. To an aqueous solution of nitrate of silver add a conside-
rable quantity of alcohol, so that an ounce of the mixtiu'e may
contain three gi-ains of the nitrate. I have tried various pro-
portions, fi-om one to six gi-ains, but perhaps thi'ee gi-ains answer
best. More experiments are here requii'ed, since the results
are much influenced by this part of the process.

3. Dip the plate into this solution, and then let it dry spon-
taneously. Faint prismatic colom-s will then be seen upon the
plate. It is important to remark, that the nitrate of silver
appeal's to form a tiiie chemical combination with the albumen,
rendering it much harder, and insoluble in liqiuds which dis-
solved it previously.

4. Wash with distilled water to remove any superfluous por-
tion of the nitrate of silver. Then give the plate a second

32 HISTORY OF PHOTOGRAPHY.

coating of albumen similar to the fii'st, but, in diying, avoid
heating it too much, which would cause a commencement of
decomposition of the silver.

5. To an aqueous solution of proto-iodide of ii'on add, first, an
equal volimie of acetic acid, and then ten volumes of alcohol.
Allow the mixture to I'epose two or thi'ee days. At the end of
that time it ^vi)l have changed coloiu-, and the odoiu- of acetic
acid as well as that of alcohol ^\'ill have disappeared, and the
liquid will have acquired a peculiar but agreeable vinous odour.
It is in this state that I prefer to employ it.

6. Into the iodide thus prepared and modified, the plate is
dipped for a few seconds. AH these operations may be per-
formed by moderate daylight, avoiding, however, the direct solar
rays.

7. A solution is made of nitrate of silver, containing about
70 grains to one ounce of water. To thi-ee pai-ts of this add two
of acetic acid. Then, if the prepared plate is rapidly dipped
once or twT.ce into this solution, it acqiiii-es a \qyj great degree
of sensibility, and it ought then to be placed in the camera
without much delay.

8. The plate is withch-awn from the camera, and in order to
bring out the image, it is dipped iuto a solution of pi'otosvdphate
of iron, containing one pai-t of the satiu-ated solution diluted
with two or thi-ee parts of water. The image appeal's very
rapidly.

9. Having washed the plate with water, it is now placed in a
solution of hyposulphite of soda, which in one minute causes the
image to brighten up exceedingly, by remo\Tng a kind of veil
which previously covered it.

10. The plate is then washed with distilled water, and the
process is terminated. In order, however, to guard against
futiu'e accidents, it is well to give the picture another coating of
albumen and of varnish.

" These operations may appear long in the description, but
they are rapidly enough executed after a little practice. In the
process which I have now described, I trust that I have effected
a harmonious combination of several previously ascertained and
valuable facts, esj^eciaUi/ of the 2)hotographic 2^ropert^ of iodide
of iron, which ivas discovered hy Dr. Woods, of Parsonstown,
in Ireland ; and that of sidphate of iron, for ivhich science is
indebted to the researches of Mr. Robert Hunt. In the time
adjustment of the proportions, and in the mode of operation,
lies the diflBculty of these investigations, since it is possible, by
adopting other ^jroportions and manipulations not very gi-eatly
difiering from the above, and which a careless reader might

Talbot's instantaneous process. 33

consider to be the same, not only to fail in obtaining the highly
exalted sensibility which is desirable in this process, but actually
to obtain scarcely any photographic result at all."

JVIr. Talbot proposed the name of Amplntype, or doubtful
image, for these pictiu'es. This name had, however, been adopted
previously, at INIr. Talbot's recommendation, by Sir John Her-
schel, and in the Collodion processes, to be by-and-by described,
we have similar phenomena, to which the name applies with
equal force.

It is not improbable but the high degree of sensibility which
is certainly obtained in this process, is rather due to the forma-
tion of an iodide of ethyle in the mixtiu'e, than to the combination,
as INIi'. Talbot supposes, of the proto-iodide and the proto-sulphate
of iron. My own researches convince me that we should seek
for the highest degrees of sensibility amidst the numerous com-
binations of the ethyle and methyle compounds with the metallic
oxides.

The last invention of Mr. Fox Talbot is one by which he
obtains etchings of objects ; the images of which are produced
by radiant action upon steel plates. This process, and analogous
ones by others, will be described in Part III.

CHAPTER IV.

DAGUERREOTYPE THE DISCOVERY OF 31. DAGUERRE.

Section I. — The Original Process of Daguerre,

It has been already stated that Niepce and Daguerre having
by accident discovered they were prosecuting experiments of the
same kind, entered into a pai'tnership. On the 5th Dec, 1829,
Niepce communicated to Daguerre the particulars of the process
employed by him, which has been already described (Chap. II.),
under the term Heliograph y. Niepce died in July 1833, but
he has left some letters which clearly show that he had been a
most industrious investigator. One extract appears of particular
importance : — " I repeat it, sir," he says, " I do not see that we
can hope to derive any advantage fi-om this process {the use of
iodine^ more than from any other method which depends upon
the use of metallic oxides," &c. Again, he says, " a decoction of
Thlaspi (Shepherd's purse), fumes of phosphonis, and particularly
of sulphur, as acting on silver in the same way as iodine, and
caloric, produce the same effects by oxidizing the metal, for from
this cause proceeded in all these instances theii" extreme sensibility
to light." After the death of M. Nicephore Niepce, a new
agreement was entered into between his son M. Isidore Niepce
and Daguerre, that they should pm-sue their investigations in
common, and shai-e the profits, whatever they might eventually
prove to be.

The discovery of DagueiTe was reported to the world early in
January, 1839 ; but the process by which his beaxitiful jjictiu'es
were produced was not made known until the July following,
after a bill was passed securing to himself a pension for life
of 6000 francs, and to M. Isidore Niepce, the son of M. Niepce
above mentioned, a pension for life of 4:000 francs, with one-half
ill reversion to their widows. It was regi-etted, that after the
French Government had thus liberally purchased the secret of
the process of the Daguerreotype, for "the glory of endoimng the
world of science and of art loith one of the most surprising

THE DISCOVERY OF M. DAGUERRE. 35

discoveries that honour tJieir tiative land," on the argiiment that
" the invention did not admit of being secured by patent, for as
soon as published all might avail themselves of its advantages,''
that its author shoiild have guarded it by a patent right in
England.

From the primaiy importance of this very beautiful branch of
the photogi'aphic aii;, I shall devote some space to a description
of the original process, reserving for the division devoted to the
manipxilatory details the descrij)tion of each improvement which
has been published, having any practical advantage, either by
lessening the labour- required, or reducing the expense.

The pictm-es of the daguerreotype are executed upon thin
sheets of silver plated on copper. Although the copper serves
principally to supj)ort the silver foil, the combination of the two
metals appears to tend to the perfection of the effect. It is
essential that the silver should be veiy pure. The thickness of
the copper should be sufficient to maintain perfect flatness, and
a smooth sm-face ; so that the images may not be distox-ted by
any wai-ping or xmevenness. Unnecessary thickness is to be
avoided on account of the weight.

The process is di\'ided by DagueiTC into five oj^erations. The
first consists in cleaning and polishing the plate, to fit it for
receiving the sensitive coating on which light foi-ms the picture.
The second is the formation of the sensitive ioduret of silver over
the face of the tablet. The thii'd is the adju.sting of the plate in
the camera obscura, for the purpose of receiving the impression.
The foui-th is the bruiging out of the photographic picture, which
is invisible when the plate is taken fi-om the camei-a. The fifth
and last operation is to remove the sensitive coating, and thus
prevent that susceptibility of change under luminous influence,
which would othei'^\'ise exist, and quickly destroy the pictiu-e.

First Operation.— A small i^hial of olive-oil — some finely
carded cotton — a muslin bag of finely le-vigated pvunice — a i^hial
< .f nitric acid, diluted in the proporfion of one part of acid to

xteen parts of water, are reqxiired for this operation. The
-ei-ator must also provide himself Av-ith a small spii'it-lamp, and
au iron wire fi'ame, upon which the plate is to be placed whilst
leuig heated over the lamj). The following figm-es represent
iliis fi-ame. The first view is as seen fiom above. The second
is a section and elevation, showing the manner in which it is
fixed.

The plate being first powdered over with piunice, by shaking
the bag, a piece of cotton dipped into the olive-oil is then
carefully nibbed over it with a continxious cii-cular motion, com-
mencing from the centre. When the plate is well polished

HISTORY OF PHOTOGRAPHY.

it must be cleaned by powdering it all over ^vitll piunice,
and then rubbing it witli diy cotton, always ronndiiig and
crossing the strokes, it being
impossible to obtain a true
siu'face by any other motion
of the hand. The surface of
the plate is now nibbed all
over with a pledget of cotton,
slightly wetted with the di-
luted nitric acid. Frequently
change the cotton, and keep
rubbing briskly, that the acid
may be equally diffused over
the silver, as, if it is permitted
to iTin into di-ops, it stains the
table. It will be seen when
the acid has been properly
diffiised, from the appearance
of a thin film equally spread over the surface. It is then to be
cleaned off with, a little pimiice and diy cotton.

The plate is now placed on the wire fi"ame — the silver upwards,
and the spirit-lamp held in the hand, and moved about below it,
so that the flame plays upon the copper. This is continued for
five minutes, when a wliite coating is formed all over the sm-face
of the silver; the lamp is then withdi'a%vn. A charcoal fii'e may
be used instead of the lamp. The plate is now cooled suddenly,
by placing it on a mass of metal, or a stone floor. When per-
fectly cold, it is again polished A\dth chy cotton and pumice. It
is necessary that acid be again applied two or three times, in
the manner before dii-ected, the diy pumice being powdered over
the plate each time, and polished off gently Tvath diy cotton.
Care must be taken not to breathe upon the plate, or touch it
with the fingers, for the slightest stain upon the sui-face ^\t11 be
a defect in the drawing. It is indispen-
sable that the last operation with the acid
be performed immediately before it is in-
tended for use. Let every pai-ticle of dust
be removed, by cleaning all the edges, and
the back also, with cotton. After the fii'st
polishing, the plate c is fixed on a board
by means of fom- fillets, b B B B, of plated
copper. To each of these are soldered two a u o d
small projecting pieces, which hold the ^•

tablet near the corners ; and the whole is retained in a proper
position by means of screws, as represented at D D d d.

THE DISCOVERY OF M. DAGUERKE.

Second Operation. — It is necessary for this opex-ation, wliich.
is really the most important of all, that a box, similar to figs. 4

and 5, be provided. Figiu-e 4
represents a section, supposed
to pass do"\\Ti the middle of the
appai'atus by the line A b in
fig. 5, which represents the box
as seen fi-om above, c is a small
lid which acciu'ately fits the in-
terior, and divides the boxes into
two chambei's. It is kept con-
stantly in its place when the
box is not in use ; the piu'pose
of it being to concentrate the
vapoui' of the iodine, that it may
act more readily upon the plate
^ when it is exjwsed to it. d is

the little capside in which
the iodine is placed, which is covered mth the ring, J, upon
which is stretched a piece of fine gauze, by which the particles

of iodine are prevented
ft'om rising and staining
the plate, while the va-
poiu", of coiu-se, passes
treely through it. E is
-B the board with the plate
attached, which rests on
the four smaller pro-
jecting pieces, F, fig. 5.
G is the lid of the box,
which is kept closed, ex-
cept when the plate is
removed or inserted. H represents the supports for the cover c.
K, tapei-ing sides aU roimd, forming a fimnel-shaped box within.
To prepare the plate : — The cover c, being taken out, the cup,
D, is charged with a sufficient quantity of iodine, broken into
small pieces, and covered -with, the gauze, J. The board, E, is
now, with the plate attached, placed feice dowTiwai-ds, in its pro-
per position, and the box carefully closed.

In this position the plate remains \mtil the vapour of the
iodine has produced a definite golden yeUow coloui-, nothing
more nor less.* If the operation is prolonged beyond the point
at which this efiect is produced, a \-iolet coloiu- is assumed,

* If a piece of iodine is placed on a silver tablet, it will speedily be sur-
rounded with coloured rings: these being the colours of thin films, as described

A —

HISTOEY OF PHOTOGRAPHY.

"which, is much less sensitive to light ; and if the yellow coating
is too pale, the picture produced will prove very faint in all its
parts. The time for this cannot be fixed, as it depends entirely
on the tem^ieratiu-e of the surrounding air. No artificial heat
must be applied, unless in the case of elevating the temperature
of an apartment in which the operation. may be going on. It is
also important that the temperature of the inside of the box
should be the same as it is without, as otherwise a deposition of
moisture is liable to take place over the surface of the plate. It
is well to leave a portion of iodine always in the box ; for, as it is
slowly vaporized, it is absorbed by the wood, and when required
it is given out over the more extended surface more equally, and
with greater i-apidity.

As, according to the season of the year, the time for pro-
ducing the requii-ed efiect may vary from five minutes to half
an hour, or more, it is necessary, from time to time, to inspect
the plate. This is also necessary, to see if the iodine is acting
equally on every part of the silver, as it sometimes happens that
the colour is sooner produced on one side than on the other,
and the plate, when such is the case, must be tm-ned one quarter
round. The plate must be inspected in a darkened room, to
which a faint light is admitted in some indirect way, as by a
door a little open. The board being lifted from the box "with
both hands, the operator tvmiing the jJate towards him rapidly,
observes the colour. If too pale, it must be retui-ned to the
box ; but if it has assiuned the "violet colour it i« useless, and the
whole process must be agaia gone thi'ough.

From description, this operation may appear veiy difficult ;

but with a Kttle practice the precise interval necessary to pro-
duce the best effect is pretty easily guessed at. When the

by Sir Isaac Newton. Close examination will show the formation of two yellow
ringSi one within and the other without the series. If we cover one-half of the
circle with a card, and expose the other part to light, the rings will be found to
change colour, the outer and the inner yellow darkening the most readily, and
to an equal shade ; thus proving the advantage of obtaining this yellow tone.

THE DISCOVERY OF M. DAGUERRE.

proper yellow coloiu- is produced, tlie plate must be put into a
fi-ame, whicli fits the camera obsciu-a, and tlie doors are instantly
closed upon it, to j^revent the access of light. The figiu-es
represent this fi-ame, fig. G, with the dooi"s, B B, closed on the
plate ; and fig. 7, with the dooi-s opened by the half cii'cles, A a.
D D are stops by which the doors ai-e fastened until the mo-
ment when the plate is requii-ed for use. The thii'd operation
should, if possible, immediately succeed the second : the longest
interval between them should not exceed an hoxir, as the iodine
and silver lose their requisite photogenic properties.* It is
necessary to observe, that the iodine ought never to be touched
with the fingers, as we are very liable to injure the plate by
touching it with the hands thus stained.

TMrd Operation. — The thii-d operation is the fixing of the
plate at the jH-oper focal distance from the lens of the camera
obscura, and placing the camera itself in the right position for
taking the view we desii-e. Fig. 8 is a perpendicular section,
lengthwise, of DagueiTe's camera. A is a gi-oimd glass by which

the focus is adjusted ; it is then removed, and the photographic
plate substituted, as in c, fig. 9. b is a mirror for observing
the efiects of objects, and selecting the best points of view.
It is inclined at an angle of io^, by means of the support, L.
To adjust the focus, the mirror is lowered, and the piece of
gi'ound glass, A, used. The focus is easily adjiisted by shding
the box, D, out or in, as represented in the plate. When the
focus is adjusted, it is retained in its place by means of the screw,
H. The object glass, j, is achromatic and peiiscopic ; its diameter
is about one inch, and its focal distance rather more than four-
teen inches, m, is a stop a short distance fi'om the lens, the

* This is contrary to the experience of the author of this volume ; and
Dr. Draper, of New York, states that he has found the plates improve by
keeping a few hours before they are used ; and M. Claudet states, that even
after a day or two the sensibility of the plates is not impaired.

HISTORY OF PHOTOGRAPHY,

object of which is to cut oS all those rays of light which do not
come dii-ectly from the object to which the camera is du-ected.
This instrument reverses the objects; that which is to the right
in natm-e being to the left in the photograph. This can be
remedied by using a mirror outside, as K J, in figure 9. This
aiTangement, however, reduces the quantity of light, and in-
creases the time of the operation one-third. It will, of course,
be adopted only when there is time to spare. After having
placed the camera in front of the landscape, or any object of
which we desire the representation, our first attention must be
to adjust the plate at such a distance from the lens, that a neat
and sharply defined picture is
produced. This is, of course,
done by the obscured glass. The
adjustment being satisfactorily
made, the glass is removed, and
its place sujDplied by the frame
containing the prepared plate, and
the whole secui-ed by the screws.
The doors are now opened by
means of the half ch-cles, and the
plate exposed to receive the pic-
ture. The length of time neces-
sary for the production of the best
efifect, varying with the quantity
of light, is a matter which re-
quires the exercise of considerable
judgment, particularly as no im-
pression is visible upon the tablet
when it is withdrawn fr-om the
camera. At Paris this varies from
three to thirty minutes. The most
favourable time is from seven to ^"

three o'clock. A di-awing which, in the months of June and
July, may be taken in three or four minutes, will require five or
six in May or August, seven or eight in April and September,
and so on, according to the season. Objects in shadow, even
dui'ing the brightest weather, will require twenty minutes to be
correctly delineated. From what has been stated, it will be
evident that it is impossible to fix, with any precision, the exact
length of time necessary to obtain photographic designs ; but by
practice we soon learn to calculate the requii-ed time with con-
siderable correctness. The latitude is, of coiu-se, a fixed element
in this calculation. In the sunny climes of Italy and southern
France, these designs may be obtained much more promptly

THE DISCOVERY OF M. DAGUEKRE,

than in tlie uncertain clime of Great Britain. It is very im-
portant that tlie time necessary is not exceeded, — prolonged
solarization has the effect of blackening the plate, and this de-
stroys the clearness of the design. If the operator has failed in
his fii'st experiment, let him immediately commence with
another plate ; coiTecting the second trial by the fii'st, he will
seldom fail to produce a good photogTaph.

Fourth Operation. — The apparatus requii-ed ia this operation
is represented by fig. 10. A, is the lid of the box ; B, a black
board with grooves to receive the plate ; c, cup contaiaing a
little mercmy, j; D, spu'it-lampj F, thermometer; G, glass
through which to inspect the oj^eration ; H, tablet as removed

from the camera ; i, stand for
the spirit-lamp. All the in-
terior of this apparatus shoxild
be covered with hard black
vamish. The board and the
affixed plate being withdi-awn
from the camera, are placed at
an angle of about 45*^ within
this box — the tablet with the
pictm-e downwards, so that it
may be seen thi'ough the glass
G. The box being carefully
closed, the spirit-lamp is to be
lighted and placed under the
cup containing the mercury.
The heat is to be applied until
the thermometer, the bulb of
which is covered with the mer-
cury, indicates a temperature
of 60° centigrade (140'" Fah.)
The lamp is then withdi-awn,
and if the thermometer has
risen rapidly, it will continue
to rise without the aid of the lamp; but the elevation ought
not to be allowed to exceed 75° cent. (167'' Fah.)

After a few minutes, the image of nature impressed, but till
now invisible, on the plate, begins to appear ; the operator
assui-es himself of the progress of this development by examin-
ing the picture thi-ough the glass, G, by a taper, taking care that
the rays do not fall too strongly on the plate, and injm-e the
nascent images. The operation is continued till the ther-
mometer sinks to i5° cent. (113*^ Fah.) When the objects

HISTORY OF PHOTOGRAPHY.

11.

have been strongly illunimated, or when the plate has been kept

in the camera too long, it will be found that this operation is

completed before the thermometer has fallen to

55'^ cent. (131'"'' Fah.) This is, however, always

known by obsendng the sketch thi-ough the

glass.

After each operation the apparatus is care-
fully cleaned in every part, and in particidar the
strips of metal which hold the plate are well
rubbed with pumice and water, to remove the
adhering mercury and iodiue. The plate may
now be deposited in the grooved box (fig. 11),
in which it may be kept, excluded from the light,
until it is convenient to perform the last fixing
operation.

Fifth Operation. — This process has for its object the removal
of the iodine from the plate of silver, which prevents the fm-ther
action of the light.

A satui-ated solution of common salt may be used for this
piu'pose, but it does not answer nearly so well as a weak solu-
tion of the hyposulphite of soda. In the fii'st place, the plate is
to be placed in a trough of watei-, plungmg and withdi-awing it
immediately ; it is then to be plunged into one of the above
saline solutions, which wovdd act upon the di^awing if it was not
previously hardened by washing in water.

To assist the effect of the saline washes, the plate must be
moved to and 6-o, which is best done by passing a wu-e beneath
the plate. When the yeUow colour has quite disappeared, the
plate is lifted out, great care being taken that the impression is
not touched, and it ls again plunged into water. A vessel of
warm distilled water, or very pure rain-water boiled and cooled,
being provided, the plate is fi:xed on an inclined plane, and the
water is poiu'ed in a continuous stream over the picture. The
drops of water which may remain upon the plate must be re-
moved by forcibly blowing upon it, for otherwise, in diying, they
would leave stains on the drawings. This finishes the di-awing,
and it only remains to presei-ve the silver from tarnishing and
from dust.

The shadows in the Daguerreotype pictiu'es are represented
by the polished svirface of the silver, and the lights by the ad-
hering mercury, which will not bear the slightest rubbing. To
preserve these sketches, they must be placed in cases of paste-
board, with a glass over them, and then framed in wood, They
are now unalterable by the sun's light.

IMPROVEMENTS IN DAGUERREOTYPE. 43

The same plate may be employed for many successive trials,
provided the silver be not polished through to the copper. It
is very important, after each trial, that the mercury be removed
immediately by polishing with pumice-powder and oil. If this
be neglected, the mercmy finally adheres to the silver, and good
drawings caimot be obtained if this amalgam is present.

Section II. — Improvements in Daguerreotype.

It was announced that the inventor of the Daguerreotype had
succeeded in improving the sensibility of his plates to such an
extent, as to render an instantaneous expostire sufficient for the
production of the best effects ; consequently, seciu-ing faithful
impressions of moving objects. In a communication with which
I was favoiu'ed from M. Daguerre, he said, — " Though the
principle of my new discovery is certain, I am detennined not
to publish it before I have succeeded in making the execution
of it as easy to eveiybody as it is to myself I have annoimced
it immediately at the Royal Academy of Paris, merely to take
date, and to ascertain my right to the priority of the invention.
By means of that new process, it shall be possible to fix the
images of objects in motion, such as 2^uhliG ceremonies, market-
places covered with 2^^02)le, cattle, dx. — the effect being instan-
taneous.''''

In 1844, M. Daguerre, in a letter to M. Arago, published this
process ; but it proved so complex in its manipulatory details,
and so veiy uncertain, that it has not been adopted. As it is,
however, c\u'ious, and involves the use of some agents not ordi-
narily employed, it is thought advisable to make some extracts
from the Coniptes Rendus for April, 1844, in which it was pub-
lished : —

" By superposing on the plate several metals, reducing them
to powder by friction, and by acidulating the empty spaces
which the molecules leave, I have been enabled to develope
galvanic actions which permit the employment of a much thicker
layer of iodide, without ha\'ing to fear, diu-ing the oj^eration of
light in the camera obscm-a, the influence of the liberated
iodine.

" The new combination which I employ, and which is com-
posed of several metallic iodides, has the advantage of gi"ving a
sensible layer capable of receiving impressions simultaneously
by aU the degrees of tone, and I thus obtain, in a very short
space of time, the representation of objects vividly enlightened
with demi-tints, all of which retain, as in natiu'e, their ti'ans-
parency and their relative value.

44 HISTOEY OF PHOTOGRAPHY.

"By adding gold to tlie metals wliicli I fii'st used, I am enabled
to avoid the great difficulty which the use of bromine, as an
accelerating substance, presented. It is known that only very
experienced persons could employ bromine with success, and that
they were able to obtaiu the maximtim of sensibility only by
chance, since it is impossible to determine this point very pre-
cisely, and since immediately beyond it the bromine attacks the
silver, and is opposed to the formation of the image.

" With my new means, the layer of iodine is always satui-ated
with bromine, since the plate may, without inconvenience, be
left exposed to the vapoiu- of this substance for at least half the
necessary time ; for the aj^plication of the layer of gold is op-
posed to the formation of what is called the veil of bromine.
The process which I am about to give may, perhaps, be found
rather complicated; but, notwithstanding my desire to simplify
it as much as possible, I have been led, on the contrary, by the
results of my experiment, to multiply the substances employed,
all of which play an important pai-t in the whole jDrocess. I
regard them all as necessary for obtaining a complete result,
which must be the case, since I have only gradually arrived at
discovering the properties of these different metals, one of
which aids in promptitude, the other in the vigoiu- of the im-
pression, &c.

" The operation is divided into two principal parts : the first,
which is the longest, may be made a long time previously, and
may be regarded as the completion of the manufacture of the
plate. This operation, being once made, serves indefinitely ; and,
without recommencing it, a great number of impressions may be
made on the same plate. The new substances employed are : —
Aqueous solution of bichloride of mercury : solution of cyanide
of mercury : white oil of petroleum, acidulated with nitric acid :
solution of chlorine of gold and platinum. These are prepared
as follows : —

"Aqueous Solution of Bichloride of Mercury. — 8 grains of
bichloride of mercury in 10,000 grains of distilled water.

'• Solution of Cyanide of Mercury. — A flask of distilled water
is satm-ated with cyanide of mercmy, and a certain quantity is
decanted, which is diluted mth an equal quantity of distilled
water.

"Acidulated White Oil of Petroleum.*— This oU is acidulated
by mixing with it one-tenth of pure nitric acid, leaving it for at

* The most suitable oil of petroleum is of a greenish yellow tint, and takes,
at different angles, azure reflections.

I have given the preference to this oil over the fixed oils, because it always
remains limpid, although strongly acidulated. My object in employing an

IMPROVEMENTS IN DAGUERREOTYPE. 45

least forty-eight haul's, occasionally agitating the flask. The oil
which is acidulated, aud which then powerfully reddens litmus-
paper, is decanted. It is also a little coloured, but remains veiy
limpid.

" Solution of Chloride of Gold and Platinnm.— In order not to
multiply the solutions, I take the ordinaiy chloride of gold, used
for fixing the impressions, and which is composed of 15 grains of
chloride of gold, and 50 graias of h\-posulphite of soda, to a
quart of distUled water. With respect to chloride of platinimi,
4 grains must be dissolved in 3 quarts of distilled water ; these
two solutions are mixed in equal quantities.

" First Preparation of the Plate. — For the sake of brevity
in the following description, I wiM abiidge the name of each
substance. Thus, I will say, to designate the aqueous solution
of bichloride of mercuiy, sublimate; for the solution of cyanide
of mercury, cyanide; for the acidulated oil of petroleum, oil; for
the solution of chloride of gold and platinum, gold and platinum ;
and for the oxide of ii-on, ro^|,ge only.

"The plate is first polished with sublimate and tripoli, and
afterwards with rouge, until a beautiful black is arrived at.
Then, the plate is laid on the hoiizontal plate, and the solution
of cyanide is poured on it and heated over a lamp, as in fixing an
impression with chloride of gold. The mercury is deposited, and
forms a whitish layer. The plate is allowed to cool a Kttle, and,
after having poured off the liquid, it is dried by rubbing with
cotton and sprinkling with rouge.

" It is now necessary to polish the whitish layer deposited by
the merciuy. With a piece of cotton steeped in oil and rouge
this layer is rubbed until it becomes of a fine black. In the last
place, it may be rubbed veiy strongly, but with cotton alone, in
order to render the acidulated layer as thin as possible. The
plate is afterwards placed on the horizontal plane, and the solu-
tion of gold and platinum is poiu'ed on. It is heated in the
ordinary manner ; it is then allowed to cool, the liquid is poured
off, and it is dried by gentle friction with cotton and rouge.
This operation must be performed Tvdth care, especially when the
impression is not immediately continued ; for, othei'WTise, some
lines of liquid would be left on the plate, which it is cUfiicult to
get rid o£ After this last friction the plates shoidd be only

acidulated oil is to reduce the metals to powder, and to retain this powder on
the surface of the plate, at the same time giving greater thickness to the layer
by its unctuous properties ; for the naphtha which results from the distillation
of this oil does not produce the same effect, because, being too fluid, it carries
away the powder of the metals. It is for the same reason that I have lately
recommended the employment of essence of lavender rather than that of essence
of turpentine.

46 HISTORY OF PHOTOGRAPHY.

dried, aud not polislied. This includes the fii'st preparation of
the plate, which may be made a long time previously.

"Second Preparation. — I do not think it fit to allow a longer
interval than twelve hours to intervene bv2tween this operation
and iodizing the plate. We left the plate with a deposit of gold
and platinum. In order to polish this metallic layer, the plate
is rubbed with a piece of cotton, and oil and rouge, xxntiL it again
becomes black ; and then with alcohol and cotton only, in order
to remove tliis layer of rouge as much as possible. The plate is
again iiibbed very strongly, and passing several times over the
same places, with cotton impregTiated with cyanide. As this
layer ch'ies veiy j^romptly, it might leave on the plate traces of
inequality; in order to avoid tliis, the cyanide must be again
passed over it, and, while the plate is still moist, we quickly iiib
over the whole sm-face of the plate with cotton imbibed with a
little oil, thus mixing these two substances; then, with a piece
of dry cotton, we nib, in order to unite, and, at the same, to dry
the plate, taking cai-e to remove from the cotton the parts which
are moistened with cyanide and oil. Finally, as the cotton stdl
leaves traces, the plate is likewise sprinkled with a little rouge,
which is removed by gently iiibbing.

"Afterwards the plate is again rubbed with cotton impreg-
nated with oil, only in such a manner as to make the bui-nish of
the metal return ; it is then sprinkled -^ith rouge, and then very
gently rubbed roimd, to remove all the rouge, which carries
with it the superabundance of the acidulated layer.* Finally,
it is strongly nibbed with a rather fii*m pledget of cotton, in
order to give the last polish, t

" It is not necessary often to renew the pledgets of cotton im-
bibed with oil and rouge; they must only be kept free from
dust.

" On Iodizing'. — The colour of the impression depends on the
tint given to the metallic iodide; it may, therefore, be varied at
will. However, I have foimd the \iolet rose colom" most suit-
able.

" For transmitting the iodine to the plate, the sheet of card-
board may be replaced by an earthenware plate deprived of
enamel. The iodine transmitted by this means is not decom-

* This must be done as gently as possible ; for otherwise the rouge would
adhere to the plate, and would form a general film.

f In operating on a plate a long time after it has received the first prepara-
tion, it is necessary, before employing the acidulated oil and red oxide, to mani-
pulate, as I indicate further on, for the plate which has received a fixed impres-
sion. This precaution is necessary for destroying the stains which time may
have developed.

IlIPROVEHEN'TS IX DAGUERREOTYPE. 47

posed. It is useless, I may even say injurious, to heat tlie plate
before exposing it to the vapoiu' of iodine.

"WasMng with Hyposulphite of Soda.— In order to remove
the sensitive layer, the solution of hyjDosiJphite of soda must not
be too strong, because it destroys the shai'pness of the impres-
sion. 60 gi-ammes of the hyposulphite are sufficient for 1 quart
of distilled water."

The elaborate nature of this process is a baiTier against its
use, since the residts are rarely equal to those obtained by the
ordinary Dagiierreotype, as it is now practised, and the laboxu- to
be expended on the preparation infinitely greater.

The advantages which have been derived from the employ-
ment of compounds of iodine and bromine, or chlorine, by which
the sensibility of the DagueiTeotype has been greatly improved,
ai-e so great, that, Avith the incidental notice that we owe the
application of bromine to Mr. Goddai-d, the consideration of
them are postponed to the Tliird Part, as more entii'ely belong-
ing to the manipulatory details.

CHAPTER V.

THE PHOTOGRAPHIC PROCESSES ON PAPER OF SIR JOHN'
HERSCHEL.

The researclies of Sir "John Herschel have been principally
directed to the investigation of the physical laws which regulate
the chemical changes we have been considering. His analyses
of the prismatic spectrum have been most complete, and, as far
as they have been earned out, go to prove the operation of forces
other than those with which we are acquainted.

At the same time, however, as this philosopher has been
encaged in investigations of this high order, he has, from the
midtitude of his experiments, been successfiil in producing
several processes of great beauty. There are not any which are
to be regarded as peculiarly sensitive — they are indeed for the
most part rather slow — but the manipulation required is of the
easiest character, and the residts are most ciuious and instructive.

The philosophy which is for ever united with the scientific
investigations of Sir John Herschel, is too valuable to be omitted
from any description of the processes which he recommends ;
the following quotations are therefore taken fr-om his communi-
cation to the E-oyal Society, and Hnked together by my own
remarks in such a manner as it is hoped will be most easily
understood by the unscientific amateui*.

Section I. — Cyanotype.

The processes in which cyanogen is employed are so called : —
Su- John Herschel makes the following remarks on the subject
of his experiments with the cyanides : —

" I shall conclude this part of my subject by remarking on the
great niimber and variety of substances which, now that atten-
tion is di-awn to the subject, appear to be i:)hotogra2jhically
impressible. It is no longer an insidated and anomalous afiec-
tion of certain salts of silver or gold, but one which, doubtless,
in a greater or less degi'ee peiwades all natm-e, and connects
itself intimately with the mechanism by which chemical combi-
nation and decomposition is operated. The general instability

SIR J. herschel's cyanottpe. 49

of organic combinations might lead us to expect the occurrence
of numerous and remarkable cases of this affection among bodies
of that class, but among metallic and other elements inorganic-
ally arranged, instances enough have ah-eady appeared, and more
are daily presenting themselves, to justify its extension to all
cases in which chemical elements may be supposed combined
with a certain degree of laxity, and so to speak in a tottering
equilibrium. There can be no doubt that the process, in a gi-eat
majority, if not in all, cases which have been noticed among in-
organic substances, is a deoxidising one, so far as the more re-
frangible rays are concerned. It is obviously so in the cases of
gold and silver. In that of the bichromate of potash it is most
probable that an atom of oxygen is parted with, and so of many
othei-s. A beautiful example of such deoxidising action on a
non-argentine compoim.d has lately occiu'red to me in the exami-
nation of that interesting salt, the feiTosesquicyanuret of potas-
sium, described by Mr. Smee in the Fhilosophical Magazine, No.
109, September, 1840, and which he has shown how to manu-
factiu'e in abundance and piu-ity, by voltaic action on the com-
mon, or yellow ferrocyanuret. In this process nascent oxygen
is absorbed, hycb-ogen given off; and the characters of the
resulting compound in respect of the oxides of iron, forming,
as it does, Pi-ussian blue with protosalts of that metal, but pro-
ducing no precipitate with its persalts, indicate an excess of
electro-negative energy, a disposition to part with oxygen, or,
which is the same thing, to absorb hych-ogen (in the presence of
moistiu-e), and thereby to return to its pristine state under cir-
cirmstances of moderate solicitation, such as the affinity of jjrotoxide
of iron (for instance), for an additional dose of oxygen, (fcc.

" Paper simply washed with a solution of this salt is highly
sensitive to the action of light. Pi-ussian blue is deposited (the
base being neces.sai'ily supplied by the destruction of one
portion of the acid, and the acid by decomposition of another).
After half an hour or an hoiu-'s exposure to sunshine, a veiy
beautiful negative photogi-ajih is the result, to fix which, all that
is necessary is to soak it in water in which a little sulphate of
soda is dissolved, to ensure the fix;ity of the Pi-ussian blue de-
posited. "While diy the impression is dove-coloiu- or lavender
blue, which has a ciuious and striking effect on the greenish-
yellow ground of the paper, produced by the saline solution.
After washing, the gi-ound coloiu- disappears, and the photogi-aph
becomes bright blue on a white groimd. If too long exposed,
it gets 'over sunned,' and the tint has a brownish or yellowish
tendency, which, however, is removed in fixing ; but no increase

50 HISTORY OF PHOTOGRAPHY.

of intensity beyond a certain point is obtained by continuance
of exposiu'e.

" If paper be waslied with a solution of ammonio-citi-ate of
iron, and dried, and tlien a wash passed over it of the yellow
ferrocyanru'et of potassium, there is no immediate formation of
true Prvissian blue, but the paper rapidly acquires a violet-
piu'ple coloiu", which deepens after a few minutes, as it dries, to
almost absolute blackness. In this state it is a positive photo-
graphic paper of high sensibility, and gives pictures of great
depth and sharpness ; but A\ith this peculiarity, that they darken
again spontaneously on exposiu'e to the air in darkness, and are
soon obliterated. The pajDer, however, remains susceptible to
light, and capable of receiving other pictiu-es, which in their turn
fade, without any possibility (so far as I can see) of arresting
them; which is to be regretted, as they are very beautiful, and
the paper of such easy preparation. If washed ^^dth ammonia
or its carbonate, they are for a few moments entirely oblite-
rated, hut presently reappear, toith reversed lights and shades.
In this state they are fixed, and the ammonia, with all that it
will dissolve, being removed by washing in water, their colour
becomes a piu-e Prussian blue, which deepens much by keeping.
If the solution be mixed, there results a very dark violet-coloured
ink, which may be kejit uninjm-ed in an opaque bottle, and will
readily fiu-nish, by a single wash, at a moment's notice, the
positive paper in question, wliich is most sensitive when wet.

" It seems at first sight natiu-al to refer these curious and
complex changes to the instability of the cyanic compounds;
and that this opinion is to a certain extent con-ect, is proved by
the photographic impressions obtained on papers to which no
ii'on has been added beyond what exists in the feiTocyanic salts
themselves. Nevertheless, the following experiments abun-
dantly prove that in several of the changes above described, the
immediate action of the solar rays is not exerted on these
salts, but on the iron contained in the ferruginous solution
added to them, which it deoxidizes or othei-wise alters, thereby
presenting it to the ferrocyanic salts in such a form as to
precipitate the acids in combination with the peroxide, or
protoxide of ii*on, as the case may be. To make this evident,
all that is necessary is simply to leave out the ferrocyanate in
the preparation of the paper, which thus becomes reduced to a
simple washing over with the ammonio-citric solution. Paper
so washed is of a bright yellow colour, and is apparently little,
but in reality highly sensitive to photogi'aphic action. Exposed
to strong sunshine, for some time indeed, its bright yellow tint

SIR J. HERSCHEL's CYA^OTYPE. 51

is dulled into an oclu-ey liiie, or even to grey, but the change
altogether amounts to a moderate per-centage of the total light
reflected, and in short exposures is such as would easily escape
notice. Nevertheless, if a slip of this paper be held for only
four or five seconds in the suji (the effect of which is quite im-
perceptible to the eye), and, when withdrawn into the shade, be
washed over with the ferrosesquicyan'ate of potash, a conside-
rable deposit of Pnissian blue takes place on the part sunned,
and none whatever on the rest ; so tliat on washing the whole
with water, a pretty strong blue impression is left, demonstrat-
ing the reduction of ii'on in that portion of the paper to the
state of protoxide. The effect in question is not, it should be
observed, pecidiar to the ammonio-citrate of iron. The am-
monio and potassia-tartrate fully possess, and the perchloride
exactly neutralized, partakes of the same property : but the
experiment is far more neatly made, and succeeds better, with
the other salts."

In further development of these most interesting processes
Sir John Herschel says : — " The varieties of cyanotype pro-
cesses seem to be innumerable, but that which I shall now
describe deserves particidar notice, not only for its pre-eminent
beauty while in progi-ess, but as illustrating the pecidiar power
of the ammoniacal and other persalts of iron above-mentioned
to receive a latent pictiu-e, susceptible of development by a
gi'eat variety of stimuli. This process consists in simply passing
over the ammonia-citrated paper on which such a latent
pictm'e has been impressed, very sjxiringly and evenly, a wash of
the solution of the common yellow ferrocyanate (prussiate) of
potash. The latent picture, if not so faint as to be quite in-
visible (and for this purpose it should not be so), is negative.
As soon as the liquid is ajDpHed, which cannot be in too thin a
film, the negative pictui'e vanishes, and by veiy slow degi'ees is
replaced by a positive one of a violet-blue colour on a greenish-
yellow gi-ound, which at a cei-tain moment possesses a high
degi-ee of sharpness, and singular beauty and delicacy of tint.
If at this instant it be thrown into water, it passes immediately
to Prussian blue, losing at the same time, however, much of its
sharpness, and sometimes indeed becoming quite blotty and
confused. But if this be delayed, the picture, after attaining a
certain maximum of distinctness, grows rapidly confused, es-
pecially if the quantity of liquid applied be more than the paper
can easily and completely absorb, or if the brush in applying it
be allowed to rest on, or to be passed twice over any part. The
effect then becomes that of a coarse and ill-printed woodcut, all

52 HISTORY OF PHOTOGRAPHY.

the strong shades being nm together, and a total absence pre-
vailing of half lights.

" To prevent tliis confusion, gum-arabic may be added to the
pnissiated solution, by wliich it is hindered from spreading un-
manageablj -witliin the pores of the paper, and the precipitated
Prussian blue allo-«-ed time to agglomerate and fix itself on the
fibres. By the use of this ingi-edient also, a much thinner and
more equable film may be spread over the siu'face ; and lohen
perfectlij dry, if not sufficiently developed, the application may be
repeated. By operating thus I have occasionally (though rarely)
succeeded in producing pictures of great beauty and richness of
efiect, which they retain (if not thrown into water) between the
leaves of a portfolio, and have even a certain degi-ee of fixity —
fading in a strong light, and recovering their tone in the dark.
The manipiilations of tliis process are, however, delicate, and
complete success is comparatively rare.

" If sidphocyanate of potash be added to the ammonio-citrate
or ammonio-tartrate of iron, the pecidiar red colour which that
test induces on persalts of the metal Ls not produced, but it
appears at once on adding a drop or two of dilute sulphiu'ic or
nitric acid. This cii'cumstance, joined to the perfect neutrality
of these salts, and theii' power, in such neuti-al solution, of
endiu'ing, undecomposed, a boiling heat, contraiy to the usual
habitudes of the peroxide of iron, together with theii* singvdar
ti-ansformation by the action of light to proto-salts, in apparent
opposition to a very strong affijiity, has, I confess, inclined me
to speculate on the possibility of their ferruginous base existing
in them, not in the ordinary foi'm of peroxide, but in one isome-
ric with it. The non-formation of Pnissian blue, when their
solutions are mixed -^dth prussiate of potash, and the formation
in its place of a deep violet-coloiu'ed liquid of singulai* instability
under the action of light, seems to favoiu' this idea. Nor is it
altogether impossible that the peculiar "prepared" state super-
ficially assumed by iron under the influence of nitric acid, first
noticed by Keii', and since made the subject of exjieriment by
M. Seohnbeia and myself, may depend on a change supei-ficially
operated on the iron itself into a new metallic body isomeric
with iron, unoxidable by nitric acid, and wliich may be consi-
dered as the radical of that peroxide which exists in the salts in
question, and possibly also of an isomeric protoxide. A combi-
nation of the common protoxide with the isomeric peroxide,
rather than with the same metal in a simply higher stage of
oxidation, would afibrd a not unplausible notion of the chemical
natm-e of that peculiar intermediate oxide to which the name of

SLR s. herschel's cyanotype. 53-

'Ferroso-fen'ic' has been given by Berzelius. If (to render my
meaning more clear) we for a moment consent to designate such
an isomeric form of ii'on by the name sideriuni, the oxide in
question might be regarded as a sideriate of iron. Both phos-
phoras and arsenic (boches remarkable for sesqui-combinations)
admit isomeric forms in theu* oxides and acids. But to retiu-n
from this digression.

" If to a mixture of ammonio-citrate of u'on and sulphocyanate
of potash, a small dose of nitric acid be added, the resvJting red
liquid, spread on paper, spontaneously whitens in the dark. If
more acid be added till the point is attained when the discolora-
tion begins to relax, and the paper when dry retains a conside-
I'able degree of coloiu-, it is powerfully affected by Hght, and
receives a positive picture with great rapidity, which appears at
the back of the paper with even more distinctness than on its
face. The impression, however, is pallid, fades on keeping, nor
am I acquainted at present with any mode of fixing it.

"If paper be washed with a mixtm-e of the solutions of
ammonio-citrate of ii'on and ferrosesquicyanate of potash, so as
to contain the two salts in about equal proportions, and being
then impressed with a picture, be tlu-own into water and di'ied,
a negative blue image "will be produced. This picture I have
found to be susceptible of a very cui'ious transformation, pre-
ceded by total obliteration. To effect tliis it must be washed
with solution of proto-nitrate of mercury, which in a little time
entirely dischai'ges it. The nitrate being thoroughly washed
out and the picture di'ied, a smooth iron is to be passed over it,
somewhat hotter than is used for ii'oning linen, but not
sufficiently so to scorch or injure the paper. The obhterated
pictui-e immediately reappears, not blue, but brown. If kept for
some weeks in this state between the leaves of a poi-tfoHo, in
complete dai-kness, it fades, and at length almost entu-ely dis-
appears. But what is very singular, a fi-esh application of the
heat revives and restores it to its fiill intensity.

" This curious transformation is instiiietive in another way.
It is not operated by light, at least not by light alone. A certain
temperature must be attained, and that temperatiu'e suffices in
total darkness. Nevertheless, I find that on exposing to a very
concentrated spectnim (collected by a lens of short focus) a slip
of paper diily prepared as above (that is to say, by washing with
the mixed solutions, exposiu-e to sunshine, washing, and dis-
charging the uniform blue colour so induced as in the last
article), its whiteness is changed to broA\Ti over the whole region
of the red and orange rays, hut not beyond the luminous sijec-
trum. Three conclusions seem unavoidable : — 1st, that it is the

54 HISTORY OF PHOTOGRAPHY.

heat of these rays,, not theii' light, which operates the change ;
2ndly, that this heat possesses a peculiar chemical quality which
is not possessed by the purely calorific rays outside of the
visible spectrum, though far more intense ; and, 3rdly, that the
heat radiated from obscurely hot ii-on abounds especially in rays
analogous to those of the region of the spectiixm above
indicated."

Sir John Hei-schel then proceeds to show that whatever be
the state of the ii'on in the double salts in question, its reduction
by blue light to the state of protoxide is indicated by many
other reagents. Thus, for example, if a slip of paper prepared
with the ammonio-citrate of iron be exposed partially to sun-
shine, and then washed with the bichi-omate of potash, the
bichromate is deoxidised, and precipitated upon the svmned
portion, just as it would be if directly exposed to the siin's
rays.

I have proved this fact with a great number of prepai-ations
of cobalt, nickel, bismuth, platinum, and other salts which have
been thought hitherto to be insensible to solar agency ; but if
they are partially sunned, and then washed with nitrate of silver,
and put aside in the dark, the metallic silver is slowly reduced
upon the sumied portion. In many instances days were required
to produce the visible picture ; and in one case, paper, being
washed -svith neutral chloride of platinum, was sunned, and then
washed ia the dark with nitrate of silver: it was some weeks
before the image made its appearance, but it was eventually per-
fectly developed. This specimen has been kept for several years,
and continues constantly to improve in clearness and definition.

Section II. — Chrysottpe.

A process of an analogous character to that which has just
been described, and in which the chloride of gold is an agent,
must be next described : this was discovered at the same time
as the cyanotype, and has been termed the chrysotyi^e.

" In order to ascertain whether any poi-tion of the iron in the
double ammoniacal salt employed had really imdergone deoxida-
tion, and become reduced to the state of protoxide, as si;pposed,
I had recourse to a solution of gold exactly neutralised by
carbonate of soda. The proto-salts of iron, as is well known to
chemists, precipitate gold in the metallic state. The effect
proved exceedingly striking, issuing in a process nowise in-
ferior in the almost magical beauty of its effect to the calotype
process of Mr. Talbot, which in some respects it nearly resembles ;

SIR J. herschel's chrysotype. 55

with this advantage, as a matter of expei'imeiital exhibition, that
the disclosiu-e of the dormant image does not requii'e to he per-
formed in the dark, being not interfered with by moderate day-
light. As the experiment will probably be repeated by others,
I shall here describe it ab initio. Paper is to be washed
with a moderately concentrated solution of ammonio-citrate of
iron, and dried. The strength of the solution should be such as
to dry into a good yellow coloiir, not at all brown. In this state
it is ready to receive a photographic image, which may be im-
pressed on it either from nature in the camera obscura, or from
an engi"aving on a frame in sunshine. The image so impressed,
however, is very faint, and sometimes hardly perceptible. The
moment it is removed from the frame or camera, it must be
washed over with a neutral solution of sold of such strenscth as
to have about the colour of sherry wine. Instantly the picture
appeal's, not, indeed, at once of its fiill intensity, but dai-kening
with great rapidity up to a certaia point, depending on the
strength of the solutions used, »tc. At this point nothing can
siu-pass the sharpness and perfection of detail of the resulting
photograph. To arrest this process and to fix the picture (so
far at least as the further agency of light is concerned), it is to
be thi-own into water very slightly acidulated with sulphiu-ic
acid, and well soaked, dried, washed ^^■ith hydrobromate of
potash, rinsed, and dried again.

" Such is the outline of a process to which I propose applying
the name of Chrysotype, in order to recall, by similarity of struc-
ture and termination, the Calotype process of Mr Talbot, to
which, in its general efiect, it affords so close a parallel. Being
veiy recent, I have not yet (Jvme 10, 1842) obtained a complete
command over all its details, but the termination of the session
of the Society being close at hand, I have not thought it
advisable to suppress its mention. In point of direct sensibility,
the chiysotype paper is certainly inferior to the calotj^e ; but it
is one of the most remarkable peculiarities of gold as a photo-
graphic ingredient, that extremely feeble impressions once made
by light go on afterwards darkening spontanemisly and very slowly,
Ojpparerdly vnthout limit, so long as the least vestige of unreduced
chloride cf gold remains in the paper. To illustrate tliis curious
and (so far as applications go), highly impoi*tant property, I shall
mention incidentally the results of some experiments made,
during the late fine weather, on the habitudes of gold in presence
of oxalic acid. It is well known to chemists that this acid,
heated with solutions of gold, precipitates the metal in its
metallic state ; it is iipon this j^roperty that BerzeUus has
founded his determination of the atomic weight of gold. Light,

56 HISTORY OF PHOTOGRAPHY.

as well as heat, also operates this precipitation ; but to rendei'
it effectual, several conditions are necessary : — 1st, the solution
of gold must be neutral, or at most very slightly acid ; 2nd, the
oxalic acid must be added in the form of a neutral oxalate ;
and 3rdly, it must be present in a certain considerable quantity,
which quantity must be greater the greater the amomit of free
acid present in the chloride. Under these conditions, the gold
is precipitated by light as a black powder if the liquid be in any
bulk, and if merely washed over paper a stain is produced, which,
however feeble at first, under a cei-tain dosage of the chloride,
oxalate, and free acid, goes on increasing from day to day and
fi'om week to week, when laid by in the dark, and especially in
a damp atmosphere, till it acqiiii'es almost the blackness of ink ;
tlie unsunned portion of the paper remaining unaffected, or so
slightly as to render it almost certain that what little action of
the kind exists is due to the effect of casual dispersed light
incident in the preparation of the paper. I have before me a
specimen of paper so treated in which the effect of thirty
seconds' exposiu-e to sunshine was quite invisible at first, and
which is now of so intense a purple as may well be called black,
while the imsunned portion has acquired comparatively but a
very slight brown. And (which is not a little remarkable, and
indicates that in the time of exposui-e mentioned the maximum
of effect was attained) other portions of the same paper exposed
in graduated progression for longer times, viz. 1 min., 2 min.,
and 3 min., are not in the least perceptible degree darker than
the portion on which the light had acted during thirty seconds
only.

" If paper prepared as above recommended for the crysotype,
either "with the ammonio-citrate or ammonio-tartrate of iron, and
impressed, as in that process, with a latent picture, be washed
with nitrate of silver instead of a solution of gold, a very sharp
and beautifid pictui'e is developed, of great intensity. Its dis-
closure is not instantaneous ; a few moments elapse without
apparent effect ; the dark shades are then first touched in, and
by degrees the details appear, but much more slowly than in the
case of gold. In two or thi'ee minutes, however, the maximiun
of distinctness will not fail to be attained. The picture may be
fixed by the hyposulphite of soda, which alone, I believe, can be
fully depended on for fixing argentine photographs.

" The best process for fixing any of the photographs prepared
with gold is as follows : — As soon as the pictm-e is satisfactorily
brought out by the aiu'iferous liquid, it is to be rinsed in spring
water, which must be three times renewed, letting it I'emain in
the third water five or ten minutes. It is then to be blotted off

PHOTOGRAPHIC PROPERTIES OF MERCURY. 57

and dried, after -which it is to be washed on both sides with a
somewhat weak solution of hvdriodate of potash (iodide of fotas-
sium). If there be any fi-ee chloride of gold present in the pores of
paper, it will be discoloured, the lights passing to a ruddy brown ;
but they speedily whiten again spontaneously, or at all events on
throwing it (after lying a minute or ttvo,) into fresh water, in
which, being again rinsed and di'ied, it is now perfectly fixed."

Section III. — Photographic Properties of Mercury.

" As an agent in the dagueiTeotj'pe process, it is not, strictly
speaking, photographically affected. It opei-ates there only in
\"ii-tue of its readiness to amalgamate with silver pr-operly pre-
pared to receive it. That it possesses direct photographic sus-
ceptibility, however, in a veiy eminent degree, is proved by the
following experiment. Let a paper be washed over with a weak
solution of periodide of ii'on, and, when dry, with a solution of
proto-niti-ate of mercury. A bright yellow paper is produced,
which (if the right strength of the liquids be hit) is exceedingly
sensitive while wet, darkening to a bro-wn coloiu- in a very few
seconds in the sunshine. Withdra^^^l, the impression fades
rapidly, and the paper in a few horns recovers its original colour.
In operating this change of colour, the whole spectrum is effec-
tive, "ss-ith the exception of the thei-mic rays beyond the red.

" Proto-nitrate of merciuy simply washed over paper, is slowly
and feebly blackened by exposui-e to sunshine. And if paper
be impregnated with the ammonio-citrate of iron, already so
often mentioned, partially sunned, and then washed with the
proto-nitrate, a reduction of the latter salt, and consequently
blackening of the paj^er, takes place very slowly in the dark over
the sunned portion, to nearly the same amount as in the dii-ect
action of the Hght on the simply nitrated paper.

" But if the mercxuial salt be subjected to the action of light
in contact vnih the ammonio-citrate or tartrate, the efiect is far
more powerful. Considering, at present, only the citric double
salt, a paper prepared by washing fii'st with that salt and then
with the mercuiial proto-nitrate (diying between) is endowed
with considerable sensibility, and dai'kens to a very deep broAvn,
nay, to complete blackness, on a moderate exposure to good
sun. Veiy sharp and intense photographs of a negative cha-
racter may be thus taken. They are, however, difficult to Jix.
The only method which I have foimd at all to succeed has been
by washing them with bichromate of potash and soaking them
for twenty-foui- hours in water, which dissolves out the chi'omate

58 HISTORY OF PHOTOGRAPHY.

of mercury for the most part ; leaving, however, a yellow tint
on the groimd, which resists obstinately. But though pretty
effectually fixed in this way against light, they are not so against
time, as they fade considerably on keeping.

" When the proto-niti-ate of mercury is mixed, in solution, with
either of the ammoniucal double salts, it forms a precipitate, which,
worked up -with a brush to the consistence of cream, and spread
upon paper, produces very fine pictures, the intensity of which it
is almost impossible to go beyond. Most unfortunately, they
cannot be preserved. Every attempt to fix them has resulted
in the destruction of their beauty and force ; and even when
kept from light, they fade with more or less rapidity, some
disappearing almost entii^ely in three or foui- days, while others
have resisted tolerably well for a fortnight, or even a month.
It is to an over-dose of tartaric acid that their more rapid deterio-
ration seems to be due, and of coui-se it is important to keep
down the proportion of this ingredient as low as jiossible. But
wthout it I have never succeeded in producing that peculiar
velvety aspect on which the charm of these pictiu'es chiefly
depends, nor anything like the same iutensity of colom* without
over-suuTiing."

Section IV. — Ferro-Tartbate of Silver.

Extending his inquiries still fiu'ther into these very remarkable
changes, the following process presented itself to Sir J. Herschel,
which is hi many respects remarkable.

If nitrate of silver, sj^ecific gra^-ity 1 -200, be added to ferro-
tartaric acid, specific gravity 1'023, a precipitate falls, which is
in great measure redissolved by a gentle heat, leaving a black
sediment, v/hich, being cleared by subsidence, a liquid of a pale
yellow coloiu' is obtained, in which a fui'ther addition of the
nitrate causes no tvu-bidness. When the total quantity of the
nitrated solution amounts to about half the bulk of the ferro-
tartaric acid, it is enough. The liquid so prepared does not
alter by keeping in the dark.

Spread on paper, and exposed wet to the sunshine (partly
shaded) for a few seconds, no impression seems to have been
made ; but by degrees (although withdi'awn from the action of
the light) it developes itself spontaneously, and at length be-
comes very intense. But if the paper be thoroughly di-ied in
the dark (in which state it is of a very pale gi'eenish-yellow
colour), it possesses the singidar property of recei\'ing a dormant
or invisible picture j to produce which (if it be, for instance, an

SIR J. HERSCHEl's AilPHITTPE. 59

engraving that is to be copied), from thirty seconds' to a minute's
exposTire in the stmshine is requisite. It should not be continued
too long, as not only is the ultimate effect less stiiking, but a
picture begins to be ^-isibly produced, which dai-kens spont-
neously after it is 'withch-a-mi. But if the exposiu-e be discon-
tinued before this effect comes on, an invisible impression is the
result, to develope which aU that is necessary is to breathe upon
it, when it immediately appears, and very speedily acquires an
extraordinary intensity and shai-pness, as if by magic. Instead
of the breath, it may be subjected to the regulated action of
aqueous vapour by laying it in a blotting-paper book, of which
some of the outer leaves on both sides have been damp, or by
holding it over warm water.

Many preparations, both of silver and gold, possess a similar
property, in an inferior degi-ee ; but none that I have yet met
with, to anything like the extent of that above described.

These pictiu-es do not admit of being pennanently fixed ; they
are so against the action of light, but not against the operations
of time. They slowly fade out even in the dark ; and in some
examples which I have prepared, the remarkable phenomenon
of a restoration after fading, but vriih reversed Kghts and shades,
has taken place.

Section Y. — The Amphitype.

The following very remarkable process was communicated by
Sir John Herschel, at the meeting of the British Association at
York. The process cannot be regarded as perfect, but from its
beauty when success is obtained, and the cui-ious nature of aU
its phenomena, it is deemed important to include it, ia the hope
of inducing some investigator to take it up.

Su- John Herschel says, alluding to the processes just de-
scribed, '•' I had hoped to have perfected this process so far as to
have reduced it to a definite statement of manipvdations which
would insure success. But, capricious as photogi-aphic processes
notoriously are, this has proved so, even beyond the ordinary
measure of such caprice. * * * Paper proper for producing
an amphitype pictiu-e may be prepared either with the ferro-
tartrate or the ferro-citrate of the protoxide or the peroxide of
mercun^, or of the protoxide of lead, by u.sing creams of these
salts, or by successive applications of the nitrates of the respec-
tive oxides, singly or in mixtiu-e, to the paper, alternating with
solutions of the ammonio-tarti-ate or ammonio-eitrate of iron,
the latter solution being last applied, and in more or less ex-

60 HISTORY OF PHOTOGRAPHY.

cess. * ■■* " Paper so prepared aud diied takes a negative
pictxire, in time varying from half an hour to five or six hours,
according to the intensity of the light; ?nd the impression pro-
duced varies in apparent force from a faint and hardly percep-
tible pictm-e to one of the highest conceivable ftilness and rich-
ness both of tint and detail, the colour in this case being a
superb velvety browna. This extreme richness of effect is not
produced except lead be present either in the ingredients used,
or in the paper itself. It is not, as I origiually supposed, due to
the presence of free tartaric acid. The pictures in. this state
are not pennanent. They fade in the dark, though with very
different degrees of rapidity, some (especially if free tartaric or
citric acid be present) in a few days; w^hile others remain for
weeks unimpafred, and require whole years for their total ob-
literation. But, though entii'ely faded out in appearance, the
pictui-e is only rendered dormant, and may be restored, chang-
ing its character fr-om negative to positive, and its colour' from
bro^-n to black (in the shadows) by the folloA\-ing process : —
A bath being prepared by ponring a small quantity of solution
of per-nitrate of merciuy into a large quantity of water, and
letting the sub-nitrated precipitate subside, the pictui-e must be
immersed in it (carefidly and repeatedly cleai'ing off the air-
bubbles) and allowed to remain till the picture (if anywhere
"\4sible) is entii"ely destroyed, or if faded, till it is judged suffi-
cient from pre\"ious experience; a term which is often marked
by the appearance of a feeble jDositive pictiu-e of a bright yellow
hue on the pale yellow gi'ouud of the paper. A long time
(several weeks) is often requii-ed for this, but heat accelerates
the action, and it is often complete in a few houi's. In this
state the pictm*e is to be veiy thoroughly rinsed and soaked in
pure warm water, and then cfried. It is then to be well ii-oned
with a smooth iron, heated so as barely not to injm-e the paper,
placing it, for better security against scorching, between smooth
clean papers. If, then, the process have been successful, a j^er-
fectly black positive pictui-e is at once developed. At first it
most commonly happens that the whole picture is sooty or
dingy to such a degree that it is condemned as spoiled, but on
keeping it between the leaves of a book, especially in a moist
atmosphere, by extremely slow degrees this dinginess disap-
pears, and the picture disengages itself A\'ith continually in-
creasing sharpness and clearness, and acquii-es the exact effect
of a copperplate engraving on a paper more or less tinted with
pale yellow.

" I ought to observe, that the best and most unifonn speci-
mens which I have procured have been on paper previously

THE COLOUKING MATTER OF FLOWERS. 61

washed -wdth ceHain preparations of vu'ic acid, which is a veiy
remarkable and powerful photographic element. The intensity
of the original negative picture is no criterion of what may be
expected in the positive. It is from the production, by one and
the same action of the light, of either a positive or a negative
picture, according to the subsequent manipulations, that I have
designated the process thus generally sketched out, by the tenn
" Ani2Jhitype f a name suggested by Mr. Talbot, to whom 1
communicated this singiilar result ; and to this process or class
of processes (which I cannot doubt when piu'sued will lead to
some veiy beautiful results) I propose to restrict the name in
question, though it applies even more ajipropriately to the fol-
lowing exceedingly curious and remarkable one in which silver
is concerned. At the last meeting I annoimced a mode of pro-
ducing, by means of a solution of silver in conjunction with
ferro-tartaric acid, a dormant picture brought out into a forci-
ble negative impression by the breath, or moist aii-. The solu-
tion then descx'ibed, and which had at that time been prepared
some weeks, I may here accidentally remark, has retained its
limpidity and photogTaphic properties quite unimpaii-ed dui'ing
the whole year since elapsed, and is now as sensitive as ever — a
property of no small value. Now, when a picture (for example,
an impression from an engTaving) is taken on paper washed
with this solution, it shows no sign of a picture on its back,
whether that on its face be developed or not ; but if, while the
actinic influence is still fresh iipon the face {i.e. as soon as it is
removed from the light), the hack be exjDosed for a very few
seconds to sunshine, and then removed to a gloomy place, a
positive pictiu'e, the exact complement of the ner/ative one on the
other side, thoiigh wanting of course in sharpness if the paper be
thick, sloioly and gradually makes its appearance there, and in
half an hour requires considerable intensity. I oiight to men-
tion that the ferro-tartaric acid in question is prepared by pre-
cipitating the feiTO-tartrate of ammonia by acetate of lead, and
decomposing the precipitate by dilute stdphiu'ic acid."

Section YI. — The Colouring Matter of Flowers.

The residts obtained by Sir John Herschel on the colouring
juices of flowers are too remaikable to be omitted in a treatise
in which it is desirable that every point should be registered up
to the date of publication, which connects itself with the pheno-
mena of chemical change applied to photography.

" In operating on the colouis of flowers, I have usually pro-
ceeded as follows: — The petals of the fresh flowers, or rather

62 HISTORY OF PHOTOGRAPHY.

STich parts of them as possessed a uniform tint, were cruslied
to a pulp in a marble moii-ar, either alone, or "with addition of
alcohol, and the juice expressed by squeezing the pulj) in a clean
linen or cotton cloth. It was then spread on paper with a flat
briLsh, and ckied in the air without artificial heat, or at most
with the gentle warmth which rises in the ascending cuiTent of
air ft'om an Amott stove. If alcohol be not added, the applica-
tion on paper must be perfoi'med immediately, since exposure
to the au- of the juices of most flowers (in some cases even but
for a few minutes) iiTecoverably changes or destroys then* co-
lour. If alcohol be present, this change does not usually take
place, or is much retarded; for which reason, as well as on
account of certain facilities afibrded by its admixture in pro-
curing an even tint (to be presently stated), this addition was
commonly, but not always, made.

"Most flowers give out their colouring matter readily enough,
either to alcohol or water. Some, however, as the Escholzias
and Calceolarias, refuse to do so, and requii'e the adcUtion of
alkalies, others of acids, &g. When alcohol is added, it should,
however, be observed that the tint is often apparently much
eufeebled, or even discharged altogether, and that the tinctm*e,
when spread on paper, does not reappear of its blue intensity
till after complete chying. The temporaiy destniction of the
colour of the blue heartsease by alcohol Ls cmious, nor is it by
any means a singular instance. In some, but in very few cases,
it is destroyed, so as neither to reappear on chying, nor to be
capable of revival by any means tried. And in all cases long
keeping deteriorates the coloxirs and alters the qualities of the
alcoholic tinctures themselves ; so that they should always be
used as fresh as possible.

" If papers tinged with vegetable colours are intended to be
presei-ved, they must be kept perfectly dry and in darkness.
A close tin vessel, the air of which is dried by quicklime (care-
fiiUy enclosed in double paper bags, well pasted at the edges to
prevent the dust escaping), is used for this pm^jose. Moisture
(as already mentioned, especially assisted by heat) destroys them
for the most pai-t rajjidly, though some (as the coloiu- of the
Senecio sjjiendens) resist obstinately. Then- destructibihty by
this agency, however, seems to bear no distinct relation to their
photographic properties.

" This is also the place to observe that the coloui' of a flower is
by no means always, or usually, that which its expressed juice
impai'ts to white paper. In many cases the tints so imparted
have no resemblance to the original hue. Thus, to give only a
few instances, the red damask rose of that intense variety of

THE COLOURIXG MATTER OF FLOWEES. 63

coloiir commonly called by florists the black rose, gives a dark
slate blue, as do also the clove cai'uation and the black holly-
hock : a fine dark bro^Ti variety of spai'axis give a dull olive
gi-een ; and a beautiful rose-coloiu'ed tulip, a dii-ty bluish green ;
but perhaps the most striking case of this kind is that of a com-
mon sort of red poppy {Papaver Kheuni), whose expressed juice
imparts to paper a rich and most beautiful Ijlue colour, whose
elegant properties as a photographic material ^vtH be further
alluded to hereafter. *

" This change of coloiu' is probably owing to different causes
in difierent flowers. In some it undoubtedly ai'ises from the
escape of carbonic acid, but this, as a general cause for the change
from red to blue, has, I am aware, been controverted. In some
(as is the case "wdth the yellow ranunculi) it seems to arise from
a chemical alteration depending on absorption of oxygen ; and in
others, especially where the expresssd juice coagulates on stand-
ing, to a loss of vitality or disorganization of the molecules. The
fresh petal of a single flower, merely ciiished by rubbing on diy
paper, and instantly dried, leaves a stain much more nearly ap-
proximating to the original hue. This, for examjjle, is the only
way in which the fine blue colovu- of the common field veronica
can be imparted to paper. Its expressed juice, however quickly
prepared when laid on with a bnish, afibrds only a dii-ty neutral
grey, and so of many others. But in this way no even tint can
be had, which is a fii-st requisite to the experiments now in ques-
tion, as well as to their application to photogi-aphy.

" To seciu'e this desirable evenness of tint, the following mani-
pulation ^"ill generally be found successful ; — The pajjer should
be moistened at the back by sponging and blotting off. It should
then be pinned on a boai-d, the moist side do-^mwards, so that
two of its edges (suppose the right-hand and lower ones) shall
project a little beyond those of the board. The board being
then inclined twenty or thii-ty degi-ees to the horizon, the alco-
holic tinctm-e (mixed ^vith a very little water, if the petals them-
selves be not very juicy) is to be applied "v\-ith a brush in strokes
fi'om left to I'ight, taking care not to go over the edges which
rest on the boai'd, but to pass cleai-ly over those which project,
and observing also to carry the tint from below upwards by
quick sweeping strokes, leaving no diy spaces between them,
but keeping up a continuity of wet sm-face. When all is wet,
cross them by another set of strokes from above downwards, so
managing the brush as to leave no floating liquid on the paper,

* A semi-cultivated variety was used, having dark purple spots at the bases
of the petals. The common red poppy of the chalk {Fajjavier hyhridum) gives
a purple colour much less sensitive aud beautiful.

64 HISTORY OF PHOTOGRAPHY.

It must then be diied as quickly as jjossible over a stove, or in
a cui-rent of "warm aii* ; avoiding, however, such heat as may
injure the tint. The i:)resence of alcohol prevents the solution
of the gummy 'principle, which, when present, gives a smeary
surfiice ; but the evenness of tint given by this process results
chiefly from that singular intestine movement which always
takes place when alcohol is in the act of separation from water
by evaporation ; a movement which dispei"ses knots and blots in
the film of liquid "s\T.th great energy, and spreads them over the
surrounding sui-face.

" Corchorus Japonica. — The flowers of this common and hardy
but highly ornamental plant are of a fine yellow, somewhat in-
clining to orange, and this is also the coloui* the expressed juice
imparts to paper. As the flower begins to fade the petals ivMten, —
ajn indication of their photographic sensibility which is amply
verified on exjjosiu-e of the stained paper to sunshine. I have
hitherto met "sv-ith no vegetable coloiu- so sensitive. If the
flowers be gathered in the height of then* season, paper so coloiu-ed
(wliicli is of a very beautiful and even yellow) begins to discoloui"
in ten or twelve minutes in cleai* sunshine, and in half an hour
is completely whitened. The colour seems to resist the first
impression of the light, as if by some remains of vitality, which
being overcome, the tint gives way at once, and the discolora-
tion, whem commenced, goes on rapidly. It does not even cease
in the dark lohen once begun. Hence it happens that photographic
impressions taken on such paper, which, when fresh, are very-
sharp and beautiful, fade by keeping, visibly fr-om day to day,
however carefully preserved fr-om light. They requii'e fr-om
half an hoiu- to an hoiu- to complete, according to the sunshine.
Hydi'iodate of potash cautiously applied retards considerably, but
does not ultimately prevent, this spontaneous dischaige.

'■'■Common Ten Weeks Stocks: 21 athiola annua. — Paper stained
with the tincture of this flower is changed to a vivid scarlet by
acids, and to green by alkalies ; if ammonia be used the red
coloui- is restored as the ammonia evaporates, proving the absence
of any acid quality in the coloiu-ing matter sufficiently energetic
to coerce the elastic force of the alkaline gas. Sulphurous acid
whitens it, as does the alkaline sulphites ; but this efiect is
transient, and the red coloiu' is slowly restored by fr-ee exposure
to ail-, especially \ni]i the aid of hght, whose influence in this case
is the more remarkable, being exactly the reverse of its ordinary
action on tliis coloui-ing principle, which it destroys irrecoverably,
as above stated. The following experiments were made to trace
and illustrate this ciu-ious change : —

" Two photogi-aphic copies of engravings taken on paper tinted

THE COLOmrS'G MATTER OF FLOWERS. 65

witli this colour were placed in a jar of sulpliiu-otis acid gas, by
which they were completely whitened, and all traces of the pic-
tures obliterated. They were then exposed to fi-ee air, the one
in the dark, the other in sunshine. Both recovered, but the
former much more slowly than the latter. The restoration of the
picture exposed to the sun was completed in twenty-foiu' hours,
that in the dai-k not till after a lapse of two or three days.

" A slip of the stained paper was wetted with liquid sulphu-
rous acid, and laid on blotting-jjaper similarly wetted. Being
then crossed with a strip of black jiaper, it was laid between
glass plates and (cTaporation of the acid being thns prevented)
was exposed to fuU sunshine. After some time the red colour
(in spite of the presence of the acid) was considerably restored
in the portion exposed, wliile the whole of the portion covered
by the black paper remained (of course) perfectly white.

" Slips of paper, stained as above, were placed under a receiver,
beside a small cajisule of liquid sulphurous acid. When com-
pletely discoloui'ed, they were subjected (on various occasions,
and after various lengths of exposure to the acid fiimes, from
half an hoiu* to many days) to the action of the spectrum ; and
it was found, as indeed I had expected, that the restoration of
colour zvas ojierated hy rays coniplenientary to those lohich destroy
it in the natural state of the paper ; the violet rays being chiefly
active, the blue almost equally so, the green little, and the yel-
low, orange, and most refrangible red, not at all. In one expe-
riment a pretty-well defined red solar image was developed by
the least refrangible red rays also, being precisely those for which,
in the unprepai'ed paper the discolovuing action is abruptly cut
off. But this spot I never succeeded in reproducing; and it
ought also to be mentioned, that, according to differences in the
preparation not ob^dous, the degree of sensibility, generally, of
the bleached paper to the restorative action of light, differed
gi'eatly ; in some cases a perceptible reddening being j^roduced
in ten seconds, and a considerable streak in two miniites, while
in others a veiy long time was requii-ed to produce any effect.
The dormancy of this colouring principle, imder the influence of
sulphiu-ous acid, is well shown by dropping a little weak sulphu-
ric acid on the paper bleached by that gas, which immediately
restores the red colour in all its vigour. In like mamier alka-
nes restore the colour, converting it at the same time into green.
" Pajxiver orientale. — The chemical habitudes of the sulphu-
rous acid render it highly probable that its action in inducing a
dormant state of the colorific principle, consists in a jiartial
deoxidizement, unaccompanied, however, with disorganization of
its molecules. And this view is coiToborated by the similar ac-

66 HISTORY OF PHOTOGRAPHY.

tion of alcohol already spoken of; similar, tliat is, in kind,
though less complete in degree. Most commonly, vegetable
colours, weakened by the action of alcohol, are speedily restored
on the total evaporation of the ingredient. But one remarkable
instance of absolute dormancy induced by that agent has oc-
curred to me in the case of Pajmver orientale, a flower of a vivid
orange coloiu-, bordering on scarlet, the colom-ing matter of
which is not extractable otherwise than by alcohol, and then
only in a state so completely masked as to impart no more than
a faint yellowish or pinkish hue to paper, which it retains when
thoroughly dry, and apparently dm-ing any length of time, with-
out perceptible increase of tint. If at any time, however, a
di'op of weak acid be appHed to jDaper prepared with this tincture,
a vivid scarlet coloiu- is inrmediately developed ; thus demon-
stating the continued though latent existence of the coloming
principle. On observing this, it occurred to me to inquire whe-
ther, in its dormant state, that principle stUl retained its sus-
ceptibility of being acted on by light, since the same powerful and
delicate agent which had been shown, in so many cases as to con-
stitute a general laAv, capable of disoi'ganizing and destroying
vegetable coloiu-s actually developed, might easily be presumed
competent to destroy the capacity for assuming colour, in such
organic matter as might possess it, under the influence of their
otherwise appropriate chemical stimuli. A strip of the paper
was therefore exposed for an hour or two to the spectiixm, but
without any sensible effect, the whole sm-face being equally red-
dened by an acid. As tlus experiment sufficiently indicated the
action of light, if any, to be very slow, I next placed a strip,
partly covered, in a south-east window, where it remained from
June 19 to August 19, receiving the few and scanty sunbeams
which that interval of the deplorable summer of 1841 afforded.
When removed, the part exposed could barely be distinguished
from the part shaded, as a trifle yellower. But on applying
acid, the exposed and shaded portions were at once distinguished
by the assumption of a vivid red in the latter, and the former
remaining u.nchaiiged.

" A mezzotinto pictiu'e was now pressed on a glazed frame over
another portion of the same paper, and abandoned on the upper
shelf of a green-house to whatever sun might occm- from August
19 to October 19. The interval proved one of almost uninter-
rupted storm, rain, and darkness. On removal, no appearance
whatever of any im^jressed pictm-e coidd be discerned, nor was
it even possible to tell the top of the pictm-e from the bottom. It
was then exposed in a glass jar to the fumes of miuriatic acid,
when, after a few minutes, the development of the dormant pic-

THE COLOUING MATTER OF FLOAATIES. 67

tiire commenced, and slowly proceeded, disclosing the details in
a soft and pleasing style. Being then laid by in a drawer, with
free access of aii*, the picture again faded, by very slow degi'ees,
and on January 2, 1842, was foimd quite obliterated. Being
then sxibjected to the acid vapoiu' the colour was reproduced.

" Viola odorata. — Chemists are familiar with the colour of this
flower as a test of acids and alkalies, for which, however, it seems
by no means better adapted than many othei-s ; less so, indeed,
than that of the Violar tricolor, the common puqjle ii'is, and
many others which might be named. It offers, in fact, another
and rather a striking instance of the simultaneous existence of
two coloiuing ingredients in the same flower, comporting them-
selves differently, not only in regard to light but to chemical
agents. Extracted with, alcohol, the juice of the violet is of a
rich blue coloiu-, which it impai-ts in high perfection to paj^er.
Exposed to sunshine, a portion of this colour gives way pretty
readily, but a residual blue, rather inclining to greenish, resists
ob,stinately, and requires a very much longer exposure (for whole
weeks, indeed) for its destruction, which is not even then com-
plete. Photographic impressions, therefore, taken on this paper,
though veiy pretty, are exceedingly tedious in their preperation,
if we would have the lights sharply made out.

" Sparaxis tricolor ?, var. — Stimulating Effects of Akcdies. —
Among a gi-eat many hybrid varieties of this genus, lately for-
warded to me from the Cape, occiu-red one of a very intense pur-
plish-brown colour, nearly black. The alcoholic extract of this
flower in its liquid state is rich crimson-bro-wni. Spread on paper,
it imparted a dark olive-green colom-, which proved perfectly in-
sensible to veiy prolonged action, either of sunshine or the
spectrum. The addition of carbonate of soda changed the colour
of this tincture to a good gi'een, slightly inclining to olive, and
which imparted the same tint to paper. In this state, to my
surprise, it manifested rather a high degree of photographic sen-
sibility, and gave very pretty pictures with a day or two of ex-
posiu-e to sunshine. When prepared with the fresh juice there
is hardly any residual tint, but if the j^aper be kept, a gi-eat
amoimt of indestiiictible yellow remains outstanding. The ac-
tion is confined chiefly to the negative end of the spectiiim; all
but the fii-st five or six parts beyond the yeUow show little more
than a trace of action. A photogi-aph impressed on this paper
is reddened by muriatic acid fumes. If then transferred to an
atmosphere of ammonia, and when super-satiu"ated the excess of
alkali allowed to exhale, it is fixed, and of a dark green colour.
Both the tint and sharpness of the picture, however, sufler
in this process.

68 HISTORY OF PHOTOGRAPHY.

" Red Poppy : Papaver Rheum 1 — Among the vegetable coloiu-s
totally destroyed by light, or wliicli leave no residual tint, at
least when fresh prepared, perhaps the two most rich and
beautiful are those of the red poppy and the double purple
groundsel (Senecio sjdendensj. The former owes its red coloiu-
in all probability to free cai'bonic acid, or some other (as the
acetic), completely expelled by drying : for the colour its tincture
imparts to paper, instead of red, is a fine blue very slightly
verging on slate-blue. But it has by no means the ordinary
chemical characters of blue vegetable colours. Cai-bonate of
soda, for instance, does not in the least degree tiu'n the expressed
juice gi-een ; and when washed with the mixtu,re, a paper results
of a light slate-grey, hardly at all inclining to green. The blue
tincture is considerably sensitive, and from the ricliness of its
tone and the absence of residual tint, paper stained with it
affords photographic impressions of great beauty and sharpness,
some of which will be found among the collection submitted
with this paper for inspection.

"Senecio sjylendens. — This flower yields a rich piirple juice in
great abimdance and of surprising intensity. Nothing can
exceed the rich and velvety tint of paper tinted while it is fresh.
It is, however, not very sensible to light, and many weeks are
necessaiy to obtain a good photographic impression."

In the progress of my own researches on this subject, I found
that the green colouiing matter of the leaves of herbaceous
plants, when spread upon paper, changed with tolerable rapidity
when exjiosed to simshine. There are, however, some very
curious points connected with the phenomena of these changes
which demand a far more extensive investigation than they have
yet received.

I find that the juices taken from the leaves in the spring,
change more rapidly than when expressed from the same plants
in the autumn ; and the juices of those flowei"ing plants which
have been cultivated under the artificial circumstances of
a store-house, or conservatory, are more readily afiected than
such as are grown in the open air. Many of the experiments
just described fimiish very instructive examples of the opera-
tions of the solar rays upon, organic bodies, from which we may
deduce important truths connected with natural phenomena.

CHAPTER VI.

miscella:nt:ous processes.

Section I. — Mr. Ponton's Process. (Bichromate of
Potash.)

Under the general term of the Chromatyije, I would propose to
include all those processes which involve the use of any of the
salts of chromium. It was originally introduced to distinguish
a particular process which I discovei'ed, and published at the
meeting of the British Association at Cork, in August 1843;
biit it ajjj^ears very convenient to adopt the princijile intro-
duced by Sir John Herschel, of grouping the phenomena of
photogi'aphy imder special terms derived from the most prominent
chemical jjrepai-ation employed.

There are many preparations which are affected by Kght in a
similar manner to the salts of silver. Several have been tried as
photogi-ajjhic materials, but as yet without much success, with
the exception of the bichromate of potash, which was &st an-
nounced as a useful photographic agent by Mr. 'Mungo Ponton,
in the Edinburgh New Philosophical Joiu-nal ; from which I
quote Mr Ponton's own accoimt.

" "When paper is immersed ia the bichromate of potash, it is
powerfidly and rapidly acted on by the sun's rays. When an
object is laid in the usual way on this paper, the portion exposed
to the light speedily becomes tawny, passing more or less into
a deep orange, accorcHng to the strength of the light. The por-
tion covered by the object retains the original bright yellow tint
which it had before exposure, and the object is thus represented
yellow upon an orange ground, there being several gradations of
shade, or tint, according to the greater or less degree of trans-
parency in the different parts of the object.

" In this state, of course, the drawing, though very beautiftil,
is evanescent. To fix it, all that is required is careftil immersion
in water, when it will be found that those portions of the salt
which have not been acted on by the light are readily dissolved
out, wliile those which have been exposed to the light are com-
pletely fixed on the paper. By the second process the object is
obtained white upon an orange ground, and quite permanent. If
exposed for many hoiu-s together to strong simshine, the colour
of the ground is apt to lose in depth, but not more so than most
other coloming matters. This action of light on the bichromate

70 HISTORY OF PHOTOGRAPHY.

of potasli differs from that upon the salts of silver. Those of the
latter which are blackened by light are of themselves insoluble
in water, and it is difficult to impregnate paper with them in a
uniform manner. The blackening seems to be caused by the for-
mation of oxide of silver.

" In the case of the bichromate of potash, again, that salt is
exceedingly soluble, and paper can be easily satui-ated with it.
The agency of light not only changes its coloiu-, but deprives it
of solubility, thus rendering it fixed in the paper. This action
appears to consist in the disengagement of fi-ee chromic acid,
which is of a deep red coloui", and which seems to combiue with
the paper. This is rendered more probable from the circum-
stance that the neutral chi-omate exhibits no similar change.
The best mode of preparing paper with bichromate of potash is
to use a satiu'ated solution of that salt ; soak the pa^^er well in
it, and then dry it rapidly at a bi-isk fii-e, excluding it from day-
light. Paper thus prepared acquii'es a deep orange tint on ex-
posiu-e to the sun. If the solution be less strong, or the chying
less rapid, the colonr will not be so deep. A pleasing variety
may be made by using sidphate of indigo along with the biclii'o-
mate of potash, the coloiu* of the object and of the paper being
then different shades of gi'een. In this way, also, the object
may be represented of a darker shade than the ground."

Paper prepared with the bichromate of potash, though as
sensitive as some of the papers prepared with the salts of silver,
is mvich inferior to most of them, and is not sufficiently sensitive
for the camera obscm-a. This paper, however, answers quite
well for taking drawings from dried plants, or for copying prints.
Its great recommendation is its cheapness, and the facility with
which it can be prepared. The price of the bichromate of potash
is about two shillings per pound, whilst the nitrate of silver is
five shillings the oujice.

As the deep orange ground of these pictures prevents the per-
meation of the chemical rays of light, it is very easy to procure
any number of facsimiles of an engraving, by transfer from the
first negative photograph. The correct copies have a beautiful
sharpness; and, if carefidly managed, but Kttle of the minute
detail of the original engraving is lost.

A photogi-aphic paper prepared with the bichromate of
potash of another kind is described by M. E. Becquerel. He
states, — It is sufficient to steep a paper prejoared in Mr. Ponton's
manner, and upon which there exists a faint copy of a drawing,
in a solution of iodine in alcohol, to wash this paper in alcohol,
and then diy it : then the parts which were white become blue,
and those which were yellow remain more or less clear.

ME. ponton's process. 71

M. E. Becquerel lias pursued liis investigation!? into tlie action
of the ckromic acid on organic compounds, and lias slio^vn that
the mode of sizing the papers influences their coloration by
light, and that with unsized paper coloration is effected only
after a long time. Perceiving that the principal reaction re-
svdted from the chromic acid contained in the bichromate of
potash, on the starch in the size of the paper, it occniTed to ]M.
E. Becquerel, that, as starch has the property of forming ^\-ith
iodine a combination of a very fine blue colour, it should pro-
duce deep shades of that tint, whilst the lights still remained an
orange-yellow.

His method of proceeding is to spread a size of starch very
uniformly over the surface of the paper. It is then steeped in
a weak alcoholic solution of iodine, and aftenvai'ds washed in a
great quantity of water. By this immersion it should take a
very fine blue tint. If this is uniform, the paper is considered
fit for the experiment ; in the contrary case it is sized again.
It is then steeped in a concentrated solution of bichromate of
potash, and pressed between folds of blotting-paper, and di-ied
near the fire. To be effective, it should be very dry.

It is now fit for use. When the copy is effected, which re-
quires in sunshine about five minutes, the photogi-aph is washed
and ch'ied. A\Tien diy, it is steeped in a weak alcoholic solution
of iodine, and afterwards, when it has remained in it some time,
it is washed in water, and carefully dried ^vith blotting-paper,
but not at the fii*e, for at a little below 100® Fahr. the combi-
nation of iodine and starch discolours.

If it be considered that the drawing is not sufficiently dis-
tinct, this immersion may be repeated several times; for by
this means may be obtained the intensity of tone that is de-
sired, which intensity can be changed at will by employing a
more concentrated solution of iodine.

When the paper is damp, the shades are of a veiy fine blue,
but when it is dry the colour becomes deep violet. If while
the di'awing is still wet it be covered with a lawyer of gum.
ai-abic, the colour of the drawing is greatly presei'ved, and more
beautiful when it is diy. When a paper is thus prepared, it
loses at first a little of its tone, but it afterwards presei-ves its
violet tint.

Section II. — The Chromatype.

This process, devised by the author, is a pleasing one in its
results : it is exceedingly simple in its manipulatory details, and

72 HISTORY OF PHOTOGRAPHY,

produces very cliai'ming positive pictures by the fii-st appKca-
tion. The chromatype is founded on the above process of
Mr. Ponton's: —

One di-achm of sulphate of copper is dissolved in an ounce of
distilled water, to which is added half an oimce of a saturated
solution of bichi'omate of potash ; this solution is applied to the
sui'face of the paper, and, when diy, it is fit for use, and may be
kept for any length of time without spoiling. When exposed
to sunshine, the first change is to a didl brown, and if checked
in this stage of the process we get a negative pictiu-e, but if the
action of the light is contiuued, the browning gives way, and
we have a positive yellow pictm-e on a white groiind. In either
case, if the paper, when removed from the sunshine, is washed
over with a solution of nitrate of silver, a very beautiful positive
pictui^e results. In practice, it will be found advantageous to
allow the bleaching action to go on to some extent; the pictiu-e
i-esulting from this will be clearer and more defined than that
which is procured when the action is checked at the brown
stage. To fijc these pictures it is necessary to remove the
nitrate of silver, which is done by washing in pure water : if the
water contains any muriates the picture sufiers, and long soak-
ing in such water obliterates it, or if a few grains of common
salt are added to the water, the apparent destruction is very
rapid. The pictui'e is, however, capable of restoration ; all that
is necessaiy being to expose it to sunshiue for a quarter of an
hour, when it revives; but instead of being of a red colour, it
becomes lilac, the shades of coloiu- depending upon the quantity
of salt used to decompose the chromate of silver which forms
the shadow parts of the pictiu-e.

Mr. Bingham remarks on this process, that if we substitute
sulphate of nickel for the sulphate of copper, the paper is more
sensitive, and the pictui-e is more clearly developed by nitrate
of silver.

The following modification of this process possesses some ad-
vantages. If to a solution of the sulphate of copper we add a
solution of the neutral chromate of potash, a very copious brown
precipitate falls, which is a tiiie chi'omate of copper. If this
precipitate, after beiug well washed, is added to water acididated
with sulphuric acid, it is dissolved, and a dichromatic solution is
formed, which, when spread upon paper, is of a pure yellow. A
very short exposure of the jjapers washed with this solution is
quite snfficient to discharge all the yellow from the paper, and
give it perfect whiteness. If an engraving is to be copied we
proceed in the usual manner ; and we may either bring out the
picture by placing the paper in a solution of carbonate of soda

THE FERROTYPE. 73

or potash, by which all the shadows are represented by the
chromate of copper, or by washing the paper with nitrate of
silver. It may sometimes happen that, o-wdng to deficient light,
the photograph is darkened all over when the silver is applied :
this colour, by keeping, is gradually removed, and the pictvu-e
comes out clear and sharp.

If the chromate of copper is dissolved in ammonia, a beautiful
gi-een solution results, and if applied to paper acts similarly to
those just described.

The chromatype pictures, under certain conditions, afibrd a
beautiful example of the changes which take place, slowly, in
the dark, li'om the combined operations of the materials em-
ployed.

If we take a chi'omatype picture after it has been developed
by the agency of either nitrate of silver, or of mercuiy, and
place it aside in the dark, it will be found, after a few weeks,
to have dai'kened considerably both in the lights and shadows.
This darkening slowly increases, until eventually the pictiu-e is
obliterated beneath a film of metallic silver or mercvuy; but,
while the pictui-e has been fading out on one side, it has been
developing itself on the other, and a veiy pleasing image is seen
on the back. After some considerable time the metal on the
front gives way again, the paper slowly whitens, and eventually
the image is presented on both sides of the paper of equal in-
tensity, in a good neutral tint upon a grey gToimd. These
results, it will be remembered, are of a very similar character
to those already described as peculiar to the amphitype process
of Sii' John Hei'schel.

Section III. — The Ferrotype.

This process, which is of remarkable sensibility, was discovered
by the author, and published in the Athenceum, under the name
of the Energiatype ; but from a desire to group all those pictiu-es
under a general head into which ii'on salts enter as an element,
the present name is preferred. The preparation of the paper
is as follows:' — Good letter-paper (Whatman's is the best) is
washed over with the following solution, viz. : Five gi-ains of
succinic acid (it is important that succinic free from any oil of
amber, or adventitious mattei-, should be obtained) are to be
dissolved in one fluid omice of water, to which are added about
five grains of common salt, and half a (frachm of mucilage of
gum ai-abic. When tlry, the paper is cfrawn over the surface of
a solution of sixty grains of nitrate of sOver in one ounce of dis-

74 HISTORY OF PHOTOGRAPHY.

tilled "water. Allowed to dry in tlie dark, the paper is now fit
for use, is of a pure white, retains its coloiu", and may be pre-
sented for a considerable time in a portfolio, until wanted for
use.

The preparation of this paper is by no means difficult, but
requires care and attention. The solutions must be apjilied very
equally over the paper, which should be immediately hung upon
a frame or clothes' horse to dry. Extreme care must be taken
that the paper be not exposed to light, after the nitrate of silver
solution has been applied, until required for use. Many of the
disappointments experienced by the experimenters on the ener-
giatype are occasioned by a neglect of this precaution ; as,
although no apparent effect may have been produced by the ex-
posure, the clearness of the subsequent picture will be seriously
injured. The succinic acid must also be very piu-e. "We shall
now briefly describe the method of applying this process to the
different purposes for which it is best adapted, premising that
the varying circumstances of time, place, and light, will render
necessaiy such modifications of the following directions as the
experience of the operator may suggest. As a general rule, an
open situation, sunshine, and, if jjossible, the morning sun,
should be preferred, as the image is sharper, and the coloiir
produced more intense, and less affected by the subsequent
fixing process.

In the camera, for a building or statue, an exposm'e of half a
minute in strong sunshine is usually sufficient ; for a portrait,
taken imder oi'dinary conditions, two or three minutes are
required.

When the paper is taken from the camera, nothing is visible
upon it ; but by attending to the following directions the latent
pictui'e will quickly dev elope itself. Having mixed together
about one ch-achm of a saturated solution oi protosulq^hate of iron
and two or three drachms oi mucilage of gu7n arabic, poiir a small
quantity into a flat dish. Pass the prepared side of the paper
taken from the camera rapidly over this mixture, taking care to
insure complete contact in every part. If the paper has been
sufficiently impressed, the picture will almost immediately appear,
and the further action of the iron must be stopped by the appli-
cation of a soft sponge and plenty of clean water. Should the
image not appear immediately, or be imperfect in its details, the
iron solution may be allowed to remain upon it a short time ;
but it mvist then be kept distiirbed, by rapidly but lightly brush-
ing it up, otherwise numerous black specks will form and
destroy the photograph. Great care should be taken that the
iron solution does not touch the back of the picture, which it

THE CATALYSOTYPE. 75

will inevitably stain, and, the picture being a negative one, be
rendered useless as a copy. A slight degi'ee of heat will assist
the development of the image where the time of exposure has
been too short.

The picture should be carefully washed to take off any super-
ficial blackness, and may then be permanently fixed by being
soaked in water to which a small quantity of ammonia, or, better
still, hyposulphite of soda, has been added. The paper must
again be well soaked in clean water, to clear it from the soluble
salts, and may then be di'ied and pressed.

Exact copies of prints, feathers, leaves, &c., may be taken on
the succinated paper by exposing them to the light in the
copying-frame, until the margin of the prepared paper, which
should be left imcovered, begins to change colour very slightly.
If the object to be copied is thick, the surface must be allowed
to assume a darker tint, or the light "w-ill not have peneti-ated to
the paper.

Positive copies of the camera negatives are prociu'ed in the
same manner as the copies of the prints, &c., just described.
Instead, however, of using the iron solution, the paper must be
exposed to the light, in the frame, a sufficient time to obtain
perfect copies. The progress of the pictiu^e may be observed by
tui'ning up the comer of the paper, and, if not sufficiently done,
replacing it exactly in the same position. They should be fixed
with hyposulphite, as before directed.

At the meeting of the British Association at York in 1844, 1
showed, by a series of photogi-aphs, that the protosuliTiliate of
iron was most efiective in developing any photographic images,
on whatever argentiferous preparation they may have been re-
ceived. Eveiy subsequent result has shown that with proper
care it is the most energetic agent for developing with which we
are acquainted. The difficulty of obtaining, and of preserving,
the salt free of any peroxide, or a basic salt which falls as a
brownish-yellow powder, has been the principal cause why it has
not been so generally employed as the gallic acid : this can be
insiu'ed by adding a few drops of sulphuric acid and some iron
filings to the solution of the protosulphate of iron.

Section IV. — The Catalysotype.

This process of Dr. Wood's is capable of producing pictures
of superior excellence. Owing to the inconstancy of the iodine
compounds, it is a little uncertain, but, care being taken to in-
sure the same degree of strength in the solutions, a very imiform

76 HISTORY OF PHOTOGRAPHY.

good result may be obtained. The process and its modifications
are thus described by the inventor.

''Let well-glazed paper (I prefer that called wove post) be
steeped in water to which hych'ochloric acid has been added in
the proportion of two drops to three ounces. When well wet,
let it be washed over with a mixtiu-e of syrup of iodide of iron
half a drachm, water two drachms and a half, tinctiu-e of iodine
one drop.

" When this has remained on the paper for a few minutes, so
as to be imbibed, dry it lightly with bibulolus paper, and being
removed to a dark room, let it be washed over evenly, by means
of a camel-hair pencil, with a sokition of nitrate of silver, ten
grains to the ounce of distilled water. The paper is now ready
for the camera. The sooner it is used the better ; as when the
ingredients are not rightly mixed it is liable to spoil by keeping.
The time I generally allow the paper to be exposed in the
camera varies from two to thirty seconds ; in clear weather,
without sunshine, the medium is about fifteen seconds. With
a bright light, the pictm^e obtained is of a rich brown colour ;
with a faint light, or a bright light for a very short time continued,
it is black. For portraits out of doors, in the shade on a clear
day, the time for sittuag is from ten to fifteen seconds.

" If the light is strong, and the view to be taken extensive, the
operator should be cautious not to leave the paper exposed for
a longer period than five or six seconds, as the picture will ap-
pear confused from all parts being equally acted on. In all
cases, the shorter the time in which the picture is taken the
better.

" When the paper is removed from the camera no pictm-e is
visible. However, when left in the dark, without any other
preparation being used, for a period which varies with the length
of time it was exposed, and the strength of the light, a negatiAje
picture becomes gradually developed, until it arrives at a state
of perfection which is not attained, I think, by photography
produced by any other process.* It would seem as if the salt
of silver, being slightly afiected by the light, though not in a
degree to produce any visible effect on it if alone, sets up a
catalytic action, which is extended to the salts of iron, and which

* The picture, when developed, is not readily injured by exposure to mo-
derate light ; it ought, however, to be fixed, which may be done by washing it
with a solution of bromide of potassium, fifteen or twenty grains to the ounce,
iodide of potassium, five grains to the ounce. It may either be applied with a
camel-hair-pencil or by immersion. The picture must then be well washed in
water to remove the fixing material, which would cause it to fade by exposure
to light.

THE CATALYSOTYPE. / /

continues after the stimulus of the light is withdi-awTi. The
catalysis which then takes place has induced me to name this
process, for want of a better word, the Catalysotype. Sir* J.
Herschel and Mr. Fox Talbot have remarked the same fact with
regard to other salts of kon, but I do not know of any process
being employed for photographic pui'poses, which depends on
this action for its development, except my own.

" My reason for using the mimatic solution previous to washing
with the iodide of iron is this : I was for a long time tonnented
by seeing the pictm-es spoiled by yellow patches, and could
not remedy it, luitU I obseiwed that they presented an appear-
ance as if that poi-tion of the nitrate of silver which was not de-
composed by the iodide of iron had flowed away from the part.
I then recollected that Sii' J. Herschel and Mr. Hunt had
proved that iodide of silver is not veiy sensitive to light, unless
some free nitrate be jDresent. I accordingly tried to keep both
together on the paper, and after many plans had failed, I suc-
ceeded by steeping it in the acid solution, which makes it fi-eely
and evenly imbibe whatever fluid is presented to it. I am sure
that its utility is not confined to this effect, but it was for that
purpose that I fii'st employed it.

" My reason for adding the tincture of iodine to the sjTup is,
that having in my first experiments made use of, with success,
a s}Tup that had been for some time prepared, and afterwards
remarking that fresh spnip did not answer so well, I examined
both, and foimd in the former a little free iodine ; I therefore
added a little tinctm-e of iodine with much benefit, and now
always use it in quantities proportioned to the age of the syrup.
" The following liints will, I think, enable any experimenter to
be successfid in producing good pictures by this process. In the
fii'st place, the paper used should be that called wove post, or
well-glazed letter paper. When the solutions are applied to it,
it should not immediately imbibe them thoroughly, as would
happen with the thinner sorts of paper. If the acid solution is
too strong, it produces the very effect it was oi'iginally intended
to ovei'come ; that is, it produces yellow patches, and the picture
itself is a light brick coloiu- on a yellow gi-oimd. When the
tincture of iodine is in excess, partly the same restxlts occiu* ; so
that if this effect is visible, it shows that the oxide of sUver
which is thrown down is partly re-dissolved by the excess of acid
and iodine, and their quantities shoidd be diminished. On the
contrary, if the silver solution is too strong, the oxide is depo-
sited in the dark, or by an exceedingly weak Hght, and in this
case blackens the yellow parts of the pictui'e, which destroys it.
"WTien this effect of blackening aU over takes place, the silver so-

78 HISTORY OF PHOTOGRAPHY.

lution should be weakened. If it be too weak, the paper ramains
yellow after exposiu-e to ligbt. If the iodide of iron be used
in too great quantity, the picture is dotted over with black spots,
which afterwards change to white. If an excess of nitrate of
silver be used, and a photograph immediately taken before the
deposition of the oxide takes place, there ^vdll be often after some
time a jjositive pictm-e formed on the back of the negative one.
The excess of the nitrate of silver makes the paper blacker where
the light did not act on it, and this penetrates the paper ;
whereas the darkening produced by the light is confined to the
sm-face. The maximum intensity of the spectinim on the paper,
when a prism of crown glass is used, lies between the indigo and
blue ray. The difference of eifect of a strong and weak light is
beautifully sho^vn in the action of the spectrum : that pai-t of
the paper which is exjiosed to the indigo ray is coloured a red-
dish brown, and this is gradually darkened towards either ex-
tremity, untn it becomes a deep black.

" I have not had many opportnities of experimenting with
the catalysotype, but it certainly promises to repay the trouble
of ftu'ther investigation. The simplicity of the process, and the
sensibility of the paper, should cause it to be extensively used.
It has all the beauty and quickness of the calotype, without its
trouble, and very little of its uncertainty ; and, if the more fre-
quent use of it by me, as compared with other processes, does not
make me exaggerate its facility of operation, I think it is likely
to be practised successfiUly by the most ordinary experimenters."
Dr. Woods subsequently made the following addition : —

" Since the jireceding paper was wi-itten, I have been ex-
perimenting with the catalysotype, and one day having had
many failiu-es, which was before quite unusual with me, I am
induced to mention the cause of them, for the benefit of sub-
sequent experimenters. The paper I used was very stiff and
liighly glazed, so that the solution fii'st applied was not easily
imbibed. The blotting paper was very dry and biblulous. When
using the latter, I removed nearly all the solution of iron from
the first, and, of course, did not obtain the desii'ed residt.

" While vaiying the process in endeavovu'ing to find out the
cause just mentioned, I discovered that the following proportions
gave veiy fine negative pictvu-es, from wliich good positive ones
were obtained : — Take of syrup of iodide of u-on, distilled water,
each two drachms ; tinctui-e of iodine, ten to twelve ch*ops : mix,
Fii"st brush this over the paper, and after the few minutes, having
dried it with the blotting paper, wash it over in the dark (before
exposure in the camera) ^dth the follo\%Tng solution, by means
of a camel-hail" pencil : — Take of nitrate of silver one drachm ;

FEEROCTAMDE OF POTASSIUM. 79

pure water one ounce : mix. This gives a darker picture than
the original preparation, and consequently, one better adapted
for obtaining positive ones ; it also requires no previous steeping
in an acid solution. To &s. the pictiu-e let it be washed fii'st in
water, then allowed to remain for a few niiautes in a solution of
iodide of potassium (five gi-ains to the ounce of water) and washed
in water again. The paper I use is the common vmglazed copy
paper, but such as has a good body. I have tried the same
paper ■v\-ith the original preparation, and find it to answer ex-
ceedingly well ; it does not requii-e in this case, either, an acid
solution. The same precautions and hints apply to the amended
as to the original process ; such as, when it blackens in the dark,
there is too much caustic used ; when it remaias yellow, or that
it is studded with yellow spots, too much iodine ; wh.en marked
with, black spots, too much iron. It is necessary to mention
these, on account of the vai-ying strength of the mateiials em-
ployed."

Sectiox Y. — Ferroctaxide of Potassium.

At the meeting of the Biitish Association at Plymouth in
184:1, I fii^t dii-ected attention to the use of the ferrocyanide of
potassiiun ia combiaation -«-ith the iodide of silver. The jjrocess
resulting from this being very important in many points, the
absti-act of the paper then read, as given in the Transactions of
the Sections, is reprinted.

The author having been engaged in experiments on those
varieties of photogi-aphic di-a wings which ai-e formed by the action
of the hychiodic salts on the dai-kened chloride of silver, and Tvith
a view to the removal of the iodide formed by the process from
the paper, was led to observe some peculiar changes produced
by the combined influences of simshine and the ferrocyanide of
potassium. It was found that the ordinary photogi-aphic paper,
if allowed to darken in sunshine, and then slightly acted on by
any hydiiodic solution, and, when cfry, washed ^dth a solution
of the ferrocyanide of potassium, became extremely sensitive to
Light, changing from a light brown to a friU black by a moment's
exposui-e to simskine. Following out this result, it was dis-
covered that pei-fectly pui-e iodide of silver was acted on with
even gi-eater rapidity, and thus it became easy to form an ex-
quisitely sensitive photogi-aphic paper.

The method recommended is the following : —

Highly glazed letter paper is washed over with a solution of
one drachm of nitrate of silver to an ovince of distiUed water ;

80 HISTORY OF PHOTOGRAPHY.

it is quickly di-ied, and a second time washed with the same so-
hition. It is then, when diy, placed for a minute in a solution
of two ch'achms of the iodide of potassium in six ounces of water,
placed on a smooth board, gently washed by allowing some water
to flow over it, and di-ied in the dark at common temperatures.
Papers thus prepared may be kept for any length of time, and
are at any time rendered sensitive by simply washing them over
with a solution formed of one di-achm of the ferrocyanide of
potassium to an ounce of water.

These papers, washed "^dth the ferrocyanide and di'ied in the
dark, are, in this diy state, absolutely insensible, but they may
at any moment be rendered sensitive by merely washing them
with a little cold clean water.

Papers thus prepared are rendered quite insensible by being
washed over with the above hydriodic solution. They are, how-
ever, best seciu'ed against the action of time by a solution of
ammonia.

Section YI. — The Fluorotype,

So called from the introduction of the salts of fluoric acid, con-
sists of the following process of manipulation : —

( Bromide of potassium, 20 grains.

\ Distilled water ... 1 fluid oimce.

( Fluoride of sodium . 5 gi-ains.

( Distilled water ... 1 fluid ounce.

Mix a small quantity of these solutions together when the
papers are to be prepared, and wash them once over with the
mixture, and, when diy, apply a solution of nitrate of silver,
sixty grains to the ounce of water. These papers keep for some
weeks without injury, and become impressed with good images
in half a minute in the camera. The impression is not suffi-
ciently strong when removed from the camera for producing
positive pictiu'es, but may be rendered so by a secondaiy process.
The photograph should first be soaked in water for a few
minutes, and then placed upon a slab of porcelain, and a weak
solution of the proto-sulphate of iron brushed over it ; the
picture almost immediately acquii-es an intense colour, which
should then be stopped directly by pkmging it into water slightly
acidulated with muriatic acid, or the blackening ■wall extend all
over the paper. It may be fixed by being soaked in water, and
then dipped into a solution of hypo-sulphite of soda, and again
soaked in water as in the other processes.

BROMIDE OF SILVER A^"D MERCURIAL VAPOUR, 81

Mr. Bingliam has tlie following remarks on tliis process, and
he gives a moditied form, into "which a new photographic element
is introdiioed: —

" We find it is better to add to the proto-snlphate of iron a
little acetic or snlphui-ic acid : this will be found to prevent the
darkening of the lights of the picture to a gi*eat extent, and it
will be found better not to prepire the paper long before it is
required for iTse, this being one reason why the picture often
becomes dusky on application of the proto-siilphate.

" Reasoning upon the principle that the action of Ught is to
reduce the salts of sUver in the paper to the metaUic state, and
that any substance which would reduce silver woidd also quicken
the action of light, we were led to the following expei-iment : —
The protochloride of tin possesses the property of reducing the
salts both of silver and of gold : a paper was prepared with the
bromide of silver, and previously to exposing it to light it was
washed over with a veiy weak sokxtion of the chloride of tin ;
the action of light upon the paper was exceedingly energetic ;
it was almost instantaneously blackened, and a copy of a print
was obtained in a few seconds."

The use of fliiorides has been recently introduced as a novelty
by some French jjhotogi-aphers, but reference to the author's
Researches on Light, published in 1844, will distinctly show
that I was the fii-st to employ these salts, as photographic agents.

SECTIO^' XI* — Bro^hde of Silver a>'d Mercxtrial Yapoltj.

In my first publication on this subject, in Griffin's Scientific
Miscellany, I introduced the following process, which, although
it has never yet been properly worked out, involves many points
of interest : — Some extremely curious residts, led me to examine
the efiect of the merciuial vapour on the piu-e precij)itated
iodides and bromides. 1 was long perplexed with exceedingly
anomalous results, but being satisfied fi'om particular experi-
ments that these researches promised to lead to the discovery
of a sensitive preparation, I persevered.

To prepare this sensitive paper we proceed as follows : —
Select the most perfect sheets of well-glazed satin post, quite free
from specks of any kind. Placing the sheet carefully on some
hard body, wash it over on one side by means of a very soft
camel's hafr pencil, with a solution of sixty grains of the bromide
of potassium, in two fluid ounces of distilled water, and then diy
it qtiickly by the fire. Being cby, it is again to be washed over

82 HISTORY OF PHOTOGRAPHY.

with tlie same solution, and cliied as befoi'e. Now, a solution of
nitrate of silver, one hundi'ed and twenty grains to the fluid
ounce of distilled water, is to be applied over the same surface,
and the paper quickly dried in the dark. In this state the
papers may be kept for use. When they are requii-ed, the above
solution of silver is to be plentifully applied, and the paper
placed icet in the camera, the greatest care being taken that no
day-light, not even the faintest gleam, falls upon it, until the
moment when we are prepared, by removing the screen, to per-
mit the light, radiated from the objects we wish to copy, to act
in producing the picture. After a few seconds, the light must
be again shut off, and the camera removed into a dark room.
It will be fotmd, on taking the paper ft-om the box, that there is
but a very slight outline, if any, as yet visible. Place it aside,
ia. perfect darkness, until quite dry, then fix it in a mercurial
vapoiu- box, and apply a very gentle heat to the mercury. The
moment the merciuy vaporizes, the pictiu-e will begin to develope
itself The spirit lamp must now be removed for a short time,
and when the action of the mercmy appears to cease, it is to be
very carefully applied again, iintil a well-defined picture is
visible. The vaporization must now be suddenly stopped, and
the photogi'aph removed fr-om the box. The diuwing will then
be very beautiful and distinct ; but much detail is still clouded, for
the develojDment of which it is only necessary to place it cau-
tiously in the dark, and allow it to remain vmdistiu'bed for some
hours. There is now an inexpressible charm about the picture,
equalling the delicate beauty of the daguerreotypes ; but beiag
still very susceptible of change, it must be viewed by the light
of a taper only. The nitrate of silver must now be removed
from the paper by well washing in soft water. When the
picture has been di-ied, wash it quickly over with a soft brush,
dipjDed in a wai-m solution of the hyposidphite of soda, and then
w^ell wash it for some time in the manner dii-ected for the
ordinaiy photogi'aphs, in order that all the hyposidphite may be
removed. The di-awing is now fixed, and we may use it to
procure positive pictures, many of which may be taken from one
original.

Section VII. — Positive Photographs by One Process.

About the same time Mr. Talbot, Sir- John Herschel, Dr. Fife,
and myself, discovered the veiy remarkable property of the
iodides iu bleaching the darkened salts of silver. Many very
beautiful results may be thus obtaiaed. The manipulatory

POSITIVE PHOTOGKAPHS BY ONE PROCESS. 83

details published by Dr. Fife were simple in their- character, but
arrived at by a long series of inqviiries. It is now quite easy to
prepare photographic papers, on which the iodine solutions shall
act ^vith perfect uniformity : —

Soak the paper for a few minutes in phosphate or mui'iate of
soda, removing -wdth a soft bi-ush any aii' bubbles which may form
on it. The supei-flvious moistui'e must be wiped off with very clean
cotton cloths, and the papers dried at common tempei'atm-es.
When dry, the paper must be pinned out on a board, and the
sUver solution spi-ead over it, boldly but lightly, with a veiy
soft sponge brush. It is to be instantly exjDOsed to sunshine,
and, if practicable, carried into the open aii', as the more speedily
evaporation proceeds the less does the silver penetrate the
paper, and the moi*e delicate it is. The fii-st surface is very
irregular, being as before described, and represented in fig. 2.
As soon as the siu'face appears dry, the silver solution must be
again aj^plied as before, and the exposure repeated. It must
now be exposed imtil a fine chocolate-brown colour is produced
equally on all parts of the surface, and then, vmtil requii-ed
for use, be carefully preserved fi"om the further influence of
light. If the paper is to be kept long, the darkening must
not be allowed to proceed so far as when it is to be speedily
made use of.

In darkening these papers, the greatest possible attention
must be paid to the quantity of light to which they are sub-
mitted, evex-thing depending on the rapidity of the blackening
process. The morning sun should be chosen, for the reasons
before stated. A perfectly cloudless sky is of great advantage.
The injurious consequence of a cloud obscui'ing the sun during
the last darkening process, is the formation of a siuface which
has the appearance of being washed with a dii-ty brush. This
is with difficidty removed by the iodides, and the resulting
pictures want that clearness which constitutes their beauty.
Papers darkened by the diffused light of a cloudy day are
scai-cely, if at all, acted on by these salts. Great care must be
taken to prevent the silver solution from flowing over the edges
of the paper, as thereby an extra quantity of darkened silver is
formed on both sides, which requires a long-continued action of
the iodides and sunshine to bleach.

The kind of paper on which the silver is spread is an object
of much importance. A paper known to stationers as satin
post, double-glazed, bearing the mark of J. Whatman, Turkey
Mill, is decidedly siiperior to every other kind I have tried. The
dax'k specks which aboimd in some sorts of paper must be
avoided, and the spots made by flies veiy carefully guarded

84 HISTORY OF PHOTOGRAPHY.

against. These are of small consequence diu-ing the darkening
process, but when the bleaching wash is apjilied, they form
centres of chemical action, and the whitening process goes on
aroimd them, independently of light, deforming the drawing
with small rings, which are continually extending theii* diameters.
The saline washes may be considerably varied, and combined
to an indefinite extent, with a continued change of efiect, which
is smgularly interesting. In theii* apj^lication we shoidd be
guided, as in the negative jjrocess, by their combining propor-
tions. The following list of the salts which will give the best
effects, selected from upwards of seven himdi'ed combinations,
will show the variety of coloui-s produced. They are placed in
the order of the sensitiveness they appear to maintain, when
used as nearly as possible imder the same circumstances : —

Colour of Picture.
Muriate of Ammonia. . Red, changing to black in the sunshine.

Chloride of Sodium Ditto, ditto.

Chloride of Strontium A fine brown.

Chloride of Barium ...A rich brown, inclining to 2Jurple.

Sol. Chloride of Lime Very red.

Sol. Chloride of Soda A brick red.

Iodide op Potassium. . . . Yelloivish broivn.

Chloride of Potas- f Variable, sometimes yellowish, ofien a

siUM \ steel blue.

Phosphate of Soda Mouse colour.

Tartrate of Soda Bark brown.

Urate op Soda Yelloioish broivn.

Chloride of Iron Deep broion, lohich blackens.

Bromide of Sodium Med brown of a p)ecidiarly rich tint.

The change mentioned in the colour of the finished pictm-e
is that which arises from a fresh exposure to the solar rays ;
where no change is mentioned, it is too slight to be worth notice.

When papers prepared with any of the above, except the
phosphates, are soaked for a Little time in water, and di-ied in
the svmshine, the picture produced, — it matters not what
iodide is used, — is rendered peculiarly red, and does not change
by re-exposm"e. By washing some of the papers with weak
solution of ammonia, this peculiarity is produced in a very
striking maimer.

In the other divisions will be foimd some farther remarks on
the very pecuHar physical phenomena presented by the action
of the compounds of iodine on these darkened salts of silver, and
details of yet more perfect forms of manipidation.

ON THE APPLICATION OP THE DAGUERREOTYPE TO PAPER. bO

Section VIII. — On the Application of the Daguerreotype
TO Paper.

The expense and. inconvenience of metallic tablets rendered
it in the highest degree desirable that paper should be employed
in theii' place. A very extensive series of experiments at length
led to the pleasing conclusion of being enabled to prepare a
paper which answered in every respect as well as the silver
plates, and in many much better.

This discovery formed the subject of a commimication to the
Royal Society, which that learned body did me the honoui* to
print in their Transactions. My memoir is entitled, — " On the
Influence of Iodine in rendering several Argentine Compounds,
spread on Paper, sensitive to Light ; and on a New Method of Pro-
ducing, with greater distinctness, the Photographic Image." This
paper contains the substance of the follo^ving remarks ; but
since the publication of the Transactions I have been successful
in simplifying the process of prejoaration.

My experiments established, in the most satisfactoiy manner,
that even on the silver talilets a semi-oxidized sm'face was j^re-
sented to the iodine. They also proved that perfectly pure
iintarnished silver was by no means readily acted on by the
iodine. From this I was led to prepare oxides of silver in many
different ways, which enabled me to spread them over paper,
and the residt was instiiictive. Any of the ordinary photo-
graphic papers allowed to darken to a fiill brown, which is a
stage of induced oxidation, become, by long exposvire to iodine,
of a steel-blue or violet colour. If exposed in this state to
sunshine for a long period, their coloiu^ changes fi'om gi'ay to a
clear olive. Now, exposui'e to sunshine for a minute, or to dif-
fused daylight for five minutes, produces no ctpparent change ;
but mercui'ial vapour speedily attacks the portions which have
been exposed to light, and a faitlifid picture is given of whatever
may have been superposed. There is, however, a want of suffi-
cient conti*ast between the lights and shadows. By allo-wang
the first darkening to proceed until the paper acquires the olive
colour, which indicates the formation of a time oxide of silver,
it wiU be found, although it is not more speedily acted on
by the iodine, that it is more sensitive, and that a better
pictui'e is formed. The kind of photographic preparations used
appears to have but little influence on the resiUts, — a chloride,
iodide, or bromide of silver, allowed to darken, answers equally
well.

There are many things, xmfortimately, which prevent oui"

86 HISTORY OF PHOTOGRAPHY.

availing oui'selves of tliis easy metliod of producing a tolerably
sensitive Dagnen-eotype paper. These are, certain in-egular
formations of oxides in different states, and the revival of
metallic silver in some parts of the siu-face.

I next spread papers with the pxu-e oxide formed by chemical
means, and also the protoxide, and many of its salts. These
papei's were not veiy readily affected by iodine, or influenced by
light dui'ing short exposiu-es.

Silver is revived fi-om its solutions by hydrogen gas ; conse-
qently, nothing is more easy than, by washing a paper ^\^th
nitrate of silver in solution, to procure a fine silver paper, by
passing a cuiTent of hydrogen gas over it.

A pictiu-e of a peculiarly delicate character may be produced
on this kind of paper ; but it has not the required sensibility,
and there is a great want of contrast in the lights and shadows.
It may be interesting to state, that the yellow-brown phosphate
of sUver is as readily acted on by iodine as the oxides, and is
quite as sensitive to luminous influence. Phosphuxetted hydrogen
gas effects the revival of metallic silver, and the sui'face produced
by means of this gas, used as the hydrogen was in the former
case, is of a fine steel-blue, which coloui* arises firom a portion
of phosphorus having entered into combination with the silver.
These kinds of paper comported themselves in eveiy respect
as the metallic tablets — were equally sensitive, and produced
pictures as deKcately beautiful. Unfortiinately, however, owdng
to the spontaneously inflammable natm-e of the phosphuretted
hydrogen gas, it is not safe to operate with it. After various
ineffectual contrivances to overcome this difficulty, I was obliged
to abandon the iise of this gas entu-ely — warned of the danger
I incui-ed, — by several violent but fortunately harmless explo-
sions. The vapour of phosphorus and of sulphur was also tried,
and many very beautiful effects were produced. At length,
however, I stopped at sulphuretted hydrogen, which answers
in eveiy respect.*

To prepare this, soak a paper of veiy fii-m textm-e, not too
much glazed, in a weak soliTtion of the muriate of ammonia. It
must then be wiped with clean cloths, and carefully dried. The
paper is then dipped into a weak solution of the nitrate of silver,
and the small bubbles which form on its surface are carefully
removed with a camel's hair pencil. When the paper is nearly,
but not quite, diy, it must be exposed in a closed vessel to sul-

A very interesting account of the revival of gold and silver from their solu-
tions by these gases, will be found in a tract on Combustion, published by Mrs.
Fulhame.

ON THE APPLICATION OF THE DAGUERREOTYPE TO PAPER. 87

phiu'etted hydrogen gas, slowly foiined fi'om the suljihiiret of
antimony and hydrochloric acid : in a few minutes it will become
of an iron-brown colour, hav^ing a fine metallic lustre. It is
again to be passed through a solution of silver, somewhat stronger
than the first, and dried, taking care that no shadow falls on
the paper whilst it is chying. It is then a second time sub-
mitted to sulphuration, and, by careful management, the process
is now generally completed. If, howevei', the paper is not
considered to be sufficiently dark, it mi;st be once more washed
in the solution of silver, and again subjected to the action of
sulphuretted hydi-ogen.

If the above paper be allowed to remain in the sulphui-etted
hydi'ogen gas after the maximiim blackness is produced, it is
again whitened with some quickness. This may be accounted
for in two ways : the gas may be mixed with a portion of
muriatic acid vapoiu", or a quantity of chlorine sufficient to pro-
duce this efiect may be liberated from the preparation on the
paper to react on the sulphm*et of silver.

The perfection of these papers consists in ha'sang a deep black
groxmd to contrast with the merciu'ial deposit, by which means
the pictures have the advantage of being seen equally well in all
^Dositions, whereas Daguerre's pictiu'es on the metal plates can
only be seen to advantage at certain angles.

The sulphuretted paper may be rendered sensitive in the
same manner as the plates by exposui-e to the vapour of iodine.
I, however, prefer di-awing the paper over a solution thus
formed : — A saturated solution of any salt of iodine is made to
dissolve as much pure iodine as possible, and of this liquid two
di-achms are mingled with foui- ounces of water. Care is re-
quii'ed that one side only of the paper is wetted, which is by no
means difficult to efiect, the fluid is so greedily absorbed by it ;
all that is necessary being a broad shallow vessel to allow of
the paper touching the fluid to its fuU "nddth, and that it be
drawn over it with a slow steady movement, ^Tien thus
wetted, it is to be quickly dried by a warm, but not too bright
fire ; of course daylight mxist be carefully excluded. Papers
thus iodidated do not lose their sensitiveness for many days if
cai'efitlly kept from Hght.

On examining the sheet after the Daguerreotype processes in
the camei*a, and of mercimalization, have been completed, a very
perfect pictiu-e is found upon it ; but it is still cajmble of vast
improvement, which is, by the follo'ndng simple plan, accom-
plished in a way which is at once magical and beautifril.

Action of Corrosive Sublimate.— Dip one of the Daguen-eo-
type pictiu'es, formed on the sulphui-etted paper, into a solution

88 HISTORY OF PHOTOGRAPHY.

of coiTosive sublimate : the drawing instantly disappears, but,
after a few minutes, it is seen unfolding itself, and gradually
becoming far more distinct than it was before ; deKcate lines,
before invisible, or barely seen, are now distinctly marked,
and a rare and singular perfection of detail given to the
drawing. It may appear, at first sight, that the bichloride
of mercury dissolves off the metal, and again deposits it in
the form of chloride (calomel). But this does not account
for the fact, that if the paper has been pre^iared with the nitrate
of silver, the mercury disappears, and the drawing vanishes, the
deposit taking place only on those parts upon which light has
acted but feebly ; as, for instance, on the venations of leaves,
leaving those portions of sui-face which were exposed to full
luminous influence wdthoxit a particle of qvucksilver. When the
I^aper has been either a chloride or iodide, the effect is as above,
and the thickness of the deposit is as the intensity of the light
has been ; consequently, the semi-tints ai-e beautifully preserved.
If the di-awdng remains too long in the solution, the precipitate
adheres to the dark parts and destroys the effect. The singvda-
rity of this operation will be more striking if the pictiu-e has
been soaked some time in the solution of the hyposuljjhite of
soda, and then dipped into the bichloride of mercmy. As the
dra^ving disappears, a series of circles, formed of a white powder,
appear to arise from the paper, generally commencing at the
centre, and slowly extending over the whole surface : the powder
is afterwards deposited, and the sheet is buried in the precipi-
tate ; but on taking the paper from the liquid, and passing a
stream of water over it, the precipitate is entii'ely removed from
all the i^arts excejit the lights of the picture. I have also foxmd
the invisible jshotographic image become evident, without the
aid of mercurial vapour, by simply soaking for some time in a
solution of corrosive sublimate.

When these papers are prepared with due care, they are ex-
tr-emely sensitive, and if used for copying engravings diu-ing
bright sunshine, the efiect is instcaitaneous. The great difficulty
is to present the paper to the sun, and withdraw it with sujfi-
cient celerity. In the weak light of the camera a few minutes
during sunshine is quite sxifiicient for the jiroduction of the best
effects. One gi-eat advantage of these pictures over those pro-
cm-ed on the plated copper is, that the merciuy does not lie
loosely as on the tablets, but is firmly fixed, being absorbed by
the paper ; therefore these pictures may be kept without injiuy
in a portfolio.

If, instead of immersing the paper in a vessel ftdl of sulphu-
retted hych'ogen gas, a stream of the gas is made to j^lay upon it,

SALTS OP GOLD AS PHOTOGRAPHIC AGENTS. 89

it asstimes a most richly iridescent siu-face ; tlie A*arious coloiu-s
are of difierent degrees of sensibility, but for siu-face di-awings
they may be used ; and in copying of leaves or flowers, beautiful
pictui'es, which appear to glow with the natiu'al coloui"s, are
procured.

Section IX. — Salts of Gold as Photographic Agents.

It is well known that gold is revived from its ethereal solution
by the action of light, and that the same effect takes place when
the nitro-miu-iate of gold is spread on charcoal. We are mainly
indebted to Hei-schel's paper, published in 1840, for the know-
ledge we possess of gold as a photogTapliic agent.

Considering it probable that the requii'ed unstable equilibrium
might be induced in some of the salts of gold, I was induced to
pm'sue a great many experiments on this point. In some cases,
where the pajier was impregTiated with a mordant salt, the salt
of gold was darkened rapidly, without the assistance of light ;
in others, the effect of light was very slow and uncertain. By
washing paper with muriate of barytes, and then with a solution
of the chloride of gold, a paper, having a slight pinky tint, is
procui'ed ; by exposiag this paper to sunshine it is at fij'st
whitened, and then, but very slowly, a darkening action is in-
diiced. If, however, we remove the paper from the light, after
an exposure of a few minutes, when a very faint impression, and
oftentimes not any, is apparent, and hold it in the steam of boil-
ing water', or immerse it in cold water, all the pai-ts which were
exposed to the light are rapidly darkened to a full piu-jjle brown,
leaving the covered portions on which the light has not acted,
a pure white, producing thus a fine negative ch-awing. If, while
such a paper, or any other paper, prepared with the chloride of
gold, is exposed to the sun, we wash it with a weak solution of
the hydriodate of potash, the oxidation is very i-apidly broxight
on, and the darkness produced is much greater than that ob-
tained by the other method ; but this plan is not often applicable.
I have not yet been enabled to produce Avith the salts of gold
any paper which should be sufficiently sensitive for use in the
camera obscura.

Sir John Hei-schel devoted much attention to the examination
of the salts of plantinum as well as gold. He found plantinum
under nearly all circumstances veiy little sensitive to Kght, but
the following were the results obtained^vith the salts of gold: —

If paper impregnated vdih oxalate of ammonia be washed with
chloride of gold, it becomes, if certain proportions be hit, pretty

90 HISTORY OF PHOTOGRAPHY.

sensitive to light ; passing rather rapidly to a violet piu-ple in
the sun. It passes also to the same piu'ple hue in the dark,
though much more slowly; so that, as a photographic combina-
tion, it is useless.

Paper impregnated with acetate of lead, when washed with
perfectly neutral chloride of gold, acquires a brownish-yellow
hue, and a sensibihty to light, which, though not great, is at-
tended with some peculiarities highly worthy of notice. The
first impression of the solar rays seems rather to whiten than to
darken the paper, by discharging the original coloiu', and sub-
stituting for it a pale grayish tint, which by slow degrees in-
creases to a dark slate coloui-. But if arrested while yet not
more than a moderate ash gray, and held in a current of steam,
the colour of the part acted on by the sunshine, and that only,
darkens immediately to a deep piu'ple. The same effect is pro-
duced by immersing it in boiling water. If plunged in cold
water, the same change comes on more slowly, and is not com-
plete till the paper is cMed by heat. A dry heat, however, does
not operate tliis singular change.

If a neuti'al solution of the chloride of gold is mixed with an
equal quantity of the solution of bichromate of potash, paper
washed with this solution, and exposed to light, speedily changes,
fii-st to a deep broA\'n, and ultimately to a bluish black. If an
engraving is superposed, we have a negative copy, blue or brown,
upon a yellow gi-ound. If this photograph is placed in clean
water, and allowed to remain in it for some hoiu's, veiy singular
changes take place. The yellow salt is all dissolved out, and
those parts of the paper left beautifidly white. All the dark
portions become more decided in their character, and according
as the solarizatiou has been prolonged or otherwise, or the light
has been more or less intense, we have either crimson, blue,
brown, or deep black negative photographs.

Section X. — The Influence of Chlorine and Iodine in

RENDERING SOME KINDS OF WoOD SENSITIVE TO LiGHT.

Having on many occasions subjected the simply nitrated pho-
tographic paper to the influence of chlorine and iodine in close
wooden boxes, I was often struck with the sudden change which
light prodviced on the wood of the box, particiilarly when it was
of deal ; changing it in a few minutes from a pale yellow to a
deep green. This curious effect frequently occiu-ring, led me to
observe the change somewhat more closely, and to pursue some

INFLUENCE OF CHLORINE ON WOOD. 91

experiments on the subject. These produced no very satisfac-
toiy result. They proved the change to depend much on the
formation of hydrochloric and hych-iodic acids, and the decom-
position of water in the pores of the wood. I found well-baked
wood quite insusceptible of this very curious phenomenon. The
woods of a soft kind, as the deal and willow, were much sooner
influenced than the harder varieties, but all the light-coloui'ed
woods appeared more or less capable of undergoing this change.
All that is necessary is, to place at the bottom of an air-tight
box, a vessel containing a mixtm-e of manganese and muriatic
acid, or simply some iodine, and fix the piece of wood at some
distance above it. Difierent kinds of wood requii-e to be more
or less saturated with the chlorine or iodine, and consequently
need a longer or shorter exposm-e. The time, therefore, neces-
saiy for the wood to remain in the atmosphere of chlorine can
only be settled by direct experiment. Wood is impregnated
very readily with iodine, by putting a small portion in a capsvde
a few inches below it. It does not appear to me at jjresent that
any practical result is likely to arise out of this peculiar pro-
perty ; it is only introduced as a singular fact, which is perhaps
worthy a little attention.

CHAPTER YII.

PHOTOGRAPHS OS GLASS PLATES, ANT) RECENT IMPROVEMENTS.

To Sir John Herschel we are indebted for tlie first use of glass
plates to receive sensitive photogi-aphic films.

Section I. — Precipitates of Silver Salts.

The interest which attaches to this is so great, and there
appear to be in the process recommended by the English experi-
mentalist so many suggestive points, fi.-om which fiitvu-e j^hoto-
gi-aphists may start, that the passages are given in Sii' John
Herschel's o'wti words : —

" With a view to ascertain how far organic matter is indis-
pensable to the rapid discoloration of argentine compounds, a
process was tried which it may not be amiss to relate, as it issued
in a new and veiy pretty variety of the photograpliic art. A
solution of salt of extreme dilution was mixed with nitrate of
silver, so dilute as to form a liquid only slightly milky. This
was poui'ed into a somewhat deep vessel, at the bottom of which
lay horizontally a very clean glass plate. After many days the
greater part of the liquid was decanted ofi" with a siphon tube,
and the last portions very slowly and cautiously draiaed away,
drop by di-op, by a siphon composed of a few fibres of hemp,
laid pai-aUel and moistened without twisting. The glass was
not moved till quite diy, and was fotind coated with a pretty
uniform film of chloride of silver, of delicate tenuity and che-
mical pvuity, which adhered with considerable force, and was
veiy little sensitive to light. On cbopping on it a solution of
nitrate of silver, however, and spreading it over by inclining the
plate to and fro (which it bore without discharging the film of
chloride) it became highly sensitive, although no organic matter
could have been introduced -wdth the nitrate, which was quite
pure, nor coidd any indeed have been present unless it be sup-
posed to have emanated from the hempen filaments, which were
bai'ely in contact %\ith the edge of the glass, and which were

PRECIPITATATES OF SILVER SALTS. 93

constantly abstracting matter from its surface in place of intro-
ducing new.

" Exposed in this state to the focus of a camera with the glass
towai'ds the incident light, it became impressed with a remark-
ably well-defined negative pictiu-e, which was direct, or reversed,
according as looked at from the front or the back. On pouring
over this cautiously, by means of a pipette, a solution of hypo-
sulphite of soda, the picture flisappeax'ed, but this was only while
wet ; for on washing in pure water and drying, it was restored,
and assvuned much the air of a dagueiTeotype when laid on a
black groimd, and still more so when smoked at the back, the
silvered portions reflecting most light, so that its characters had,
in fact, changed from negative to positive. From such a pic-
ture (of course before smoking) I have found it practicable to
take photogi-aphic copies ; and although I did not, in fact, suc-
ceed in attempting to thicken the film of silver, by connecting
it. Tinder a weak solution of that metal, with the reducing pole
of a voltaic pile, the attempt afibrded distinct indications of its
practicability with patience and perseverance, as here and there,
over some small portions of the sm-face, the Kghts had assumed
a full metallic brilliancy tmder this process. I would only men-
tion further, to those who may think this experiment worth
repeating, that all my attempts to secure a good result by diy-
ing the nitrate in the film of chloride have faded, the crystalliza-
tion of the salt disturbing the uniformity of the coating. To
obtain delicate pictm-es the plate must be exposed wet, and when
withdi-awn must immediately be plunged into water. The
rdtrate being thus abstracted, the plate may then be dried, in
which state it is half fixed, and it is then ready for the hypo-
sulphite. Such details of manipulation may appear minute, but
they cannot be dispensed with in pi-actice, and cost a gi'eat deal
of time and trouble to discover,

" This mode of coating glass with films of precipitated argen-
tine or other compoimds, afibrds, it may be observed, the only
efiectual means of studj-ing then- habitudes on exposui-e to light,
free from the powerful and ever-vaiying influence of the size in
paper, and other materials used in its manufactm-e, and estimat-
ing thefr degree of sensibility and other particidars of their
deportment under the influence of reagents. I find, for example,
that glass so coated with the iodide of silver is much more sen-
sitive than if similarly covered with the chloride, and that if
both be washed with one and the same solution of nitrate, there
is no comparison in respect of this valuable quality; the iodide
being far superior, and of coiu-se to be adopted in preference for
the use of the camera. It is, however, more difficult to fix:, the

94 HISTORY OF PHOTOGRAPHY.

action of the hyposulpliites on this compound of silver being
comparatively slow and feeble.

" When the glass is coated %vith the bromide of silver, the action,
per se, is very slow, and the discoloration ultimately produced
far short of blackness ; but when moistened with nitmte of silver,
sp. gr. ri, it is still more rapid than with the iodide, tui'ning
quite black in the covu-se of a very few seconds' exposure to
sunshine. Plates of glass thus coated may be easily preserved
for the use of the camera, and have the advantage of being ready
at a moment's notice, requiring nothing but a wash over with
the nitrate of silver, which may be delayed imtil the image is
actually thrown on the plate, and adjusted to the coi-rect focus
with all deliberation. The sensitive wash being then applied
with a soft flat camel-hau* brush, the box may be closed and the
picture impressed, after which it only requires to be thrown into
water, and dried in the dark, to be rendered comparatively in-
sensible, and may be finally fixed with hyposidphite of soda,
which must be applied hot, its solvent power on the bromide
being even less than on the iodide."

Sir John Herschel suggested a trial of the fluoride of silver
aipon glass, w^hich, he says, if proved to be decomposable by
light, might possible efiect an etching on the glass, by the cor-
roding property of the hydrofluoric acid.

The metallic fluorides have been found to be decomposable,
and a veiy sensitive process on paper, called the fluorotype, will
be described in the chapter on Miscellaneous Processes. I am
not aware that any experiments have been made dii-ectly upon
glass, but it is certainly worthy of a careful trial.

Herschel has remarked that we cannot allow the wash of
niti-ate to dry upon the coating of the chloride or iodide of silver.
If, however, we dip a glass which has one film of chloride upon
it into a solution of common salt, and then spread upon it some
nitrate of silver, we may veiy materially thicken the coating,
and thus produce more intense efiects. IVIr. Towson employed
gla.ss plates prepared in this manner with much success. The
mode adopted by that gentleman was to have a box the exact
size of the glass plate, in the bottom of which was a smaU hole ;
the glass was placed over the bottom, and the mixed solution,
just strong enough to be milky, of the salt and silver poui-ed in.
As the fluid finds its way slowly arovmd the edges of the glass,
it filters out ; the pecidiar surface action of the solid glass plate,
probably a modified form of cohesive force, separating the fine
precipitate, which is left behind on the sm-face of the plate. By
this means the operation of coating the glass is much quickened.
Another method by which films of any of the salts of silver can

ALBUMEN COLLODION. 95

be produced VL])on glass plates, is tlie foUo-ttT.ng modification of
the patent processes of Di"ayton and of Thompson for silvering
glass: —

Take a very clear plate of glass, and having put around it an
edging of wax about haLf-an-inch in depth, poiu- into it a solu-
tion of niti'ate of silver made alkaline by a few di-ops of am-
monia, taking care that no oxide of silver is precipitated ; mix
with this a small quantity of spirits of wine, and then add a mix-
ture of the oils of lavender and cassia, or, which is perhaps the
best process, a solution of gi'ape sugar. In a short time the
glass will be covered Avith a veiy beautiful metallic coating. The
solution is now poiu'ed oft", the edging of wax removed, and the
silver is exposed to the action of diluted chloi-ine, or to the vapour
of iodine or bromine, until it is converted into a compound of one
of these elements, after which we may proceed as recommended
by Sir John Herschel.

Section II. — Albumen.

In the Technologiste for 1848, M. Is'iepce de Saint- Victor
published his mode of applying albumen to glass plates. M.
Blanquart Evei-ard followed, and successively albumen, gelatine,
and serum were employed. Messi-s. Eoss and Thomson, of
Edinbm-gh, have been eminently successftJ operatoi-s with
albumen on glass plates, many of their pictvu-es leaving little to
be desired. The manipulatory details of the albumen process
will be found in the technical division of this work.

Section III. — Collodion.

The successful application of a solution of gun-cotton in
ether, to form the film for receiving the sensitive sivrface on
glass, has been claimed respectively by Mr. Fiy and 'Mr. Archer.
There is some difficulty in fixing precisely this point, since there
was no actual publication of the process until long after it was
generally in use. Mr. Fiy certainly introduced the use of gutta
percha in combination with collodion.

The last novelty to which reference need be made is the icax
paper process of M. Le Gray, which will be fully described.

CHAPTER VIII.

PORTRAITURE BY THE DAGUERREOTPYE.

When Dagiien-e published his pi"ocess, a period of twenty
minutes was reqiiii'ed to obtain a good copy of any external
object : hence this period was far too long to admit of its being
employed for portraiture.

Mr. Towson, of Devo?ipoi-t, in a veiy raluable paper wliich
appeared in the Philosopliical Mayazine in 1839, offered several
suggestions on the use of large lenses, <tc., which he sup^Dosed
might lead to the use of the daguerreotype for the pui-poses of
portraiture.

Dr. Draper, of New York, acting on the suggestions of Mr.
Towson relative to the adjustment of the focus, succeeded in
accelerating his process so far as to obtain portraits from the
life. He published his process in the London and Edinburgh
Philosophiccd JfagaciJie for September 1840. From this paj3er
I shall take the liberty of making copious extracts. It was
first stated that it was necessaiy, to procure any impression of
human features on the daguerreotype plate, to paint the face
white, or dust it over with a white powder, it being thought that
the light reflected from the flesh would not have sufiicient jDower
to change the iodized surface. This has been shown to be an
error, for, even when the sun shines but dimly, there is no
difficulty in obtaining a connect delineation of the featiu-es.

"When the sun, the sitter, and the camera, are situated in
the same vertical plane, if a double convex non-achromatic lens
of four inches diameter and foiu-teen inches focus be employed,
jjerfect miniatures can be prociu-ed in the open air in a period
varying with the character of the Kght from 20 to 90 seconds.
The dress also is adminibly given, even if it should be black ;
the slight difierences of illumination are sidficient to characterize
it, as well as to show each button and button-hole, and every
fold. Partly owing to the intensity of such light, which cannot
be endiu'ed without a distortion of the featm'es, but chiefly
owing to the cii'cmnstance that the rays descend at too gTcat
an angle, such pictiu-es have the disadvantage of not exhibiting
the eyes with distinctness, the shadow from the eyebrows and
forehead encroaching on them. To procm-e fine proofs, the best
position is to have the Line joining the head of the sitter and

PORTRAITURE BY THE DAGUERREOTYPE. 97

the camera so arranged as to make an angle with the incident
Tays of less than ten degrees, so that all the space beneath the
eyebrows shall be illuminated, and a slight shadow cast from
the nose. This involves, obviously, the use of reflecting mirrors
to direct the ray. A single mirror would answer, and would
economise time, but in practice it is often convenient to employ
two ; one placed, with a suitable mechanism, to direct the rays
in vertical lines, and the second above it, to direct them in an
invariable course towards the sitter.

" On a bright day, and with a sensitive plate, portraits can be
obtained in the course of five or seven minutes, in the diffused
day-light. The advantages, however, which might be supposed
to accrue from the features being more composed, and of a
natm'al aspect, are more than counterbalanced by the difficulty
of retaining them so long ia one constant mode of expression.
But in the reflected sunshine, the eye cannot bear the effulgence
of the rays. It is therefore absolutely necessary to pass them
through some blue medium, which shall abstract from them
their heat and take away theu* offensive brilliancy. I have used
for this purpose blue glass, and also ammoniaco-sulphate of
copper, contained in a large trough of plate glass, the intei'stice
being about an inch thick, and the fluid diluted to such a point,
as to permit the eye to bear the light, and yet to intercept
no more than was necessaiy. It is not requisite, when coloured
glass is employed, to make use of a large surface ; for if the
camera operation be carried on untU the proof almost solarizes,
no traces can be seen in the portrait of its edges and boun-
daries ; but if the process is stopped at an earlier interval, there
will be commonly found a stain coiTesponding to the figure of
the glass."

"The chair in which the sitter is placed has a staff at its
back, terminating in an iron ring, that supports the head, so
arranged as to have motion in directions to suit any stature
and any attitude. By simply resting the back or side of the
head against this ring, it may he kept sufficiently still to allow
the minutest mai'ks on the face to be copied. The hands should
never rest upon the chest, for the motion of respiration disturbs
them so much as to bring them out of a thick and ckmisy
appearance, destroying also the representation of the veins on
the back, which, if they are held motionless, are copied with
surprising beauty.

" It has already been stated, that certain pictorial advantages
attend an arrangement in which the light is thrown upon the
face at a small angle. This also allows us to get rid entirely of
the shadow fi-om the background, or to compose it more grace-

98 HISTORY OF PHOTOGRAPHY.

fully in the picture ; for this, it is weU that the chair should be
brovight from the back-ground, from three to six feet.

"Those who undertake daguerreotype portraitvires will of
course arrange the back-grounds of their pictiu'es according to
their own tastes. When one that is quite imiform is required,
a blanket, or a cloth of a ch-ab coloui', properly suspended, will
be found to answer very well. Attention must be paid to the
tint : wliite, reflecting too much light, would solarize ujoon the
proof before the face had time to come out, and, owing to its
reflecting all the rays, a blur or irradiation woidd apjjear on all
edges, due to cliromatic aberration.

" It will readily be imdei'stood, that if it be desrred to intro-
duce a vase, an imi, or other ornament, it must not be aiTanged
against the back-ground, but brought forward imtil it appears
perfectly distract upon the obscured glass of the camera.

" Different paii-s of the dress, for the same reason, require
intervals, differing considerably, to be faii'ly copied ; the white
parts of a costume passing on to solarization before the yellow
or black tints have made any decisive representation. We
have, therefore, to make use of temporary expedients. A person
dressed in a black coat and open waistcoat of the same colour,
must put on a temporary front of a drab or flesh colom% or, by
the time that his face and the fine shadows of his woollen clothing
are evolved, his shirt will be solarized, and be blue, or even
black, with a white halo around it. Where, however, the
white parts of the dress do not expose much surface, or expose
it obliquely, these precautions are not essential ; the white
collar will scarcely solarize until the face is passing into the
same condition.

" Precautions of the same kind are necessary in ladies' dresses,
which should not be of tints contrasting strongly.

"■ It will now be readily understood, that the whole art of
taking daguerreotype miniatures consists in directing an almost
horizontal beam of light, throvigh a blue-coloui-ed medium,
upon the face of the sitter, who is retained in an unconstraiaed
posture by an appropriate but simple mechanism, at such a
distance from the back- ground, or so arranged with respect to
the camera, that his shadow shall not be copied as a part of
his body."

Professor Draper used a camera having for its objective two
double convex lenses, the united focus of which for parallel rays
was only eight inches ; they were four inches in diameter in
the clear, and were mounted in a barrel, in front of which the
aperture was narrowed down to tliree and a half inches, after the
manner of Daguerre's. He also adopted the principle of bringing

PORTRAITURE BY THE DAGUERREOTYPE. 99

the plate forward out of the best visible fociis, into the focxis
of the violet rays, as was first suggested by Mr. Towson, of
Devonpoi-t, who also made many experiments, about the same
period, with camei-as having mirrors instead of lenses. A patent
was taken out by Mr. Woolcott, a philosophical instniment-maker
of New York, for a camera for portraiture, with an elliptical
mirror; which form of apparatus was also patented by Mr. Beard,
in England, who having somewhat modified Dr. Draper's arrange-
ments, succeeded still better in obtaining copies of " the human
face divine."

A camera obscura of this description is constructed as follows.
Fig. 12 is a sectional view of the apparatus. At one end of a

-----

,--::-

-^^^EEH^^^-SSs^

fu , "'" ^^^^-v-'^^-^— °^

L--:rr:

box shaped as in the figure, and having an opening at D, is placed
an elliptical mirror, A. The prepared plate B is fixed to the
sliding frame c, by which it is adjusted to the best focus. The
rays of light, radiating from a figui-e placed at F, will, it must be
evident, pass through the opening at d, and fall on the muTor,
as represented by the dotted lines, and will be thence reflected
to the plate B.

The miri-or has certainly the advantage of throwing a greater
quantity of light upon the plate, but it has the great dis-
advantage of limiting the size of the picture. With a mii-ror
of seven iaches diameter, we only procure pictui-es which will
be perfect over two square inches ; whereas, with a lens of three
inches diameter and foiu-teen inches focal leng-th, pictures of a
foot squai'e may be worked. From this it will be seen that the
mirror is only applicable where single objects are to be copied.

Eventually the sensibility of the siu-face of the plates was
gi'eatly increased. Mr. Goddard appears to have been the first
to employ bromine in combination with iodine ; and it was sub-
sequently found by M. Claudet and others, that chloriue had an
accelerating power, but not to the same extent as the bromine.

These discoveries led to that amazing degree of sensitiveness
which now enables us, in good light, to take a picture in less
than a second of time.

CHAPTER IX.

GENERAL SUMMARY OF THE HISTORY OF PHOTOGRAPHY,

It is tliouglit that it may prove of some interest to a^ipend tlie
following table, compiled witli much care for the British Asso-
ciation, by the author, and printed by that body in their
Reports for 1850, and to which now niimex'ous additions are
made. It is believed that the dates of discovery are accurately
giveii, the date of publication being, of com-se, in all cases, taken
where there was the slightest doubt : —

Silver.

Nitrate of

(photographically employed)

with organic matter .

with salts of lead . . . .

Ritter . . . . 1801
Wedgwood & Davy 1802

Chloride of

(photogi'aphically employed

J. F. Herschel
J. F. Herschel
C. W. Scheele

( Wedgwood

\ Talbot . . .
darkened, and hydi'iodic salts Fyfe, Lassaigne

{ Herschel .

( Ryan .
Himt . . .
J. B. Reade .
Talbot . .

Iodide of (photogi-aphically used)

with ferrocyanate of potash

■ with infusion of galls .

with gallic acid (Calotype) .

with protosulphate of iron

(Fen-otype)

• with iodide of iron (Catalyso-

■fcype)

Bromide of

Fluoride of

Fluorotype

Oxide of

■ ■ with ammoni;i

Phosphate of

Tartrate — Urate — Oxalate — Bo-
rate, &c. ,

Hunt . .

Woods
Bayard
Channing
Hunt . .
Davy . .
Uncei'taiu.
Fyfe . .

Herschel .

1839
1839
1777

1802
1839
1839
1840
1840
1841
1839
1841

1844

1844
1840
1842
1844
1803

1839

1840

GKN'EEAL SUililABY.

101

SiL"\T2i — continued.

Benzoates of Hunt

Fomiiates of Do.

Fulminates of Do.

. 1844
. 1844
. 1842

Silver Plate.

With vapoiu' of iodine (Daguerreo

type)

With bromine and iodine .
"With chlorine and iodine
With vapoiu- of sulphur .
With vapour of phosphorus

Daguerre .

. 1839

Goddai'd .

. 1839

Claudet

. 1840

Niepce

. 1820

Niepce

. 1820

Ctl.\ss Plate.

Precipitates of silver Herschel . . . 1839

Albxmien on Xiepcede St. Victor 1848

Collodion Uncertain . . 1850

Gold.

/-,! 1 • , c i Rumford . . . 1798

^^"^^""^ 1 Herschel . . . 1840

Etherial solution of Eumfbrd . . . 1798

Etherial solution of, with percya-

nide of potassium . . . Hunt . . . . 1844
Etherial solution of. ^vith protocja-

nide of potassium . . . Do 1844

Chromate of Do 1844

Plate of gold and iodine vapour . Goddard ... 1 842

Platinum.

Chloride of

. . Herschel .

. 1840

Chloride of, in ether . . .

. . Do. . . .

. 1840

Chloride of, with lime . .

. . Do. . . .

. 1832

Iodide of

. . Do. . . .

. 1840

Bromide of

. . Hunt . .

. 1844

Percyanide of

. . Do. . .

. 1844

Mebcurt.

Protoxide of Uncertain.

Peroxide of Guibourt.

Carbonate of Himt . .

1844

102

HISTORY OF PHOTOGRAPHY.

Mercury' — continued.

Chromate of Hunt .

Deutioclide of Do.

Nitrate of Herschel

Protonitrate of Herschel

Cliloride of Boulky

Bichloride of Yocrel .

1843
1843'
1840
1840
1803
1806

Iron.

Protosulpliate of Hunt . .

Perstdphate of |

Ammonio-citi'ate of V Herschel .

Tartrate of j

Attention was tii'st called to the

very peculiar changes produced

in the iron salts in general, by Herschel .
Cyanic compounds (Prussian blue, J Scheele

&c.) \ Desmortiers

Ferrocyanates of Fischer

Iodide of Dr. Woods

Oxalate of Hunt .

Cliromate of Do.

Several of the above combined with

mercury Herschel .

Copper.

Chromate of (Chromatype)

Sulphate of

Carbonate of ... .

Iodide of

Copper-plate iodized .

1844
1840

1845
1786
1801
1795
1844
1844
1844

1843

Manganese.

Permanganate of potash
Chloride of ... .

Lead.

Oxide of (the puce-coloured) .
Red lead and cyanide of potassium.

Hunt . .
Do. . .
Do. . .
Do. . .

Talbot . .

. . 1843
. . 1844
. . 1844
. . 1844
. . 1841

Frommherz
Himt . .

. . 1824
. . 1844

Davy . .
Hunt . .

. . 1802
. . 1844

Nickel.
Nitrate of

Iodide of . : : : : : ; :}^^^^

1844

GEJfERAL SUMMARY.

103

Tin.

Purple of cassius Uncei-tain.

Cobalt Hunt . .

Arsenic sulphuret of .... Sage . ,
Arsenical salts of

Antimony

Bismuth

Cadmium

Rhodiuji ......

Chromium.

Bichromate of potash
with iodide of starch

Chlorine and Hydrogen.

Chlorine (tithonized)
and ether

Glass, manganese, reddened
Cyanogen, solution of . .

Phosphorus

1844
1803

[. Hunt

1844

Mungo Ponton
E. Becquerel .

1838
1840

J Gay-LussacandThe-
) nard . . . 1809
Draper . . . 1842
Cahoiu-s . . . 1810

Faraday . . . 1823
Pelouse & Richard-

Schiilze
Hitter .

in nitrogen Beckman

and ammonia Vogel .

Nitric Acid decomposed by light . Scheele

Methyle Compounds Cahours

( Petit
Crystallisation of salts influenced J r<i-.„,-,4.ai
by light ^j).J

1838

1727
1801
1800
1806

1786

1846
1722

1788
1789

104

HISTORY OF PHOTOGRAPHY.

E-Esmous Bodies {Hdiography)

Asplialtum

Eesin of oil of lavender .

Guaiacum

Bitumens all decomposed
All residua of essential oils .
Flowers, colours of, expressed,

spread upon paper .
Yellow wax bleached

and

KieiDce . . . 1814
Niepce . . . 1814
Niepce & Daguerre 1830
WoUaston . . 1803
Da^Tierre . . . 1839

Daguerre

Herschel
Seneljier
Licetas
Kirclier
Canton
Biot .
^ E. Becquerel
Influence of light on electrical

phenomena E. Becquerel

Phosphorescent influences of solar
rays

1839

1842
1791
1646
1646
1768
1840
1839

1839

In the foregoing chapters every thing has been included which
appeared necessaiy to the complete illustration of the history of
Photography. It may be thought by many that the manipulatoiy
details included in this division should have been reserved for
that which is more strictly technical. The difficulty of doing
this without an annoying repetition, has led me to adopt what I
consider to be the clearer coui-se.

PART II.

SCIENTIFIC IMESTIGATIONS

PHOTOGRAPHY.

CHAPTEK I.

GENERAL BEMAEKS ON THE SOLAR AGENCY PRODUCING
CHEMICAL CHANGE.

Previously to explaining the practice of photogi'apliy, to wliicli
a separate division is given, it appears impoi-taut that the phy-
sical conditions of the elements with which we have to work
should be understood.

The sim-beam is oiu^ pencil, and cex"tain delicate chemical
l^reparations form our drawing-board. Every beam of light
which flows ft'om its solar source is a bundle of rays, haAdug
each a veiy distinct character as to coloui' and its chemical
functions. These rays ai-e easily shown by allowing a pencil of
sunlight to fall on one angle of a prism : it is bent out of its
path, or refracted, and an elongated image is obtained, present-
ing the various coloiu*s of which light appeal's to be constituted
— i"ed, orange, yellow, green, blue, indigo, and violet. This
coloured image is called the solar or the prismatic spectrum.
The red ray, being the least refracted, is fovmd at the lower edge,
ajid the violet, being the most so, at the other extremity of this
chromatic series. Below the ordinarily \dsible red, another i"ay
of a deeper red, distinguished as the extreme red, or crimson ray

108

SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

may be detected, by examining the whole tlirougli a cobalt blue
glass ; and, by throwing the spectram upon a piece of yellow

Violet.

Indigo.

Blue,

Green.
Yellow.
Orange.
Red.

papei', another ray appeal's at the violet extremity, named by
Sii' John Herschel the lavender ray. Yet more recently, Mr.
Stokes has proved a most remarkable extension of the luminous
rays. By throwing a prismatic spectrvim into a solution of qui-
nine in diluted sulphiuic acid, or an infusion of the bark of the
horse-chestnut tree, a set of extra spectral rays, extending far be-
yond the violet, make their appearance, proving the existence of
light over a space which has hitherto been thought incapable of
producing any luminous phenomena.

The original spectiTim of seven bands of colour was examined
by Sir Isaac Newton, and that eminent philosopher determined
that a given degree of refrangibihty indicated a given colour ;
that the colour of a ray at once indicated its angle of refraction.
Since the days of Newton, until our own time, this position had
never been called in question ; the seven rays were regarded as
the primary colours of white light, and the law of Newton
received as tinith ujDon his authority. Sir David Brewster has,
however, argued that this law will not stand the test of exami-
nation. He imagines that the prismatic spectrum consists of
three chromatic spectra overlapping each other, and that those
three colours — red, yellow, and blue — can be detected in every
part of the image. Sii' John Herschel has added two rays to
the luminous or visible spectrum, — thus making the number
nine instead of seven ; but these may, equally with the others, be
but combinations of the three primaries. There is, however,
much reason to doubt if the new extra spectral rays can be
comprehended within these three.

The colours of light will be rendered most femiliar by calling
to memory the conditions of that very beautiful natural pheno-
menon, the rainbow. The primary bow is usxxally accompanied

THE AGENT PRODUCIXG CHEMICAL CHAffGE. 109

by a secondary image, in whicli the oi'cler of the colours is re-
vei'sed. From close examination of the prismatic spectrum, I
am dLsposed to believe that -n-henever we obtain this chromatic
division of white light, it is accompanied by a secondaiy spectnim,
and that the real conditions of the coloiu-s are as follows : —

The yellov.^ is the most luminous ray, and the illuminating
power diminishes on either side of it ; on one side it blends with
the blue, to form the green, and on the other with the red, giving
rise to the orange ray. The blue diminishing in intensity sinks
towards blackness, and thus produces the indigo, the extreme
edge of which represents the limit of the ordinary spectrum at
that end ; as the outer edge of the red forms its limits, as far as
the human eye is concerned, on the other. Beyond the indigo
we have the violet taj : this would appeal' to be the blending of
the red of the supplementary specti-um with the blue of the
ordinaiy one, the lavender mj resulting from the intercombina-
tion of the less Iviminous I'ays with the coloui'ed sm-face upon
which it is thrown. Then the extreme red or crimson ray '^'ill
be seen to result from the blending of the extreme blue of the
extraordinary with the red of the ordinary spectral image. This
passage is still retained ; but I have eveiy reason to believe that
it will before long requii-e some modification, the discoveiy of
]Mr. Stokes materially altei-ing the conditions.

Sir William Herschel, and Sii' Henry Englefield, determined
the heating powei'S of these rays to be very varied. A thermo-
meter was placed in each, and the following restdts obtained : —

In the blue ray, in 3' the thermom. rose from 00° to oGi^, or 1°

green '•

yeUow '■'■

' full red "

•' edge of red "

'•' 58

' 4

'• 62

' 6

'•■ 72

■ 16

'• 731

•151

•• 79

• 18

Quite out of visible light in 2^' '•

Sir John Herschel, by another form of experiment, has fully
confirmed these results, and shown that the calorific, or heat-
produciug radiations, being less refi-acted by the prism than the
light-excitmg rays, exist a considerable distance further fr-om the
\'isible rays than has been hitherto suspected. Light and heat
have not, therefore, the same degi-ees of refi-angibdity ; their
influences are not coincident, theu* maxima in the solar spectrum
ai-e wide asunder. Melloni has shown that, by the use of
coloured media, these agencies can be, to a considerable extent
separated from each other. Glass stained ^\-ith oxide of copper,
and washed on one side -with a colomdess solution of alum,
admits the light rays most freely, but obstructs 95 per cent, of

110

SCIENTIFIC KfVESTIGATIOXS ON PHOTOGRAPHY.

the heat rays. On the contrary, a sKce of obsidian or black
mica obstructs nearly all the light radiations, but offers no im-
pediment to the passage of heat.

The chemical influences of the prismatic rays vary as their
heating powers, but in the contraiy direction.

If we place a jjiece of photogTaphic paper in such a position
that the spectrum falls upon it, it will be found to be very
unequally impressed by the various rays. Some very extraordi-
naiy peculiaiities have been observed by Sir John Herschel and
myself ; but it will be sufficient for oiu" present piu-jiose to state
the general featm-es of the impression under ordinary condi-
tions. For some distance below the visible red ray, the paper
will be found uncoloured ; on the part where the red ray falls,
a tinting of red or jAnk will be evident. The orange and yellow
rays leave no stain, and the green in general but a faint one. In
the place occupied by the blue ray, the first decided darkening
is evident, which increases through the indigo and violet rays.

Over this space the extra")
spectral or fluorescent >
rays become visible. )

and extends some distance beyond them. The shaded wood-
engra^-ing (Fig. 14) will serve to assist the reader in compre-

THE AGENT PRODUCING CHEMICAL CHAIfGE. Ill

hencling the phenomena. The chemical radiations have a higher
refrangibility than the ordinaiy luminous i^ays, and consequently
they extend in fuE action to a considerable space beyond the
lavender rays, where no light exists which can jjroduce excite-
ment on the optic nerve of the hujuan eye, unless aided by some
peculiar conditions of transparent media, or of optical an-ange-
ments.

Whenever we thi'ow a prismatic spectrum upon any photo-
graphic sui-face, it is always accompanied by a sufficient quantity
of difiiised light to produce some chemical change, which shows
itself in darkening, over the parts beyond the coloured image.
However, there are two points where this change does not take
place, and where the paper is preserved positively white; these
are the points of maximum light and heat — the yellow and
crimson rays. Here we have the evidence of the interference
of these agencies with the chemical radiations. As we can
separate heat and light from each other by the use of coloured
media, so can we isolate the chemical and limiinous principles of
the Sim's rays. By a yellow glass stained with silver we may
cut off the agency producing chemical change so completely, that
the most sensitive photogi'aphic material may be exjDOsed to a
full flood of sunshine, without its undergoing any alteration in
colour. If, however, we take a dark-blue glass, such as is usu-
ally prepared with the oxide of cobalt, of so deep a colovu* that
it obstiiicts a considerable quantity of Hght, and place imder it
the same, or any photographic preparation, it will be found to
darken as rapidly as if no glass had been interposed between it
and the sim.

Thus we obtain conclusive proof that it is not light, luvmwiis
power, which produces the chemical change. That it is not
HEAT is shown in the same manner by the protecting influence
exerted by the maximum calorific raysj and therefore we ai-e
driven to the hypothesis of the existence of a new agency — a
new imponderable element — or a noble form of force which is
broadly distinguished from these principles or forces in its effects.
To mark this the term Actinism has been proposed, and it is
now very generally adopted. The word signifies nothing moi-e
than 7X11/ power, and therefore, as involving no theoiy, it is fi-ee
from many of the objections which Avoidd apply to any other
term adopted from preconceived ideas.

Photogi'aphy ajipears to be a misnomer, since the pictures so
called are not cbawn by hght. It is, however, too finuly rooted
in the public mind to admit of the hope that any other may be
adopted. If I might venture a suggestion, I would advocate a
retm'n to the term introduced by Niepce, whose processes are

112

SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

described iu the historical section — Heliography, Sun-drawing,
which most clearly expresses the fact, leaving the questioaof the
particular agent effecting the chemical change still open for exa-
mination.

The annexed figiire (15) shows the conditions as they are at
present known.

f Space of extra spectral
\ rays.

b Lavender

Actinism, or chemi- "|_
cal radiant power, j

Light, c

Heat. D

Violet.
Indigo.

Blue.

Green.

Yellow.

Orange.

Red.

rt Extreme Bed.

Prom A to B exhibits the Ne"wi;onian spectrum, a and b being
the rays which belonged to modern discoveiy previously to the
recent observation that the peculiar blue rays seen in solution
of quinine and some minei'al oils belong to a yet higher order of
refrangibility. The curves c, D, and e, represent the relative
maxima of heat, light, and actinism, F being a second apparent
maximum — indicated in the red i-ay — of the chemical powei-s.
This may, however, be proved eventually to be a function of
heat, since we know that calorific power will produce chemical
change even when it is exercised as a radiant force.

The operation of these antagonistic forces is somewhat re-
mai'kably sho^vn over different regions of the earth. Advancing
from our owTi lands towards the tropics, it is found that the
difiiculties of obtaining pictures by the solar influences increase ;
and, under the action of the glowing light of equatorial climes,
a much longer period is required for impressing a photograph
than is occupied in the process either in London or Paris. It

THE AGENT PRODUCING CHEMICAL CHANGE. 113

has been stated by Dr. Di-aper, that in his progi-ess from New
York to the Southern States he found the space protected fi'om
chemical change by the yellow rays regularly increasing..

The same result is apparent in the differences between the
spring and summer. Usually in March and April photographs
are more readily obtained than in Jvme and July.

It is worthy of notice, that the moi-ning sun, between the
hours of eight and twelve, produces much better effects than
can be obtained after the hom* of noon : this was observed at a
veiy early period by Daguerre. For drawings by appUcatiou,
this is but slightly, if at all, felt, but with the camera it is of
some consequence to attend to this fact. We are not yet in a
position to record more than the fact, — the cause of the diffe-
rence is not detennined ; probably it may be found to exist in a
greater absorptive action of the atmosphere, caused by the eva-
poration of aqueous vapour from the earth. In the neighbour-
hood of large towns it might be accounted for by the circumstance
of the air becoming, during the day, more and more impregnated
with coal smoke, &c., which offers very powerful interruption
to the free passage of the chemical rays. This will, however,
scarcely account for the same interference being found to exist
in the open countn', some mUes from any town. Until our
meteorological obseiwers adopt a system of registering the varia-
tions of light and actinic power by means of some well-devised
instniment, we cannot expect to an-ive at any very definite
results. The subject involves some matters of the first import-
ance in photometiy and meteorolog}', and it is to be desu-ed that
oiu' public obseiwatories should be fiu-nished with the required
instruments for carrying out a series of observations on the
clii;mal and monthly changes in the relative conditions of the
solar radiations. We have now e\ddence which proves that
changes, almost inappreciable, in the condition of the atmo-
spheric media, thi'ough which the solar rays traverse, ai-e capable
of producing a most remarkable influence upon the colours of
the specti*um and their chemical power.

Many of the phenomena of vegetable life will be found to be
directly dependent upon the operation of these principles ; and
it would be important to mark any abnonnal states of gi'owth —
such as not unfrequently occur — and to be enabled to refer them
to peculiar solar conditions."

^ See Researches on Liglit. 2nd Edition, by the Author.

CHAPTEE II.

CHEMICAL CHANGES ON SENSITIVE PKEPAEATIONS.

It is of some moment to tlie pliotograpliic artist that he is
acquaiuted with the changes which occtu- in the several agents
which he employs. A few of these are therefore selected.

Section I. — Nitrate of Silver.

1. The crystallised salt, in a pure state, should be procured.
The commercial salt often contains nitrate of potash. The fused
nitrate, which is sold in cylincMcal sticks, is yet more liable to
contamination. A preparation is sometimes sold for nitrate of
sUver, at from sixjience to ninepence the ounce less than the
ordinary price, which may induce the luiwary to purchase it.
This reduction of price is effected by frising with the salt of
silver a projjortion of some other metalHc salt. The fi-aud is
readily detected by observing if the salt becomes moist on expo-
sm-e to the air, — the adulterated niti-ate of silver being delique-
scent. The evils to the photographer are, want of sensibility
upon exposm-e, and the perishability (even in the dark) of the
finished drawing.

As all the silver salts are jDrepai^ed from the nitrate, it is of
consequence that its character and changes be clearly understood.

Experiment 1. — Dry nitrate of silver, free of organic matter,
will not blacken by sunshine ; and, when dissolved in perfectly
pure distilled water, it may be exposed for a long time to solar
influence without imdergoing any visible change. Add, how-
ever, to the solution the smallest appi-eciable quantity of any
organic matter, and it wiU almost immediately begin to blacken.
This is so certain, that nitrate of silver is the most sensitive
test that we have for the presence of organic matter in water.

Exjjeriment 2. — Place a stick of charcoal in pure water con-
taining nitrate of silver, most beautiful crystals of silver will form
around the charcoal. We here see that carbonaceous matter
has the power of effecting the decomposition of the silver salt.
In the first examj^le, we have the metal precipitated as a black

NITRATE OF SILVEE. 115

powder — oxide of silver and metallic silver ; in the last,, it is
revived as a piu'e wliite metal, the ciystals being of exceeding
brilliancy. Thus we leaiii that the organic matter of the j)aper
or of the size, is necessary to deteimine the change on which
the photogi-aphic phenomena depend.

These salts have veiy
remarkable colorific

Section IL — Chloride of Siltek.

For the formation of the chloride of silver, any of the follow-
ing salts may be added to a solution of nitrate of silver : —

1. Chloxide of Sodium (Common Salt).

2. Chloride of Potasium.

3. of Baiium.

4. of Strontium.

5- . of Calcium.

6. Hydrochloride of Ammonia. [ properties.

7. of Peroxide of iron. J

8. Hydrochloric Acid (Spirits of Salts).

9. Solution of Chlorine in water.

The above salts are only necessaiy for the piu-pose of giving
a variety of coloiu^ to the artist's productions ; either one of
them may be used. Tliis is a point of much interest, as the
result of iising these different materials as the base deter-
mining the tone of the finished pictui'e, enables us to produce
effects which are in accordance with the subject which we desire
to represent.

A few experiments of an easy character will be instructive,
as pointing out the character of those changes which sensitive
surfaces undergo.

Experiment 3. — Pour some of the solution of common salt into
the solution of niti-ate of silver ; immediately, a very copious
white precipitate takes place. Pour off the supernatant liquor,
and well wash it, by the dim light of a candle, with piu-e dis-
tilled water ; then expose it to daylight : it will change colour
very slowly, passing fi'om white to grey. Drop a little niti*ate
of silver upon the white pi-ecipitate, it avlU dai'ken much more
rapidly than before ; add a little organic matter, and the
change occui-s still qiiieker ; and the degree of darkness
which it eventually attains will be cousidei'ably deeper than
before.

In this experiment we prove that, although the white salt of
silver changes coloiu' alone, the addition of nitrate of silver and
organic matter considerably quickens the operation ; therefore,

116 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

in preparing the papers, it is always necessary for tlie nitrate of
silver to be in excess.

Scheele, in his " Experiments on Air and Fire^'' has some ex-
periments which are remarkably to the point. " I precipitated
a solution of silver by sal ammoniac ; then I edulcorated and
dried the precipitate, and exposed it on a piece of paper to the
beams of the sun for the space of two weeks, when the siu'face
of the white powder grew black ; after which I stirred the
powder, and repeated the same several times. Hereuj)on I
poured some caustic spii-it of sal ammoniac on this, in all ap-
pearance, black powder, and set it by for digestion. This men-
struum dissolved a quantity of luna cornua (horn silver), though
some black powder remained undissolved. The powder having
been washed, was for the greater part dissolved by a pure acid
of nitre, which, by the operation, acquired volatility. This solu-
tion I precipitated again by means of sal ammoniac into horn
silver. Hence it follows that the blackness which the luna
cornua acquii'es is silver by reduction.''''

Experiment 4. To determine the character of the change set up
by sunshine. — A solution, No. 1, is nitric acid and oxide of silver
dissolved in water, and a solution, No. 2, is chlorine and sodium.
When these solutions are mixed, a white precipitate — chloride
of silver — falls. The chlorine of the common salt seizes the
silver, and as this is nearly insoluble, it is precipitated : the
nitric acid combines at the same time with the soda, and this
remains in solution. The chloride of silver being carefully
washed, is placed in very piu-e distilled water, to which a minute
portion of organic matter has been added, and then exposed to
sunhine. After it has darkened, I'emove the water, and it will
be found to contaia chlorine ; by adding some nitrate of silver,
we shall obtain a fresh precipitate, and we may thus determine
exactly the amount of decomposition which has taken place.

In the process, the strong affinity existing has been broken
up. Metallic silver, in a state of very fine division, is produced ;
and the clilorine set free dissolves in the water, from which we
can precipitate it again as chloride of silver, and consequently
readily ascertain its quantity.

It is necessary now to direct attention to the efiects of
organic matter in accelerating the blackening process. Sii' John
Herschel, whose researches in this branch of science are marked
with his usual care, has given jjarticidar attention to this matter.
As it is impossible to convey the valuable information that Sir
John has published, more concisely than in liis own language, I
shall take the liberty of extracting rather freely from his memoir,
published in the Philosophical Transactions.

CHLORIDE OF SILVER. 117

" A great many experiments were made by precipitating
organic liquids, both vegetable and animal, "witli solutions of
lead ; as also, after adding alum, with aUcaHne solutions. Botli
alumina and oxide of lead are well-kno^Ti to have an afiinity to
many of these fugitive organic compounds which cannot be con-
centrated by evaporation A^athout injiuy, — an aflinity suificient
to cany them down in combination, when precipitated, either
as hych'ates or as insoluble salts. Such- precipitates, when col-
lected, were applietl, in the state of cream, on paper, and, when
diy, were washed with the niti^ate. It was here that the fii-st
prominently successfid re&Tilt was obtained. The precipitate
thrown dowTi from a liquid of this description by lead, was found
to give a far higher degi'ee of sensitiveness than any I had before
obtained, receiving an equal depth of impression, when exposed,
in comparison with mere nitrated paper, in less than a fifth of
the time ; and, moreover, acquiring a beautiful ruddy broM-n
tint, almost amomitiiig to crimson, with a peculiarly rich and
velvety effect. Alumina, similarly precipitated from the same
liqiud, gave no such result. Struck by this difference, which
manifestly refeiTed itself to the precipitate, it now occiu-red to
me to omit the organic matter (whose necessity I had never
before thought of questioning), and to operate with an alkaline
precipitant on a mere aqueous solution of niti-ate of lead, so as
to produce simply a hych-ate of that metal. The result was in-
structive. A cream of this hydrate being applied and dried,
acquired, when washed with nitrate of .silver, a considerable in-
crease of sensitiveness over what the niti-ate alone would have
given, though less than in the experiment where organized
matter was present. The rich crimson hue aLso acquired in that
case under the influence of Hght, was not now produced. Two
peculiarities of action were thus brought into view ; the one,
that of the oxide of lead as a Ttwrdant (if we may use a tei-m
borrowed from the art of dyeing), the other, that of organic
matter as a colorific agent.

" Paper washed with acetate of lead was impregnated with
various insoluble salts of that metal — such as the sidphate, phos-
phate, miuiate, hycModate, borate, oxalate — and others, by wash-
ing with their appropriate neutral salts, and, when chy, applying
the nitrate of silver as usual. The results, however, were in no
way striking, as regards sensitivenes.s, in any case but in that of
the muriatic applications. In all cases where such appheations
were used, a paper was produced infinitely moi'e sensitive than
any I had at that time made. And I may here obseiwe, that in
this respect the mmiate of stroutia appeared to have decided
advantage."

118 SCIE^-TIFIC mVESTIGATIOXS ON PHOTOGKAPHY.

It -would oe tedious and useless to mention all tlie combrna-
tions of alkaliae and earthy niimates which have been de^dsed to
vary the effect, or increase the sensitiveness of the silver pre-
pai-ations : the veiy considerable differences produced through the
influence of these salts will afford peculiarly interesting results
to any inquii'er, and furnish him with a curious collection of
photogTaphic specimens. As a general nile, all the solutions
should be made in the combining proportions of the material
used. With a scale of chemical equivalents at hand, the photo-
graphic experimentalist need not eiT, taking care that a slight
excess of pure nitrate of silver prevails upon his paper or plate.

These changes should be well understood, before the photo-
gTaphic amateurs proceed to the prepai-ation of the sensitive
papers employed for the oixlinary processes.

Section III. — Iodide of Silver.

If iodide of silver is precipitated by mixing together solutions
of iodide of potassiu^m and nitrate of sUver in a concentrated
state, a heavy yellow powder falls, which "wdll scarcely change in
colour by an exposure of many days to svmshine. But if the
solutions are infinitely diluted, so that on mixing they only
become milky, and the Kght powder which occasions the opacity
falls but slowly to the bottom of the vessel, it will be found that
it is sensitive to the weakest solar radiations. There does not
appear to be any chemical difi'erence between the iodides thus
obtained ; but there are some remarkable physical pecuKarities,
and it is believed that attention to these wiU be found eventually
to be of the utmost importance.

Section IV. — Bromide of Silver.

In many of the works on chemistry, it is stated that the
chloride is the most sensitive to light of all the salts of silver •
and, when they ai'e exposed in a perfectly fonned and piu-e state
to solar influence, it wUl be found that this is nearly con-ect.
Modem discovery has, however, shown that these salts may
exist in peculiar conditions, in which the afiinities are so
delicately balanced as to be distiu-bed by the faintest gleam ;
and it is singular that, as it regards the chloride, iodide, and
bromide of silver, when in this condition, the order of sensibility
is reversed, and the most decided action is evident on the
bromide before the eye can detect any change in the chloride.

BBOMIDE OF SILVER. 119

The slight additional expense of the bromides is not "worthy
eonsidei'ation, particularly as their use may be confined to
papers for the camei*a obscura, the pictures on "which are of
course of the negative character, and the positive photogi-aphs
can be foimed by transfer on the chloridated papei"s of a highly
sensitive kind. Since there has been some question as to the
use of the iodide of silver "nrithout an infringement of patent, it
is not a little svu-j^jrising that the bromide has not been more
generally employed.

It "will be foimd that the bromide and iodide are much alike
in the singular "want of sensibility "which they sometimes exhibit
under the circiimstances ah'eady alluded
to, "which are not easy of exi^lanation.

If a paper first "washed "with a solution
of nitrate of silver has bromide of potas-
sium applied to it in different propor-
tions, say 20 grains, 15 gi^ains, and
10 gi-ains each, in t"wo ch-achms of "water,
and, when cby, be again "washed over
■with the silver solution, it will be foxmd,
unless, as is occasionally the case, some
organic combination interferes, that the 16

order of sensitiveness ^vill begin "with the

weakest solution, the stronorest bein? the least influenced by
light. The difierent degrees of darkness induced are faii-ly repre-
sented in the mai'gin. (Fig. 18.) As the different bromides give
to photogi'aphic paper varieties which much resemble those
enumerated under the muriates, I have thought it unnecessary
to give an account of any of them. The paper prepared "with
the bromide of potassium is the kind I have adopted, after
ha"v-ing tried upwards of two hundred combinations of silver
"with the other bromides.

To prepare a highly sensitive paper of tliis kind, select some
sheets of veiy superior glazed post, and wash it on one side
only "with bromide of potassium — forty gi-ains to one ounce of
distilled water, over wliich, when cby, pass a solution of one
hundi-ed grains of nitrate of silver in the same quantity of water.
The paper must be cbied as quickly as possible without exposing
it to too much heat ; then again washed "with the silver solution,
and, when dry, carefully preserved for use.

It "n"ill be perceived that I adopt a slightly difierent manipu-
lation fi-om that recommended by Mr. Talbot. Instead of wash-
ing the paper with the solution of silver first, and applying the
bromide or the mmiate over tliis, and then the silver wash ag;iin,
I use the alkaline salt fii-st, and apply the metallic washes one

120 SCIENTIFIC rS'VESTIGATIOXS OX PHOTOGRAPHY.

on tlie otlier. I liave been induced to this from obsei'ving that
the photographic prepariition penetrates less deeply into the
paper than when laid on as originally prescribed, and, con-
sequently, the seiLsibility of it is increased. It will be found
that an addition of about one-twelfth of spirits of wine to the
solution of sUver will much increase the blackness of the paper
when solarised ; and I think we may safely say that the sensi-
bility is also improved by it, — at all events it is not impaii'ed.

M. Biot has expressed his opinion that it is not possible tr»
find any substance more sensitive to Light than the bromide of
silver : this is tme to a ceriain extent, but in combination \vith
deoxidizing agents other prepai-ations will be named which have
a decided superiority over the pm-e bromide of silver.

Section V. — IMiscellaxeous Salts of Silver

Fluoride of Silver.— The use of this salt appears to have been
first suggested by Sir John Hei-schel ; it forms the basis of a
process by the author, aheady described. It has lately been
claimed as a new photogi-aphic agent by the French, but the
date of publication determines this question in the author's
favour.

Phosphate of Silver. — Dr. Fyfe appeai-s to have been the
fii'st to suggest the use of the phosphate of silver as a photo-
graj)hic material, but I am obliged to confess it has not, in my
hands, proved anytliing like so successful as, from Dr Fyfe's
description, it was in his own. Indeed, he himself observes, in
speaking of its use in the camera obscura : — " Though repre-
sentations may be got in this way, yet, so far as I have found,
they have not the minute distinctness of those got by the method
afready mentioned (i. e. by application). Owing to the inter-
ference of the lens, the Light does not act neai'ly so powerfully
on the paper, as when it has to permeate merely a frame of
glass."

For all practical piu'poses, the method which Dr. Fyfe has
given of prepai'ing these papers is, perhaps, the best : — " The
paper is first soaked in the phosphate of soda, and then ch-ied,
after which the nitrate is spread over one side by a brush ; the
paper again di-ied, and afterwards again put tlu'ough the salt, by
which any excess of silver is converted to phosjihate. As thus
prepared, it acquires a yellow tinge, which becomes black by
exposure to light." It will be evident from these dii'ections,
that what was formerly said about the necessity of ha\TLng the
nitrate of sUver in excess, is here, according to Dr. Fyfe,

inSCELLAXEOUS SALTS OF SILVER. 121

oLjectionahle. It certainly does not appear to be so essential
in tliis preparation, that anything but piu-e phosphate of silver
should be vised ; yet I cannot help fancying that a slight advan-
tage is gained, even here, by allowing a little excess of nitrate.
Dr. Fjfe has given a process for applying the phosphate of
silver, mixed as a paint, on metal, glass, or paper. It, however,
reqnii'es the skill of an artist to produce an even siu-face, and
unless a uniform gi-ound is given, the .pictiu-e is defoiTned by
waving lines of different shades. A method of jirecipitating
argentine salts on smooth surfaces will be given in the following
pages, by which means the most unifoiTa face is procured, and
many beautiful effects produced.

Fulminate of Silver. — Notwithstanding the extraordinaiy
degi'ee of sensibility which has been given to paper and to
the metallic plates by the industrious experiments of chemists,
I am convinced that we may hope to obtain agents of far
higher natiu-al sensibility than those we now possess ; and I
look with much anxiety to some of the combinations of organic
radicals with metallic bases. The fidminates and the ethyle
compounds present a veiy promising line of inquiiy. Mr. John
Towson, of Devonport, who piu-sued, conjointly with myself,
a most extensive series of researches on photogi-aphic agents,
was endeavoming to foiTQ a solution of silver, in which the
elements shoidd be so delicately balanced as to be overturned
by the action of the faintest light. To do this, he dissolved
some very pure silver in nitric acid, to which spirits of wine
was added somewhat suddenly in proportions equal to the
acid used, and the precipitation of the fidminate prevented
by a quick effusion of cold water, siifficient to bring the specific
gi'avity of the solution to 1-17, and to this a few cb-ops of am-
monia were added. Pieces of Bank post paper dipped in this
solution became, the instant they were presented to the declin-
ing Hght of an autumnal evening, a beautifid black having a
pm'ple tinge. This effect did not seem to come on gradually,
but, as by a sudden impulse, at once. Both this gentleman and
myself have often endeavoured to rejieat this, but in no one
instance have either of us succeeded in producing ami;hing
neai'ly so sensitive. It shoidd be stated, that the solution pre-
pared in the evening had become, by the foUo'w'ing morning,
only ordinai-ily sensitive, and that papers prepared with it were
deliquescent and bad. In repeating any modification of this
experiment, the gi-eatest care shoidd be taken, as explosions of
considerable violence ai-e othei'wise likely to occur.

Another series of expeiiments on the fulminates of silver have
produced veiy pleasing photographic residts, but I am not

122 SCIENTIFIC IXVESTIGATIOXS OX PHOTOGRAPHY.

enabled to specify any particular method of preparing them,
whicli may be certain of reproducing the results to which I allude.
Nothing can be more capricious than they ai'e ; the same salt
darkening rapidly to-day, which ^vill to-morrow appear to be
absolutely insensible to radiation, and which wiW again, in a few
days, recover its sensitiveness, to lose it as speedily as before.

The beautiftd researches of Professor Frank land, of Owen's
College, Manchester, however, most satisfiictorily prove that a
great many of the metals will combine with organic radicals in
the sunshine which will not so eomliiue in darkness.

Organic and other Salts of Silver. — AVith the exception of
the cai'bonate, tartrate, acetate, citi-ate, oxalate, and one or two
others, the salts of sUver, besides those already described, do not
appear to be sensibly influenced by Kght. Many have been
mentioned by authors as absolutely insensible to its influence ;
but recent expeiiments have produced modifications of these
salts, which are delicately sensitive to the solar ray. Amongst
othei-s, the chi'omate has been named, and certaiuly it has not yet
been rendered sensitive to an exposiu-e of some hom-s to dayhght ;
but one experiment of mine has proved that the solar beam will,
in a few days, produce a fine revival of metallic silver fi.-om its
chromate ; and another expeiiment ^vith it, has the most pleasiag
result of biingiag ■K\'ithkL the range of probabilities, the j^iroduction
of photogi-aphic pictiu-es yd. their natiu-al colours.

Reseai'ches having this object in view led to the discoveiy of
the chromatype ; but this beautiful salt (chromate of silver) has
not yet been applied directly as the photographic agent. In
the present state of oiu- knowledge, we cannot ventui'e to affirm
that any salt of silver, or, indeed, of any of the other metals,
exists, having an absolute insensibility to light, or in which the
requii'ed unstable equilibriimi may not be induced, so that the
Sim's beam might change the character of its combinations. I
am, indeed, con-sinced that no body in natm-e is entii-ely unin-
fluenced by the action of the sun's rays. Pajjei-s washed with
either of the alkaline carbonates, and then "nn-th a solution of
nitrate of silver, resemble in theii* character those prepai'ed with
the miuiates, but are not darkened so readily.

The tartrate of sil%-er possesses some very extraordinaiy pecu-
liaiities. Papers may be prepared, either by spreading the tar-
ti-ate at once over the surface, or better, by soaking the ^mper
in a solution of Rochelle salt (the tartrate of potash and soda),
and then applying two washes of the solution of niti-ate of silver.
The fia-st action of Hght is very feeble, but there gradually comes
on a stronger discoloiu-ation, which eventually proceeds with
rapidity, and at length blackens to an extent beyond almost

ORGA^^c co5ipot::nt>s with silver. 123

every other paper. This discoloiu-ation may be Tvonderfiilly
accelerated by washing over the tartrated paper with a veiy
dilute solution of the hydiiodate of potash, diu-ing the process
of darkening. It is not easy to use this when copying anything,
but there are cases in which the extreme degi'ee of dai'kness
which this prepai-ation acquii'es, rendei's it valuable. The acetate
of silver comports itself in the same manner as the tartrate.
The citi'ate, oxalate, &c., are only interesting as forming part
of the series of argentine preparations which exhibit decisive
changes when exposed to light. The methods of rendering them
available will be sufficiently undei-stood from the foregoing
details, and it would only be an imnecessary waste of words to
give any more particiUar dii-ections as it regai"ds them.

CHAPTER III.

THE THEORY OF THE DAGUERREOTYPE.

Numerous speculations having been ventm-ed as to the pecu-
liar chemical changes which light produces on the iodidated
silver tablets, I shall make no apology for introducing a few
remarks on this very interesting subject.

Numerous exj^eriments on plated coj)per, pure silver plates,
and on silvered glass and paper, have convinced me that tlie fii'st
operations of polishing with nitric acid, &c., are essential to the
production of the most sensitive surface. All who will take the
ti'ouble to examine the subject "will soon be con\'inced that the
acid softens the silver, bringing it to a state in which it is ex-
tremely susceptible of being either oxidized or iodized, according
as the cu'cumstance may occur, of its exposure to the atmosphere
or to iodine. The process, adopted, I believe, first in America,
of producing a deposit of chemically pure silver on the plated
metal, by means of the voltaic batteiy, wliich certainly gives
rise to some peculiar conditions, appears to prove that the soft
surface of silver is of advantage.

The sensitive siu-face is a combination of iodine, or of iodine
and bromine, with the silver. When exposed to radiant in-
fluences in the camera, a molecular change is efiected, and there
is much doubt if any iodine or bromine is removed from the
surface. Some have thought that tlie superficial film being
decomposed, the iodine and bromine attack a lower s\u"face of
the plate ; but experiments are still wanting.

I have discovered that all the rays of the prismatic spectrum
act on the Daguerreot\'pe plate, except the yellow, and a circle
of Kght of a peculiar and mysterious character, which surrounds
the visible spectrum. The light acting on a prepared tablet,
appeal's to decompose the film of iodide of silver to difierent
depths, according to the order of refit'angibility of the rays : the
violet ray and extra-spectral rays efiecting the . deepest decom-
position, whilst the red acts to a depth inappreciably slight.
Thus it is that the spectrum impressed on a Daguerreotype
plate reflects natiu'al tints of the same kind as Sir* Isaac
Newton's thin films ; the thickness of each film of reduced silver

.* A

ON THE THEORY OF THE DAGUERREOTYPE. 125

on the plate being in exact proportion to tlie chemical agency of
the coloiu-ed ray by "which it was decomposed.

On photogTajjhic papei-s, the decomposed ai-gentine salt exists,
in all probability, in a state of oxide, mixed with revived silver ;
but on the silver tablet the iodide is changed over all the parts
on which the light acts, and pure silver in a state of extreme
division results. The depth to which the decomposition has
been effected being in exact relation to the intensity and colour
of the light radiated from the object which we desu-e to copy, the
merciu'ial vapoiu- iinites with diiferent ijroportions of silver, aids
in encreasing the decomposition of the silver salt, and thus are
formed the lights and middle tints of the picture. The shadows
ai'e produced by the unchanged silver fi-oni which the iodine is
removed by the hyposulphite of soda.

Daguerre himself laid much stress uj^on the necessity of ex-
posing the plate to receive the vapoiu- of merciuy at an angle of
4-5''. This, perhaps, is the most convenient position, as it enables
the oi^ei-ator to view the plate distinctly, and watch the develop-
ment of the design : but beyond this, I am satisfied there exists
no real necessity for the angular position. Both horizontally
and vertically, I have often produced equally efiective daguerreo-
types. Looking at a daguerreotype pictiire in such a position
that the light is incident and reflected at a large angle, the
th-awing appears of the negative character ; the silver in such a
• position appearing white, and the amalgam of mercvuy and silver
a pale grey. Yiew the plate in an}- position which admits of
but a small angle of reflection, and we then see the design in
all its exquisite beauty, correct in the ai-rangement of its lights
and shades, — the silver appearing black, while the amalgam, by
contrast- in part, and partly in reality, appeal's nearly white.

The cause leading to the unifomi deposition of the mercurial
vapour is diSicult of solution. It does not appear to me that
any one of the hypotheses put forth, satisfies all the conditions
of this peculiar phenomenon.

Few papers have been published which so completely investi-
gate the phenomena of the chemical change in the daguen-eotype,
as that of Mr. George Shaw. As giving a lai-ge amoimt of
valuable information, I transfer it fi-om the Philosophical
Magazine.

" It is well kno%vn that the impression produced by light on
a plate of silver rendered sensitive by M. Daguen-e's process, is
wholly destroyed by a momentary exposure of the plate to the
vapour of either iodiQe or bromine. Although this fact has
long been known, the natm-e of the action by which so extra-
ordinary an efiect is produced has not yet been satisfactoiily

126 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

explained. In tlie hope of elucidating this subject, a series of
experiments was instituted, the results of wliich are recorded in
the following remarks.

" A silver plate prepared by exposure to iodine or its com-
pounds with bromine, may be exposed to the vapour of merciuy
without being in any way affected by the exposure. If, howevei',
the prepared plate be previously exposed to light, or made to
receive the luminous image formed in the camera ohscura, the
merciu-ial vapour attacks it ; forming, in the former case, a white
film, and in the latter, a pictm-e corresponding to the kiminous
imasre which had been allowed to fall on it.

" If a prepared plate, after receiving a vertical impression by
light, be exposed to the vapoiu' of iodine or bromine, it is found
that the vapom- of mercuiy no longer attacks it ; or, in other
words, the impression produced by light is destroyed.

" The first experiments made for the pm'pose of arriving at
the cause of this phenomenon had reference to the relation
between the time of the exposure to light and the time of ex-
posvu-e to the vapour of iodine or bromine necessaiy to destroy
the effect produced by light. Prepared plates were exposed in
the camera ohscura for a length of time, which previous experi-
ment had determined to be sufficient for a full development of
the pictiu-e ; some of those plates were exposed dui-ing two
seconds to an atmosphere feebly charged with the vapour of
bromine, while others were carefully preserved from contact with
the vapoiu"s of iodine or bromine. The atmosphere of bromine
employed, was pi'oduced by adding thu'ty drops of a satiu-ated
solution of bi'omine in water to an ounce of water : the solution
was pom"ed into a glass vessel, and the plate was exposed to the
vapoiu' in the vessel dm-ing the time specified. The plates were
then introduced into the mercury box, and by volatilizing the
metal, pictui'es were developed on all those which had not been
exposed to the vapoxu' of bromine, while those which had been
exposed to it exhibited no trace of a pictiu"e under the action of
mercury.

" The same experiments were repeated -with iodine, with
exactly similar results.

" Prepared plates were exposed to diffused light in the shade,
and others were exposed to the direct rays of the sun ; the object
being in both cases the production of a more intense impression
than that prodviced by the feeble light of the camera ohscura.
Some of these plates were exposed to the vapour of bromine,
and others to the vapoiu" of iodine, while others were carefully
preserved from the vapours of these substances. On subsequent
exposiu-e to the vapoiu' of merciuy, those plates which had not

ON THE THEORY OF THE DAGUEEROTYPE. 127

beeu exposed to iodine or bromine, exhibited, by the large quan-
tity of merciuy wbich condensed on tbem, the effects of expo-
sure to intense light : v/hile those which had been subjected to
the action of either bromine or iodine were in no way affected
by the vapoiu- of mercvuy. Many repetitions of these experi-
'ments demonstrated that the effect of exposure to the most
intense light, was completely destroyed by the shortest exposui'e
to the vapour of bromine or iodine.

" Experiments were now instituted for the piui:)0se of ascer-
taining in what condition the prepared plate was left, after having
been fii-st exposed to light and aftei-wards exposed to the vapoiu'
of bromine or iodine. In these experiments a method of treat-
ment somewhat different from, and more convenient than that
described, was resorted to, as in practising that method efiects
occasionally presented themselves which interfered with the re-
sults, and rendered it difficidt to determine with certainty, how
far some of the appearances produced were due to the action of
light. It is well known, that a prepared plate has a maximum
of sensitiveness when the iodine and bi-omine are in a certain re-
lation to each other : if there be a deficiency of bromine, the
maximiun sensitiveness is not obtained, and, if there be an excess,
the plate is no longer sensitive to light ; bxit when ■exposed to
the vapoiu' of merciuy, loithout having been exposed to light, be-
comes white all over, by the condensation of merciuy thereon ;
that is to say, it exhibits the appearance of a plate which had
been lyroperly prepared, and which had been exposed to light.
From this it will be evident, that a plate properly prepared in
the fii'st instance, and then exposed to light, may, by subsequent
exposiue to the vapour of bromine, have the impression produced
by light wholly destroyed ; and yet, by the accumulation of bro-
mine, may exhibit, on exposure to merciuy, an appearance
similar to that due to light. In other words, it is impossible
(in the case supposed) to distinguish between an effect produced
by light and an effect due to excess of bromine. By using
iodine in the place of bromine, there is no risk of producing the
appearance which accompanies excess of bromine ; but, on the
other hand, by auginenting the quantity of iodine, the sensitive-
ness of the plate is diminished. These difficidties were over-
come by using a solution containing both iodine and bi'omine,
in such proportions that the evaporation of each should take
place in the proportion in which they produce on silver the most
sensitive siu-face. The solution employed was made by adding
alcoholic solution of iodine to a solution of chloi-ate of potash,
until the latter woidd take up no more of the former ; and to
each oimce, by measure, of this solution, ten di-ops of a saturated

128 SCIENTIFIC IWESTIGATIONS ON PHOTOGRAPHY.

sokition of bromine in watei' were added. Tlie solution of
chlorate of potash was made by diluting one part of a saturated
solution of the salt with ten parts of water. The use of the
chlorate is simply as a solvent of iodine. In the subsequent ex-
periments, the plate was exposed to the vapour of this mixture
of iodine and bromine with precisely the same effect as when '
either was used separately, and without the inconvenience, or
uncertainty, which attended then* use.

" A number of preliminary experiments, the detail of which
would be xuiinterestiug, appeared to indicate, that not only is
the effect of light on a dagiierreotype plate destroyed by iocUne
or bromine, l)ut that the plate is restored to its original condi-
tion ; in other words, that its sensitiveness to light is restored.
In order to determine this point, the following experiments were
made.

" A prepared plate was exposed to light, and afterwards to
the mixed vapour ; * mercurial vapour produced no effect upon
it after a long exposure ; the plate on removal from the mercury
box was a second time exposed to light, and again introduced
into merciu-ial vapom\ The appearance of this plate was very
little changed, and it was conckided that no effect, or, if any,
very little, -was produced by the second exposiu-e to light. This
conclusion was, however, erroneous, as the following experiments
proved : —

" A prepared plate was exposed to light, and afterwards to
the mixed vapour : merciu-ial vapour was found to have no effect
upon it ; the plate was then partly covered with a metallic
screen, fixed close to, but not in contact with it, and the whole
was exposed to Kght. On placing the plate in the merciuy box,
a broad white band, nearly corresponding to the edge of the
defended part, made its appearance ; the whole of the defended
part (excepting the band in question) was unaffected, and the
exposed part exhibited very little change. By a careful exami-
nation of the plate after it was removed from the mercury box,
the white band in the middle appeared to be produced by the
feeble light wliich had passed imder the edge of the metal i:)late
which had screened the light fr-om part of the prepared surface ;
and the veiy dai-k, and apparently unaltered appearance of the
exposed part, was occasioned by an excess of action, for mercury
was found to have condensed on that part in large quantities,
and to have produced the dark lead colour which is commonly
called solarisation ; but which effect, in the case in question,

* " I shall hereafter call the mixed vapours of iodine and bromine produced
in the way described in the last paragraph but one, mixed vajjour, in order to
avoid circumlocution. — G. S."

ON THE THEORY OF THE DAGUERREOTYPE. 129

was SO excessive, that the colour of the part on which merciiry had
condensed differed but very slightly 6'om that on which no light
had fallen. It was now e\'ident that the apjiarent absence of
effect in the last experiment was in reality occasioned by an ex-
cess of action ; and by repeating that experiment, and making
the time of the second exposure to light much shorter than
before, the plate assumed, under the action of mercury, an in-
tense and beautiful whiteness.

" From these experiments, then, it was perfectly clear that
the impression produced by the light on a Daguen-eotype plate
is wholly destroyed by the mixed vapom', and that its sensitive-
ness to light is restored.

" It now remained to discover to what extent the sensitive-
ness is restored by the treatment in question. It was not at
first expected that the sensitiveness to light was as great after
tills treatment as after the original preparation of the plate ;
but experiments aftei-wards proved that the sui"face lost none of
its sensitiveness by this treatment, nor even by numerous repe-
titions of it. A prepared plate was exposed to light ; the im-
pression was destroyed and sensitiveness restored by the mixed
vapour ; the plate was a second time exposed to light and a
second time to bromine ; still its sensitiveness appeared unim-
paii'ed, for a fourth or fifth exposure gave, on treatment with
mercmial vapoui', a vivid impression. In order to determine
with the greatest accui-acy if the sensitiveness of the prepared
surface was at all impaired by these repeated exposures to light,
the camera ohscura was resorted to. A series of plates was jire-
pared with the utmost attention to uniformity ; some of these
were exposed in the camera ohscura, and pictiu^es obtained by
the subsequent exposure to vapour of merciuy : the time re-
quisite for the proper development of the picture was noted ;
others were first exposed to the direct rays of the sim, and after-
wards to the mixed vapour, and these were exposed in the
camera ohscura for the same length of time as those which had
not been exposed to light. On treatment with mercurial vapour,
perfect pictvu-es were produced, which could not be distiuguislied
fi-'om those taken on plates prepared by the ordinary method.
So completely does the mixed vapoiu- restore the sensitiveness
of prepared plates after exposiu^e to light, that the most beau-
tifid impressions were obtained in the camera ohscura in two
seconds on plates which had previously been four times exposed
to the direct light of the svm, and after each such exposure
treated ^vith the mixed vapour.

" As the ] Jates experimented on, to this stage of the inquiry,
had been wholly exjDosed to the sun's light previous to exposure

130 SCEENTiriC tS'VESTIGATIOXS ON PHOTOGRAPHY.

in tlie caviera obscura, it was tlioiight that possibly some sHglit
effect was produced, which, from being the same on aU parts of
the plates, escaped observation ; and in order to avoid the possi-
bility of error from this cause, the impressions of light which it
was intended to destroy by bromine were afterwai'ds made in the
camera ohscura. Prepared plates were impressed ydxk vri-tual
images of different kinds, the camera ohscura being pointed first
at a house, afterwai-ds to a bust, next to a tree, and finally to a
living figTu-e, the plates after each impression, excepting the last,
beiQg momentarily exposed to the mixed vapom". In every in-
stance, the most perfect impressions of the objects to which the
camera ohscura was last dii-ected were obtained, and no trace of
the previous impressions was left.

'' Experiments were next instituted for the purjiose of ascer-
taining if the prepared sui'face, after the process of merciuiali-
zation, could be made to receive another impression by treatment
with mixed vapoiu-. Impressions were taken with the camera
ohscura, and after the full development of the picture by vapour
of merciuy, the plates were exposed to bromine and again placed
in the camera ohscura, the instrument being dii-ected in different
experiments to diflerent objects : on exposui'e to merciuial
vapoiu", other pictm-es made their appeai-ance, and althovigh con-
fused fi'om superposition on the fii'st pictiu'es, could be clearly
traced, and were found perfect in every part. This production
of pictm-e upon pictiu-e was repeated, imtil, by the confrision of
the supei-}3osed images, the effects of further exposui-e could be
no longer distingiushed.

"In all the experiments hitherto described, the destniction of
the impressions by bromine was effected in the dark, the ap-
paratus being situated in a room into which only a very feeble day-
light was admitted. It remained to be discovered if the mixed
vapoiu' had the power of destroying the effect of light while the
plate was still exposed to light, or if the vapoiu- had the power
oi susjoending or 2)reveiiti/ig the action of light on a daguerreotype
plate. In order to determine this point, the apparatus was
placed near the window of a well-lighted room, and so ari-anged
that, dui'ing the whole time of the preparation of the plate, by
exposiu'e fii'st to iodine and afterwards to bromine, it was ex-
posed to full daylight, and by a mechanical aiTangement, of too
obvious a natm-e to render description necessaiy, the plate was
withdi-awn from the bromine vessel into a dai'k box ; that is to
say, it was withch-awn at the same moment from the influence of
both light and bromine : on being placed in the camera ohscura,
plates so prepai-ed received impressions which by mercimaKzation
produced excellent pictures, and there was no trace of the action

ox THE THEORY OP THE DAGUERREOTYPE. 131

of any Hglit save that of the carniera ohscura. It follows, then,
that light is incapable of exerting any appreciable influence on
daguerreotype plates during the time they are receiving their
coatings of iodine and bromine.

" Although these experiments afford no information on the
subject in reference to which they were originally undertaken,
they are yet not without interest, both in their theoretical
bearing and in theii- practical application. They demonstrate
not only that the change (whatever it may be) effected by light
on silver plates prepared by DagueiTe's process, is completely
siispended in the presence of the vapoiu- of either iodine or bro-
mine, but that after that change has been produced the impres-
sion may be destroyed, and the plate restored to its original
condition, by a momentaiy exposru'e to either of these vapours.
In their practical application, these experiments show that all
the care which has been taken to exclude Kght from daguerreo-
type plates diu'ing theii* pi-eparation is unnecessaiy ; that so far
from a dark room being essential to the operations of the da-
guerreotype artist, the Kght of day may be allowed to fall on
the plate dui'ing the whole time of its preparation ; and that it
is only necessaiy to -^dthdraw it at the .same moment fi'om the
action of bromine and light by sKding it fr-om the bromine vessel
into the dai'k box in wliich it is can-ied to the camera ohscura ;
and where, fr-om the situation or otherwise, there is a difficulty
in obseiwing the colour of the plate during the process of iodizing,
it may be removed fr-om the iodine vessel, and its^colour examined
by the direct light of the sun, without risk or injiuy : for when
returned to the iodine or bromine vessel for a moment, the effect
of light is wholly destroyed.

" Perhaps the most valuable pi-actical application of these facts
is in the use of the same plate for receiving several impressions.
When, on taking the portrait or picture of any object liable to
move, there is reason to suppose that the motion of the person
or object has rendered the operation useless, it is not necessary
to thi'ow aside the plate on which the imperfect impression has
been taken, and resort to the tedious process of cleaning and
prepaiing another ; it is only necessaiy to treat the plate in
the manner already pointed out, and it is again equal in every
respect to a newly-prepared plate ; and this treatment may be
repeated, until, by the slow accumulation of too thick a film of
iodide of silver, the plate no longer possesses the same degree of
sensitiveness to Ught."

The researches of M. Claudet are ot considerable importance,
particularly as being those of a thoroughly practical photo-
graphic artist.

132 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY,

The plienomena which M, Claudet considers have not yet
been satisfactorily explained, and of which he treats, are those
referj'ing to the following points : —

1. Which is the action of light on the sensitive coating 1

2. How does the mei"curial vapour produce the daguerreotype
image 1

3. What are the particular rays of light that impai-t to the
chemical siu-face the affinity for mercuiy ?

4. Which is the cause of the difference in achromatic lenses
between the visual and photogenic lenses 1 why do they con-
stantly vary ?

5. What are the means of measuring the photogenic rays,
and of finding the true focus at which they produce the image 1

At the meeting of the British Association at Swansea, M.
Claudet expressed his opinion that the decomposition of the
chemical sxu'face of the dagTierreotype plate, by the action of
certain rays of light, produced on that sui-face a white precipi-
tate, insoluble in the hyposulphite of soda, which, when examined
by the microscope, had the appearance of crystals reflecting
light, and which, when seen by the naked eye, were the cause of
a positive dagTierreotype image. These were probably particles
of pure white silver.

The opinion of DagueiTe himself, and other writers, was, that
the action of light on the iodide of sUver had only the effect of
darkening the siu-face, and consequently of prodiicing a negative
image. But it escaped them, that, under the darkened iodide
of silver, another action could take place after a continued ex-
posure to light, and that the hyposulphite of soda washing could
disclose a positive image. M. Claudet proved this fact in ob-
taining, by the action of light only, and without mercury, images
having the same appearance as those developed under the action
of mercurial vapour. This direct and immediate effect of light
is certainly remarkable ; but the dagaxerreotype process is not
founded on that priuciple, on account of the slowness of its
action ; and it is fortunate that, long before light can produce
the white precijntate alluded to, it operates another effect,
which is the wonderful property of attracting the vapour of mer-
ciuy. This vapour is condensed in the form of a white powder,
having also, when examined by the microscope, the appearance
of reflecting crystals.

It is pi'obable that light exei'cises a two-fold action on the iodide
of silver, whether it is combined or not with chlorine or bromine.
By one, the iodide is decomposed, and the silver set free is pre-
cipitated on the surface in the form of a white powder or small
crystals ; by the other, which begins long before the former,

ON THE THEORY OP THE DAGUERREOTYPE. 133

the parts affected by light have been endowed with an affinity
for mercm'ial vapoui-.

By means of his photographometer, this investigator has been
able to ascertain that the pxu-e light of the sun perforins in
about two or three seconds the decomposition of the bromo-
iodide of silver, which is manifested by the white precipitate ;
while the same intensity of light determines the affinity for mer-
ciu'ial vapour in the short space of about i Vo o^^ P^^ '^^ ^ second.
So that the affinity for mercury is imjmrted by an intensity of
light 3000 times less than that which produces the decomposi-
tion manifested by the white precipitate.

For this reason it is difficult to suppose that the two actions
are the same. We must admit that they ai-e different. Long
before it can effect the decomposition of the siu-face, light im-
parts to the sensitive coating the affijoity for mercui'ial vapour ;
and this ajjpears to be the j^i'inciple of the formation of the
image in the daguerreotype process.

In a paper communicated to the Royal Society on the 17th
of June, 1847, M. Claudet stated that the red, orange, and
yellow rays were destroying the action of white light, and that
the siu'face was recovering its former sensitiveness or unaffected
state after having been submitted to the action of these rays.
It was inferred fi-om that ciu-ious fact that light coidd not have
decomposed the surface ; for if it had, it wovdd be difficult to
imderstand how the red, oi-ange, or yellow rays could combine
again, one with another, elements so volatile as bromine and
iodine, after they had been once separated from the silver.
These experiments have much in common ^vith those of M.
Edmond Becquerel, who has been led to a division of the spec-
trum into exciting rays and continiiating rays. But he had not
yet been able to ascertain that, when light has decomposed the
bromo-iodide of silver, the red, orange, or yellow rays cannot
restore the siu'face to its former state. The action of light,
which can be destroyed by the red, orange, or yellow rays, does
not determine the decomposition, which woidd requii'e an inten-
sity 3000 times greater ; it is the kind of action produced by an
intensity 3000 times less, giving the affinity for mercmy, which
is completely destroyed by the i-ed, orange", or yellow rays.
White light, or the chemical rays which accompany it, communi-
cate to the surface the affinity for mercmy ; and the red, orange,
or yellow rays withdi~aw it. This is in effect the same pheno-
menon as Dr. WoUaston observed with the tinctiu'e of giun
guaiacum ; one set of rays restoring the colour which another
set had removed. A singular anomaly requires notice : viz. that
when the sensitive svu'face is prepared only with iodine without

134 SCIENTIFIC IJfVESTIGATIONS ON PHOTOGRAPH Y,

bromine, the red, orange, or yello-w rays, instead of destroying
the action of white light, continue the effect of decomposition
as well as that of affinity for merciuy. Still there is a double
compound of iodine which is far more sensitive than the simple
compound, and on which the red, orange, or yellow rays exercise
their destiiictive action as in the case of the bromo-iodide.

The phenomenon of the continuing action of the red, orange,
or yellow rays, on the simple compound of iodide of silver, was
discovered by M. Ed. Becquerel ; and soon after M, Gaudin
found, that not only those rays continue the action by which
mercmy is deposited, but that they develope without mercury
an image having the same appearance as that produced by mer-
cm'ial vapoiu'.

M. Gaudin, not having observed the fact of the white precipi-
tate, which is the result of the decomposition by the action of
light, coidd not explain the cause of the image brought out under
the influence of the yellow ray.

M. Claudet states that the iodide of silver without bromine
is about 100 times more sensitive than the bromo-iodide to the
action of the rays which produce the decomposition of the com-
pound foiming the white precipitate of silver, while it is 100
times less sensitive for the effect which gives the affinity for
mercury. It may be that, in the case of the iodide of silver
alone, the decomposition being more rapid, and the affinity for
merciuy slower than when bromine is added to the comjiound,
the red, orange, or yellow rays having to act upon an incipient
decomposition, have the power, by theii' o^vn photogenic influ-
ence, of continuing the decomposition when it has begvm. This
may explaiu the development of the image under red, orange, or
yellow glasses, according to M. Gaudin's discovery. But in the
case of the bromo-iodide of silver, the red, orange, or yellow rays
have to exert their action on the affinity for mercury, begun a
long time before the decomposition of the compound ; and they
have the property of destroying that affinity.

So that it would appear that all the rays of light have the pro-
perty of decomposing the iodide of silver in a longer or shorter
time, as they have that of producing the affinity for mercury on
the bromo-iodide of silver : with the difference, that, on the
former comjDound, the separate actions of the several rays con-
tinue each other, and that on the second compoimd these separate
actions destroy each other. We can understand that, in the fii-st
case, all the rays are capable of operating the same decomposi-
tion ; and that in the second, the affinity for mercuiy when im-
parted by one ray is destroyed by another. This would explain
the various phenomena of the formation of the two different de-

ON THE THEORY OP THE DAGUERREOTYPE. 135

posits, and also the anomaly of the continuation of the action by
the red, orange, or yellow rays, according to M. Ed. Becquerel's
discovei'ies on the iodide of silver ; and of the destruction of that
action by the same rays, according to M. Claudet's observations
on the bromo-iodide of silver.

The red, orange, and yellow rays, when acting on an unaffected
sixrface, are considei'ably less capable than the most refrangible
rays, of imparting the affinity for mercurial vapour on both the
iodide and bromo-iodide of silver ; and they destroy that affinity
when it has been produced on the bromo-iodide of silver by the
photogenic rays. It follows from this fact, that when the red,
orange, or yellow rays are more abundant in the light than the
most refrangible rays, the photogenic effect is retarded in pro-
portion to the excess of these antagonistic rays. This happens
when there exist in the atmosphere some vaporirs which absorb
the most refrangible I'ays. In these circumstances the light ap-
pears rather yellow ; but it is very difficult to judge by the eye
of the exact colour of the light, and of the propoi'tion of photo-
genic rays existing in the atmosphere at any given moment.

The vapours of the atmosphere which I'ender the light yellow,
act as any other medium intercepting the blue rays, and those
which have the same degree of refrangibility.

If we cover an engraving one-half with light yellow glass, and
place it before a camera obscura, in order to represent the whole
on a daguerreotype plate, we shall find that during the time
which has been necessaxy to obtain the image of the half not
covered, not the slightest effect has been produced on the half
covered with the yellow glass.

Now, if we cover one-half with deep blue glass, and the other
with the same light yellow glass, the engi-aving will be seen very
distinctly through the yellow glass, and not at aU through the
blue. In I'epresenting the whole, as before, on the daguerreotype
plate, the half which was clearly seen by the eye has produced
no effect ; and in the other, which could not be seen, is as frdly
i-epresented, and in nearly as shoii; a time, as when no blue glass
had been interposed.

Thus we might construct a room lighted only throiigh an inclo-
sure of pale yellow glass, in which light woidd be very dazzling
to the eye, and in this room no photographic operation coidd be
performed ;* or in a room inclosed by deep blue glass, which
would appear very dark, and in which the photographic opera-
tion would be nearly as rapid as it would be in open air.

* I have recently proved that this statement requires some modification ; the
rays permeating many yellow glasses act powerfully on the sensitive surfaces
of collodion and iodine.

136 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

Thus we may conceive certain states of the atmospliere under
which thei'e will be an abundance of illuminating rays, and very
few actinic rays ; and some others, under which the I'everse will
take place. Considering how difficult it is to judge by the eye
alone of the chemical state of light, we can understand why the
photographer is constantly deceived in the effect he tries to pro-
duce, having no means to ascertain beforehand, with any degree
of certainty, the intensity of light. For these reasons M. Clau-
det tiu-ned his attention to contrive an apparatus by which he
could test at the same time the sensitiveness of the daguerreo-
type plate and the intensity of light. This instrument he called
a Photographometer.

" By this instiniment," says the inventor, " I have been able to
discover at what degree of intensity of light the effect called
solarization is produced : on well-prepared plates of bromo-iodide
it does not begm under an intensity 512 times greater than that
which determines the first effect of mercury ; and also at what
degree the decomposition producing the white precipitate without
mercury manifests itself, both on iodide and on bromo-iodide of
silver. On the first, it is 100 times quicker than on the bromo-
iodide : and on the last it is produced by an intensity 3000 times
greater than that which developes the first affinity for^ mercury.

" In the course of my experiments I noticed a ciu-ious fact,
which proved very puzzling to me, until I succeeded in assigning
a cause to it. I shall mention it here, because it may lead to
some further discoveries. I observed that sometimes the spaces
under the round holes, which had not been affected by light
dm-ing the operation of the photogi-aphometer in a sufficient
degree to determine the deposit of merciuy, were, as was to be
expected, quite black ; while the sj^aces suiTounding them were
in an unaccountable manner slightly affected by mercury. At
first I could not explain the phenomenon, except by siqjposing
that the whole plate had been previously by accident slightly
affected by light, and that the exposure through the holes to
another sort of light had destroyed the former effect. I was
naturally led to that explanation, having before observed that
one kind of hght destroys the effect of another; as, for example,
that the effect of the light from the north is destroyed by the
light ft'om the south, when certain vapoui's existing in the latter
portion of the atmosphere impart a yellow tint to the light of
the sun. But after repeated experiments, taking great care to
protect the jjlate from the least exposui-e to light, and recol-
lecting some experiments of M. Moser (see Chapter on Ther-
mography), I fomid that the affinity for mercury had been im-
parted to the su.rface of the Daguerreotype plate by the contact

ON THE THEORY OF THE DAGUERREOTYPE. 137

of the metallic plate having the roimd holes, while the space
under the hole had received no similar action. But it must be
observed that this phenomenon does not take place every time ;
some days it is frequent, and in some others it does not manifest
itself at all. Considering that the plate fiu-nished with roimd
holes is of copper, and that the Daguerreotype plate is of silver
plated on copper, it is probable that the deposit of mercmy is
due to an electric or galvanic action deterjained by the contact
of the two metals ; and perhaps the circumstance that the action
does not take place every time, will lead to the supposition that
it is developed by some peculiar electric state of the ambient
atmosphere ; and by a degree of dampness ia the air which
would increase the electric ciuTent. JMay we not hope that the
conditions being known in which the action is produced, and by
availing ourselves of that property, it ^\all be possible to increase
on the dag-uerreotype plate the action of light ? for it is not
improbable that the affinity for mercury imparted to the plate
is also due to some electrical influence of light. How could we
othei"wise explain that affinity for mercury given by some rays
and withdi'a"svTi by some others, long before light has acted as a
chemical agent 1

" The question of the actiaic focus is involved in another kind
of mystery, which requires some attention. I have found that
with the same lenses there exists a constant valuation in the
distance between the two foci. They are never in the same
relation to each other : they are sometimes more or less sepa-
rate ; in some lights they are very distant, and in some others
they are veiy near, and even coincide. For this reason I con-
stantly try their position before I operate. I have not been able
to discover the cause of that singular phenomenon, but I can
state positively that it exists. At first, I thought that some
variations in the density or dispersive power of the atmosphere,
might produce the alteration in the distance between the two
foci; or that when the yellow ray^ were more or less abundant,
the visual rays were refracted on diffisrent points on the axis of
the foci, according to the mean refrangibHity of the rays com-
posing white light at the moment. But a new experiment has
proved to me that these coidd not be the real causes of the
variation. I generally employ two object-glasses; one of shorter
focus for small pictures, and the other of longer focus for larger
images. In both, the actinic focus is longer than the visual
focus ; but when they are much separated in one they are less
so in the other : sometimes, when they coincide in one, they are
very far apart in the other, and sometimes they both coincide.
This I have tried every day during the last twelve months, and

138 SCIEKTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

I have always found the same variations. The density of the
atmosphere, or the colour of Hght, seems to have nothing to do
with the phenomenon, otherwise the same caiise would produce
the same effect in both lenses. I must obsei've, that my daily
experiments on my two object-glasses are made at the same
moment and at the same distance for each, other^vise any
alteration in the focal distance would disperse, more or less,
the actinic rays, whiaii is the case, as it is easy to prove. The
lengthening or shortening the focus, according to the distance
of the object to be represented, has, for effect to modify the
achromatism of the lenses. An optician, according to M. Lere-
bours' calculation, can at will, in the combination of the two
glasses composing an achromatic lens, adapt such curvatures
or angles in both that the visual focus shall coincide with
the actinic focus ; but he can obtain this result only for one
length of focus. The moment the distance is altered, the two
foci separate, because the visual and actinic rays must be re-
fracted at different angles in coming out of the lens, in order
to meet at the focus given for one distance of the object. If
the distance is altered, the focus becomes longer or shorter ;
and as the angle at which different rays are refracted remains
nearly the same, they cannot meet at the new foctis, and they
fonn two images. If the visual and actinic rays were re-
fracted parallel to each other, in coming out of the lens they
would always coincide for every focus ; but this is not the case.
It seems, therefore, impossible that lenses can be constructed in
which the two foci will agree for all the various distances, until
we have discovered two kinds of glasses in which the densities
or the refractive jiower wiU be in the same ratio as the dispersive
power."

CHAPTER IV.

ON THE PHOTOGRAPHIC EEGISTRATIOX OF PHILOSOPHICAL IXSTRU-
MENTS AND THE MEANS OF DETERMINTNG THE VARIATIONS OF
ACTINIC POWER, AND FOR EXPERI3IENTS ON THE CHEMICAL
FOCUS.

Section I. — Photographic Registration.

There are so many advantages attendant on self-registration, as
to make the perfection of it a matter of much interest to eveiy
scientific enqiiii'er. Tlie fii-st wlio suggested tlie use of photo-
graphic paper for this purpose was Mr. T. B. Jordan, who
brought the subject before a committee of the Royal Cornwall
Polytechnic Society, on the 18th of Febx'uary, 1839, and exhi-
bited some photographic registers on the 21st of March of the
same year. The plan this gentleman adopted was to fiimish
each instrument with one or two cylinders containing scrolls of
photographic paper. These cylinders are made to revolve slowly
by a very simple connection with a clock, so as to give the paper
a progressive movement behind the index of the instiiiment,
the place of which is registered by the representation of its own
image.

The application of this principle to the barometer or thermo-
meter is most simple ; the scale of either of these instiiiments
being perforated, the paper is made to revolve as close as possible
to the glass, in order to obtain a well-defined image. The
cj'Hnder being made to revolve on its axis once in forfy-eight
hovu's, the paper is divided into forty-eight parts by vertical lines,
which are figiu*ed in correspondence with the hoiu" at which they
respectively arrive at the tubes of the instiimients. The
graduations on the paper con^espond to those on the dial of the
barometer or scale of the thermometer, and may be printed on
the i^aper fi-om a copperplate, or, what is much better, may be
printed by the light at the same time from opaque lines on the
tube, which would of course leave a light impression on the
paper : by this means we shoidd have all that part of the paper
above the mercury darkened, which would at the same time be
gi-aduated with white lines, distinctly marking the flvictuations
in its height for every minute diu'ing daylight, and noting the
time of eveiy passing cloud.

140 SCIENTIFIC nrV'ESTIGATIONS ON PHOTOGRAPHY.

Mr. Jordan lias also published an account of his veiy inge-
nious plan of applying the same kind of paper to the magne-
tometer or diiirnal variation needle,* and several other philo-
sophical instriunents ; but as these applications were not at the
time eutii-ely siiccessful, owing principallj^ to the difficulty of
finding a suitable situation for so delicate an instiiiment, it is
thought unnecessary to occupy these pages with any particular
description of the aiTaugements adopted, which, however, were
in all essential points similar to those employed by Mr. Ronalds,
and adopted in some of om- magiietic and meteorological obser-
vatories. Those of Mr. Brooks are of somewhat more refined a
character, and require special notice.

A reflector is attached to the end of a delicately suspended
magnet ; this reflects a pencil of strong artificial light upon
photographic paper placed between two cylindei-s of glass, which
are kept in motion by a small clock arrangement. As the paper
moves in a vertical direction whilst the magnet oscillates ia a
horizontal one, a zigzag line is mai'ked on the paper ; the extent
of movement on either side of a fixed line showing the deviation
of the magnet for eveiy horn- of the day. By means of this
arrangement many most remarkable phenomena connected with
terrestrial magnetism have been discovered, and since the methods
of adjustment have been rendered more perfect, and the inven-
tion applied to a gi'eat variety of instruments, we may hope for
yet more important results.

The registration of the ever-varying intensity of the light is
so important a subject, that it has occiapied the attention of
several eminent scientific observers. Su' John Herschel and
Dr. Daubeny have applied their talents to the inquiry, and
devised insti-uments of much ingenuity for the purpose. The
instrument constructed by Su' John Herschel, which he has
named an actinograph, not only registers the direct efiect of
solar chemical radiation, bvit also the amoimt of general actinic
power in the visible hemisphere ; one portion of the apparatus
being so arranged that a sheet of sensitive paper is slowly
moved in such a direction, that the dii-ect rays of the sun, when
unobscured, may fall upon it through a small slit made in an
outer cylinder or case, while the other is screened fi.'om the
incident beam. The paper being fixed on a disc of brass, made
to revolve by watch-work, is afiected only by the light which
" emanates from that definite circumpolar region of the sky to
which it may be considered desii'able to limit the obseiwation,"
and which is admitted, as in the other case, through a fine sht in
the cover of the instrument.

* See the Sixth Annual Eeport of the Eoyal Cornwall Polytechnic Society.

PHOTOGRAPHIC REGISTRATION.

141

Mr. Jordan has devised an instrument for numerically regis-
tering the intensity of the incident beam, which appears to
have some peculiar advantages ; a description of which I shall
take the Kberty of transcribing. Figau'e 17 is an elevation of
the instrument ; it consists of two copper cylinders supported

on a metal frame : the interior one is fixed to the axis and does
not revolve, being merely the support of the prepared paper ;
the exterior cylinder is made to revolve about this once in
twenty -foiu* houi-s by a clock movement. It has a triangular
aperture cut down its whole length, as showTi in the figui-e, and
it carries the scale of the instrument, which is made to spring
closely against the prej^ared paper. This scale or screen is
composed of a sheet of metal foil between two sheets of var-
nished paper, and is divided into one hundred pai-ts longitudinally,
every other part being cut out, so as to admit the light to the
prepared paper Avithout any transparent medium intervening.
The lengths of the extreme divisions, measm'ing round the
cylinder, are proportioned to each other as one to one hundred ;
consequently the lower di\'ision will be one hiindi-ed times longer
passing over its own length than the upper one over its own
length, and the lines of prepared paper upon these divisions will,
of coiu'se, be exposed to the light for times bearing the same
proportion to each other.

Now, as the sensitiveness of the paper can readily be adjusted,
so that the most intense light vnU only just tint it through the
ujiper division diiring its 2)(issage imder the opening, and the
most feeble light will produce a similar tint through the lower
division dui'ing its passage, the nmnber of lines marked on the
paper at any given time ^\dll furnish a comparative measiu'e of
the intensity of solar light at that time, and may be registered
as so many degrees of the Heliogra^yh, the name Mr. Jordan

142 SCIENTIFIC INTESTIGATI0X3 OS PHOTOGRAPHY.

has given liis instrument, just as "sve now register tlie degrees of
the thermometer.

An instrument of this kind was made by me for the British
Association, and experiments carried on with it, at intervals, for
some years. Many of the results were very cui-ious, but the
instiiunent being placed at the Obsenatory at Kew, the obser-
vations were unfoi-tunately discontinued.* It is believed that,
with an instmment properly constiiicted, the details of the
one employed were capable of much improvement ; many very
remarkable altei-ations in the relative chemical power of the
solar i-adiations would be detected. From the indications I
have obtained, I believe there exists a constant law of change,
and that the correct expression of the phenomena is given in
the following pa.ssages — the concluding summary of my Report
on this subject to the British Association at Edinburgh : —

" It will be evident that the question which assumes the ipost
prominence in our consideration of these remarkable phenomena
is that of the identity or othei'wise of Kght and actinism.

" Fresnel has stated that the chemical effects produced by the
influence of light are owing to a mechanical action exerted by
the molecules of fether on the atoms of bodies, so as to cause
them to assume new states of equilibrium dependent on the
nature and on the velocity of the vibrations to which they are
subjected.

" Arago says, it is by no means proved that the photogenic
modifications of sensitive siu'faces result fi-om the action of solar
light itself. These mocUfications are perhaps engendered by
invisible radiations mixed with light properly so called, proceed-
ing with it, and beuig similarly refi'acted.

" These views fairly represent the condition in which the argu-
ment stands, and a yet more extensive set of experiments seems
to be necessaiy before we can decide the question. It appears,
however, important that we should dismiss, as completely as
possible, from our minds, all preconceived hypotheses. The
phenomena were all iinknown when the theories of emission and
of undulation were fi-amed and accepted in explanation of
luminous effects ; and it will only retard the discovery of the
truth, if we prosecute oiu' researches over this new gi'oimd, with
a determination to bend all our new facts to a theory which was
framed to explain totally dissimilar phenomena."

* A new instrument has been constructed, and a regular series of observations
are now in progress.

THE PHOTOGBAPHOMETER.

143

Section II. — Instbuments for Measuring Actinic
Variations, &c.

the photogbaphojieter.

M. Claiidet has devised tlie Photograpliometer and the
Dynactinometer for measiu^iug the intensity of the actinic
radiations. These are both most ingeidoiis instiniments, the
operations of which will be rendered intelligible by the following
description : —

The accompanying figure (18) shows the photogxaphometer

complete. The sensitive plate or paper is placed in a dai'k box,
which is placed in an independent frame, as shown in Figs. 20
and 21, and as placed in its position at F in the adjoining cnt.
A black silk webbing being fixed to the moveable plate seen at

19.

the head of the instrument, and strained over two rollers, r, r,
it will be evident that the sensitive plate is screened from light

144

SCIE^^"IFIC Ds^'ESTIGATIOXS ON PHOTOGRAPHY.

until the moveable slide falling down tlie inclined plane passes
over it. The openings in this moveable plate are parallel to
each other. They are seven in number, each opening being
one-half of the following one, and double that of the preceding
one. Thus, after the operation of the light, we have seven
separate images, the different intensities of which represent the

20.

action of light during the intervals of time in the geometric
progression of — 1 ; 2 : 4 : 8 : 16 : 32 : 64 :

The box in which the plate or paper is placed for experiment,

is pierced with holes, and these correspond with the slits in A B.
(fig. 22). By inclining the instrument, which can be verj'

THE FOCIMETER.

lio

readily done to any degree by means of the curved arms c D E,
any A^elocity can be given to tlie falling screen, and thus the
plate be exposed to the action of the chemical rays for any period
of time we please. Fig. 20 shows the result obtained on a plate
by this instrument ; the lettei"s corresponding with the holes iu
the other woodcuts. In fig. 2 1 the screen with the vei-tieal slits
is shown at the moment it is supposed to be passing over the
holes A B c D. In this example the plate had been exposed to
the vapour of iodine, in such a manner that one zone had
attained the fii'st coating of yeUow coloiu* ; a second zone had
reached the red ; a third the blue and gi*een ; and a fourth
having passed through all these tints, had obtained the second
yellow coating. The number of white circular spots on each
vertical zone indicates the degree of sensitiveness of the various
coatings ; the less sensitive being the first coating of yellow, d,
and the most sensitive the second yellow coating, a. This is
sho^vn by the deposit of mercury on the plate i*epresented by
the increased whiteness of the spots corresponding with the
holes, each foui' vertical spots having been exposed for the same
time to solar influence.

THE FOCIMETER.

M. Claudet has also devised a veiy ingenious uistniment for
focusing, which he calls his Focimeter. (Fig. 22.) This it will
be seen from the accompanying woodcut consists merely of
segments of a circle, numbered and placed at fixed distances
apart, upon a movable axis. This is copied by the camera on
a plate or paper, and the result is shown in the annexed figure
(23), in which it will be seen difierent degrees of eSect are sup-
posed to have been produced. These determine the best focal

146 SCIENTIFIC IKVESTIGATIONS ON PHOTOGRAPHY.

point for any lens very readily, and it is really a most useful
piece of apparatus in the hands of the photogi-apher.

THE DYNACTINOMETER.

The Djmactinometer is thus described bj- the inventor : — It
consists of a thin metallic disc, perfectly black, having a slit ex-
tending fi-om its centre to the cii'cumference, fixed on an axis
revolving tlirough a permanent metallic disc perfectly white.
The white disc has also a slit from its centre of the exact length
of the radius of the black disc ; and by means of these two sHts,
which are so adjusted that the lilack disc can intersect the white
disc, and by revolving, gradually cover the whole white area,
the space of the wliite surfxce on which the black disc can be
superposed forms itself a sort of dial, which is divided into any
number of equal segments, all niimbered. The inventor has
adopted the number of twenty segments for a large circle in-
scribed on the dial, and of eight segments for a smaller circle,
after the manner of the divisions of the Focimeter, but on the
same plane. These eight segments are niimbered in geometrical
progression, 1, 2, 4, 8, 16, 32, 64.

The black disc may be made to revolve in such a manner that
it shall cover a new segment of the large circle dvu-ing each
second, or any other equal fraction of time. By that means the
last segment will have received eight times more light than the
first, the black disc having moved over the whole in eight
seconds.

The difierences of photogenic intensities are hardly observable
when they follow the ai'ithmetical progression : the instrument

THE DTNACTINOMETER. 147

is SO constructed that it may indicate the intensities in the
geometrical progi-ession. The fii-st segment remains always
covered, in order to be represented black on the dagiien'eotype
plate and mark the zero of intensity : the second is exposed to
li»ht dm-ino; V, the thii-d diu-inw 2". the fom-th during 4", the
fifth diu'ing 8", the sixth during IG'', the seventh dui'ing 32",
and the eighth diu-ing 64". This series, which could be extended
by dividing the circle into a greater number of segments, is quite
sufficient for all observations intended for practically measuring
the intensity of the photogenic light, and for comparing the
power of object-glasses.

The instrument is made to move by applying the hand on a
handle fixed on the back at the extremity of the axis on which
the disc revolves. An ojjerator accustomed to count seconds
by memory, or by following a seconds' beater, can perform the
experiment with sufficient regularity ; but in order to render
the instiTunent more exact and more complete, it can be made
to revolve by clock-work, which gives it, at "s\t11, either the ai-ith-
metical or the geometrical progi-ession. This last movement
presented some difficulty; but the inventor has been able to
obtam it without mnch complication in the machinery, and the
apparatus is within the reach of the gi'eater number of operators
having establishments on a complete footing.

For the ins'tiiimeut moving by hand, it is necessary that a
second person should open and shut the object-glass at a given
signal. But in adapting before the object-glass a flap connected
with a cord and pulley, the operator, holding the cord in the
left hand, can open the flap at the moment that with the right
hand he makes the disc revolve, and shut the apparatus when
the revolution is complete.

When the instrument acts by clock-work, the object-glass
may be opened and shut by the same means, at the signal given
by a bell which strikes at the commencement and at the end of
the revolution.

If a dagueiTeot}"iDe plate receive the image of the dynactino-
nieter during its revolution, it is ob^-ious that each segment in-
dicates an efifect in proportion to the intensity of light and to
the time that it has remained uncovered; also that the number
of seconds marked on the fii-st segment visible is the measure of
the intensity of light at the moment of the experiment ; the
efiect of each segment being in reality the degi-ee of intensity
which can be obtained diu'ing the coiTCsponding time.

^Tien we want to compare two object-glasses, they are adapted
to two camerae obscm-ae placed before the dyuactinometer.
After ha-ving set the focus of the two apparatus, they are

148 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

charged each with a daguen-eotype plate or a photogenic paper.
When all is ready, the flaps are opened at the moment that the
dynactinometer commences its revolution, and they are shut
when it is completed. The plates are removed and the images
brought out. In comparing the result produced on each, it is
easy to see which object-glass is the most rapid, and in what pro-
portion. For instance, if the aritlimetical progression has been
followed, and on one of the plates or papers the number 4 of
the great cii-cle is the first \'isible, the conclusion is that it has
been necessary for the intensity of the light at that moment to
operate diu-ing four seconds in order to produce an efiect in the
camera obscui'a; and if, on the other plate or paper, the first
seven seomients have remained black, and the eighth segment is
the first upon which the light has operated, the conclusion will
be that the object-glass which has produced the efiect on the
fii-st plate or paper has double the photogenic power of the
other.

But if the geometrical progression has been followed, the
same experiment will show the image of the segment No. 3, re-
presented on one plate, and that of the segment IsTo. 4 on the
other, as having each the fii'st degi-ee of intensity : and we have
to draw the same conclusion as regai'ds the power of each object-
glass.

However, this conclusion woidd be exact only on the supposi-
tion that the two plates were endowed with the same degree of
sensitiveness : for if they had not been prepared identically in
the same manner, we coidd not have the exact measure of the
comparative power of the two object-glasses. The difierence
might be due, not to any difference in the power of the object-
glasses, but to the inequality in the sensitiveness of the two
plates; although, in repeating the experiment several times, the
mean result might be sufficiently conckisive. But this difficidty
has not escaped the inventor, and he has tried to avoid it.
Being able, by means of the photogi-aphometei-, to compare the
sensitiveness of two plates under the action of the same in-
tensity of light, and dimng the same space of time, he availed
himself of this instrument to determine beforehand the compa-
rative sensitiveness of the plates which are to be used in the
experiment with the dynactinometer. By this means we can
try beforehand several couples of plates, and keep them as it
were stamped with their degree of sensitiveness imtil we want
to apply them to test the power of two lenses. The impression
is made on one-half of the plate, leaving the other half for the
image of the dynactinometer.

After having operated in the two camerse obscurae; each sup-

THE DTNACTIXOilETER.

149

])lied with the lenses the power of which we wish to compare,
we submit the two plates, each impressed with both the photo-
graphometer and dynactinometer, to the vapour of mercury,
which developes the two images on each plate.

The number of sj:)ots given by the photogi'aphometer, Fig. 20,
will indicate the sensitiveness of the plate; and in comparing
the two images given by the dynactinometer. Fig. 24, 25,
accounting for the difference of sensitiveness of each plate, if
there is any, we are able at once to determine the comparative
power of the two lenses.

For the practical investigation of this veiy important photo-
graphic question, IMr. Knight has devised an apparatus that will
be most valuable as affording the means of adjusting readily to
the best focal distance.

jVIr. Knight's apparatus consists in a frame having two
grooves; one vertical, in which he places the gi'ound glass, and

ii-lj c'— ^M

•1
'01 -m

the other forming an angle with the first destined to receive
the plate : the planes of the grooves intersect each other in the
middle. After having set the focus upon the ground glass, this
last is removed, and the plate is placed in the inclined gi-oove.
Now, if a newspaper, or any large piinted sheet, is put before
the camera, the image will be represented on the inclined plate ;
and it is obvious in its inclination the various points of the
plate will meet a different focus. The centre of the plate will
coincide with the visual focus ; by its incKnation it will in one
direction meet the photogenic focus at a point more or less
distant from the centre, if the photogenic focus is shorter than
the visual focus, and in the other dii-ection if it is longer. The
frame is furnished with a scale of division, having the zero in
the centre. When the image is represented on the daguerreo-
type, by applying against it another movable scale of division
similar to the other, the operator can find what is the division

150 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

above or under zero, kt which the image seems best defined ;
and after having removed from the camera the experiment frame,
and set the focus as usual on the ground glass, he has only to
move the tube of the object-glass by means of the rack and
pinion, and to push it in or out ; a space corresponding with the
division of the scale indicating the deviation of the true photo-
genic focus: the tube of the object-glass is for that purpose
marked with the same scale of division.

CHAPTEE Y.

THERMOGRAPHY,

Since the ciirious natiu-e of the results obtained, as I believe, by
heat ratliations, have been supposed by some to belong to the
same class of phenomena as those we have particularly under con-
sideration, I am induced to introduce the subject in this treatise
on photography, merely reprinting my original communication
on the subject, as the investigations have not been continued.

The. Journal of the Academy of Sciences of Paris, for the 18th
of July, 1842, contained a communication made by M. Regnault
fi'om M. Moser, of Konigsberg, " Siu' la Formation des Images
Dagueri'iennes;"* in which he announced the fact, that " v^hen
two bodies are sufficiently near, they impress their images upon
each other." The Jounud of the 29 th of August contained a
second communication from M. Moser, in which the residts of
his researches are summed up iu twenty-six paragraphs. From
these I select the following, which alone are to be considered on
the present occasion : —

" 9. All bodies ludiate light even in complete darkness.

" 10. This light does not appear to be allied to phospho-
rescence, for there is no difference perceived whether the bodies
have been long in the dark, or whether they have been just
exposed to daylight, or even to direct solar light.

"11. Two bodies constantly impress theii* images on each
other, even in complete darkness.

•' 14. However, for the image to be appreciable, it is neces-
sary, because of the divergence of the rays, that the tlistance of
the bodies should not be very considerable.

" 15. To render the image \'isible, the vapom- of water, mer-
cuiy, iodine, &c., may be used.

'•' 17. There exists latent light as well as latent heat."

The anuoimcement at a meeting of the British Association
of these discoveiies, natiu-ally excited a more than ordinary
degree of interest. A discovery of tliis kind, changing, as it
did, the featui-es, not only of the theories of light adopted by

* Comptes Hendus, tome xv., No. 3, folio 1 19.

152 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

pliilosopliers, but also the commonly received opinions of man-
kind, was more calculated to awaken attention tlian anything
which has been brought before the pubhc since the publication
of Daguerre's beaiitiful photogi-aphic process. Having instituted
a series of experiments, the results of which appear to prove
that these phenomena are not produced by latent light, I am
desirous of recording them.

I would not be understood as denying the absorption of light
by bodies; of this I think we have abundant proof, and it is a
matter well deserving attention. If we pluck a nastui^tium when
the sun is shining brightly on the flower, and carry it into a
dark room, we shall still be enabled to see it hj the light which
it emits.

The human hand will sometimes exhibit the same phenomenon,
and many other instances might be adduced in proof of the
absorption of light ; and I believe, indeed, of the principle that
light is latent in bodies. I have only to show that the conclu-
sions of M. Moser have been formed somewhat hastily, being led,
no doubt, by the striking similarity which exists between the
effects produced on the daguerreotype plates imder the influence
of light, and by the juxtaposition of bodies in the dark, to con-
sider them as the work of the same element.

1. Dr. Draper, in the PMlosopliical Magazine for September
1840, mentions a fact which has been long known, — "That if a
piece of very cold clear glass, or, what is better, a cold polished
metallic reflector, has a little object, such as a piece of metal,
laid on it, and the surface be bi'eathed over once, the object
being then carefully I'emoved, as often as you breath on it
again, a spectral image of it may be seen, and this phenomenon
may be exhibited for many days after the fii'st trial is made."
Several other similar experiments are mentioned, all of them
going to show that some mysterious molecular change has taken
place on the metallic surface, which occasions it to condense
vapours unequally.

2. On repeating this simple experiment, I find that it is
necessary for the production of a good effect to iise dissimilar
metals; for instance, a piece of gold or platina on a plate of
copper or of silver "will make a very decided image, whereas
copper or silver on their respective plates gives but a very faint
one, and bodies which are bad conductors of heat, placed on
good conductors, make decidedly the strongest impressions when
thus treated.

3. I placed upon a well-poUshed copper plate a sovereign, a
shilling, a large silver medal, and a penny. The plate was
gently warmed by passing a spirit-lamp along its under siu-face :

THERMOGRAPHY. ] 53

when cold, the plate was exposed to the vapour of merciuy :
each piece had made its impression, but those made by the gold
and the large medal were more distinct ; not only was the disc
marked, but the lettering on each was copied.

4. A bronze medal was supported upon slips of wood, placed
on the copper, one-eighth of an inch above the plate. After
mercui'ialization, the space the medal covered was well marked,
and, for a considerable distance aroimd, the mercury was un-
equally deposited, giving a shaded border to the image ; the
spaces touched by the merciuy [?] were thickly covered ^dth the
vapoui".

5. The above coins and medals were all placed on the plate,
and it was made too hot to be handled, and allowed to cool
without theii" being removed ; impressions were made on the
plate in the following order of intensity, — gold, silver, bronze,
copper. The mass of the metal was found to influence ma-
terially the result ; a large piece of copper making a better
image than a small piece of silver. When this plate was
exposed to vapour, the results were as before. On rubbing oflT
the vapoui-, it was found that the gold and silver had made per-
manent impressions on the copper.

6. The above being repeated with a still gi-eater heat, the
image of the copper coin was, as well as the others, most faithfully
given, but the gold and silver only made permanent impressions.

7. A silvered copper plate was now tried with a moderate
warmth. Mercuiial vapoui^ brought out good images of the
gold and copper ; the silver marked, but not well defined.

8. Having repeated the above experiments many times with
the same results, I was desii-ous of ascertaining if electricity had
any similar effect : powei-ful discharges were passed through and
over the plate and discs, and it was subjected to a long-continued
ciu-rent without any effect. The silver had been cleaned off
from the plate ; it was now waiTaed with the coins and medals
upon it, and submitted to discharges from a very large Leyden
jar : on exposing it to mercui'ial vapoiu*, the impressions were
veiy prettily brought out, and, strange to say, spectral images
of those which had been received on the plate when it was
silvered — thus proving, that the influence, whatever it may
be, was exerted to some depth in the metal.

9. I placed upon a plate of copper, blue, red, and oi-ange-
coloured glasses, pieces of crown and flint glass, mica, and a
square of tracing paper. These were allowed to remain in con-
tact half an hoiu-. The space occupied by the red glass was well
marked ; that covered by the orange was less distinct, but the
blue glass left no impression ; the Shapes of the flint and crown

154 SCIENTIFIC INVESTIGATIOXS ON PHOTOGRAPHY,

glass were weU made out, and a remarkably strong impression
where the cro^vn glass rested on tlie tracing paper, but the mica
had not made any impression.

10. The last experiment repeated. After the exposm-e to
mercmial vapoiu', heat was again applied to dissipate it : the
impression still remained.

11. The experiment repeated, but the vapour of iodine used
instead of that of mercury. The impressions of the glasses ap-
peared in the same order as before, but also a very beautifid
image of the mica was developed, and the paper well marked
out, showing some relation to exist between the substances used
and the vapours applied.

12. Placed the glasses used above, with a piece of well-smoked
glass, for half an hour, one-twelfth of an inch below a polished
plate of copper. The vapour of mercury brought out the image
of smoked glass only.

13. All these glasses were placed on the copper, and slightly
warmed : red and smoked glasses gave, after vaporisation,
equally distinct images, the orange the next, the others left but
faint max'ks of theu* forms ; polishing with Tripoli and putty
powder would not remove the images of the smoked and red
glasses,

1 4. An etching, made upon a smoked etching ground on glass,
the copper and glass being placed in contact. The image of the
glass only could be brought out.

15. A design cut out in paper was pressed close to a copper
plate by a j)iece of glass, and then exposed to a gentle heat ; the
imjiression was brought out by the vapour of mercury in beauti-
fid distinctness. On endeavouring to rub off the vapour, it was
found that all those parts which the paper covered amalgamated
with mercmy, which was rubbed j&'om the rest of the plates :
hence thei-e resulted a perfectly white pictiu'e on a polished
copper plate.

16. The coloured glasses before named were placed on a plate
of copper, with a thick piece of charcoal, a copper coin, the mica,
and the paper, and exposed to fervent simshine. Mercuidal
vapour brought up the images in the following order : — Smoked
glass, crown glass, red glass, mica beautifully delineated, orange
glass, paper, charcoal, the coin, blue glass — thus distinctly
proving that the only rays which had any influence on the metal
were the calorific rays. This experiment was repeated on
different metals, and "svith various materials, the jDlate being ex-
posed to steam, mercuiy, and iodine : I invariably found that
those bodies which absorbed or permitted the permeation of the
most heat gave the best images. The blue and violet rays

THERMOGRAPHY, 155

could not be detected to leave any evidence of action, and as
spectra imprinted on pliotogi-apliic papers by light, wliicli had
permeated these glasses, gave evidence of the lai"ge quantity of
the invisible rays which passed them freely, we may also consider
those as entirely without the power of effecting any change on
compact simple bodies.

17. In a paper which I pubhshed in the Philosophical Maga-
zi7ie for October, 1840, I mentioned some instances in which I
had copied printed paper and engravings on iodized paper by
mere contact and exposm-e to the influence of calorific rays, or
to artificial heat. I then, specvilating on the probability of our
being enabled, by some such process as the one I then named,
to copy pictiu'es and the like, proposed the name of Thermo-
graphy, to distiuguish it from Photography.

18. I now tried the effects of a print in close contact wdtli a
weU-polished copper plate. When exposed to mercvuy, I found
that the outline was very faithfidly cojiied on the metal.

19. A paper ornament was pressed between two plates of
glass, and wai-med j the impression was brought out with toler-
able distinctness on the imder and warmest glass, but scarcely
traceable on the other.

20. Rose leaves were faithfully copied on a piece of tia plate,
exposed to the full influence of sunshine ; but a much better
impression was obtained by a jDrolonged exposure in the dark.

21. With a view of ascertaining the distance at which bodies
might be copied, I placed upon a plate of polished copper a thick
piece of plate-glass, over this a square of metal, and several other
things, each being larger than the body beneath. These were
all covered by a deal box, which was more than half an inch
distant fi'om the plate. Things were left in this position for a
night. On exposing to the vapoiu- of mercury, it was found that
each article was copied, the bottom of the deal box more faith-
fully than any of the others, the grain of the wood being imaged
on the plate.

22. Having found, by a series of experiments, that a black-
ened paper made a stronger image than a white one, I very
anxiously tried to effect the copying of a printed page or a print.
I was partially successful on several metals ; but it was not
imtil I used copper plates amalgamated on one siirface, and the
mercury brought to a very high polish, that I produced any-
thing of good promise. By carefully preparing the amalga-
mated surface of the copper, I was at length enabled to copy
from papei', line-engravings, woodcuts, and lithographs, with
surprising accui-acy. The first specimens produced exhibited a
minuteness of detail and shai-pness of outline quite equal to

156 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

tlie early daguerreotypes and the pliotogi-aphic cojiies prepared
with the chloride of silver.

The following is the process adopted by me, which I consider
far from perfect, but which affords us very delicate images : —

A well-poHshed plate of copper is rubbed over with the nitrate
of mercury, and then well washed to remove any nitrate of
copper which may be formed ; when quite dry, a little mercury
taken up on soft leather or linen is well rubbed over it, and the
sm-face worked to a perfect mirror.

The sheet to be copied is placed smoothly over the mercurial
surface, and a sheet or two of soft clean pajjer being placed upon
it, is pressed into equal contact with the metal by a piece of
glass, or flat board : in this state it is allowed to remain for an
hour or two. The time may be considerably shortened by
apjjlying a very gentle heat for a few minutes to the under
surface of the plate. The heat must on no account be so great
as to volatilise the mercury. The next process is to place the
plate of metal in a closed box, prepared for generating the vapoirr
of mercury. The vapour is to be slowly evolved, and in a few
seconds the picture will begin to appear ; the vapoiu- of mercury
attacks those pai-ts which correspond to the white parts of the
pi'inted page or engraving, and gives a very faithful but some-
what indistinct image. The plate is now removed from the mer-
curial box, and placed into one containing iodine, to the vapour
of which it is exposed for a short time : it will soon be very
evident that the iodine vapour attacks those parts which are
free from mercurial vapour, blackening them. Hence there
results a perfectly black picture, contrasted ■wdth the gray ground
formed by the merciu-ial vapour. The picture being formed by
the vapom's of mercury and iodine, is of course in the same state
as a daguerreotype picture, and is readily destroyed by rubbing.
From the dej^th to which I find the impression made in the
metal, I confidently hope to be enabled to give to these singular
and beautiful productions a considerable degree of permanence,
so that they may be used by engravers for working on.

It is a cm-ious fact that the vapours of mercury and of iodine
attack the plate difierently ; and I believe it will be found that
vajjours have some distinct relation to the chemical or thermo-
electrical state of the bodies upon which they are received.
Moser has observed this, and attribvites the phenomena to the
colours of the rays, which he supposes to become latent in the
vapour on its passing from the solid into the more subtile form.
I do not, however, think this explanation will agree with the
results of experiments. I feel convinced that we have to do
with some thermic influence, and that it will eventually be fovmd

THERMOGRAPHY. 157

that some purely calorific excitement produces a molecular
change, or that a thermo-electric action is induced which effects
some change in the polarities of the idtimate atoms of the solid.
These are matters which can only be decided by a series of
well-conducted experiments ; and, although the subject will not
be laid aside by me, I hope the few curious and cei-tainly im-
portant facts which I have brought before you, "s\-ill elicit the
attention of those whose leisure and well-known experimental
talents qualify them in the highest degree for the interesting-
research into the action of those secret agents wliich exert so
powerful an influence over the laws of the material creation.
Although attention was called to the singailar manner in which
vapours disposed themselves on plates of glass and copper, two
years since, by Dr. Draper, Professor of Chemistry at New
York, and about the same time to the calorific powers of the
solar spectrum, by Sir John Hei'schel,* and to the influence of
heat artificially applied, by myself, yet it is certainly due to M.
Moser, of Konigsberg, to acknowledge him to be the first who
has forcibly called the attention of the scientific world to an
inquiry which promises to be as important in its residts as the
discovery of the electropile by Yolta.

As to the practical utility of this discovery, when we reflect
on the astordshing progress made in the ai't of Photography
since Mr. Fox Talbot published his fii-st process, what may we
not expect from Thermography, the fii-st mde specimens of
which exhibit far greater perfection than the early efforts of the
sister art ?

Asa subject of purely scientific interest, thermography promises
to develop some of those secret influences which operate in the
mysterious arrangements of the atomic constituents of matter,
to show us the road into the yet hidden recesses of nature's
works, and enable us to pierce the mists which at present
envelope some of the most striking jjhenomena which the
penetration and industry of a few " chosen minds" have brought
before oui' obsciu'ed visions. In connection with photogi-aphy,
it has made us acquainted with subtile agencies working slowly
but surely, and indicated physical powers beyond those which
are already known to us, wliich may possibly belong to a more
exalted class of elements, or powers, to which Light, Heat, and
Electricity are subsidiary in the great phenomena of Natm-e.

* Philosophical Transactions, Part I., 1840, p. 50.

CHAPTER VI.

ON THE POSSIBILITY OF PRODUCING PHOTOGRAPHS IN THEIR
NATURAL COLOURS.

Few speculations are more replete with interest than that of the
jjrobability of onr succeeding in the production of photogi-aphic
images in their local colours. M. Biot, a great authority, says, —
" Substances of the same tint may present, in the quantity, or
the uatm*e of the radiations which they reflect, as many diversities,
or diversities of the same order, as substances of a different tint ;
inversely, they may be similar in their property of reflectiug
chemical radiations when they are dissimilar to the eye ; so that
the difference of tint which they px'esent to the eye may entirely
disappear in the chemical pictui-e. These are the difficulties in-
herent in the fonnation of photogi'aphic pictui'es, and they show,
I think, evidently, the illusion of the experimenters who hope to
reconcile, not only the intensity, but the tints of the chemical
impressions produced by radiation, with the colours of the objects
fi-om which these rays emanate." It may be remembered that
two years since, Sii- John Herschel succeeded in procm-ing upon
photogi'aphic paper a coloured image of the solar spectrum ; and
that emment inquirer has commimicated to me a recent discoveiy
of great interest, which I have his permission to publish. " I
have got specimens of paper," says Sii" John Herschel, " long
kept, which give a considerably Ijetter rej^resentation of the
spectrum in its natural colours than I had obtained at the date
of my paper (Febrviary 1840), and that light on a dark ground ;
but at present I am not prepared to say that this will prove an
available process for coloured photogTaphs, though it brings the
hope ne((/rer." Here we have the speculations of one philosopher
representing the production of such pictures as hopeless, while
the experiments of another prove these to be within the range
of probabilities.

My ovm experiments have, in many instances, given me
coloui'ed pictiu-es of the prismatic spectrum, dark upon a light
ground, but the most beautiful I have yet obtained has been
upon the daguerreotype iodidated tablets, on which the colours

NATURALLY-COLOURED PHOTOGRAPHS. 159

have, at the same time, had a peculiar softness and brilliancy.
Daguerre himself has remarked, that when he has been copying
any red brick or painted building, the photograph has assumed
a tint of that character. I have often obsei-ved the same thing
in each variety of photogi*aphic material, i.e., where a salt of
silver has been used. In the Philosojyhical Magazine for April
1840, will be found a paper, — " Experiments and Observations
on Light which has permeated Coloured Media," — in which I
describe some ciu'ious x-esults on some of those photogTaphs
which are prepai'ed with the hycModic salts exposed to limiinous
influence with coloiu'ed fluids superimposed ; permitting, as
distinctly isolated as possible, the permeation of the violet and
blue, the gi'een, the yellow, and the red rays, under each of Avhich
a complementaiy coloui" was induced. Diu-ing January of the
present year, I prepared some papers with the bichromate of
potash and a very weak solution of nitrate of silver : a piece of
this paper was exposed behind foiu- coloiu-ed glasses, which
admitted the passage respectively of, 1st, the violet, indigo, and
blue rays ; 2d, the blue, the green, and a portion of the yellow
rays ; 3d, the green, yellow, and orange rays ; and, 4th, the
orange and red rays. The weather being extremely foggy, the
aiTangement was unattended for two days, being allowed to lie
upon a table opposite a -ttTiidow having a southern aspect. On
examining it, it had, imder the respective colour's, become tinted
of a blue, a green, and a red : beneath the yelloAv glass the
change was uncertain, from the pecuKar coloiu' of the paper, and
this without a single gleam of sunshine. My numeroiis engage-
ments have prevented my rei^eating the observations I desii^e on
this salt, which has hitherto been considered absolutely insensible
to light.

The baiytic salts have nearly all of them a peculiar caloiific
efiect ; the miuiate, in particidar, gives rise to some most rich
and beautiful crimsons, particularly under the influence of light
which has permeated the more delicate gi-een leaves ; and also in
copymg the more highly-colom^ed flowers, a variety of tintings
having been observed. We may always depend on producing a
photograpliic copy of a leaf of a green colovir by the following
an-angement : — HaA-ing silvered a copper plate, place it in a
shallow vessel, and lay thereon the leaf of which a copy is
desu-ed, maintaining it in its position by means of a piece of
glass j poiu' upon it, so that the plate beneath the glass may be
covered, a solution of the hydriodate of potash, containing a little
fi'ee iodine : then expose the whole to simshine. In abottt half
an hour, one of the most beautiful photographic designs which
can be conceived is produced, of a fine green yellow. The fluid

160 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

is yellow, and cuts off nearly all tlie " chemical" rays, allowing
only of the free passage of the less refrangible rays ; the most
abundant being the yellow. This retards the process of solariza-
tion, but it produces its complementary coloiu- on the plate.

These facts will, I think, prove that the possihility of our being
enabled to produce coloiu-ed photographs is decided, and that
the probabiliti/ of it is brought infinitely nearer, particularly by
Sfr John Herschel's very important discovery, than it was
supposed to be.

M. Edmond Becquerel has recently succeeded in obtaining
bright impressions of the spectrum in colonics, and copying highly
coloured drawings on metallic plates prepared with chlorine.
The results of M. Niepce de St.-Victor have been of a satisfactory
character : the main particulai-s thereof we select from a memoir-
entitled " Upon the Relation existing between the Coloiu-s of
certain colom-ed Flames with the Heliographic Images colom'ed
by Light."

When a plate of silver is plunged into a solution of sulphate
of copper and chloride of sodium, at the same time that it is
rendered electro-positive by means of the voltaic battery, the
chloride fonned becomes susceptible of colouration, when, having
been withdrawn from the bath, it i-eceives the influence of
light.

M. Niepce de St.-Victor, from obsei'ving that when chloride
of sodium (common salt) was employed, the plate became more
susceptible of receiving a yellow colour than any other, and
knowing that it impai'ted a yellow colour to flame, was led to
believe that a relation existed between the colour communicated
by a body to flame, and the coloiu- develojjed upon a plate of
silver, which should have been cHoridated with the particular
body.

To avoid complexity, it may be briefly stated that the bath in
which the sensitive sm*face is obtained is prepared with water,
holding free chlorine in solution, to which has been added the
salt which is essential to give a ^predominance to any particular
colour.

It is well known that strontiau gives a purple colour to flames
in general, and to that of alcohol in particular. If we prepare a
plate of silver and pass it into water saturated with chlorine to
which is added some chloride of strontian, and when thus pre-
pared we place upon it a coloured design of red and other colom-s,
and then expose it to the sunsliine, after six or seven minutes
we shall perceive that the colours of the image are reproduced
upon the plate, but the reds much more decidedly than the
others. When we would produce successfully the other rays of

X ATUR ALLY-COLO L'RED PHOTOGRAPHS. 161

the solar spectram, we operate in the same manner as we have
indicated for the red i-ay — employing for the orange the chloride
of calcium, or that of uranium for the yellow, or the hyjiochlorite
of soda, or the chlorides of sodium or potassium. Yery fine
yellows have been obtained with a bath composed of water
slightly acidulated with muriatic acid and a salt of copper.

The green rays are obtained with boracic acid or the chloride
of nickel ; also with all the salts of copper: The blue and indigo
I'ays are obtained with the double chloride of copper and ammonia.
The violet rays are obtained with the chloride of strontian and
sulphate of copper. Those substances which give white flames,
as the chloride of antimony, the chloride of lead, and the chloride
of zinc, yield no colour by kmiinous action. All the colours of
a pictm-e have been produced by preparing a bath composed of
the deuto-chloride of copper ; and M. Niepce states that this
salt thrown into burning alcohol produces a variegated flame
according to the intensity of the fire, and it is nearly the same
with all the salts of copper mixed with cliloriue. ZS^iepce says —
" If we put a salt of copper in liquid chlorine we obtain a very
sensitive sm-face by a single immei'sion in the bath ; but the
colorific result of this mixture is seldom good. I prefer a
mixture of equal parts of chloride of copper and of chloride of
iron, with thi-ee or four parts of water ; the cliloride of iron has,
as those of copper, the property of being impressed on the plates
of silver and of producing many colours, but they are infinitely
more feeble, and the yellow always predominates ; and this agrees
with the yellow colour produced in flame by this salt."

It should be understood that when the plate of silver, being
previously connected with a voltaic battery, is phmged into the
bath and the circuit completed, it becomes covered with a dark
coating, probably of a sub-chloride of silver mixed ^vith the salt,
on wluch the colour to be produced by solar radiation depends.

If we form a bath composed of all the substances which
separately give a dominant colom-, we obtain veiy lively colours ;
but the great difficulty is the mixing of the salts in such propor-
tions as to give an equality to the tints, as it commonly happens
that some colom's are excluded by others. We cainiot always
depend upon obtaining the same results with the same materials,
owing principally to the diflicrJty of preserving the solution at
a uniform strength. Liquid chlorine is necessary — the application
of dry chlorine ^\t11 not produce the same results, and the volatile
chlorine is continually escaping from the water.

Niepce de St.-Victor has made many experiments to produce
the colours upon salts of silver and copper spread on paper, but
without success; the metallic plate appears absolutely necessary,

162 SCIE^TTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

and the pui-er the silver the more perfect and intense is the
impression. The following is recommended as the most effectual
mode of manipulating. The sUver plate is highly polished with
the best tripoli powder and ammonia ; being perfectly cleaned it
is connected with the battery, and then plunged into the bath
prepared in any of the ways stated. It is allowed to remain in
the bath for some minutes, taken from it, washed in a large
quantity of watex*, and dried over a spu-it-lamp. The surface
thus produced is of a dull neutral tint, often almost black ; the
sensibility of the plate ajjpears to be increased by the action of
heat, and when brought by the spiiit-lamp to the cerise red
colour it is in its most sensitive state. The sensibility, how-
ever, of the plates is low, two or three hours being requii'ed to
produce a decided effect in the camera obscura. The name of
Heuochromes has been given to these naturally coloiu'ed
photographs, some of which, the personal gift of the inventor to
Mr. Malone, I have inspected. These, when I fii-st saw them,
were perfectly coloiu-ed in correspondence with the drawings of
which they were copies ; but the colours soon faded, and it does
not appear as yet that any successfid mode of fixing the colours
has been discovered.

CHAPTEK VII.

ox LEN'SES FOR THE PHOTOGRAPHIC CAMERA.

It is to the photographic artist, a matter of considerahle
momeut that he undei-stands the principles upon which his
instmments are constnicted. It has, therefore, been thought
advisable to add a short chapter which should give a sufficiently
popular explanation of the dioptrical phenomena '«dth which we
have especiaUj to deal.

Upon the refractive power of the media employed, depends
the perfection of the results we obtain ; thei'efore, some of the
phenomena of refraction, or breaking back, as the term implies,
should be clearly understood.

A ray of light passing though a vacuum progi-esses in a
perfectly straight Hne, and we shoidd, if we lookecl at a brilliantly
illuminated point — were it possible — imder such conditions, see
it in its ti-ue position, the numerous rays coming undisturbed
directly to the eye. But all matter, however attenuated it may
be, has the property of refracting, or bending the ray of light ;
consequently we do not see the stai-s in their ti-ue position, owing
to the refractive power of the atmosphere.

The most simple illiLstration of refraction is to allow a sun-
beam, a, passing through a small hole in the window-shutter of
a dark room, to fall upon the siuface of a fluid contained in a
glass vessel, b b : instead of proceeding onward to a, it will be
found to alter its course at the siu--
face of the fluid, and pass along the
line to a a. Every substance has
different refractive powers in virtue
of its physical constitution ; but a
ray of light incident perpendicularly
on a refecting medium, as the ray c,
(Fig. 27) sufiei-s no refraction. If we
float, one upon the other fluids, b,c,d,
having cUfierent powei-s of refi-action
we shall then see the relative phenomena exhibited by the

164

SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

bending of the ray a a, in passing tln-ough tliem (Fig. 28).

It will be evident tbat no great
difficulty exists in measuring tbe
refractive powers of diflerent trans-
parent bodies : and that hence we
are enabled to tabulate those which
have the highest and lowest refrac-
tive indices. A few of the most
impoi-tant are given in the following
table : —

Air

Water

Alcohol

Oil of Cloves

Crown glass

Plate glass

Flint glass

Do. containing much lead
Diamond

1-000294

1-336

1372

1-535

1-534

1-542

1-830

2-028

2-439

This knowledge enables us to trace a ray of light through
transparent bodies of any form, provided we can find the incli-
nation of the incident ray to the sui'face, where it either enters
or quits the body.

If parallel rays fall upon a plane sm-face G, of glass, they will
retain their parallelism after passing through it as the ray A
(Fio-. 29). The rays diverging from the point A, will be refracted

by the first surface into the

directions h b, and by con-
tinuing a a, and b b, back-
wards, we shall find they
meet at a point beyond A :
so that supposing the eye
to be placed within the
body G, the point A would
appear removed to B. But
when the rays undergo a second refraction by passing out of
the second surface, we shall find by continuing the lines back-
wards that they meet at c ; therefore a plane glass diminishes
the apparent distance of the point of the diverging rays. If,
instead of a plane glass, we employ a piece equally curved,
like a watch-glass, it produces very little change in the form
and position of objects.

Lenses are glasses ground to difiei-ent foi-ms, their- sui-faces
being segments of spheres, and it is in obedience to the i-efrac-

LENSES FOR THE PHOTOGRAPHIC CAMERA.

165

tory power of the surfaces so produced that their peculiarities
belong. The adjoining figui-es represent the varieties.

30.

1 is termed a plano-convex lens.

2 is a double convex lens.

3 is formed of parts of two circles of different diameters,
and is called a meniscus lens, or concavo-convex.

4 is a plano-concave lens.

5 is a double concave lens. And

6 is a concavo-convex lens, fonned of parts of the inner sur-
faces of two dissimilar circles.

It is not necessary to examine the laws of refi'action for all
these forms ; the phenomena will be fiilly understood by an
examination of a few leading points. Whatever may be the
form of a lens, the incident rays pai-aUel to its axis pass
through without suffering refraction, as a a a, a a a, Fig. 31.

All other rays mxxst have a certain amoxmt of obliquity, and
these all consequently suffer refraction, as the rays a a. Now

16G SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

tlie rays B B, and the ray c c, are refi-acted, and meet at D d' ;
tlie line h h represents the focal image produced of the body
from which the light proceeds.

In the last figiire the image produced l^y the lens is repre-
sented as cm'ved : a little consideration Avill show that it is not
possible that such a cm-ved surface as that represented could
produce an image of equal distinctness over every ^yart of a
plane surface : the rays cannot meet, as they are refracted from
curved sm-faces along any straight line, as F F F ; and supposing
we receive on the sui"face of a lens a bright circular image, it
will be brilliant and well defined around the centre, the light
becoming fainter towards the edge, and at length passing into a
cloudy halo, exhibiting the prismatic colours. This is called
spherical aberration, and to it is due that want of distinctness
which commonly is found around the edges of pictures taken in
the camera obscvira.

It is therefore important, in the selection of lenses, that we
look for sharpness of definition over the tvhole of a perfectly
flat field. To manufacture a lens which shall effect this, is a
task of some difficulty ; but by attention to the two facts, that
a lens, one surface of which is a section of an ellipse, and the
other of a circle struck from the farthest of the two foci of that
ellipse, as in Fig. 32, produces no abeiTation, much may be

» effected. A meniscus

lens, therefore, with
a convex sui'face, part
of an ellipsoid, the
focal distance of which
coincides with its far-
ther focus, and a con-
cave svu"face, part of a
sphere, whose centre
is that focus, will
meet all oxvc require-
ments. The mecha-
nical difficulties of producing such lenses are great, but they
may, by cautious manipulation, be to a great extent over-
come. There are other methods by which the aberration of
sphericity may be corrected, but for a description of these the
reader is referred to Su- Jolin Herschel's Treatise on Light, in
the Encyclopxjedia Metropolitana.

If we take such a lens as we have been describing, and stop
its centre with a blackened disc, leaving only a small portion of
the edge for the light to pass through, and throw its image on
a screen, we shall find it will be bordered with fringes of

LENSES FOR THE PHOTOGRAPHIC CAMERA. 1G7

coloiu'. At one distance red will ijrevail, at another violet.
This is the result of chromatic aberration, and arises from the
unequal refrangibility of the dissimilar rays. The red rav is
less bent than the \'iolet ; consequently, supposing the rays r r
to fall on the edge of a lens, they will converge to a point

33.

at F, whereas if the rays v v fall along the same circular line,
they will, being more refracted, meet at f. Now if we place a
disc at e, just the size of the cone of light, it will be edged -svith
■violet, but if we move it to a, the coloured border will be i-ed.

The indices of refraction for the several rays have been most
carefully determined by Fraimhofer, and for a standard medium,
a flint glass prism, they are respectively

Red 1-627749 dark Hue B

Orange 1-629681 C

YelloV 1-635026 D

Green 1-642024 E

Blue 1-648260 F

Indigo 1-660285 G

Violet 1-671062 H

Fraunhofer has determined the absolute values from the fixed
dark lines which he observed in the spectrum : they represent,
however, very closely the rays distinguished by their colom*s.

By refen-ing to the table of the refractive powers of trans-
parent bodies (page 164), it will be seen that for a beam of
white light, the difference between the most refr-actory flint
glass and crown glass, in their refracting powers, is as 2-028 is
to 1.5-34, and this j^roportion is maintained nearly, but not
exactly, for all the coloui-ed rays : if, therefore, we have a cro-wna

glass lens, the refractive power of which will place the focus
at a, for the violet rays, and at b, for the red rays, and we grind

IGS SCIENTIFIC rS'VESTIGATIOXS OX PHOTOGRAPHY.

to ±ii it a riint glass lens, the refractory power of -wliich wotild
place the foci of the rays at c, d, it will be seen that the result
of such a combination would be the formation of a colourless
image, at a mean point between them, by recombining the rays
into white light. Such as is represented in the figure Ls the
achromatic lens of a camera obscura.

There is, however, a point to be examined in connection with
the lens for photogi-aphic purposes, which is of the fii-st im-
portance, and which has not hitherto been sufficiently attended
to. It is this. The luminous and coloured rays of the spectmm,
and the chemical rays, are not coincident at any point of the
.spectral image, and the relation between the chemical power,
and the illuminatiug power, of a ray, Ls subject to constant
variations.

It is often stated that the violet and blue rays are the
chemical rays, and hence it is infeiTed, if the glass of a camera
is corrected so as to make these rays, and the less refrangible
red, to correspond, all is done which can be desired.

It must be distinctly undei-stood that the coloiu' of any par-
ticular ray has no direct relation to its chemical character. It
is true, if the more refrangible rays are made to coiTCspond
with the more luminous rays, we approach the desired point,
but we do not neeessai^ily reach it. The following remarks from
a paper read by the author before the Photographic Society of
London will explain this point.

I need not go pailiculai-ly into the conditions necessary to
ensure the recombination of the particular rays produced by
refraction. When we look thi-ough an ordinary single lens, or
a bad telescope, we find all objects are fringed with colour — we

have chromatic dispersion — and this arises from the circumstance
that every one of the coloured rays has a distinct and separate

LENSES FOR THE PHOTOGRAPHIC CAMERA. 169

focal distance. Hence tlie object of a combination of lenses is to
bring the coloured rays to one point, wbere they are reunited
into white light. If we bring the face of one prism up against
the face of another, we i-ecombine those prismatic rays which
would be produced by refraction in one du'ection, and obtain a
spot of white light by such recombination. I have endeavoured to
show, in the accompanying figure, something like the resxilt that
takes place. We start with a coloured image whose parallel
rays fall upon a lens of flint glass, that lens representing virtually
two prisms placed with two of their faces together. Supposing
we use a double convex lens, representing the conditions of two
prisms placed edge to edge, we should vii*tually produce achro-
maticity.

In this way the lenses have been corrected for colovir, but this
correction for colour does not include the necessary correction
for ensuring the coincidence of the chemical and the luminous
rays. Supposing the correction has been made for all the
chromatic rays from the dark line A to the line H in the violet,
there still remains an outlying set of radiations, nearly equal in
leng-th to the space between A and H, and over this space the
actinic force is more particularly active. Now, in ordinary
achromatic lenses, the focus of these rays of high refrangibility
must fall nearer the inner surface of the lens than the true
luminous focus. This was fii-st noticed in achromatic lenses by
Mr. Claudet. The fact for ordinary meniscus lenses, was in the
first instance pointed out by Mr. Towson, who, in a paper
published in the Philosojyhical Magazine, showed, that if, having
obtained the best visual image, with a non-achromatic lens in
the camera, we then put the prepared plate or paper about a
quarter of an inch nearer the glass, when the focal distance is
about 12 inches, an infinitely better photograph is obtained than
that produced by the best visual image. Mr. Claudet's observa-
tions on this point were also published in the Philoso2)hical
Magazine for 1844, and may be referred to with advantage, as
containing some very imporfant observations on this matter.
He showed that even wdth achi'omatic lenses the visual focus and
the chemical focus were not coincident. He states that with
some lenses he fovmd the focus was nearer the glass, and sometimes
it was fiu'ther from the glass, than the visual focus. It is rather
difficult to understand why this shoiild be, u.nless the glasses
had in one case been " over-corrected," and in the other that they
had been " under-corrected." Suppose we have the lens corrected
accurately for chromatic dispersion so as to bring the line H and
the line A coincident the one with the other, the residt would
be that the chemical rays would fall still nearer the inner surface

170 scirvriFic rs'A'ESTiGATioxs on photography.

of the lens. And it will be found by a very cai-eful adjustment
of tlie camei-a, jDai-ticiilaiiy wliere single aclii-omatic lenses are
employed, if we tiy two expeiiments, obtaining one picture at
the plane giving the finest visual image, and then another at a
very short distance nearer the lens, that a finer definition in the
detailed part of the picture will be produced in a shoiter time
in the last experiment, all the other conditions being the same,
than in the fii'st. Let it be clearly iinderstood that we have, and
require, the achi'omatic combination of glasses for producing
white light : the coincidence of the rays from the coloiu'ed bodies,
as sho-wn along the slightly cui'ved line, in fig. 3-5. But the
chemical i-atUations from the same object would fall upon the line,
which cuts the cui've of liuninous intensity, so that the best
chemical efiect would be produced along that line. Though in
ordinary cases, this is but a very minute distance neai-er the
inner sui-fece of good achromatic lenses, it is quite certain that
a difference does exist, and to this it is most important that
attention shoidd be given. With regai'd to the piinciple, there-
fore, that should regulate the constmction of the lenses for
photogi-aphic purjwses, we should, to a certain extent, set aside
the idea of achi-omaticity. We shoidd not only correct oui*
lenses for coloiu', but we should coiTect them for the pecvdiar
principles with which, combined in action, we have to deal.
The chemical radiations do not aflect the eye as the colom-ed
i"adiations do ; we can detect them only by the phenomena
of chemical change. They come to the eye associated with
colovu' from the coloiu-ed objects external to \is or to oui' camei-a.
TNTiatever they may be, I have afready shown on several
oc<jasions that Lifjht acts as a positively retarding agent in all
the changes which take place on oiu- photogi'aphic prepai'ations ;
therefore, by throwing the colom-ed rays even out of focus, we
may reaUy produce a perfect photogi-aphic picture in less time
than when the light-rays intei'fere. It appears to me clear that
■we ought to consider the chemical sj^ectinun as distinct and
separate in all respects from the luminous spectnim, Ijiit, like
that limiinous spectiimi, as made up of bands of different re-
fi'angibility. In this manner M. Melloni distingTushes the
calorific spectnim as possessing a set of rays which he calles ther-
mochroic rays, or rays coloui'ed for heat ; those heat-i^ays,
which ai-e indicated in Sii' John Hei'schel's experiment, being
analogous to those fr'om coloui*ed rays produced fr'om the spectral
image of a round hole, through which the luminous pencil is
admitted. The chemical spectnim, on what ever material obtained,
exhibits an analogous order of refi-angibility. — ^the same degrees
of variation of intensitv, and a similar an^ansjement of fonn. It

LEXSES FOR THE PHOTOGRAPHIC CAMERA. 171

becomes of course important, and it is the only means by wliich
photographers can work effectively, that they should obtain
lenses which ttlU give, at the same time, the most perfect visual
image that can be produced (so far achromaticity is an object to
be desired, and one "we always mitst attain), and be correct for
the actinic focus. Indeed, in doing this, we must of necessity
produce a lens which is not merely achromatic, but really
orer-achromatic. Now instead, therefore, of correcting for the
line H in the violet ray, or any particular line in the ordinary
spectrum, it becomes, from all that we know, essential to correct
for those lines recently discovered by 3Ir. Stokes, beyond the
extreme lavender ray of the Xewtonian spectitmi, or else we
leave a considerable portion of the chemical rays out of the
sphere of action. Therefore, it appears to me, under considera-
tion of the conditions I have endeavoui-ed to explain, that it
would be convenient that we should adopt for the chemical
principle of the sunbeam, Acttstsm, that which MeUoni has
done for Heat. We should establish a nomenclature by which
we should get rid of terms that we do not correctly express things
that we have to deal with. It was Mellonis j>roposition, that
the body which allows radiant heat to pass through it freely
should be dis-tinguished as diathermic, from the Greek words,
cto, through (in the same way as we use the term dio. in
diameter), and BepyLoq, icarmth. or heat : — ^that a body which is
opaque to heat should be called consequently adiothermic, fit)m a
priv. and hadepfxiKog, transcolescent, destitute of calorific trans-
parency. Coloured rays, possessing a different heat-character,
are called, upon the same principle, thermochroic, that is coloured
for heat*. Xow, adopting a nomenclature of this order, which
may be indicative of oiu' scientific results, we should speak of a piece
of yellow glass which does not allow the chemical rays to pass as
an o/J.iactinic body, or a body opaque to the actinic or chemical
rays ; then we may speak of a piece of blue glass as being diactinic.
The correct mode of expression woiild be to say dia-actinic. but
it will be more convenient, and possibly allowable, for us to dix)p
one a, and use the expression diactinic. Then we might, on the
same principle, use, when speaking of the combined influence of
colour and actinism, the term actinochrosis. from olktiv, a ray,
and \poa, colo^j.r, signifying '• o^loured for actinism." In future,
therefore, instead of asking for an achromatic lens, meaning a
lens destitute of colour transparency, we should ask for a diactinic
lens, meaning one which is transparent to the chemical rays.
I am quite satisfied that until we disassociate from our minds

* Scientific Memoirs, vol. iii., p. 535.

172 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

the idea that we are dealing with the luminous princij^le of the
svinshine in the production of om- pictm-es, we shall not arrive
at that perfection in Photography which it is desii'able we
should obtain. We certainly shall not succeed in representing
nature as natiu'e presents herself to ovir eye, until we careftJly
examine all the phenomena which are involved. We desire to
produce images equal to those beautifid ones whicli are impressed
by the physical radiations upon the human retina, and to do this
we must arrive at the same conditions in oiu' dark chamber, as
those which obtain in the visual camera obscura of the eye,
which is not strictly achromatic.

We commonly hear of a lens being slow or quick ; this is
purely accidental, arising entirely from the uncertainty in which
all our optical instrument-makers remain as to the relation of
the chemical and luminous forces to each other.

If the lenticular correction does not extend to the point of
bringing the rays beyond the violet, upon the field of vision, the
lens will be slow in action, because the light rays interfere, as
is explained in a previous page ; but if it is ti-uly diactinic it
will be a quick lens.

For portraitm-e, and all purposes requiring great distinctness
of outUne and rapidity of operation, two achromatic lenses are
usually employed. By this arrangement the focal distance is
diminished ; the image is much reduced in size, but then it is
concenti-ated in every respect, and hence improved in all the
necessary particulars. These lenses are, however, still open to
the objection that they produce some distortion, which is only
to be avoided by greatly reducing the size of the apertm^e
through which the light falls on the lens, and this necessarily
involves increased sensibility in the preparations we employ.
The distortion is not to the extent which has been represented,
but it may, by careful examination, be discovered in the finest
photographic portraits to a greater or less extent.

PART III.

PRACTICE or PHOTOGRAPHY.

CHAPTER I,

SELECTION OF PAPER FOR PHOTOGRAPHIC PURPOSES.

It is natural to suppose, that a process which involves the most
delicate chemical changes requires more than ordinaiy cai-e in
the selection of the substance upon which preparations of a pho-
tographic character are to be spread. Tliis becomes the more
evident as we proceed in oiu* experiments to produce increased
conditions of sensitiveness. As the material, whatever it may be,
is rendered more susceptible of change under solar influence, the
gTeater is the difficulty of producing pei-fectly uniform sui-faces,
and with paper this is more particularly experienced than with
either metal or glass plates. Paper is, however, so convenient
and so economical, that it is of the first importance to ovei'come
the difficulties which stand in the way of its use, as the tablet
on which the photogTaphic picture is to be delineated.

The principal difficulty we have to contend with in using paper
is, the diflTerent rates of imbibition which we often meet with in
different pai'ts of the same sheet, arising from trifling inequalities
in its textm-e and rmequal sizing. This is, to a certain extent,
to be overcome by a very careful examination of each sheet by

176 PRACTICE OF PHOTOGRAPHY.

the light of a lamp or candle at night. By extending the paper
between the light and the eye, and slowly moving it ujd and
down, and from left to right, the variations in its textiu-e will be
seen by the diflerent quantities of light which permeate it ; and
it is always the safest course to reject every sheet in which any
inequalities are detected. By day it is mure difficult to do tliis
than at night, owing to the interference of the reflected with the
transmitted light. It will, however, often happen that paper
which has been carefully selected by the above means will imbibe
fluids unequally. In all cases where the pa^Der is to be soaked
in saline solutions, we have another method of discovering those
sources of annoyance. Having the solution in a broad shallow
vessel, extend the paper, and gradually ch-aw it over the surface
of the fluid, taking care that it is wetted on one side only. A
few trials will render this perfectly easy. As the fluid is absorbed,
any irregularities are detected by the difierence of appearance
exhibited on the upper part, which will, over well-defined spaces,
remain of a dull-white, whilst other portions will be shining vdth
a reflective film of moisture. Where the importance of the use
to which the paper is to be applied, — as, for instance, to copying
an elaborate piece of architecture, or for receiving the portrait
of an individual — will repay a little extra attention, it is recom-
mended that the paper be tried by this test with ^^ure water,
and dried, before it is submitted to the salting operation. It
will be sometimes found that the paper contains minute fibres of
thread, arising from the mass of which it is formed not having
been reduced to a perfect pulp. Such paper should be rejected,
and so also should those kinds which are found to have many
brown or black specks, as they materially interfere with some
of the processes. Many of the spots in j^aper are formed by
particles of brass wu-e which have separated from the machinery
employed in its manufactiu-e, or they may be fragments of buttons
derived from the rags, and minutely divided in pulping. Some
specimens of paper have an artificial substance given to them by
sulphate of lime (plaster of Paris) ; but, as these are generally
the cheaper kinds of demy, they are to be avoided by purchasing
the better sorts. The plaster can be detected by fusing a sheet
of the paper and examining the quantity of ash. Pure paj)er
leaves less than I per cent, of ash. If plaster is present the ash
wUl be much more considerable : the increase of weight is, how-
ever, sometimes due to kaoHn. No really sensitive photographic
paper can be prejjared when sulphate of lime is jiresent ; and it
has the singular property of reversing the action of the Iodides
on the darkened chloride of silver, producing a negative in the
place of a positive photograph. It is the custom for paper-makers

SELECTION OF PAPER FOR PHOTOGRAPHIC PURPOSES. 177

to fix their names and the date on one leaf of the sheet of writing
paper. It is wise to reject this leaf", or to select paper which is
not so marked, as in many of the photographic processes which
will be described, these marks are brought out in most annoying
distinctness. From the vax'ious kinds of size which the manu-
facturers use ui theu' papers, it wall be found that constantly
varying effects wtII arise. A well-sized paper is by no means
objectionable ; on the contrary, organic" combinations exalt the
darkening property of the nitrate and chloride of silver. But
unless we are careful always to use the same variety of paper
for the same purpose, we shall be much perplexed by the con-
stantly varying results which w^e shall obtain. No doubt, with
the advancing importance of the art, the demand for paper for
photographic pui-poses wdll increase ; manufactiu-ei-s will then
find it worth the necessary care to prejDare paper agreeably to
the directions of scientific men. Several of our paper-makers
are now paying much attention to the preparation of photo-
graphic jiaj^er, and are considerabl}' improA-ing it. I have been
most obligingly furnished by Mr. Sandford, of Paternoster Row,
with specimens of a great variety of Foreign and English papers,
and from the care that gentleman is bestowing on this subject,
the most important advantages may be expected. All who desire
to make any progTess in photogi-aphy must take the necessary
precautions, or be content to meet ^vith repeated failures.

The photographic peculiarities of paper mainly depend upon
the sizes employed. The English paper manufacturers very
commonly employ gelatine, and this in very different conditions.
The French, on the contrary, use starch, and this, from the strong
aflinity existing between stai'ch and iodine, appears to be one
reason why the French paper is superior for the calotype in some
of its modifications.

Resin soap is largely employed as a size. The soap is applied
to the bibulous papei', and then decomposed by an acid w^ater,
leaving a fine film of resia spread upon the sui'face, susceptible
of the highest polish.

The following tables wall exhibit the results of an extensive
series of experiments, which were undertaken after the publica-
tion of Sir J. Herschel's memoir, On the Chemical Action of tlve
Rays of the Solar Spectrum, in which he has given a table of
results, obtained Avith different preparations on various kinds of
paper ; but as he has not established the influence of the paper,
except in a few instances, independent of the preparation, it be-
came desirable to do so ; and the residt of several years' ex-
perience has proved the correctness of the conclusions then
arrived at.

178 PRACTICE OF PHOTOGRAPHY.

In pursuing this inquiry, it was found that the same descrip-
tion of paper, from different manufacturers, gave rise to widely
different effects ; so that the most carefu.lly conducted experi-
ments, several times repeated, have only given approximations
to the truth. The foi"m of experiment was to select a number
of specimens of paper, — prepare them with great care in precisely
the same mannei', and expose them to the same solar influences.
They were partly covered with a piece of coloiu-less glass,^the
object of this being to determine whether under it the action
was quicker or slower than when exposed uncovered. Sii- John
Herschel has shown that there are some peculiar differences in
this respect ; and this method offered a very correct mode of
determining the relative effects.

I. — Papers prepared v-ith Chloride of Sodium and Nitrate of

Silver.

a. Supei-fine satin post Considerable exalting effect.

h. Thick wove post Depressing influence.

c. Superfine demy Slight exalting effect.

d. Bath di'awing card Changes slowly.

e. Thick post Slight exalting effect.

f. Common bank post Ditto.

g. Thin post Very tardy.

h. Tissue paper Considerable exalting effect.

II. — Papers prepared loith Chloride of Barium and Nitrate of

Silver.

a. Superfine satin post Slight exalting influence.

h. Thick wove post Ditto, but stronger.

c. Superfine demy Similar to a.

d. Bath tli-awing card Similar to a.

e. Thick post Considerable exalting influence.

f. Common bank post Similar to a.

(/. Thin post Similar to e.

h. Tissue paper Results uncertain.

III. — Papers prepared with Muriate of Ammonia and Nitrate
of Silver.

a. Superfine satin post Strong exalting influence.

h. Thick wove post Results uncertain — dependent on

the size employed.

c. Superfine demy Slight exalting effect.

d. Bath di'awing card. Results uncertain.

SELECTION OF PAPER FOE PHOTOGRAPHIC PURPOSES. 179

e. Thick post Results uncertain.

f. Common bank post Very slow.

g. Thin post Ditto.

h. Tissue paper Strong exalting influence.

IV. — Papers prepared with Iodide or Bromide of Potassium and
Nitrate of Silver..

a. Superfine satin post Darkens slowly.

h. Thick wove post Results uneei-tain.

c. Superfine Demy Strong exalting influence.

d. Bath Di-awing card Very slowly changes.

e. Thick post Depressing influence.

f Common bank post Slight exalting efiect.

g. Thin post, Ditto.

h. Tissue paper Results imcertain.

Unsized paper has been recommended by some, but in no in-
stance have I found it to answer so well as paper which has been
sized. The principal thing to be attended to in preparing sensi-
tive sheets — accoi'ding to my experience, extended over a period
of ten years, although it is opposed to the views of some yoimg
but able photogi'aphers — is to prevent, as far as it is possible, the
absorption of the solutions into the body of the paper — the
materials should be retained as mxich as possible vipon the very
surface. Therefore the superficial roughness of unsized sheets,
and the depth of the imbibitions, are serious objections to their
use. It must not, however, be forgotten, that these objections
apply in their force only to the silver preparations ; in some
modifications of the processes, with the bichromate of potash,
the common bibulous paper, used for filtering liquids, has been
fovmd to answer remarkably well, on account of the facility with
which it absorbs any size or varnish.

Great annoyance often ai'ises fi-om the rapid discoloiiration of
the more sensitive kinds of photographic cU'awing paper, inde-
pendent of the action of light, which appears to arise from the
action of the nitrate of silver on the organic matters of the size.
Unsized paper is less liable to this change. If we spread a pm*e
chloiide of silver over the papei*, it may be kept for any length
of time without any change of its whiteness taking place in the
dark. Wash it over with a solution of nitrate of silver, and,
particularly if the paper is much sized, a very rapid change of
colotu" will take place, however carefully we may screen it from
the light. From this it is evident that the oraranic matter of

180 PRACTICE OF PHOTOGRAPHY.

the size is the principal cause of the spontaneous darkening of
photographic papers prepared Math the salts of silver.

The most curious part of the whole matter, is, that in many
cases this change is earned on to such an extent that a revival
of metallic silver takes place, to all appearance in opposition to
the force of affinity. This is very difficult to deal with. I can
only view it in this light — the nitric acid liberates a quantity of
carbonaceous matter from the paper, which, acting by a function
peculiarly its own, will at certain temperatures effect the revival
of gold and silver, as proved by Count Rumford's experiments.

Having been informed that the paper-makei-s are in the habit
of bleaching their papers with svdphui- and suli^hites, I have sub-
mitted a considerable quantity of the browned papers to carefid
examination. lu all cases where the paper, covered with chloride
of silver, has suddenly hlackened, I have detected the presence
of suljihm-. Consequently, when the darkening goes on rapidly,
and terminates in blackness, we may, I think, correctly attribute
it to the fonnation of a sulphuret of silver.

It is, however, certain that the slow action of organic matter
is sufficient, under certain cu'cumstances, to set uji a chemical
change, which, once started, piogresses slowly, but certainly,
until the compound is reduced to its most simple form.

China clay — kaolin — has of late years been much used by the
paper manufactiu-ers, for the double piu-pose of giving weight to
the paper, and of enabling them to produce a smooth surface
upon all the finer varieties of paper ; such as the enamelled satin
post. This compound of alumina and silica would not, if the
finest varieties of clay were employed, be likely to do much mis-
chief in the papers used for photography; but the less pure
varieties of the Cornish gIaj are employed, and this commonly
contains the oxides of iron and other metals in a state of very
fine division; and these, where they come to the surface, form
little centres of action, from which dark cu'cles spread in rather
a curious manner. Thin papers have been tried, and many
varieties would answer exceedingly well, but that nearly all
kinds are found penetrated with small holes, which, though of
minute dimensions, suffer light to pass freely, and consequently
produce a spottiness on the resulting picture. Sir John Hei"schel
found that this eAol could be remedied by fastening two pieces
of such paper together; but this method is troublesome and un-
certain.

Retiu-ning to the consideration of size in the paj^er, the above-
named authority — who employed the lead salts in some of his
photograpliic processes — has the follo^^-ing remarks : —

■' The paper with a basis of lead turns yeUow by keeping in

SELECTION OF PAPER FOR PHOTOGRAPHIC PUKPOSES. 181

the dark, and the tint goes on gradually deepening to a dark
brown. But what is very singular, this change is not equally
rapid upon all kinds of paper — a difference depending, no doubt,
on the size employed; wliich, it may be observed here once for
all, is of the utmost influence in all photographic processes. In one
sort of paper (knowTi by the name of blue tvove ]}ost) it is instan-
taneous, taking place the moment the nitrate (if abundant) is
applied. And yet I find this paper to resist discoloiu'ation, by
keeping, better than any other, when the mordant base is silver
instead of lead. On the other hand, a paper of that kind called
smooth demy, rendered sensitive by a combination of lead and
silver, was found to acquire, by long keeping, a lead or slate
colour, which increases to such a degree as might be supposed to
render it useless. Yet, in this state, when it is impressed with
a photographic image, the process of fixing with hyposrdphite of
soda destroys this colour completely, leaving the gi-ound as white
as when fii'st prepared. This fortimate restoration, however,
does not take place when the paper has been browned as above
described. Some of the muriatic salts also are more apt to
induce this discolouration than others, especially those with the
earthy bases."

It will be evident from these remarks that it is of the utmost
importance to secure a paper which shall be as chemically piire
as possible. Experience has proved that recently-manufactui-ed
paper does not answer equally well with that which has been
made for a year or two. It has been thought by many that this
was an unfounded statement, but it is not so ; and the causes
operating to the improvement of paper by age are evident. The
organic matter of the size is liable to a spontaneous change :
this goes on for a considerable time, but at length the process
becomes so exceedingly slow that it may, for all pi-actical pur-
poses, be said vii-tually to rest. Paper changes its colour by
keeping from this cause, and I have found that such as I have
selected from the shop-ivorn stocks of stationers has been generally
superior to that which has been more recently manufactured.

Select, therefore, paper of a uniform textiu-e, free from spots,
and of equal transparency, choosing the oldest rather than the
newest varieties.

Where the process is highly sensitive for which the paper is
desired, it is important to treat it in the following manner : —
Ha\Tng a shallow dish sufficiently large to receive the sheets of
paper without in any way ci-umpling them, it is to be filled with
A-ery clear filtered water, to wliich a sufficient quantity of nitric
acid has been added to make it slightly som- to the taste. Taking
a sheet of paper, it should be laid on a porcelain slab, and sponged

182 PRACTICE OF PHOTOGRAPHY.

with clean water on both sides, after which it should l^e placed
in the acidulated water, and allowed to remain in it for several
hours. Too many sheets should not be placed in the vessel at
the same time. After a time they should be removed in mass,
placed on the slab, and left for half an hour under gently flowing
water, — this removes all the acid, and all those metallic and
earthy matters which it has removed fi-om the paper. After
this it is to be dided, and it is then fit for photographic use.

Tliis chapter has been deemed unnecessarily long; but, upon
renewing many of the experiments, I have become so convinced
of its importance in all particulars, that I could not induce my-
self to curtail it. All who aim at excellence in photography
should repeat the experiments and closely observe the results.

CHAPTEE II.

ox THE APPARATUS JsTCESSARY FOR THE FIRST PRACTICE OF PHO-
TOGRAPHY OX PAPER.

The most simple method of obtaining sim-pictures, is that of
placing the object to be coj^ied on a sheet of prepared paper,
pressing it close by a piece of glass, and exposing the arrange-
ment to sunshine; all the parts exposed darken, "while those
covered are protected fi-om change, the resulting pietui-e being
ivhite upon a darh ground.

It sho\ild be here stated, once for aU, that such pictures, how-
soever obtained, are called negative photograjplis ; and those
which have their lights and shadows correct as in nature — dark
upon a light ground, ?ccq jiositive photographs.

The accompanying woodcut. Fig. 36, represents a negative
copy of a cun-ant leaf, and Fig. 37, the positive copy obtained
from it.

If a cojiy is made by means of the camera, of any illuminated
object, the picture being produced by the darkening of a white

184

PRACTICE OF PHOTOGRAPHY.

or yellow paper, it will be evident that the highest lights will be
represented as dark portions, and the shadows as lights. Thus
we obtain a negative image. The female figure in the adjoining
woodcut, Fig. 38, is copied from a calotype negative portrait ; and
this negative being used by superposition on another prepared
piece of paper, produces a positive in which the lights and shades
are natural, as in the second figure, Fig. 39.

Let us commence by supposing the experimentalist to be sup-
plied with paper prepared by some one acquainted with the
manipulation, and that he is now to observe for the first time
the effects produced.

For the production of photographic drawings, it is necessary
to be provided with a copying frame and glass, the most conve-
nient size for which, is something larger than a single leaf of
quarto post wi'iting paper. The glass must be of such a thick-
ness as to resist some considerable pressure, and it should be
selected as colourless as possible, great care being taken to avoid

APPARATUS NECESSARY FOR PHOTOGRAPHY ON PAPER. \85

such glasses as have a tint of yellow or red, these colours pre-
venting the permeation of a large proportion of the most effi-
cient rays. Figures 40 and 41 represent such a frame in its most
simple form ; the fii'st showing it in front, as it is employed in
taking a copy of leaves, and the other the back, with its piece
of stout tinned iron, or board, which presses on a cushion, secur-
ing the close contact of the paper with every part of the ob-
ject to be copied, and its bi"ass bar, which, when pressed into
angular apertm-es in the sides of the frame, gives the required
pressiu'e to the paper. To copy leaves we proceed thus : —

Having placed the frame face downwards, carefully lay out on
the glass, the object to be copied, on which place the photo-
gi'aphic paper veiy smoothly.
Then cover this with the cushion,
which may be either of flannel
or velvet, fix the metal back,
and adjust it by the bar, until
every part of the object and
paper are in the closest contact.
For all ordinary uses, this frame
answers exceedingly well ; but a
more convenient pressure frame
is constructed in the manner
represented by Fig. 42. This ^^

contains two bars, one of them

movable, and both of them may be fixed in any required posi-
tion by binding screws.

In arranging botanical specimens, the imder sm-face of the
leaves should be next the glass, their upper and smooth siu-face ,
in contact with the paper. Although very beautiful copies may
be taken of di-ied specimens, they bear no comparison with those
from fresh-gathered leaves or recently collected plants, of which,
with the most delicate gradations of shades, the veins of the

186 PRACTICE OF PHOTOGRAPHY.

leaves, and the do-rni clothing the stems, are exhibited with in-
comparable fidelity. In the event of the j^lant ha'V'ing any thick
roots or buds, it vriH be best to divide them with a sharp knife,
for the pui-pose of equalizing the thickness in all parts, and in-
suring close contact.

Eugl'a^■ings are to be placed with theii' faces to the prepared
side of the photographic paper, laid veiy smoothly on the
glass, and then ■s'sith the cushion aud back pressed into the
closest contact possible : the least difference in the contact, by
peiTuitting dispersion, occasions a cloudiness and want of sharp-
ness in the photograph.

Of coiu-se a co-pj of anything taken by means of the i-ays
which have passed through it, must present all the defects as
well as all the beauties of the article, whatever it may be.
Thus, in copying a print, we have, besides the lines of the en-
graving, all the imperfections of the paper : This renders it
necessary that those engi'avings should be selected which are
on tolerably perfect paper. If the preservation of the engi-aving
is not a matter of much moment, by washing it over the back
with a vaiTush of Canada balsam and spuits of turpentine, it is
rendered highly transparent, and the residting impression is
ranch improved. Cai-e must, however, be taken to use the var-
nish very thin, that it may not impart any yellow tinge to the
paper. An exposiu-e of a few minutes only is sufficient to
produce strong aud faithful copies dui'ing sunshine ; but in dif-
fused daylight a longer period is necessaiy.

Some kind of copying fi-ame is an indispensable requisite to
the photographer: it is used for copying all objects by trans-
mission, and for multiplying the original pictures obtained by
means of the camera obscura from nature : it is, indeed, the
printing-press of the artist. Some prefer two plates of stout
plate-glass pressed very closely together with clamps and screws ;
but, as the intention is to bring the object to be copied and the
sensitive paper into the closest jDossible contact, niimerous me-
chanical contrivances will suggest themselves for this purpose to
the ingenious.

A great niunber of experiments should be made with the
copying frame before there is any attempt at using the camera
obscura.

The Camera Obscm-a, or Dark Chamber, was the invention of
Baptista Poiia, of Padua. Its piinciple will be best under-
stood by the very simple experiment of darkening a room by
closing the window-shutters and admitting a pencil of light
thi'ough a small hole in them. If a piece of paper is held at a
little distance from this hole, the figui-es of external objects will

APPARATUS >rECESSARY FOR PHOTOGRAPHY OX PAPER. 187

be seen delineated upon it ; and, by putting a small lens orer the
bole, they are rendered much more evident, from the condenisa-
tion of the rays by the spherical glass.

If, instead of a darkened room, we substitute a darkened box
(Fig. 43), the same effect will be seen. Suppose, in the first
place, the box to be without the lens, the rays would pass from
the external aiTOw in nearly right lines thi'ongh the opening,
refracted only in passing the solid edges of the hole, and foiTu
an image on the back of the dark box. The lens refracts the
rays still more, and a smaller, but a more perfectly defined pic-
tiu'e, is the result.

As in the phenomena of ^-ision, so in the camera obscura, the
image is produced by the radiations proceeding from the external
object; and as these radiations progress from Tai'ious parts, more
or less illuminated, so are the high lights, the middle tints and
shadows, most beautifully preserved in the lenticular image.
The colours, also, being in the fii"st instance the effect of some
physical modification of the primary cause, are repeated under
the same influence ; and the definition, the coloiu", and soft gra-
dation of light and shadow, are so perfect, that few more beauti-
ful optical effects can be exhibited than those of the camera obscvira.

Now as every ray of light producing the coloured image is
accompanied by the chemical principle actinism, and as this is
regulated in action by the luminous intensity of the rays, the
most luminous (j/ellow) producing the least chemical effect, while
this increases with the diminishing illuminating power of the
coloured rays of the radiating source, we have the impression
often opposed, in its relations of light and shade, to the colom-s
of the object we would copy. By referring to the fi-ontispiece
to the present volume, the effects produced by copying a coloured
image will be seen. The yellows, reds, and those colours usually
regarded as lights, are copied as shadows : hence the importance
of attention to the coloui-s of the di-ess, when a porti-ait is to be
taken by any photographic process.

188

PRACTICE OF PHOTOGRAPHY.

In the ordinary cameras used by artists for sketching, a mir-
ror is introduced, wliich throws the image on a semitransparent
table.

Fig. 44 is a section of one form of such an instimment : a a
represents the box, in one end of which is fixed the lens b.
The lenticular image falls on the mir ror c, placed at such an
angle that it is reflected on the plate of ground-glass d. e is
a screen to prevent the overpowering influence of daylight,
which would render the picture almost invisible. This form of
the apparatus, though very interesting as a philosophical toy, and

extremely useful to
the artist, is by no
means fitted for pho-
togi'ajihic purposes.
The radiations from
external obj ects sufier
considerable diminu-
tion of chemical power
in penetrating the
lens, and the reflec-
tion from the mirror

44-.

SO far reduces its in-
tensity, that its action on photographic agents is slow. To obviate
the objection of the reflected image, it is only necessary to place
the photographic paper in the place of the mirror, but not in an
angular position.

A great variety of these instruments have been introduced to
the notice of students of the art, many of them so unnecessarily

expensive that they are beyond the reach of the humble amateur.
It is conceived that a few examples of mechanical contrivances
by which the instrument is rendered portable, and in all respects
convenient will not be out of place. Fig. 4-5 represents one box
sliding within the other for the purpose of adjusting the focus,
the lens being fitted into a brass tube, which screws into the
front of the camera. The woodcut (Fig. 46) is but one box.

APPARATUS NECESSARY FOE PnOTOGRAPHY ON PAPER. 189

the lens being fitted into one brass tube sliding in anotber, like
a telescope tube, the movable part being adjusted by a screw
and rack. The mouth of the tube is contracted,
bj which any adventitious radiations are ob-
structed, and a brass shade is adjusted to close
the opening if requii-ed ; the paper is placed
in a case fitted with a glass fi-ont, as in Fig. 47,
and a shutter, by which it is protected from
the light until the moment it is required to
throw the image upon it.

In the fii'st edition of this work, a form of
camera was described, which possesses the ad-
vantages of extreme cheapness, and of being in
most respects convenient. It is, therefore, here
described in the language I employed in 1841 : — *7.

A photographic lens should possess, according to Sir John
Herschel, "the three qualities of a flat field, a sharp focus at great
inclinations of the visual ray, and a perfect achromaticity." There
can be no doubt but these qualifications are very essential, —
the two fii-st particularly are indispensable, and there is but one
objection to the latter. We can only produce perfect achroma-
ticity by a combination of glasses, and experiments prove that
by increasing the thickness of the object-glass, *and the number
of reflecting and refracting surfaces, we intennipt a considerable
portion of actinism, and consequently weaken the action on the
photographic material, whatever it may be; but ovir opticians
have succeeded to a great extent in overcoming this ditficulty.
We may, to a considerable degree, get rid of the defects arising
from chi-omatic dispersion, without having recourse to a combi-
nation of glasses of difiereut refracting powers. I long used my-

self, and constructed for others, a camera obscura which answered
well, with a non-acliromatic lens. Fig. 48 a represents the aper-

* The recent experiments of the Eev. Mr. Stokes most fully confirm this view.

190

PRACTICE OF PHOTOGRAPHY.

ture of tlie lens ; i i', a box sliding into an outer case, h h' ; k k,
a third division, containing a ground glass at the back, and a
door which caai be raised or lowered by the screw g, the whole
fitting into the frame h h'.

Figui-e 49 is a section of the camera, a is a lens of a perisco-
pic form, whose radii of cui'vature are in the proportion of 2 to 1.

^t.

This meniscus is placed with its convex surface towards the
plane of representation, and with its concavity towards the
object.

The apertui-e of the lens itself is made large, but the pencil
of rays admitted is limited by a diaphragm, or stop, constructed
as in the figure at b, between it and the plane of representation,
at about one-tenth of the focal length from the lens. By this
arrangement objects are represented with considerable distinct-
ness over every part of the field, but little
difterence being observable between the
edges and the centre, c is the plate of
ground glass at the back, which serves to
adjust the focus by, and also to lay the
photographic paper on, when we desire to
copy any object; d, a door to shut off the
light from the paper or plate until the
moment we desii-e to expose it to luminous
agency. Fig. 50 represents this screen or
jS door more perfectly, in the act of falling;
e is a d,oor at the back, through which the
picture formed on the opaque glass is examined ;y5 a pin, keeping
the door, d, in its place.

The following figures (Figs. 51, 52, 53) represent a more per-
fect arrangement. Its conveniences are those of folding, and
thus packing into a very small compass, for the convenience of

50.

APPAKATUS NECESSARY FOR PHOTOGRAPHY OX PAPER. 191

travellers. It is, however, only adapted for views, and not for
portraits.

With the camera obsciu-a properly arranged, and the copying
frame, the photogi'aphic student who confines his attention to
the processes on paper
has nearly all he re-
quires. For the con-
venience of adjusting
the instrument to dif-
ferent heights, and to
different angles of ele-
vation, tripod stands
are convenient, but
not altogether indis-
pensable. They are
made in several ways;

the two figures, 54 and 55, representing those which appear
best adapted to the use of the traveller. The arrangement of
compound legs shown in Fig. 54 insxu-es greater steadiness than
the other ; but the range of movement in Fig. 55 gives it some
advantages.

Beyond these things, a few dishes, such as are represented in
Fig. 56, A A ; and a feme b, upon which a photograph can be
placed for the jKirpose of being washed, are the only things
required for the practice of photography, except those pieces of
apparatus which, belonging to special processes, "wdll be found
described in the chapters devoted to them. The object has been
in this chapter to enumerate those only which are necessary for
an amateiu" to make his rudimentary experiments.

My object in describing tliis camera is to enable the yoimg
photographer to pui^ue his earlier studies, without ha-viug to in-
cur the cost of a perfect lens. An inferior, and consequently

192

PRACTICE OF PHOTOGRAPHY.

low-priced ackromatic lens, slioiild ever be avoided, tlie results
with sucli being far less perfect than those obtained by means of
a good periscopic lens.

CHAPTER III.

ON THE MODES OF MANIPUI^TION ADOPTED IN THE PREPARATION
OF SENSITIVE PAPERS.

The only apparatus required by the pliotogi'aphic ai-tist for the
prepai-ation of his papers, are — some veiy soft sponge brushes
and Jarge camel haii' pencils (no metal should be employed in
moimting the brushes, as it decomposes the silver salts), a wide
shallow vessel capable of receiving the sheet without folds, a
few smooth planed boards, sufficiently large to stretch the paper
upon, and a porcelain or glass slab. He must supply himself
with a quantity of good white blotting-paper, and several pieces
of soft linen or cotton cloth ; a box of pins ; a glass rod or two ;
some porcelain capsides; and some beaker glasses, graduated
measui-es, scales, and weights.

Section I. — Xitr.\te of Silver.

The most simple kind of photographic paper which is pre-
pared with the silver salts is that washed with the niti-ate of
silver only; and for many purposes it answers well, particularly
for copying lace or feathers ; and it has this advantage over
every other kind, that it is perfectly fixed by well soaking in
warm water, free from chloride of sodiiim.

The best proportions in which this salt can be used are 60 grs.
of it dissolved in a fluid ounce of distilled water. Care must be
taken to apply it equally, with a quick but steady motion over
every part, of the paper. It will be found the best practice to pin
the sheet by its four cornel's to one of the flat boards above-men-
tioned, and then, holding it with the left hand a little inclined, to
sweep the bnish, from the upper outside corner, over the ■\^•hole of
the sheet, remo^'ing it as seldom as possible. The lines in fig. 51
will represent the manner in which the brash should be moved
over the paper, commencing at a and ending at h. On no ac-
count must the lines be brushed across, nor must we attempt to

194

PEACTICE OP PHOTOGRAPHY.

cover a spot wliicli has not been ■wetted, by the application of
fresh solution to the place, as it "will, in darkening, become a
well defined space of a different shade from
the rest of the sheet. The only plan is,
"when a space has escaped our attention in
the fii'st washing, to go over the whole
sheet "with a more dilute solution. It is,
indeed, always the safest com'se to give the
sheet two washings.

The nitrated paper not being very sen-
sitive to luminous agency, it is desu'able to
increase its po"wer. This may be done to
some extent by simple methods.

By soaking the paper in a solution of
isinglass or parchment size, or by rubbing it over with the white
of egg, and diying it prior to the application of the sensitive
"wash, it will be found to blacken much more readily, and assume
different tones of colom*, "which may be varied at the taste of the
operator.

By dissolving the nitrate of silver in rectified spii-its of "wine,
instead of water, we produce a tolerably sensitive nitrated j^aper,
"which darkens to a very beautiful chocolate brown; but this
"wash must not be used on any sheets prepai'ed "with isinglass,
parchment, or albiunen, as these substances are coagulated by
alcohol, and "wash up forming streaks.

The nitrate of silver is not sufliciently sensitive to change
readily in difiused light ; consequently it is unfit for use in the
camera obscura, and it is only in strong sunshine that a copy of
an engraving can be taken in any moderate time.

Ammonio-Nitrate of Silver,— This is an exceedingly useful
preparation for many 23urposes. It is prepared by adding
ammonia to a solution of nitrate of silver : a deep olive preci-
pitate of oxide of silver takes place ; more ammonia should then
be added, di*op by drop, untd this precipitate is redissolved,
great care being taken that no more ammonia is added than
is necessary to effect a perfect solution of the oxide of silver.
This solution is more sensitive than the nitrate, and may be used
"with advantage for copying by superposition; but it is not fitted
for the camera obscm^a.

Section II. — Chloride of Silver.

This is one of the most important salts employed in photo-
graphy ; it therefore demands especial attention.

Muriated Papers, as they are termed, are formed by producing

CHLORIDE OF SILVER.

195

a chloride of silver on their surface, by washing the paper with
the solution of chloride of sodium {muriate of soda), or any other
chloride, and when the paper is dry, with a solution of nitrate
of silver.

It is a very instnictive practice to pi-epare small quantities of
solutions of common salt and nitrate of silver of difierent strengths,
to cover slips of paper mth them in various ways, and then to
expose them altogether to the same radiations. A cui-ious variety
in the degi-ees of sensibility, and in the intensity of coloui-, will
be detected, showing the importance of a veiy close attention to
proportions, and also to the mode of manipulating.

A knowledge of these preliminary but important points having
been obtained, the preparation of the paper should be proceeded
with ; and the following method is recommended : —

Taking some flat deal boards, perfectly clean, pin upon them,
by theu' foui- corners, the paper to be prepared ; observing the
two sides of the paper, and selecting that side to receive the pre-
paration which presents the hardest and most uniform siu'face.
Then, dipping one of the sponge brushes into the solution of
chloride of sodium, a sufficient quantity is taken up by it to
moisten the siuface of the paper without any hard rubbing;
and this is to be applied with great regularity. The papers being
" salted," are allowed to dxy. A great number of these may be
prepared at a time, and kept in a portfoho for use. To render
these sensitive, the pajiers being pinned on the boards, or care-
fully laid upon folds of wliite blotting paper, are to be washed
over with the nitrate of silvei', applied by means of a camel-haii"
pencil, obsei-ving the instructions pre\'iously given as to the
method of moving the brush upon the paper. After the first
wash is applied, the paper is to be di'ied, and then subjected
to a second application of the silver solution. Thus prejoared,
it will be sufficiently sensitive for all piu-poses of copying by
application. The second wash is applied for the purpose of
insuring an excess of the nitrate of silver in combination, or,
more properly speaking,
mixed with the chloride.
Mr. Cooper, with a view
to the production of a uni-
form paper, recommends
that it be soaked for a con-
siderable length of time in
the saline wash, and, after "^
it is ch-ied, that the sheet ss

should be dipped into the silver solution ; while the opei-ator
moves over its siu'face a glass rod held in two bent pieces of glass.

^^^

196 PRACTICE OF PHOTOGEAPHT.

as in fig. 58 j the object of "^liicli is to remove the small air-
bubbles forming on the sui-face of the paper, -n-hich protect it
fix>m the action of the fluid. This process, however ■well it may
answer in preparing p>aper for copying engravings, ■vrUl not yield
paper sufficiently sensitive for camera purposes; and it is objec-
tionable on the score of economy, as a larger quantity of the silver
solution is required to decomjx)se the cominon salt than in the
process previously described.

The young photographer -svould find it interesting to study the
more striking pectdiarities of a few of the preliminary washes,
such, indeed, as those noticed in the scientific division. It will
be found that nearly every variety of paper exj)Osed to the fiill
action of the solar beams will pass through varioas shades of
brown, and become at last of a deep olive colour : it must there-
fore be tmderstood that the process of darkening is in all cases
stopped short of this point. Remembering this, it will be found
that very peculiar and often beautiful tints are produced by the
chloride of barium, the hydro-chloride of ammonia, and some
of the organic acids.

Papei-s prepared with the chloride of sodium have, however,
been more extensively used than any others for positive pic-
tures, owing to the ease with which this material is always to be
procured ; and for most purposes it answers as well as any other,
but it does not produce the most sensitive photographic .siu-face.

The proportions in which this salt has been used are exceed-
ingly various ; in general, the solutions have been made too strong ;
but several chemists have recommended washes that are as much
too weak. For different uses, solutions of various qualities should
be employed. It will be found well in practice to keep papers
of three orders of sensitiveness prepared : the proportions of salt
and silver for each being as f jllows : —

A paper may be prepared vrith. the chloride as follows : —

Chloride of sodium, thirty graius to an ounce of water.

!Mitrate of silver, one hundred and twenty grains to an ounce
of distilled water.

The paper is first soaked in the saline solution, and after being
carefully pressed between folds of blotting paper and dried, it is
to be w^ashed twice with the solution of silver, drying it by a
warm fire between each washing. This paper is very liable to
become brown in the dark. Although images may be obtained
in the camera on this paper by about half an hour's exposure,
they are never very distinct., and may be regarded as rather
curious than useftiL

A less sensitive paper for copies of engravings — botanical or
entomological sj^ecimens, should have the following proportion : —

IODIDE OF SILVER. 197

Cliloride of sodium, t'.venty-five gi-ains to an ounce of water.

Nitrate of silver, ninty-nine grains to an ounce of distilled
water.

And a jet more common sensitive paper, for copying lace-
work, feathers, patterns of watch-work, &c., may be thus pre-
pared at less expense : —

Chloride of .sodium, twenty grains to an ounce of water.

Nitrate of .silver, sixty grains to an ounce of distilled water.
Applied as above directed.

This paper keeps tolerably well, and, if carefiilly prepared,
may always be depended upon for darkening equally.

Section III. — Iodide of Silver.

This salt was employed very early by Talbot, Herschel, and
others, and it enters as the principal agent into Mr. Talbot's
calotype process. Paper is washed with a solution of the
iodide of potassium, and then with nitrate of silver. By this
means papers may be prepared which are very sensitive to
himinous influence, provided the right proportions are hit ;
but, at the same time, nothing can be more insensible to the
same agency than the pure iodide of silver. A singular dif-
ference in precipitates to all appearance the same, .led to the
belief that more than one definite compound of iodine and silver
existed ; but it is now proved that pure iodide of silver will not
change colour in the sunshine, and that the quantity of nitrate
of silver in excess regulates the degree of sensibility. Experi-
ment has shown that the blackening of one variety of iodidated
paper, and the preservation of another, depends on the simple
admixture of a very minute exce.s.s of the nitrate of silver. The
papers prepared with the iodide of silver have all the peculiari-
ties of those prepared with the chloride, and although, in some
in.stance.s, they seem to exhibit a much higher order of sensi-
tiveness, they cannot be recommended for general purposes
with that confidence which experience has given to the chloride.
It may, however, be proper to state the best proportions in
which the iodidated papers can be prepared, and the most
approved method of applying the solutions.

The fijiest kind of paper being chosen, it should be pinned by
its four comers to a board, and carefully washed over with a
solution of six grains of the nitrate of silver to half an oimce of
water : when this is dry, it is to be wa.shed with a solution of
iodide of potassium, five grains in the same quantity of water,
and di'ied by, but at some little distance from, the fire j then,

198 PRACTICE OF PHOTOGRAPHY.

some short period before the paper is required for use, it must
be again washed with the silver solution, and quickly dried, with
the same precaution as before. If this paper is warmed too
mvich in drying, it changes from its delicate primrose colour to
a bright pink or a rosy brown, which, although still sensitive to
solar influences, is not so readily changed as when in an unaltered
state. The peculiar property of this salt to change thus readily
by calorific influence, and some other very remarkable efiects
produced on already darkened paper when washed with a
salt of iodine, and exposed to artificial heat, or the pure calorific
rays of the spectrum, appears to promise a process of drawing of
a new and peculiar character.

The few simple directions here given will be sufficient to
guide the young experimentalist in his earliest essays ; and it
is particidarly recommended that the first experiments shovdd
be confined to the salts named in this chapter. The minute
details requii'ed for the more highly sensitive processes are
described in immediate connection with the process to which
they refer.

I woidd advise the amateur to start upon his studies with
but three solutions — 1st, Chloride of sodium ; 2d, Nitrate of
silver ; 3d, Hyposulphite of soda. With the first he carefully
washes several sheets of good letter paper on one side only, and
dries ; with the second he, by another washing of the paper,
forms the required chloride of silver, on which he may obtain
pictures by simple exposure in the copying frame ; with the
third he gives permanency to the pictm-es which he produces.

CHAPTER IV.

ON FIXING THE PHOTOGRAPHIC PICTURES.

The power of destroying the susceptibility of a photographic
ageut to the fiu-ther action of light, when the picture is com-
pleted by its influence, is absokitely necessary for the perfection
of the art. Various plans have been suggested for accomplishing
this, which have been attended with very difierent results ; few, if
any, of the materials used prodiicing the required effect, and, at the
same time, leaving the pictiu-e unimpaired. The hyposulphite
of soda is decidedly superior to every other fixing material ; but
it will be interesting to name a few other preparations, which
may be used with advantage in some instances.

The pictiu'es formed on papei"s prepared with the nitrate of
silver only, may be rendered permanent by washing them in
very pui-e water. The water must be quite free from any
chlorides, as these salts convert the nitrate of silver into a
chloride, and attack the pictiu-e with considerable energy, and
soon destroy it, by converting the darkened silver itself slowly
into a chloride. Herschel remarks — " If the paper be prepared
^vith the simple nitrate, the water must be distilled, since the
smallest quantity of any muriatic salt present attacks the pictui-e
impressed on such paper with singular energy, and speedily
obliterates it, unless very dark. A solution containing only a
thousandth part of its weight of common salt suffices to efiect
this in a few minutes in a picture of considerable strength."

The gi-eat point to be aimed at in fixing any of the sun-
pictures is the removal of all that portion of the preparation,
whatever it may be, which ha.s not undergone change, without
distiu'bing those parts which have been altered in the slightest
degi'ee by the chemical radiations. When a picture has been
obtained upon paper prepared with the nitrate of silver, or the
ammonio-nitrate of silver, the best mode of proceeding Ls to
wash it first ^svith warm rain water, and then with a diluted
solution of ammonia : if the ammonia is too strong it dissolves
the oxide of silver, which in these processes is formed in the
fainter parts of the pictm-e, and thus obliterates the more
delicate portions.

Photogi-aphs on the muriated papers are not, however, so
easily fix:ed. Well soaking these in water dissolves out the

200 PRACTICE OF PHOTOGRAPHY.

excess of nitrate of silver, and tlius the sensibility is somewliat
diniinislied ; indeed, they may be considered as half fixed, and
may in this state be kept for any convenient opportunity of
completing the operation.

Chloride of sodium (common salt) was recommended by Mr.
Talbot as a fixing material, but it seldom is perfectly successful :
as a cheap and easy method, it may be occasionally adopted, when
the pictiu'e to be preserved is not of any particidar consequence.

It may appear strange to many that the same material which
is used to give sensitiveness to the paper should be applied to
destroy it. This is easily explained : In the first instance, it
assists in the formation of the chloride of silver ; in the other, it
dissolves out a large portion of that salt from the paper, the
chloride of silver being soluble in a strong solution of chloride
of sodium. The picture being first washed in water, is to be
placed in the brine, and allowed to remain in it for some little
time ; then, being taken out, is to be well washed in water, and
slowly di'ied. After fixing by this process, the white parts of
the photograph are often changed to a pale blue — a tint which
is not, in some cases, at all unpleasant.

I have in my possession some pictures which have been pre-
pared more than eight years, which were then fixed with a
strong solution of salt, and subsequently washed A\dtli warm
water. They have become slightly blue in the white portions,
but otherAvise they are very permanent ; and they have lost but
little of their original character.

The chloride of silver being soluble in a solution of ammonia,
it has been recommended for fixing photographs. The ammonia,
however, attacks the oxide, which forms the darkened parts
in some preparations, so rapidly that there is great risk of its
destroying the pictui-e, or, at least, of impairing it considerably.
The only photographs on which I have used ammonia with com-
plete success are those prepared with the phosphate of silver ; to
many varieties it imparts a red tinge, which is fatal to their use
for transfers. Still ammonia afibrds a ready means of 2:)artially
fixing a photograph, and thus preserving it until a more con-
venient period for giving it permanence.

The ferrocyanide of potassium, or, as it is more commonly called,
the 2Dru>ssiate of potash, converts the chloride into a cyanide of
silver, which is not susceptible of change by light ; consequently
this cheap salt has been employed as a fixing agent, but, most
unfortimately, photographs which have been subjected to this
preparation are slowly, but sm*ely, obliterated in the dark.

The iodide of silver, which is readily formed by washing the
photograph with a solution of the iodide of potassium, is scarcely
sensitive to light ; and this salt, used in the proportions of five

ON FIXING THE PHOTOGKAPHIC PICTURES. 201

or six gi'ains to four or five ounces of water, ans-vrers tolerably
■well "where transfers are not required. It tinges the Avhite lights
of the pictui'e of a pale yello-w, — a colour which is extremely
active in absorbing the chemical rays, and is therefore quite
inapplicable where any copies of the original photograph are
required. Bromide of potassium may be employed as a tempo-
rary fixing agent, and for this purpose is strongly recommended
by Mr. Talbot, and constantly employed by many of the con-
tinental travelling photographers ; since it will insure the per-
manence of an imjiression until an opportunity presents itself
for giA*ing the final peraianence to the pictiire.

Of all the fixing agents, the hyposulphite of soda is decidedly
the best. This was first j^ointed out by Sir John Hei-schel, who
also recommended that it should be used warm in some cases,
which was the plan adopted by ]Mi\ Fox Talbot in the improve-
ments of his calotype process.

To use the hyposidj^hite of soda with efiect, there are several
precautions necessary. In the fii'st place, all the free nitrate of
silver must be dissolved out of the paper by well washing. The
photograph being next spread on a plane sui'face, is to be washed
over on both sides with a saturated solution of the hyposulphite
of soda. The pictiire must then be washed, by allowing a small
stream of water to flow over it, at the same time dabbing it vdih
a piece of soft sponge, until the water passes ofi" perfectly taste-
less. This opei'ation should be repeated twice, or, in particulai"
cases, even thi'ee times. The hyposidphite of soda has the pro-
perty of dissohdng a large quantity of several of the salts of
silver, but paii;icul:Ti'ly of the chloride, with which it combines,
forming a triple salt of an exceedingly sweet taste. This salt is
liable to spontaneous decomposition, accompanied with separa-
tion of silver in the state of sidphuret : hence the necessity of
freeing the paper, by washing, of every trace of it, the sulphuret
of silver being of a dirty bro'wn. It might appear that the use
of warm water would more efiectually cleanse the paper ; but it
often occasions the immediate formation of the sulphuret of silver.

Some operators prefer leaviDg the picture in a bath of the
hyposulphite of soda for some time, and then removing the salt
by simple immersion in water, frequently changing it. The
advantages of this appear to be, that the surface of the paper is
not distiu'bed by any i-ubbing action, or by the mechanical ac-
tion of water flowing over the siuiace. For fixing the calotype
pictm-es, Mr. Cimdell, to whom we are miich indebted for im-
provements in this particiilar process, recommends the following
mode of manipulation : —

The picture, or as many of them as there may be, is to be
soaked in warm water, but not warmer than may be borne by

202 PRACTICE OF PHOTOGRAPHY.

the finger; this water is to be changed once or twice, and the
pictures are then to be well drained, and either dried altogether,
or pressed in clean and dry blotting-paper, to prepare them to
imbibe a solution of the hyposulphite of soda, which may be
made by dissolving an ounce of that salt in a quart of watei\
Having poured a little of the solution into a flat dish, the
pictm^es are to be introduced one by one; daylight ^\dll not now
injure them : let them soak for two or three minutes, or even
longer, if strongly j^rinted, turning and mo^dng them occasion-
ally. The remaining um-educed salts of silver are thus thoroughly
removed by soaking in water and pressing in clean blotting-
paper alternately ; but if time can be allowed, soaking in water
alone will have the effect in twelve or twenty-four hours, ac-
cording to the thickness of the paper. It is essential to the
success of the fixing process, that the paper be in the first place
thoroughly penetrated by the hyposulphite, and the sensitive
matter dissolved; and next, that the hyposidphite compoimds
be effectually i-emoved. Unless these salts are completely
washed out, they induce a destructive change upon the pictm-e;
they become opaque in the tissue of the paper, and unfit it for
the operation of being copied.

Mucli has been said and written about improving the tone of
the pictiu'e by the use of old hyposulphite of soda, and of
the hyposulphite in which chloride of silver has been dissolved.
My own experience, which is corroborated by that of many of
the most successfid photographic artists in England and on the
continent, convinces me, that in aiming at pecuHarity of tint by
these methods, the permanency of the photogi'aph is injured.
I have pictm-es, produced by different ai-tists, fiied after this
method, and scarcely one of them remains free from change.

The object is to remove all the chloride or iodide of silver;
and to secui-e this, as much hyposulphite of soda should be un-
conibhied, with chloride or ioclide of silver, in the solution as
possible.

The hyposulphite of silver being formed, it has to be dissolved
out of the paper, the fibres of which hold it with a strong capil-
lary force ; and it is only by very long continued soaking that
all can be removed. The slight mechanical aid afforded by
dapping the siu-face of the paper ^vith a soft sponge well filled
with water, gTeatly accelerates the removal of the salt ; and
when the paper ceases to taste sweet, we may depend ujjon the
permanence of the photograph.

The hy|50sidphite of soda has been used for almost every pho-
tographic process, from the facility it affords for removing the
silver salts. The following is the process of Gustave le Gray,
of Paris, chiefly applicable to the calotype process on waxed

ON FIXING THE PHOTOGRAPHIC PICTURES. 203

paper, "wliicli is valuable as being the directions of one who has
produced most beaxitiftil pictiu-es : but it does not differ, in any-
important particulars, from the j^rocess already given : —

" Make in a bottle the following solution : — Filtered Avater,
about a pint and a half; hyposulphite of soda, about three
ounces ; cover the bottom of a dish with this, and pkmge in
your negative proof, taking care to avoid au--bubbles : this dis-
solves the bromo-chloro-iodide of silver, but does not attack the
gallo-nitrate of silver, which foinns the blacks. — [Le Gray is in
eiTor here, and in a succeeding pai'agraph — the darkened por-
tions of these photogi-aphs being metallic, silver in a state of
extreme division.]

" Never put more than one proof at a time in the bath ; but
you may use it for several proofs one after the other.

" If you examine the proof as a transparency after it has re-
mained some time in the bath, you may be tempted to think it
is lost, as in some places spots will appear from the iodide of
sUver not being completely taken away ; but if you wait until
it is removed, which you will know by the disappearance of the
yellow tint, you will be astonished at the whiteness and trans-
parency of the paper, as well as at the beauty of the blacks in
the image.

" It will require for this, to remain in the bath from half an
hovu- to three quarters ; you will then wash it ia several waters,
and leave it ia a basin of clear water for three quarters of an
hour ; then let it dry spontaneously by hanging it up ; the proof
is then quite unalterable by light, as thei'e remains nothing more
in the paper than the gallo-nitrate of silver, which is black.
(See above.)

" Fixing by means of the bromide of potassium is not so
dvu'able, because it does not remove any of the materials used
in prepariug the pajier. It may, nevei-theless, be of great use
in travelling, and when it is required to make several proofs one
after the other; because then you avoid touching the hyposiil-
phite in preparing the negative paper, which spots at the least
contact -with it.

" You may thus place the whole of yoiu' negative proofs
together in this bath.

"Water, a jiint and tlu-ee quarters; bromide of potassium,
360 grains.

" In taking the proof oiit of the bath, yovi must wash it in
several waters, and dry it; it should be kept in the bath at
least tlu-ee quarters of an houi-, but if you leave it in two or
three hoiu"s, it will not injiu-e it."

Such is M. le Gray's statement, and so it is rendex'ed by his

204 PRACTICE OF PHOTOGRAPHY.

English translator, Mr. Cousins; but I believe tlie quantity of
the bromide of potassium to be by far too large, and that the
pictures would sustain less injury by using a solution of one
half the strength indicated. His process for fixing the positive
pictures contains some important hints.

"Dissolve in a bottle hyposulphite of soda, 1500 grains ;
" Filtered water, nearly a quart.
In another bottle dissolve 75 grains of nitrate of silver in a
wine-glass or two of water; when well dissolved, you add to
it a satiirated solution of chloride of sodium, until the white
precipitate ceases to fall ; allow it to repose a shoi-t time, and
then decant the clear liquor, and gather the precipitate of chlo-
ride of silver, which you dissolve in the other bottle of hypo-
sulphite of soda; by means of this solution you obtain directly
black tints upon the picture. The older the hyposulphite of
soda is, the better; when it gets thick, you must add a fresh
solution of hyposulphite alone, without the chloride of silver,
the old containing an excess, which it has taken from the proofs
already immersed in it. You must not filter it to take away
the deposit, but only let it repose in a large bottle, and decant
the clear liquid for use, leaving the sediment to be re-dissolved
by fresh solution.

" By leaving the proofs a longer or shorter period in the bath,
you can obtain all the tints from the red to the black, and clear
yellow; with a little practice, you will be siu-e to get the tint
you desire. You must not leave a proof less than an hour
in the bath for it to be sufficiently fixed, and it can remain
three or four days to obtain the sepia and yellow. By heating
the hyposulphite of soda I accelerate the operation; but we
must not then leave the proof for an instant to itself, as the
rapidity of action is so great, that the picture might be com-
pletely efiaced.

" By adding to the preceding solution about one fluid ounce
of liquid ammonia, I obtain pretty bister tints, and very pure
whites. The English paper is exceedingly good for these tints.

" I obtain also fine velvet-like tints by putting the photo-
gi'aph (when taken out of the hyposulphite of soda) upon a bath
of a salt of gold, using 15 grains of the chloride of gold to oiie
pint and a half of distilled water.

" Fine yellow tints are obtained by placing the proof (if too
vigorous) first in a bath of hyposulphite, and then in a bath
composed of one pint and a half of water, and one fiuid ounce
and a half of hydrochloric acid ; washing it perfectly in water.
Liquid ammonia, employed in the same quantity as last men-
tioned, gives remarkably fine tints.

ON FIXIXG THE PHOTOGRAPHIC PICTURES. 205

" When the proof is the colour you desire, wash it in several
waters, and leave it two or three hom"s in a basia of water, until,
touching it v\'ith the tongue, you perceive no sweet taste, which
indicates the presence of hyposulphite of silver; then diy it by
hanging it up, and it is tinished. The bath may contain as
many proofs as can be conveniently placed in it."

Experience has shown, that however beautiful may be some
of the tones given to a photograph by the methods recommended
by M. Le Gray, these are obtained with some sacrifice of per-
manency. Many choice productions prepared by this photo-
grapher, which I have had for but a few months in my
possession, are showing indications of decay : the change taking
place first at the edges, and gradually creeping over the whole
picture.

The following fixing processes are rather more cuiious than
useful : they were fii'st indicated by Su- John Herschel, from
whose memoir on the Chemical Agency of the Bays of the ^'olar
^'jjectrum, I quote: —

" By far the most remarkable fixing process vdth wliich I am
acquainted, however, consists in wasliing over the pictui'e with
a weak sokition of corrosive sublimate, and then laying it for
a few moments in water. This at once and completely obliter-
ates the picture, reducmg it to the state of perfectly white
paper, on which the nicest examination (if the process be per-
fectly executed) can detect no trace, and in which it may be
used for any other purpose, as ch-a^dng, A\Titing, &g., being com-
jjletely insensible to light. Nevertheless, the jDictui'e, though
invisible, is only dormant, and may be instantly revived in all
its force by merely brushing it over with a solution of a neutral
hyposTilphite, after which, however, it remains as insensible as
before to the action of light. And thus it may be successively
obliterated and revived as often as we please. It hardly requires
mention that the property in question furnishes a means of
painting in mezzotinto (i.e., of commencing on black paper and
working in the lights), as also a mode of secz'et writing, and a
variety of similar applications."

, There is a remark which ought not to be omitted in regard
to this part of our subject, viz., that it makes a gi-eat difierence,
in respect of the injuiy done to a photographic picture by
the fixing process, whether that picture has been impressed by
the long-continued action of a feeble light, or by the quick
and vivid one of a bright sun. Even supposing the pictures
originally of equal intensity, the half-tints are much less power-
fully corroded or washed out in fixing in the latter case than in
the former.

CHAPTER V.

Section I. — The Calotype as now Practised, and its
Modifications.

In the historical section, the description of the calotype, as pub-
lished by Mr. Fox Talbot, is given. Mr. Hemy Fox Talbot,
duiing 1852, made the country a free gift of all his patents,
reserving only the right of a patentee over that portion which
includes the practice of taking portraits for sale. (See the
letters in the Api:)endix.)

The fii'st important published improvement on the calotype
was due to ]Mi\ Cundell, whose process appeared in the Philo-
sophical Magazine for May, 1844, from which we extract the
following : —

1. To produce a calotype picture, there are five distinct pro-
cesses, all of wliich, except the third, must be performed by
candle-light : they are all very simple, but, at the same time,
they all requii*e care and caution. The first, and not the least
important, is —

3. The Iodizing of the Paper. — Much depends upon the
paper selected for the purpose ; it must be of a compact and
uniform texture, smooth and transparent, and of not less than
mediiim thickness. The best I have met Avith is a fine satin post
paper, made by " P. Tui-ner, ChafFord Mill." Having selected
a half-sheet without flaw or water-mark, and free from even the
minutest black specks, the object is to spread over its surface a
perfectly uniform coating of the iodide of silvei-, by the mutual
decomposition of two salts, nitrate of silver and iodide of potas-
sium. There is a considerable latitude in the degree of dilu-
tion in which these salts may be used, and also in the manner
and order of their application ; but as the thickness and regu-
lai'ity of the coating depend upon the solution of nitrate of
silver, and upon the manner in which it is applied, I think it
ought by all means to be applied first, before the surface of the
paper is disturbed. I use a solution of the strength of seventeen
grains to the ounce of distilled water.

3. The paper may be pinned by its two upper comers to a

jiE. cuxdell's calottpe process. 207

clean thy board a little larger than itself; and, holding this nearly
upright in the left hand, and commencing at the top, apply a
wash of the nitrate of silver thoroughly, evenly, and smoothly,
with a large soft binish, taking care that every part of the sui*-
face be thoroughly wetted, and that nothing remain unabsorbed
in the natiu'e of free or running solution. Let the paj:)er now
hang loose from the board into the aii* to dry, and by using sevei-al
boards time will be saved.

4. The nitrate of silver spread upon the paper is now to be
saturated with iodine, by bringing it in contact with a solution
of the iodide of potassium : the iodine goes to the silver, and the
nitric acid to the potash.

5. Take a solution of the iodide of potassium of the strength
of 400 grains to a pint of water, to which it is an improvement,
analogous to that of M. Claudet in the daguerreotype, to add
100 grains of common salt. He found that the chlorinated
iodide of silver Ls infinitely more sensitive than the simple iodide;
and by this addition of common salt, a similar, though a less re-
markable, modification is obtained of the sensitive compound.
Pom* the solution into a shallow flat-bottomed dish, sufficiently
large to admit the paper, and let the bottom of the vessel be
covered to the depth of an eighth of an inch. The prepared side
of the paper, having been previously marked, is to be brought in
contact with the surface of the solution, and, as it is desirable to
keep the other side clean and diy, it ^vill be found convenient,
before putting it in the iodine, to fold upwards a naiTow margin
along the two opposite edges. Holding by the uptm-ned margin,
the paper is to be gently di-awn along the surface of the liquid
untn its lower face be thoroughly wetted on every part ; it will
become plastic, and in that state may be sufiered to repose for a
few moments in contact with the liquid : it ought not, however,
to be exposed in the iodine dish for more than a minute
altogether, as the new compound, just fonned upon the paj)er,
upon farther exposure, would gTadually be rechssolved. The
paper is therefore to be removed, and, after diipping, it may be
placed upon any clean surface with the wet side uppeiTaost imtil
about half dry, by which time the iodine solution "wtJI have
thoroughly penetrated the paper, and have found out and
saturated every particle of the silver, wliich it is quite indis-
pensable it should do, as the smallest portion of undecomposed
nitrate of sdver would become a black stain in a subsequent part
of the process.

6, The paper is now covered with a coating of the iodide of
silver ; but it is also covered, and indeed satm-ated, with saltjjetre
and the iodide of potassium, both of which it is indispensable

208 PRACTICE OF PHOTOGRAPHY

should be completely removed. To effect the removal of these
salts, it is by no means sufficient to " dip the paper in water ; "
neither is it a good plan to wash the paper with any considei'able
motion, as the iodide of silver, having but little adhesion to it, is
apt to be washed off. But the margin of the paper being still
upturned, and the unprepared side of it kept dry, it will be found
that by setting it afloat on a dish of clean water, and allowing it
to remain for five or ten minutes, dra^\dng it gently now and
then along the surface to assist in remo\"ing the soluble salts,
these will separate by their own gravity, and (the iodide of silver
being insoluble in water) nothing will remain upon the paper
but a beautifully perfect coating of the kind required.

7. The paper is now to be dried ; but, while wet, do not on
any account touch or disturb the prepared surface with blotting
paper, or with anything else. Let it merely be suspended in the
air ; and, in the absence of a better expedient, it may be planed
across a string by one of its corners. When dry, it may be
smoothed by pressure. It is now " iodized" and ready for use,
and in this state it will keep for any length of time if protected
from the light. The second process is that of exciting or

8. Preparing the Paper for the Camera.— For tliis purpose
are i-equiied the two solutions described by j\Ir. Talbot ; namely,
a satui-ated solution of ciystallized gallic acid in cold distilled
water, and a solution of the nitrate of silver of the sirengih of
50 grains to the ounce of distilled water, to which is added one-
sixth part of its volimie of glacial acetic acid. For many
]:)ui'poses these solutions are unnecessarily strong, and, unless
skilfully handled, they are apt to stain or embroA\Ti the paper :
where extreme sensitiveness, therefore, is not required, they may
with advantage be diluted to half the strength, in which state
they are more manageable and nearly as effective. The gallic
acid solution will not keep for more than a few days, and only a
small quantity, therefore, shoidd be prepared at a time. When
these solutions are about to be applied to the iodized paper, they
are to be mixed together, in equal volumes, by means of a gra-
duated di-achm tube. This mixtm-e is called "the gallo-nitrate of
sUver." As it speedily changes, and ■wdll not keep for more than
a few minutes, it must be used without delay, and it ought not
to be prepared untU the operator is quite- ready to apply it.

9. The application of this "gallo-nitrate" to the paper is a
matter of some nicety. It ^v'ill be found best to apply it in the
following manner : — Poiu* out the solution upon a clean slab of
plate-glass, diffusing it over the sm-face to a size corresponding
to that of the paper. Holding the paper by a narrow upturned
margin, the seuaitive side is to be applied to the liquid upon the

MK. cundell's process. 209

slab, and brought in contact with it by passing the fingers gently-
over the back of the paper, which mvist not be touched with the
solution.

10. As soon as the paper is ivetted with the gallo-nitrate, it
ought instantly to be removed into a dish of water ; five or ten
seconds at the most is as long as it is safe at this stage to leave
the paper to be acted upon by the gallo-nitrate ; in that space
of time it absorbs sufficient to render it exquisitely sensitive.
The excess of gallo-nitrate must immediately be washed off by
di"a^ving the paper gently several times under the surface of
water, which mvist be perfectly clean ; and being thus washed,
it is finished by di-a-wTng it through fresh water, two or thi-ee
times, once more. It is now to be diied in the dark, in the
manner described in § 7 ; and, when siu*face-diy, it may either be
placed, while .still damp, in the camera, or in a portfolio, among
blotting-paper, for use. If properly prepared, it will keej) per-
fectly well for four-and-twenty hoiu-s at least, preserving aU its
whiteness and sensibility.

11. The light of a single candle will not injure the paper at a
moderate distance ; but the less the paper, or the exciting solu-
tion, is unnecessarily exposed, even to a feeble candle-light, the
better. Common river or spring water answers perfectly to
wash the paper, distilled water being requii'ed for the silver solu-
tions only.

Stains of "gallo-nitrate," while recent, may be removed from
the fingers by a little strong ammonia, or by the cyanide of
potassium.

The tliu'd process is that of

12. The Exposure in the Camera, for which, as the operator
must be guided by his ovn\ judgment, few directions can be
given, and few are requii-ed. He must choose or design his ovm
subject ; he must detex-mine upon the aperture to be used, and
judge of the time requii-ed, which will vary from a few seconds
to three or four minutes. The subject ought, if possible, to have
a strong and decided efiect ; but extreme lights, or light-coloiu-ed
bodies, in masses, are by all means to be avoided. When the
paper is taken from the camei'a, very little, or more commonly
no trace whatever, of a picture is visible until it has been sub-
jected to the fourth process, which is

13. The Bringing-out of the Picture, which is effected by
again applying the " gallo-nitrate " in the mamier directed in
§ 9. As soon as the paper is wetted all over, unless the pictvu-e
appear immediately, it is to be exposed to the radiant heat from
an iron, or any similar body, held within an inch or two by an
assistant. It ought to be held vertically, as well as the paper ;

210 PRACTICE OF PHOTOGRAPHY.

and the latter ouglit to be movecl, so as to prevent any one part
of it becoming diy before the rest.

As soon as the pictiire is snffieiently brought out, wash it
inxmecliatelj in clean "water to remove the gallo-nitrate, as
directed in § 10 ; it may then be placed in a dish, by itself, ii.nder
water, until yon are ready to fix it. Tha most perfect pictures
ai*e those which '•' come out " before any part of the paper
becomes dry, which they will do if sufficiently impressed in the
camera. If the paper be allowed to diy before washing off the
gallo-nitrate, the lights sink and become opaque ; and if exposed
in the dry state to heat, the paper will embrown ; the diying,
therefore, ought to be retarded, by wetting the back of the
paper, or the picture may be brought out by the vapoiu' from
hot water, or, what is better, a horizontal jet of steam. The
fifth and last process is

14. The Fixing of the Picture, which is accomplished by
removing the sensitive matter from the paper. The picture, or
as many of them as there may be, is to be soaked in warm water,
but not wamier than may be borne by the fiuger ; this water is
to be changed once or twice, and the pictiu-es ai-e then to be well
di-ained, and either diied altogether, or pressed in clean and diy
blotting-paper, to prepare them to imbibe a solution of the
hyposulphite of soda, which may be made by dissolving an ounce
of that salt in a qiiart (forty ounces) of water. Having poiu-ed
a little of the solution into a flat dish, the pictures are to be
introduced into it one by one ; daylight wiU not now injure them;
let them soak for two or three minutes, or even longer if strongly
printed, tm-ning and mo^-ing them occasionally. The remaining
uni'educed salts of silver are thus thoroughly dissolved, and may
now, with the hyposulphite, be entirely removed by soaking in
water and 2)ressrn(/ in clean white blotting-paper alternately;
but if time can be allowed, soaking in water alone vriR have the
effect in twelve or twenty-four hoiu-s, according to the thickness
of the paper. It is essential to the success of the fixing process
that the paper be in the fii-st place thoroughly penetrated by the
hyposulphite, and the sensitive matter dissolved ; and next, that
the hyposulphite compounds be eftectually removed. Unless
these salts are completely removed, they inditce a destiiictive
change upon the pietiu-e ; they become opaque in the tissue of
the paper, and entirely imfit it for the next, which is

15. The Printing Process. — The pictm-e being thus fixed, it
has merely to lie dried and smoothed, when it will undergo no
further change. It is, however, a negative pictm-e, and if it have
cost some trouble to produce it, that trouble ought not to be
gi'udged, considering that you are now possessed of a matrix

ME. cumdell's process. 211

which is capable of yielding a vast niunber of beautiful im-
jDressions. I have had as many as fifty piinted ii*om one, and I
have no doubt that as many more might be obtained from it.

1 6. The manner of obtaining these impressions have been so
often described, and there are so many different modes of
jDroceeding, that it may be siifficient to notice veiy briefly the
best process with which I am acquainted. Photography is
indebted for it to Dr. Alfi-erl Taylor. His solution is made by
dissolving one part of nitrate of silver in twelve of distilled
water, and gradually adding strong Hquid ammonia until the
precipitate at fii-st produced is at length just redissolved.

17. Some paper is to be met with, containing traces of bleach-
ing chlorides, which does not require any previous preparation ;
but in general it will be foimd necessary to prepare the paper
by slightly impregnating it with a minute quantity of common
salt. This may be done by dipping it in a solution in which the
salt can barely be tasted, or of the strength of from thirty to
forty grains to a j)int of water. The paper, after being pressed
in clean blotting-paper, has merely to be cfried and smoothed,
when it will be fit for use.

1 8. The ammonia-nitrate of silver is applied to the paper in
the manner described in § 3 ; and, when perfectly dry, the
negative pictm-e to be coj^ied is to be applied to it, with its face
in contact with the sensitive side. The back of the negative
picture being upj^ermost, they are to be pressed into close
contact by means of a plate of glass ; and, thus seciired, they are
to be exposed to the fight of the sun and sky. The exposed
parts of the sensitive paper will speedily change to lilac, slate-
blue, deepening towards black ; and the light, gradually pene-
trating through the semi-transparent negative picture, will
impiint upon the sensitive paper beneath a positive impression.
The negative picture, or matrix, being, slightly tacked to the
sensitive paper by two mere particles of wafer, the progress of
the operation may from time to time be observed, and stopped
at the moment when the pictvire is finished.

19. It ought then, as soon as possible, to be soaked in warm
water, and fixed in the manner described in § 1 4.

20. In theses pictiu-es there is a cmious and beautifrxl variety
in the tints of colour they M-iU occasionally assimie, varying from
a rich golden orange to pm-ple and black. This effect depends
in a great degree upon the paper itself ; but it is modified con-
siderably by the strength of the hyposulphite, the length of the
time exposed to it, by the capacity of the paper to imbibe it,
and partly, perhaps, by the nature of the light. "Warm sepia-
coloured pictures may generally be obtained by cfrying the

212 PRACTICE OF PHOTOGRAPHY.

paper, by pressure, and maldng it imbibe tbe liyposulpliite
supplied iu liberal quantity.

The paper of " I. Whatman, Tm-key Mill," seems to give
pictures of the finest colour, and, upon the whole, to answer best
for the purpose.

If the chemical agents employed be piu'e, the operator, who
keeps in view the intention of each separate process, and either
adopting the manipulation recommended, or improving upon it
fi-om his own resom-ces, may rely with confidence upon a satis-
factory result.

This calotype paper is so exceedingly sensitive to the influence
of light, that very beautiful photographic copies of lace, feathers,
leaves, and such like articles, may be made by the light of a
common coal-gas flame, or an Argand lamp. The mode of pro-
ceeding is precisely that described for obtaining the ordinary
photographic drawings by daylight, only substituting the calotype
paper, which shoidd be damp, for the common photographic.

When exposing the prepared paper to the light, it should be
held about four or five inches from the flame, and the time
required will be about three minutes.

Bvit little remains to be added to this very clear and satisfac-
tory description of the calotype process, — to which, indeed, is
mainly due the perfection to which it has arrived both at home
and abi'oad.

There are, however, a few modifications which must be noticed,
as tending to simplify the details in some cases, and to improve
the general effects in others. In the main, however, it will be
foimd that Mr. Cimdell's process of manipulation is almost as
good as any that can be adopted : and that gentleman certainly
merits the thanks of the patentee, and of all photographic artists.

Many modifications of Mr. Talbot's mode of manipulating
have been introduced with very variable advantages. I have,
however, found that nearly every variety of paj^er requires some
peculiar method to excite it to its maximiun degree of sensi-
bility. A few of the published methods may be noticed, as,
mider dilferent circumstances, they may prove useful.

Mr. Robert Bingham, who has operated with such success,
adopts the following process : —

Apply to the paper a solution of nitrate of silver, containing
100 grains of that salt to 1 ounce of distilled water. When
nearly, but not quite diy, dip it into a solution of iodide of
potassium, of the strength of 2o grains of the salt to 1 ounce of
distilled water, ckain it, wash it, and then allow it to dry. Now

MODIFIED PROCESSES. 213

Lnish it over with aceto-niti-ate of si]\^er, made hj dissolving
50 grains of nitrate of silver in 1 ounce of distilled water, to
Avhich is added one-sixth its volnme of strong acetic acid. Dry-
it with bibulous paper, and it is now ready for receiving the
image. When the impression has been received, it must be
washed with a satiu-ated solution of gallic acid, and exposed to
a steam heat, a jet of steam from the spout of a tea-kettle, or
any convenient vessel. The image will be gradually brought
out, and may be fixed with hyposulphite of soda. It will be
observed that in this process the solutions of nitrate of silver
and of gallic acid are not mixed before application to the paper,
as in Mr. Talbot's process.

Mr. Chaiming, of Boston, very much simplified the calotype
process. He dii-ects that the paper should be first washed over
with 60 grains of crystallized nitrate of silver, dissolved in 1 ounce
of distilled water, and when dry, with a solution of 10 grains
of the iodide of potassium in 1 ounce of water : it is then to
be washed with watei-, and dried between folds of blotting paper :
the sensibility of the paper is correctly said to be much im-
proved by combining a little chloride of sodium with the iodide
of potassium : 5 grains of the latter salt, and rather less than
this of the former, in an oimce of water, may be employed
advantageously.

To vise this paper of Mr. Channing's, where time is an object,
it is necessary to wash it immediately before it is placed in the
camera obscura, with a weak solution of nitrate of silver, to
which a drop or two only of gallic acid has been added. The
picture is subsequently developed by the gaUo-nitrate of silver,
as already described.

Blanquart Everard, Sagnez, and some others, have recom-
mended that in the preparation of the highly sensitive photo-
gTaphic papers no brushes should be employed. They pursue
the following plan : the solutions are pom-ed upon a perfectly
flat piece "of glass, and the paper carefully drawn over it, and, if
necessary, jjressed closer by another plate of glass.

A plan of iodizing paper has been proposed by , Mr. Jordan,
which offers many advantages. Iodide of silver is pi-ecipitated
from the solution of the nitrate by iodide of potassium, and this
precipitate being lightly washed, is redissolved in a strong
solution of the latter salt. This solution is applied to the paper,
and the paper allowed to chy ; after this it is placed face down-
wards upon some clean water ; the iodide of potassium is
removed by this, and a pure iodide of silver left on the paper.

If the paper carefully and properly iodized is washed with a
very dilute solution of the aceto-nitrate of silver, that is to say,

214 PRACTICE OF PHOTOGRAPHY.

with a solution composed of 10 grains of nitrate of silver to
1 fluid ounce of distilled water, and 10 drops of a concentrated
solution of gallic acid be added to anotlier ounce of distilled
water and the two mixed, it will keep for three weeks or a
month. It may be used dry in the camera, and afterwards
developed with the gallo-nitrate in the usual manner. It will,
however, require an exposure in the camei'a of from ten to
twenty minutes, and is, therefore, only usefid for still objects ;
but for buildings, landscapes, foKage, and the like, nothing can
be more beautifrd.

Le Gray recommends as a highly sensitive paper for portraits
the following : —

Distilled water 6200 grains.

Iodide of potassium .... 300 „

Cyanide of potassium ... 30 „

Fluoride of potassium ... 1 j>

Papers are washed with this, and then ^dth his strong solution
of aceto-nitrate of silver, which is described in the section
devoted to the wax paper j^rocess.

M. A. Martin, who is aided by the Imperial Academy of
Sciences of Vienna in his endeavours to improve the jihoto-
graphic processes, and render them available to the pm*poses of
art, has published the following as the best proportions in which
the solutions should be made, and the order of their apjjlication.

For the negative pictiu'e —

First. Iodide of potassium g oz.

Distilled water 10 fluid oz.

Concentrated solution of cvanide ) „ -,
n . ■ - - 7 droiis.

oi potassium j '■

Second. Nitrate of silver 7 di'achms.

Distilled water 10 fluid oz.

Strong acetic acid 2 drachms.

Third. A concentrated solution of gallic acid.

Fourth. Good spirits of wine.

Fifih. Hyposulphite of soda . . . . 1 oz.

DistUled water 10 fluid oz.

THE ROM.OT PROCESS. 21-5

For the positive pictures —

First. Chloride of sodium .... 168 grains.
Distilled water 10 oz.

Second. Nitrate of silver . . . . 1 oz.

Distilled water 10 oz.

Third. Hjrposulphite of soda .... 1 oz.

Distilled water 40 oz.

Nitrate of silver 30 grains, dissolved in ^ oz. of distilled
water, to be pouied into the solution, in a small stream, while
it is constantly stnred with a glass rod.

Martia particularly recommends the application of the iodine
salt fii'st to the paper, drying this, then applying the argentine
solution, and drying rapidly. I have m-ged the necessity of
this on several occasions : the advantages are, that the iodide of
silver is left on the very sm-face of the paper ready for the
influence of the sHghtest chemical radiation.

The prodiictions of M. Flacheron, which were seen in the
Great Exhibition, excited much interest, and the process by
which these were obtained in the Eternal City was eagerly
sought for by photogi-aphic amateiu-s. In the Art Journal for
May, Mr. Thomas has communicated the process by which the
photographers of Rome pi'oduce their best eflects ; and as this is
important, as being useful in hot climates, a sufficient portion
of that communication is transfen-ed to these pages.

"\st. Select old and thiu Euglish paper — I prefer "Whatman's:
cut it in such a maimer that the sheet shall be the sixteenth of
an inch smaller than the glass of the paper-holder on every side,
and leave two ends at diagonal comers to the sheet by which to
handle it.

" 2dly. Prepare the following solution : —

"Saturated solution of iodide of potassiiun 2| fluid di-achms:
pure iodide 9 grains: dissolve.

"Then add, distilled water 11^ ounces, iodide of potassiimi 4
drachms, bromide of potassium 1 gi'ains, and mix. Now, filter
this solution into a shallow porcelain vessel somewhat lai'ger
than the sheet of paper to be prepared. Take a piece by the two
diagonal ends, and gently place the end of the marked side
nearest to you, upon the surface of the bath ; then carefully
incline the surface of the sheet to the liquid, and allow it to rest
two minutes ; if French paper, one minute, or until the back of

216 PRACTICE OF PHOTOGRAPHY.

tlie paper (not \rettecl) becomes tiiited iinifomily by the action
of tbe dark-coloiu'ed solution. Raise it up by means of the two
ends occasionally, in order to cliase away any au*-bubbles, which
would be indicated by white spots on the back, showing that the
solution in these parts has not been absorbed. Hold the paper
by one of the ends for a minute or so, in order that the siiper-
fluous moistui"e may run off, then hang up to diy, by pinning
the one end to a string run across a room, and let the excess di'op
off at the diagonal corner. When dry the paper is ready for
use, and quite tinted with iodine on both sides. It will keep
any length of time, and is much improved by age.

" 2idly. I T\T.ll presume that four sheets are to be excited for
the camera, and that the operator has two double paper-holders,
made without a wooden partition, the interior capacity of which
is sufficiently large to admit of three glasses, all movable. The
third, as will be seen, is to prevent the two pieces of excited
paper coming in contact with each other.

" Prepare the following solution : —

'• Take nitrate of silver 2^ di-achms ; acetic-acid 4| drachms ;
distilled water 3^ omices : mix and dissolve.

" Now take four of the glasses of the liaper-holders perfectly
clean, and place each upon a piece of common blotting-paper to
absorb any little excess of liquid. Pom" about 1^ drachms, or
rather more of the solution just prepared, into a small glass
funnel, into which a filter of white bibulous paper has been
placed, and let the solution filter drop by di-op upon glass No. 1,
imtil about 1| drachms have been filtered in detached drops,
regularly placed upon its siu'fice : then, with a sHp of jDaper,
cause the liquid to be diffused over the whole siu-face of the
glass. Take a piece of prepared paper, and place it marked, side
downwards, uj)on a glass just prepared, beginning at the end
nearest you, and thus chasing out the air. Draw it up once or
twice by its two diagonal corners ; allow it to rest, and jirepare
glass No. 2 in a similar manner. Now look at glass No. 1, and
it will be pez'ceived that the \-iolet tint of the paper has become
mottled ^vith patches of white, which gradually spread, and in a
few seconds the paper resumes its original whiteness, which is an
indication that it is ready for the camera. It ^vill be found to
adhere firmly to the glass. Do not remove it ; but hold the
glass np to allow the excess of fiuid to iim off at one comer. It
must not be touched with blotting-paper, but replaced flat on
the table. Sex-ve Nos. 2, 3, and 4, in like manner. Take foiir
pieces of common white paper, not too much sized, free from ii'on
spots, and cut a trifle smaller than the prepared sheet ; soak
them in distilled water ; draw out one piece, hold it up by the

ME. muller's process. 217

fingers to drain off suiierfluous moistiire, and place it gently upon
the back of the prepared paper. AVith another piece of glass
kept for the purpose, having the edge rounded, and large enoixgh
to act uniformly upon the paper, scrape off gently the excess of
liquid, beginning at the top of the sheet, and removing viith the
rounded edge of the scraper the liquid to one of the corners.
Repeat this operation t^^-ice. Both the excited and superim-
posed paper are thus fixed to the glass. • Two glasses and papers
being thus prepared, take the clean glass Ko. 5, and place upon
No. 1 : press gently : the moist paper will cause it to adhere.
Take up the two glasses thus afiixed and place them upon glass
'No. 2, in such a manner that the supernumeraiy glass No. 5
shall be in the centre. The whole will form a compact body,
and having polished the sm-faces, and wiped the edges, may at
once be put in the paper-holders. * * *

" ithly. With a Ross's, ChevaKer's, or Lerebours' single lens,
three inches diameter, and half an inch diaphi-agm, the object
to be copied, well lighted by the sun, the paper will require from
four to six minutes' exposure.

" 5thly. Take out the three glasses, which will still firmly
adhere, sepai-ate them gently, and remove the piece of moistened
paper, which must not be vised again. Now lift up the prepared
paper by one comer to the extent of half the glass, and pour
into the centre about one drachm of a satiu-ated solution of
gaUic acid, which will immediately difluse itself Raise also the
other corner to facilitate its extension ; and sei-ve the othei-s in
like manner. The image takes generally from ten to twenty
minutes to develope. Hold iip the glass to a candle to watch
its intensity. When sufficiently developed remove the negative
from the glass. Wash it in two or thi-ee waters for a few hours,
(by with blotting-paper, and immerse each separately for ten
minutes in a bath of bromide of potassium in solution : then
wash and dry. ,

" The iodide may be removed by means of hyposulphite of soda
in the usual way, twelve months afterwards, or when convenient.
If," says Mr. Thomas, " the process has been carefidly conducted,
four beautiful negatives must be the result. I was ten days
working incessantly at Pompeii, and scarcely ever knew what a
failure was."

IVIr. MuUer, a gentleman who has been practising photography
with great success at Patna, in the East Indies, has communi-
cated his process to me, which is as follows : —

A solution of hydi'iodate of iron is made in the proportions
of 8 or 10 grains of iodide of iron to 1 ounce of water; this

218 PRACTICE OF PHOTOGRAPHY.

solution is prepared in the ordinary way, with iodine, iron turn-
ings, and water. The ordinary paper employed in photogpraphy
is washed on one side with a solution of nitrate of lead (15 grains
of the salt to 1 ounce of water) ; when dry, this paper is iodized
either by immersing it completely in the solution of the hydi-io-
date of iron, or by floating the leaded sm-face on the solution.
It is removed after a minute or two, and lightly di-ied with
blotting-paper. The paper now contains iodide of lead and j)roto-
nitrate of iron : while still moist it is rendered sensitive by a
solution of nitrate of silver (100 grains to the ounce of water)
and j^laced in the camera. After the ordinaiy exposui'e it may
be removed to a dark room ; if the image is not ah-eady deve-
loped, it will be found speedily to appear in great sharpness
without any further application. It may then be fixed with the
hyposulphite of soda in the usual manner.

In December 1852 Sir John Herschel communicated to the
pages of the AthencEum a letter from his brother-in-law, Mr.
Stewart, a resident at Pau, in the Pyrenees. In this he states,
that, at the stiggestion of Professor Regnault, he was induced to
adopt a process of manipulation which gave some charming re-
sults, and which he thus describes : —

" The following observations are confined to negative paper
processes divisible into two — the toet and the dry. The solu-
tions I employ for both these processes are identical, and are as
follows : —

Solution of iodide of jjotassium, of the strength of 5 parts of
iodide to 100 of piu-e water.

Solution of aceto-nitrate of silver, in the following proportions :
— 15 parts of nitrate of silver, 20 of glacial acetic acid, 150 of
distilled water.

Solution of gallic acid for developing — a satiu'ated solution.

Solution of hyposulphite of soda, of the strength of 1 part of
the salt to from 6 to 8 parts of water.

For both the wet and dry process I iodize my paper as fol-
lows : — In a tray containing the above solution I plimge, one by
one, as many sheets of pajier (twenty, thirty, fifty, &c.) as are
likely to be required for some time. This is done in two or
three minutes. I then roll up loosely the whole bimdle of sheets
while in the bath ; and picking up the roll by the ends, ch'op it
into a cylindrical glass vessel with a foot to it ; and pour the
solution therein (enough to cover the roU completely) ; in case it
should float up above the surface of the solution, a little piece
of glass may be pushed down to rest across the roll of paper and

MR. Stewart's process. 219

prevent its rising. The vessel with the roll of paper is placed
under the receiver of an air-pump and the air exhausted ; this is
accomplished in a very few minutes, and the paper may be left
five or six minutes in the vacuum. Should the glass be too
high (the paper being in large sheets) to be inserted under the
pneumatic pump-receiver, a stiff lid lined with India-rubber,
with a valve in the centre communicating by a tube with a
common dii'ect-action air-pump may be employed with equal
success. After the paper is thus soaked in vacuo it is removed,
and the roll dropped back into the tray with the solution, and
thus sheet by sheet picked off and hung up to dry, when, as "svith
all other iodized paper, it will keep for an indefinite time.

"I cannot say that I fidly imderstand the rationale of the action
of the air-pump, but several valuable advantages are obtained by
its use : — 1st, The paper is thoroughly iodized, and with an equa-
lity throughout that no amount of soaking procures, for no
two sheets of paper are alike, or even one perfect thi'oughout in
texture, and air-bulbs are impossible. 2d, The operation is
accomplished in a qtiarter of an hoiu", which generally occupies
one, two, or more hoxu'S. 3d, To this do I chiefly attribute the
fact that my paper is never solarized even in the brightest sun ;
and that it will bear whatever amoiuit of exposure is necessary
for the deepest and most impenetrable shadows in the view
without injuiy to the bright light."

" Wet Process. — To begin with the %vet process. Having pre-
pared the above solution of aceto-nitrate of sdver, float a sheet of
the iodized paper upon the sm-face of this sensitive bath, leaving
it there for about ten minutes. During this interval, having
placed the glass or slate of your slider quite level, dip a sheet of
thick clean white printing (imsized) paj^er in water, and lay it
on the glass or slate as a wet lining to receive the sensitive
sheet. An expert manipulator may then, removing the sensi-
tive sheet from the bath, extend its sensitive side uppei-most on
this wet paper lining, without allo-\ving any air-bubbles to inter-
vene ; but it is difficult ; and a very simple and most effectual
mode of avoiding au'-globules, particulai-ly in handhng very large
sheets, is as foUows: — Pom' a thin layer of water (just sufficient
not to flow over the sides) upon the lining paper after you have
extended it on your glass or slate, and then lay down youi* sen-
sitive paper gently, and by degrees, and floating, as it were, on
this layer of water, and when extended take the glass and
papers between the finger and thumb by an upper corner, to
prevent their slipping : tilt it gently to allow the interjjosed
water to flow off by the bottom, which will leave the two sheets
adhering closely and perfectly, without the slightest chance of

220 PEACTICE OF PHOTOGRAPHY.

air-bubbles ; it may then be left for a minute or two standing
ixpright in tbe same position, to allow every drop of water to
escape ; so that when laid flat again, or placed in the slider, none
may return back again and stain tbe paper. Of course tbe
sensitire side of tbe sheet is thus left exposed to the uninter-
rupted action of the lens, no protecting plate of glass being
interposed ; and even in this dry and warm climate, I find the
humidity and the attendant sensitiveness preserved for a couple
of hours.

'• Dry Process, — In preparing sheets for use when cZri/ for tra-
velling, &c., I have discarded the use of previously waxed paper,
thus getting rid of a troublesome operation, and proceed as fol-
lows : — Taking a sheet of my iodized paper, in place of floatiag
it (as for the wet process) on the sensitive bath, I plunge it faMy
into the bath, where it is left to soak for five or six minutes ;
then removing it, wash it for about twenty minutes in a bath,
or even two of distilled water, to remove the excess of niti-ate of
silver, and then hang it up to dry (in lieu of drying it Avith
blotting-paper.) Paper thus prepared possesses a greater degree
of sensitiveness than waxed paper, and preserves its sensitive-
ness, not so long as waxed paper, but sufficiently long for all
practical pui'poses, say tliirty hour.s, and even more. The Eng-
lish manufactiu'ed paper is far superior for this purjDose to the
French. To develope these views a few drops of nitrate of silver
ai"e requii-ed in the gallic acid bath, and they are finely fixed and
waxed as usual."

It will be apparent to the carefid reader, that all the processes
given, are in all essential particulars the same. To a few simple
alterations in the manipiilatory details are due all the variations
in sensibility, and in the general eifect of the residting pictiu-e.

The main principles ai-e : —

1st. To iodize the paper — that is, to secure a uniform coating
of iodide of silver over every part of the surface of the paper,
and an entii-e absence of either the alkaline or the metallic
nitrate.

2nd. To have an excess of nitrate of silver spread over the
iodide a short time previously to using the paper ; and if a high
degree of sensibility is requu-ed, the combination of such an
organic decomposing agent as gallic acid.

In all the photographs obtained by this process, the impres-
sions from gi-een leaves are very imperfect. Tliis is only to be
obviated by adopting the advice of Herschel, and substituting
the bromide for the iodide of silver. The following remarks are
so much to the point, that I have transferred them from the
Journal of the Photographic Society : —

ME. Stewart's process. 221

" I hare read, " says Sir J. F. W. Herschel," with considerable
interest the remarks of Sir W. J. Newton, On Photography in,
its Artistic view, d-c, conimnnicated to the Photographic Society,
and by them printed in the Journal of their Society (Xo. 1).
These remai'ks are in perfect consonance with my own impres-
sions as to the absence, in the veiy best photographic landscape
I have seen, of a true artistic representation of the relative in-
tensities of light and shade, the consequence of which is usually
a most painfid want of keeping ; — a straggle to come forward of
parts which natiu'e s\;ppresses, and a want of working ov.t, in
features which, to the eye, are palpably distinct.

" Sii' W. J. Newton strikes with the true eye of the painter
on the more prominent evil of the whole case ; he sa}^ (p. 6),
' Wondei-fid as the powei-s of the camera are, we have not yet
attained that degree of perfection as to represent faithfully the
effect of colours, and consequently of light and shade. For in-
stance, a bright red or yellow, which woidd act as a light in
natirre, is always represented as a dark in the camera, and the
same with green. Blue, on the contrary, is always lighter.
Hence the impossibdity of representing the true efiect of nature,
or of a pictui-e, by means of photography.'

" No one can have viewed the exquisite pictures of M. Eeg-
naidt or Mr. Stewart, ^\-ithout feeliug that vegetation is unduly
black and wanting in artistic production. No one who has
studied colours, not as an artist, but as a photologist, can for a
moment be ignorant of its cause. The red and yellow rays, and
especially the former, xchich form so large a portion of vegetable
greens, are suppressed. They affect not the materials at present
used in the photogi^aphic art in its highest development.

" One word suffices for the key of the difficulty — Iodint:. It is
to the practically exclusive use of this element that the whole
evil complained of (most justly) is attributable. T have shown
(see my papers, Phil. Trans. 1840, 1842, Articles 129, 217, and
' On the Action of the Rays of the Solar Spectrum on the Da-
guerreotype plate,' Load. Ed. and Dv.hl. Phil. Journal, 184.3,
Art. xix.) that iodized silver is insensible, or nearly so, to the
red and orange rays — that its range of sensibility begins, with
astonishing abruptness, beyond the medium yellow and within
the blue region — is powerful at the indigo, and extends fai- into
rays which have absolutely no effect in producing vision. No
wonder then that iodine produces pictm-es imsatisfactory to the
artistic eye. Iodine then mxist be thl■o^^^l overboard or Hmited
in its use, coute qui coute (and the sacrifice is a formidable one),
if photogi-aphy shaU ever satisfy the desires of the artists.

" What then are we to have recom-se to ? Bromixe. A new

222 PRACTICE OF PHOTOGRAPHY.

photograpliy has to be created, of wliicli bromine is tlie basis.
This I have proved in. my experiments on this substance {Phil.
Trans. 1840, Art. 77 ; also Art. xix. Lond. Ed. d-c. Journal,
above cited). The action of every luminous ray, so far as can
be traced, is equable throughout the spectiiim ; but the I'ays
beyond the luminous ones act powerfully, and these must be eli-
minated. A glass screen, with a very slight yellow tinge applied
close to the focal picture, oi-, still better, a glass cell, with opti-
cally true surfaces, containing a weak sokition of sulphate of qui-
nine, according to the recent results of Prof. Stokes, will effec-
tually cut oif these, and reduce the action of the rays within the
limits which ai"t recognizes. I believe M. Becquerel has used
the latter liquid with a similar view. I will only add, that it
were much to be wished that artists would study the siJectrum
and its habitudes in relation to theii* pigments."

Section II. — Calotype Process on Gelatine or Albumen.

The use of organic matter in facilitating the change of the
silver salts very early engaged the attention of Sir John Herschel ;
and from time to time, following his sxiggestions, others have
employed various organic mattei-s, albumen and gelatine being
the favouiite substances. These have been principally used for
the pm-j)Ose of spreading photogra^ihic preparations on glass,
which we shall have particularly to describe : at the same time,
they are stated to have been employed with much advantage
on paper by some photographists. For the negative pictm-es,
Gustave Le Gray gives us the following directions and parti-
cular information : —

First Operation. — Dissolve three hundred grains of isinglass
in one pint and three quarters of distilled water (for this purpose
use a water bath).

Take one-half of this preparation while warm, and add to it
as under : —

Iodide of potassium 200 grains.

Bromide of ditto 60 „

Chloride of sodium 34 „

Let these salts be well dissolved, then filter the solution through
a piece of linen, put it, still warm, in a large dish, and plunge
in your paper completely, leaf by leaf, one on the other, taking
care to prevent the air-bubbles fi-om adhering to the paper.

Put about twenty leaves at a time into the dish, then turn
the whole, those at the top to the bottom, then take them out

CALOTTTE PROCESS OX GFXATIST: OR ALBrilEX. I'l'o

one by one, and Lang them by one comer vrixh a jiin bent like
the letter S, to dry spontaneously.

When hung up, attach to the opposite comer a piece of
bibulous paper, which vnR facilitate the drying.

WTien the piper is dry cut it the .size required, and preserve
it in a folio for use ; this paper may be made in the day-time, as
it is not sensitive to light in this state.

The bromide does not, in this case, act as an accelerator, as it
does on the silver plates of the daguerreotype, because, instead
of quickening, it retards the operation a little ; its action is to
preserve from the gallic acid the white of the paper, which
would blacken more rapidly if you employed the iodide of
potassium alone.

Second Operation. — Prepare, by the light of a taper, the fol-
lowing solution in a stoppered bottle : — Distilled water, 6 fluid
oiinces; ciystallized nitrate of silver, 250 grains.

When the nitrate is dissolved, add 1 ounce of crystallizable
acetic acid : be careful to exclude this bottle from the light, by
covering it with bhick paper. This solution will keep good until
the whole is used.

When you wish to operate, pour the solution upon a porcelain
or glass slab, sun-ounded with a glass or paper border to keep
the liqidd fr-om running off. I usually take the solution out
of the bottle by means of a pipette, so as to prevent the distri-
bution of any pellicle of dust or other impuiity over the glass
skb.

Take a sheet of the iodized paper by two of the comers,
holding them perpendicularly, and gently lower the middle of
the paper upon the centre of the slab ; gradually depress until
the sheet is equally spread j repeat this operation several times
tmtil the air-bubbles disappear ; take also the precaution to keep
the upper side of the paper dry.

In order to prevent the fingers from spotting the paper, pass
a bone paper knife under the comer of the sheet, to lift it from
the slab between that and the thumb.

Let the sheet remain upon ths slab until the fomiation of the
chloro-bromo-iodide of silver is perfect.

This may be known by the disappeai-ance of the violet colour
which the back of the paper at fii^st presented ; it must not be
left longer, otherwise it would lose its sensitiveness.

The time required to effect this chemical change is from one
to five mi nutes, depending upon the quahty of the paper.

Spread upon a glass, fitted to the fi-ame of the camera, a piece
of white paper well soaked in water ; upon this place the pre-
pared sheet, the .sensitive side upwards.

224: PRACTICE OF PHOTOGRAPHY.

Tlie paper wliich you place imdei'neath must be free from
spots of ii'on and otlier impm-ities.

It is also necessary to mark tlie side of the glass "syhicli ouglit
to be at the bottom of the camera, and to keep it ahyays in-
clined in that dii'ection when the papers are applied ; if this
precaution is neglected, the liquid collected at the bottom, in
falling oyer the prepared paper, would not fail to produce spots.
The paper thus applied to the glass "vnll remain there for an
hour -without falling off, and can be placed within that time in
the camera.

When I am going to take a proof at a distance, I moisten the
sheet of lining paper with a thick solution of gum ai-abic, and
can thus preserye for a longer time its humidity and adhesion. I
can also in this case make use of two glasses between which the
paper is placed, according to the dii-ection of M. Blanquai-t Eyer-
ard ; but it is necessaiy to take great care that the plates of glass
are perfectly clean, and to haye them re-polished if scratched.

I employ for this pui'pose blotting-paper to clean them, as
well as my plates : it is much superior to linen, and absorbs
liquids and impm-ities that adhere to it. I neyer spare the
blotting-paper, for I would rather use a leaf too much than be
uncertain about the cleanness of my glass.

When fhe sheet of lining paper adheres well to the glass, it
should not be remoyed, but only moistened afresh with water,
after which you may apply another sheet of the sensitiye paper.

In preparing seyeral sheets of the sensitiye paper at a time, it
is not necessary to wash the slab for each sheet ; you need only
draw oyer it a piece of white paper to i-emoye any dust or pellicle
formed.

When yom- operations are finished, you may pom- back the
aceto-nitrate of silyer into a bottle, and reserye it for another
time.

The necessity of employing M. Le Gray's papers in a wet state
is their most objectionable quality, but certainly the results ob-
tained by strict attention to his directions are often exceedingly
beautiful. For deyeloping the image the following is recom-
mended, which does not, howeyer, differ essentially fi*om the
deyeloping processes afready described.

Make about a pint bottle of satimited solution of g-allic acid,
haying acid in excess, and using distilled water ; decant a por-
tion into a smaller bottle for general iLse, and fill up the other
bottle ; you will thus always haye a clear satiu-ated solution.

Pour upon a slab of glass, kept horizontal, a Little of this
liquid, spreading it equally with a slip of paper, then apply the
paper which has been exposed in the same manner as described

CALOTYPE PROCESS OX GELATINE OR ALBTJJIEK. 225

for the negative paper, being careful to keep the back cliy.
Watch its development, which is easily observed thi-ough the
back of the paper ; you may leave it thus as long as the back of
the image does not begin to spot.

When it is rendered veiy \igorous, remove it quickly to
another clean slab, and well wash it in several waters, occa-
sionally turning it, and gently passing the finger over the back;
by this means you remove any ciystals of gallic acid which
might spot the picture.

The appeai-ance of the image at the end of this process will
enable you to judge if it was exposed in the camera the proper
time.

If it becomes z, blueish grey all over, the paper has been ex-
posed too long ; if the strongest lights in the object, which
should be very black in the negative, are not deeper than the
half-tints, it has stiU been too long exposed; if, on the contrary,
it has been exposed too short a time, the lights ai'e but slightly
marked in black.

If the time has been just right, you will obtain a proof
which will exliibit well-defined contrasts of black and white, and
the Kght pai-ts will be very ti-anspai-ent. The operation is
sometimes accelerated by heating the galHc acid, and by this
process the dark parts of the picture are rendered very black.

To fix these negative proofe, a very strong solution of hypo-
sulphite of soda, — about 1 ounce of the hyposulphite of soda to 8
fluid ounces of water, — is employed, and the picture is allowed to
remain in it untU every trace of yellowness is removed from the
paper.

The VLse of albumen on paper has not been extensive, nor do
I conceive that it offers any peculiar advantages. The best
mode of proceeding is to beat the white of eggs into a froth ;
then set the fluid aside, at rest, and, when perfectly cleai', make
with it the following solution : —

White of Eggs, 2 fluid ounces and a half.
Iodide of Potassium . . .56 grains.
Bromide of Potassium . . .15^ „
Chloride of Sodium ... 4 „

Pour the solution into a dish placed horizontally, taking care
that the froth has entirely disappeared ; then take the paper
that you have chosen, and wet it on one side only, beginning at
the edge of the dish which is nearest to you, and the largest side
of the sheet, placing the right angle on the liquid, and inclining
it towards you ; advance it in such a manner as to exercise a
pressm-e which will remove the au-bubbles. Place before you a

22Q PRACTICE OF PHOTOGRAPHY.

liglit, SO as to be able to perceive tbe bubbles, and to push
tliem out if tliey remain.

Let tlie leaf imbibe for a minute at most, without touching it ;
then take it up gently, but at once, with a very regular move-
ment, and hang it up by the corner to diy.

You prepare thus as many leaves as you wish in the same
bath, taking care that there is always about a qxiarter of an inch
in depth of the solution in the dish; then place your sheets
(thus prepared and dried) one on the other between two leaves
of white paper, and pass over them several times a very hot iron,
taking out a leaf each time ; you will thus render the albumen
insoluble.

The iron should be as hot as it can be without scorching the
paper.

Then use this negative paper exactly like the first paper
named ; only great attention must be obsei-ved that the im-
mersion in the aceto-nitrate bath is instantaneous, and that
the air-bubbles are immediately di'iven out ; for every time you
stop you will make stains on the paper. It is also necessary to
warm moderately the gallic acid.

One of the best services rendered by the albumen to photo-
graphy is, without doubt, its application to the preparation of the
positive paper, to which it gives a brilliancy and vigom- difficidt
to obtain by any other method. It is prepared thus : —

Take white of eggs, to which add the fifth part by volume of
saturated solution of chloride of sodium ; then beat it into a
frotli, and decant the clear liquid after it has settled for one
night.

With this the paper is first washed, and then a strong solution
of nitrate of silver applied.

The following is the process adopted by M. Blanquart
Everard : —

Method of i^reparing i^aper with albumen, so that it may he
employed dry. — The paper prepared by means of albumen pos-
sesses properties analogous to those prepared by means of serum,
but in a much less degree ; the former, like the latter, may be
kept for an indefinite time after its preparation with the iodide
of potassium, but after having been submitted to the action of
the aceto-nitrate of silver, it "wt.11 not keep good beyond the next
day. The imi^ressions obtained by means of the following pre-
pax-ation are adnm-able : though not so well-defined as those on
glass, yet they are more beautiful, as the outline is less harsh,
and they possess more harmony and softness. It must not be
forgotten that the preparations "with albumen change with
comparative slowness.

CALOTYPE PROCESS OX WAXED PAPER. 227

Beat into a froth the whites of eggs, to which a satm-ated
sohition of iodide of potassium and bromide of potassiiun Las
been added in the j^roportion of thii-ty di'ops of the former and
two drops of the latter for the white of each egg ; let the mix-
ture stand until the froth returns to a liquid state, filter thi'ough
clear muslin, and collect the albtimen in a large flat vessel. On
this lay the paper to be prepared, and allow it to remain there
some miniites. When it has imbibed the albumen, lift it up by
one of its coniers ; let it it di-ain, and lastly dry, by suspending
it with pins to a hne or cord across the room. The subsequent
}3reparation with the aceto-nitrate of silver is in eveiy i-espect
similar to that for the ordinaiy Calot}q;je paper; care being
taken not to dry it between the two folds of blotting-paper
until it has become perfectly transparent. The exposure of the
pre^iared paper to the light in the camera is done in the same
way, and the same treatment with gallic acid is followed: it
will, however, be foimd that the time required for exposure
"will generally be four or five minutes.

Preparation of albuminous paper for receiving a positive image.
— The positive paper prepai'ed with albumen gives impressions
somewhat shining, but of a very rich tone, well defined, and of
perfect transparency ; it is prepared in the following manner : —
To any quantity of white of eggs add 25 per cent, by weight of
water, saturated with cliloride of sodium ; beat into a froth, and
filter as in the previous operation — only in this case leave the
paper in contact with the albumen for half a minute ; hang it
ujj to dry, which it usually does in six to eight minutes; then
lay it on a vessel containing a solution of lb parts of nitrate of
silver in 100 parts of water. Leave the paper on the solution
for at least six minutes, then place it on a plate to dry.

The serum of milk has also been employed on paper as a
quickening agent, and some of the French authorities speak
highly of it ; but I am not enabled, from my own experience, to
speak of its advantages.

Section III. — Calotype Process on Waxed Paper.

The most successful operators with waxed paper have been
M. Le Gray on the Continent, and Mr. Fenton in England. In
a work lately published by Le Gray, he has entered into the
question of the physical agencies which are active in producing
the chemical changes on the various preparations employed.
Throughout the essay, he evidently labours imder an entii'e

228 PRACTICE OF PHOTOGRAPHY.

misconception of tlie wliole of the phenomena, curiously enough
giving a false interpretation to every fact His manipulatory
details are very pei-fect, but his scientific explanations are only
so many soiirces of error.

First Process : To Wax the Paper,— This process divides itself
into several parts, waxing the jxiper being the first. For this pur-
pose he takes the paper prepared by Lacroix d'Angouleme, or
that of Canson brothers of Annouay. A large plate of silvered
copper, such as is employed for the daguerreotype, is obtained
and placed upon a tripod, with a lamp underneath it, or upon a
water-bath. The sheet of paper is spread upon the silver plate,
and a piece of pui-e white wax is passed to and 6'o upon it until,
being melted by the heat, it is seen that the paper has unifonnly
absorbed the melted wax. When this has thoroughly taken
place, the paper is to be placed between some folds of blotting-
paper, and then an iron, moderately hot, being passed over it,
the bibulous paper removes any excess of wax, and we obtain a
paper of perfect transparency.

Second Process; To Prepare the Negative Paper. — In a
vessel of porcelain or earthenware capable of holding 5 pints and
a quarter of distilled water, put about 4000 gTains of rice, and
allow them to steep until the gi'ains are but slightly broken, so
that the water contains only the glutinous portion. In a little
less than a quart of the rice solution thus obtained, dissolve : —

Sugar of Milk 620 grains.

Iodide of Potassium .... 225 „

Cyanide of Potassium ... 12 „

Fluoride of Potassium ... 7 „

The liquid, when filtered, will keep for a long time without
alteration.

When you would prepare the paper, some of tliis solution is
put into a large dish, and the waxed paper, sheet by sheet, is
plunged into it, one over the other, removing any aii'-bubbles
which may form. Fifteen or twenty sheets being placed in the
bath, they are allowed to soak for half an hour, or an hour, ac-
cording to the thickness of the paper. Tm^ning over the whole
mass, commence by removing the first sheet immersed, and
hooking it up by one corner with a pin bent in the shape of the
letter S, fix it on a line to dry, and remove the drop from the
lower angle by a little bundle of blotting-paper. M. Le Gray
then remarks that French and English paper slioidd never be
mixed in the same bath, but prepared separately, as the "English
paper contains a free acid, which immediately precipitates an
iodide of starch in the French papers, and gives to them a violet

CALOTYPE PROCESS ON WAXED PAPER. 229

tint." The paper being diy is to be preserved for use in a
portfolio.

Third Process : To render the Waxed Paper Sensitive. —
Make a solution of

Distilled water 2325 gi-ains.

Ciystallized niti-ate of sUver . . 77^ „
and when this is dissolved, add of

Ciystallized acetic acid -. . . 186 grains.

Papers prepared with this solution will keep well for a few
days. M. Le Gray, however, recommends for his waxed paper,
and for portraits, that the quantity of nitrate of silver be in-
creased to 15-5 gi'ains, and that it should be used moist.

The method of preparing these papers is to float upon an
horizontal plate of glass either of the above solutions, and taking
a piece of the iodized j^aper, to carefully place it upon the fluid,
taking gi'eat care that no air-bubbles interpose. The paper must
remain a short time in contact with this sensitive fluid until
chemical combination is effected. Four or five minutes are
requii'ed for some papei-s, and eight or ten seconds are sufficient
for other kinds. When a violet tint appears the paper should
be removed.

For those papers which it is desirable to keep for some time,
as during a journey, it is recommended that into one vessel of
porcelain you put about five or six miUiHtres of the strong aceto-
nitrate above described, and into another some distilled water;
you plunge completely both sides of the waxed and iodized paper
in the fii'st fluid, and allow it to remain about foiu- or five
minutes ; withdi-aw it, and plunge it immediately into the bath
of distilled water, in which let it soak for not less than four
minutes. When these papers are carefully di-ied they may be
l)resei"ved for some time for use, and by lessening the dose of
nitrate of silver this period may be considerably prolonged. It
viill of coui-se be understood by all who have followed the pro-
cesses described up to this point, that the jDapei-s which are pre-
j)ared for keeping are not those which are the most sensitive ;
hence it is necessary to expose such a much longer time in the
camera than those prepared by the stronger solution of silver.
The more sensitive variety, imder ordinary cii'cumstances of
light, vnR require an exposiu'e in the camera of about 20 seconds,
the less sensitive demanding about 10 or 15 minutes, according
to the cii'cumstances of light.

Fourth Process : The Development of the Image. — The pic-
tiu'e is developed by the aid of gallic acid dissolved in distilled

230 PRACTICE OF PHOTOGRAPHY.

water. Le Gray finds the folloAving to be tlie best propor-
tions : —

Distilled water 40 fluid ozs.

Gallic acid 60 grains.

The paper is to be plunged into this solution, and allowed to
remain until it is fidly developed. The time "svoll vaiy from ten
minutes to two hoiu's or more, according to the intensity of the
rays incident on the paper when in the camei'a. The develop-
ment of the image is much accelerated by the addition of 15 or
20 ch'ops of the aceto-nitrate of sUver.

Fifth. Process : Fixing. — It is fomid convenient often, when
on a journey, to give a temporary fixedness to the pictui-es
obtained, and to complete the process "with the hyposulphite at
any time on youi* return home. A wash of 360 grains of bromide
of potassiiun to two quarts of water is the strength which should
be employed. The process of fixing with hy]50sulj)hite consists,
as in other pi'eparations, simply in soaking the paper imtil the
yellow tint of the iodide has disappeared ; the details are parti-
cularly given in the chapter on Fixing Photographs.

CHAPTER YI.

THE DAGUERREOTYPE.

In the first di'S'ision of this work, all the details of the original
processes are given with considerable minuteness, and the vig-
nette heading to that section exhibits all the ajipai-atvis required
for even the improved modern practice.

Section T. — Daguerre's Improved Manipulation.

The following remarks bj ]\I. Daguerre on polishing and pre-
paring the plates, from the Comptes Eendus of March 13, 1843,
shoidd be carefully attended to, as the preliminary process upon
which the success of every subsequent state depends.

" Since the publication of my process, I have not been able
to occupy myself much with it. The investigations to which I
devoted myself have been in an entirely new dii-ection, and the
experiments which they reqinred were analogous with the pre-
ceding ones, only inasmuch as they were made on a metallic
plate. However, I have lately been so much stmick with the
\mequal resxdts which the impressions generally present — even
those of persons who are especially occupied with them — that I
determined to seek some means of remedying this serious incon-
venience, which I attribute to two principal causes.

" The fii'st relates to the operation of polishing, which it is
physically impossible to effect without leaving on the siu-face
of the plate traces of the liquid and of the other substances
used in this operation : the cotton alone which is employed,
however clean it may be, is sufficient to leave a film of dirt on
the silver. This first cause constitutes a very gi-eat obstacle to "^
the success of the impression, becaxise it retards the photogenic I
action by preventing the iodine from coming in direct contact /
with the silver.

"The second consists in the altemtions of the temperatiu'e of
the air with wliich the plate is in contact, fr'om the tu-st opera-
tions to the mercurial operations. It is kno"\\Ti that when a cold
body is sm'rounded with warmer air it condenses its moistiu'e.
To this effect must be attributed the difficidty which is expe-

232 PRACTICE OF PHOTOGRAPHY.

rienced in operating in a humid medium, especially when we
come to the mercurial operation, wliich requires, to raise a suit-
able vapour, a temperatvxre of 122° F.

" This vapour, which first heats the air contained in the
apparatus, produces on the metal a dew which weakens the
image. It is very evident that this humid layer is very injui-ious ;
since if, for example, the plate, on leaving the camex-a obscui-a,
be breathed on two or tliree times, the mercurial vapour can no
longer cause the impression to apj)ear.

" The water which is condensed, even at the slightest difference
of temperature between the surface of a body and the surrounding
air, contains in solution, or in susjoension, a non-volatUe sub-
stance, which might be called atmospheric dust ; and as soon as
the equilibrium of temperatui-e is estal Wished between the air
and the surface of the body, the humid vapour wliich was con-
densed on it is volatilized, and depositing on it the dust which
it contains, goes on to be re-saturated in the air with a fresh
quantity of this impure substance.

" In order as much as possible to neutralize this efiect, the
temperature of the plate may be kept higher than that of the
surrounding air, during each of the operations. But it is im-
possible to cause tliis hea,t to reach to 122° F., in order for it to
be of the same temperature as the vapour of mercury, since, if
the plate be exj^osed to that degree of heat after the operation
of light in the camera obscura, the image will be altered.

" I fij'st tried to absorb the humidity of the ail' in the mercu-
rial box by the usual means, such as lime, &c. ; but these means
are insufficient, and only complicate the process, without giving
a good result. Another means which has been proposed con-
sists in vapourizing the mercury under the pneumatic machine ;
by this process, truly, the dew on the plate is avoided, but the
pressiu'e of the air, which is indispensable to the impression,
is suppressed. The results thus obtained, also, are always
wanting in purity.

" The following is the process at which I have stopped,
because it is very simple, and because it obviates the two incon-
veniences above mentioned ; that is to say, it frees the silver as
much as possible from all dirt or dust, and neutralizes the
humidity produced by the elevation of temperatiu-e in the mer-
curial box. By the first of these two eflfects it increases the
joromptitude, and by the second it renders the lights much
whiter (especially by the application of ]\I. Fizeau's chloride of
gold) : these two eficcts are always certain. The 2:)romptitude
given by this process is to that hitherto obtained as 3 to 8 : this
proportion is accurate.

daguerre's improved manipulation. 233

" This process consists in covering the plate, after having
polished it, with a layer of very pure water, and heating it very
strongly with a spirit-lamp, and in aftersvards pouring off this
layer of water in such a manner that its upper part, where the
dust which it has raised floats, does not touch the plate.

" It is necessary to have a fi'ame of iron wire of the size of
the plate, having at one of its angles a handle,- and in the mid-
dle, on the two opposite sides, two small cramp-irons, to retain
the plate when it is inclined. After ha\TJig placed this frame
on a hoi-izontal plane, the plate is placed on it, which is covered
with a layer of very piu'e water, and putting as much water as
the surface can retain. The bottom of the plate is afterwards
veiy strongly heated, and veiy small bubbles are formed at the
sui'face. By degrees these biibbles become larger, and finally
disappear ; the heat must be continvied to ebvJlition, and then
the water must be poiu'ed ofi". The operator should commence
by placing the lamp under the angle of the fi-ame where the
handle is ; but, before removing the frame, this angle must be
very powerftilly heated, and then, by gradually remo\Tng it by
means of the handle, the water immediately begins to run off.
It must be done in such a way that the lamp shall follow, under
the plate, the sheet of water in its progress, and it must be
only gradually inclined, and just suflicient for the layer of water,
in retiring, not to lose in thickness ; for, if the water were
dried up, there would remain small isolated di'ops, which, not
being able to flow off, would leave on the silver the dust which
they contain. After that, the plate must not be rubbed : very
pui-e water does not destroy its polish. — - ,^

" This operation should be performed only just before iodizing
the plate. Whilst it is yet warm, it is placed in the iodizing
box, and, without allo-sving it to cool, it is submitted to the
vapour of the accelerating substances. Plates thus prepared
may be kept one or two days (although the sensibility diminishes
a little), provided that several plates be placed opposite to one
another, at a very short distance apart, and careftdly enveloped
to prevent change of aii- between the plates.

" The plates cannot be too well polished. It is one of the
most important points to obtain a fine polish ; but the purity
often disappears when substances which adhere to the siu'fage
of the silver are used, — such as the peroxide of iron, which has
been very generally made use of for giving the last polish.
This substance, indeed, seems to biu-nish the silver, and to give
it a more perfect polish ; but this polish is factitious, since it
does not really exist on the silver, but in fact on a very fine
layer of oxide of iron. It is for this reason that there is re-

234 PRACTICE OF PHOTOGRAPHY.

quired for polishing tliem a substance wliich. does not adliere to
tlie silver ; pumice, which I recommended at the commencement,
leaves less residue.

" As regards the liquid to be employed : in the first operations
nitric acid of five degrees must be employed, as I stated in the
first instance ; bvit for the last operations it must be reduced to
one degree.

" The polishing with oil and the heating may be suppressed.

" I take the opportunity afibrded by this communication to
lay before the Academy the following observations, which I owe
to experience : —

" The layer produced by the descending vapoiu's of the iodine
and of the accelerating substances forms Avith silver a more sen-
sible compound than is obtained with the ascending vapours.
I make this observation only to lay down a fact, for it would be
difficult to employ descending vapours, on account of the dust
which might fall dm-ing the operation, and from stains.

" The resistance which light experiences in passing through a
white glass is well known. This resistance is even greater
than it appears, and may be attributed not only to the dust
which is left on the glazing in cleaning it, but also to that
which is natxu'ally deposited on it. The object-glass of the
camera obscui'a is certainly in the same case. To ascertain this,
I put the object-glass in cold water, which I boiled ; I knew
that it was impossible to remove it without the sides. This
operation had, therefore, no other object than to raise the tem-
perature of the glass to 212° F. C, and I then immediately
poiu-ed on the two sides of the object-glass very piu-e boiling
water to remove the dust. By operating dii'ectly Avith the
object-glass, thus cleansed, I still further increased the prompti-
tude. This means presents too many difficulties to be j)ut in
practice ; only care should be taken to clean the object-glass
every day.*

" The atmospheric dust, which is the scoiu-ge of the photo-
genic images, is, on the contrary, favourable to images wliich are
obtained by contact or at a very short distance. To be con-
vinced of this, we have only to clean the two bodies which we
wish to put in contact with the boiling water, as T have just in-
dicated, and to keep them both at the same temperature as the
air ; there will then be no impression, which evidently proves
that these images have no relation with the radiation which gives
photographic images."

* Professor Stokes has recently confirmed the truth of this by some very
conclusive experiments.

POLISHING THE PLATE. 235

Section II. — Polishing the Plate.

Upon this subject but Httle remains to be added to what is
stated in DagueiTe's earliest form of manipulation, and the few
remarks just quoted.

It is of the utmost importance that a very perfect mirror sur-
face should be produced, and to ensure the utmost freedom from
all organic matter during the polishing; the plate-holder repre-
sented in two positions by e e, in Fig. 63, has been devised.
The plate-holder is secured to a table by a clamp, and the plate
to be polished is fixed upon the horizontal svu-face of the plate-
holder by means of four binding-screws placed at its comers.
The plate having undergone the preliminaiy rubbing, which, as
being a comjjaratively coai'se operation, need not be fui-ther de-
tailed than it is in the earlier section, and having been fixed on
the holder, the last poHsh is to be given to it. The hand-
buff, i, in Fig. 63, is to be dusted over with animal charcoal,
and moistened with a little spirits of ^vine : some opeiutors em-
ploy tripoli in a state of impalpable powder mixed -vv-ith essential
oil of lavender. If, however, any essential oil is used, it must
be ascertained to be quite free from castor oil, — with which it is
very commonly adulterated, — ^by placing a di'op on a piece of
paper: if it is a piu-e essential oil, it ■s\-ill, when wai-med, entirely
evaporate, but if not, a greasy spot will remain.

In jNI. Claudet's establishment, where, from long experience,
the best modes of manipulation are introduced, the last b uffin g
is efi"ected in a somewhat different manner.

In a box on a roller, to which there is a handle, Fig. 59, is

il!ll!illllliili;!l1ii!li!^llillii!

placed a long piece of drab-coloured velvet, which can be di"awn
out and extended, by means of a second roller, upon the per-
fectly flat table. The fii'st foot or two, for example, is di-a^vn out :
the plate, which has already received its preliminaiy polishing, is
placed face downwards, and being pressed close with the fingers,
a rapid cii-cidar motion is given to it, and in a few minutes it

236 PRACTICE OF PHOTOGRAPHY.

receives its highest kistre. As the velvet becomes blackened
by use it is rolled off, the portion remaining in the box being
always perfectly clean, and ready for use. The plate is now
ready for receiving its sensitive coating, and, to avoid the chance
of the surface touching any other object, M. Claudet adopts
the simple but most effective mode of pushing it from the buff
into a spherical wooden bowl, in which the plate rests by its
four corners in perfect security.

As the edges of the metallic plates are generally sharp, they
would often cut the buffs, were that accident not jjrevented by
a suitable precaution. Fig. 60 re-
^^=^^>— — presents an apparatus called a plate-

hender. The surface a is perfectly
horizontal, and has a steel border near
the bar h : upon the bar h runs a press
that carries a steel knife edge so rounded as to be able to bend a
plate but not to cut it. The silver plate that is to be buffed is
placed on this appai'atus with an edge close to the back bar, and
the press is then run along it from end to end, by which means
the edge of the silver plate is bent downwards in a very slight
degTee, but sufficient to prevent any cutting action on the buffs.
All the four edges of each plate are bent in the same manner.

Section TII. — To Give the Sensitive Surface to the Plate.

Various compounds, called accelerating liquors, have been in-
troduced, in all of which we have combinations in various pro-
portions of either bromine and iodine, or chlorine and iodine,
and sometimes of the three. These are known by the names of
Eau Bromee, or Bromine Water, Bromide of Iodine, Redman's
Sensitive Solution, Himgarian Liquid, and Woolcott's Accelerat-
ing American Fluid. These accelerating compounds are em-
ployed after the plate has been subjected to the vapoui- of iodine.
They all require to be diluted with water until about the colour of
pale sheriy. The plate is exposed to the influence of the vapour
which is escaping from the solution used in the same manner as
■with the iodine, but the colom- to be attained differs according to
the solution employed. An iodizing box is shown at c. Fig. 63 :
at the bottom of this some iodine is strewed, and in general it is
covered with a little sand or a card; — the object of this is to
avoid an irregular action on any part of the plate : the box being
adjusted with a cover, the iodine is preserved from evaporation
and lasts a long time. When the plate has assumed its fine

TO GIVE THE SENSITIVE SURFACE TO THE PLATE. 237

straw yellow in the iodine box, it is removed to the action of the
accelerating agents, liquid or otherwise, as the case may be. The
following rules will guide the experimenter in using the different
preparations. If bromide of iodine be used as the accelerating
agent, the plate should remain over the iodine until it is of a
pure yellow tint, and over the bi-omine till of a deep rose colour.
By observing the time of exposure necessary to render a plate
sensitive, any number of plates may be prepared exactly alike,
provided that the same quantity of th'e solution, always of an
uniform strength, be put into the pan. By using a much weaker
solution a longer exposure is then necessary, but the plate be-
comes more evenly covered, and there is less danger of having it
too much or too little acted upon by the accelerator. The same
remark will apply to other accelerating solutions. If Redman's
solution, or the Hungarian liquid, a pale yellow and light rose
will be found most sensitive. As a general rvile, if the yellow
colour produced by the iodine be pale, the red should be pale
also; if deep, the red must incline to violet. When several
plates are to be prepared at one time, the same solution will
serve for all ; but it seldom answers to preserve the mixtvire for
any long time ; and its use, after keeping, is one great cause of
the failures which so annoy amateiu's. The bromine contained
in these solutions is veiy subtile, and escapes, leaving little else
but iodine remaining, which Avill, after some time, give a red
colour to the plate, without rendering it sensitive, entirely dis-
appointing the expectations of the operator. Eau bromee, or
bromine water, which is very easily prepared, is extensively used
on the Continent, and is simple in its use. If a certain quantity
of an uniform solution be placed in the pan, for each plate pre-
pared, one observation will suffice to determine the time of ex-
posui'e; if not, the coloTir must guide the operator, varying
according to the degree of colour obtained over the iodine : thus,
if the fii'st colour obtained be a light yellow, the plate should
attain a full golden tint over the iodine, and may then be retained
over the bromine until it acquires a rose colour. If iodized of a
golden yellow, then, in the second operation, it is taken to a pale
rose, and in the third to a deep rose. If in the first of a full red,
in the second to a deep red, and lastly to a grey ; if the first to
a deep red, in the second to a light blue, and in the third, to a
white, or nearly the absence of all colour.

Experience, however, must invariably guide the operator, as
scarcely any two solutions, though professedly the same in cha-
racter, possess the same properties.

In a pamphlet published by M. Fizeau, bromine-water is
recommended to be prepared as follows: — "To prepare a solution

233 PRACTICE OF PHOTOGRAPHY.

of bromine, of a fixed proportion and convenient strength to
operate "witli, I, iu the fii'st place, make a satiu-ated solution of
bromine in water; this is prepared by putting into a bottle of
piu-e w-ater a great excess of bromine, agitating strongly for
some minutes, and before using allo'wing the bromine to sepa-
rate. Now, a definite quantity of this f:atm-ated water ls to be
mixed with a definite quantity of plain water, which will give a
solution of bromine always of the same strength : this mixtui'e
is conveniently made in the following manner : — The apj)aratus
necessary is a dropping tube, which is also requii'ed for another
part of the process, cajDable of holding a small definite quantity,
and a bottle having a mark to indicate a capacity equal to thii'ty
times that of the di-opping tube : fill the bottle with pvu-e water
to the mark, then add, by means of the di'opping tube, the pro-
per quantity of the satiu'ated solution of bromine.

" The pui'ity of the water is of some impoi-tance : the foregoing
proportions refer to the pure distilled water, and it is well known
that the water of rivers and springs is not pui'e ; but these
different varieties can be used as if they were absolutely pure
water by adding a few di-ops of nitric acid till they taste slightly
acid ; two,or thi-ee drops to the pint is generally sufficient.

"The liquid produced, which is of a bright yellow coloiir,
ought to be kept in a well-stopped bottle ; it is the noi'mal
sokition, and I shall call it simply bromine water, to distinguish
it from the satiu-ated solution.

'•' Bromine Box. — The box I employ for subjecting the plate
to the vapoui' of the bromine water is constructed in the follow-
ing manner : — It consists of a box lined with a varnish, which is
not acted on by bromine ; its height is about foui- inches ; the
other dimensions ai"e regvdated by the size of the plate, which
ought to be at least half an inch all round, short of the sides of
the box; it is composed of three separate portions — the cover,
which is the £i"ame holding the plate, the body of the box, and
the bottom, upon which is placed the vessel for the bromine;
this moveable bottom is slightly hollowed, so that the bromine
vessel may always be placed in exactly the same position."

Few men have done more for photography than M. Fizeau, and
in nearly all his siiggestions he has been exceedingly happy : the
bromine water thus prepared is used with the best effect by our
most eminent daguerreotype artists.

Bromide of iodine is best prepared by the method of "SI. de
Valicoui-s, which is as follows : — " Into a bottle of the capacity
of about two oimces, pour thu-ty or forty di'ops of bromine, the
precise quantity not being of importance. Then add, gi-ain by
grain, as much iodine as the bromine will dissolve till quite

TO GIVE THE SEKSITIVE SURFACE TO THE PLATE. 239

saturated. Tliis point is ascertained wlien some gi-ains of the
iodine remain undissolved. Tliey may remain in the bottle, as
they ■will not interfere with the success of the preparation.

" The bromide of iodine thus prepared, fi'om its occujiying so
small a space, can very easily be carried, but in this state it is
much too concenti-ated to be iLsed. When it is to be employed,
pour a small quantity, say fifteen di-ops, by means of a th-op-
piug-tube, into a bottle containing about half an ounce of filtered
river water. It will easily be understood that the bromide of
iodine can be iised with a greater or less quantity of water, with-
out altering the proportion which exists between the bromine
and iodine."

Mr. Goddard, on December 12, 1840, published a letter in the
Literary Gazette on the use of bromide of iodine as an acceler-
ating agent, but chloride of iodine was fii-st employed by M. Clau-
det in 18-41, and is prepared by merely placing iodine in an
atmosphere of chloiine. Chloride of bromine is made by mixing
two drachms of a sattu'ated solution of bromine with fifteen
di'ops of strong mm'iatic acid and about nine or ten ovmces of
water. The Himgarian mixtui'e appears to be a similar com-
pound to this.

For the following exceedingly convenient preparations we are
indebted to Mr. R. J. Bingham, who has for some time, with
much siTccess, devoted his attention to the improvement of
photographic processes. The following extracts are fi'om the
Philosopltical Jfagazine for Octobei", 1846 : —

"An Improvement in the Dacjuerreotype Process hy the appli-
cation of soTne new compounds of bromine, cldorine, and iodine,
vjith lime. — All pei-sons who have practised the dagueiTeotype
must have remarked that in warm weather a con.siderable depo-
sition of moistiu'e takes place upon the glass or slate cover used
to confine the vapoui' in the bromine or accelerating pan. This
moisture must also necessaiily condense upon the cold metallic
siirface of the plate dm-ing the time it is exposed to the bromine
vapour. In fact, I have been informed by a number of profes-
sional daguerreotypists (and I have experienced the difficulty
myself), that they were unable to obtain perfect pictures during
the excessive heat of the late season ; and a veiy clever and
enterprising operator, who last year made a tour on the Con-
tinent, and brought home some of the finest proofs I have ever
seen, entii-ely failed this season in obtaining clear and perfect pic-
tiu'es, fi'om the constant appearance of a mist or cloud over the
prepared surfoce. This appeai-s to be caused by the deposition
of moistm-e u^^on the plate, arising from the water in which the
bromine is dissolved. To obviate this, some have recommended

240 PRACTICE OF PHOTOGRAPHY.

the pan to be kept at a low temperature in a freezing mixture ;
and M. Daguerre, in a communication to tlie French Academy
of Sciences, recommends the plate to be heated : but in practice
both these are found to be unsuccessful. (See Lerebours' Traite
de Photographie?)

" It appeared to me, that if we could avoid the use of water
altogether in the accelerating mixtiire, not only would the diffi-
culty I have mentioned be avoided, but a much more sensitive
sui'face would be obtained on the plate. With this view T
endeavoured to combine bromine with lime, so as to form a
compound analogous to bleaching powder. In this I was suc-
cessful, and find that bromine, chloride of iodine, and iodine,
may be united with lime, forming compounds having properties
similar to the so-called chloride of lime.

" The bromide of lime* may be produced by allowing bromine
vapour to act upon hych-ate of lime for some hours : the most
convenient method of doing this is to place some of the hydrate
at the bottom of a flask, and then put some bromine into a glass
capsule supported a little above the lime. As heat is developed
dimng the combination, it is better to place the lower jiart of
the flask in water at the temperature of about 50° Fah. : the
lime gradually assumes a beautiful scarlet coloiu", and acquires
an appearance very similar to that of the red iodide of mercury.
The chloro-iodide of lime may be formed in the same manner :
it has a deep brown colour. Both these compoimds, when the
vapour arising from them is not too intense, have an odour
analogous to that of bleaching powder, and quite distinguishable
from chlorine, bromine, or iodine alone.

" Those daguerreotypists who use chlorine in combination
with bromine, as in Woolcott's American mixtui^e, or M.
Guerin's Hungarian solution, which is a compound of bromine,
chlorine, and iodine, may obtain similar substances in the solid
state, which may be used with great advantage. By passing-
chlorine over bromine, and condensing the vapours into a liquid,
and then allowing the vapour of this to act upon lime, a sohd
may be obtained having all the j^roperties of the American
accelerator ; or by combining the chloro-iodide of lime with a
little of the bromide, a mixture similar to that of M. Guerin's
may be produced : but I greatly prefer, and would recommend,

* " I call this substance bromide of lime, although there is a diificulty as to
the composition of bleacliing powder, and which would also apply to the com-
pounds I describe. Some chemists regard the chloride of lime to be a compound
of lime, water, and chlorine. Balard thinks it is a mixture of hypochlorite of
lime and chloride of calcium ; and the view of Millou and Prof. Graham is, that
it is a peroxide of lime, in which one equivalent of oxygen is replaced by one
of chlorine."

TO GIVE THE SENSITIVE SUEFACE TO THE PLATE. 241

tlie piire bromide of lime, it being, as I believe, tlie quickest
accelerating substance at present kno-vru. Ey slightly colouring
the plate with the chloro-iodide, and then exposing it for a pro-
per time over the liromide, proofs may be obtained in a fraction
of a second, even late in the afternoon. A yellow coloiu- should
be given by the use of the fii-st substance; and the proper time
over the bromide is readily obtained by one or two trials.*
With about a drachm of the substance in a shallow pan, I give
the plate ten seconds the whole of the first day of using the pre-
paration, and add about thi-ee seconds for eveiy succeeding one.
The compomid shoidd be evenly strewed over the bottom of the
pan, and will last, with care, about a fortnight.

" The gi'eat advantage of this compound is, that it may be
used continuously for a fortnight without renewal ; and, unlike
bromine water, its action is unafiected by the ordinary changes
of temperatm-e."

The advantages of a dry material are so great, that the bro-
mide of lime is now commonly used.

By the emplo}Tnent of these agents a sensitive coating is pro-
duced, upon which actinic changes are almost instantaneously
made. The modes of proceeding to prepare the plates are
similar to those already named.

The time necessary for the jilate to be exposed to the action
of the bromine water, if it be used, must be determined by expe-
riment, for it will vaiy according to the size of the box and the
quantity of liquid used. It is ordinarily between thii-ty and
sixty seconds, the time varying with the temperatui'e of the
atmosphere: when once determined, it will be constant with
the same box, the same strength of solution, and the same
temperature.

The method of coating the plate with bromine from the water
which is most approved is as follows : — Place a pan in a properly
prepared box, fill a pipette with bromine water, and pour it
careftilly from this into the pan, then close the vessel with a glass
plate : the liquid must cover evenly the bottom of the pan ;
if not level, it must be adjusted: the level will be easily seen
through the glass slide. When everything is thus aiTanged, the
plate, previously iodized, is to be placed in its fi-ame over the
pan, the slide withdi-awn, and the necessary time counted ; after

* " It is better to count time both over the iodine and the bromide of lime :
the exposure of the plate to the iodine, after it has received its proportion of
bromine, should be one-third of the time it took to give it the first coating of
iodine. We have found that if less iodine than this be allowed to the plate it
will not take up so much mercury, neither will the picture produced be so bold
and distinct."

242 PRACTICE OF PHOTOGRAPHY.

this has elapsed, the slide should be shut, and the plate imme-
diately placed in the dark box of the camera.

For a second operation, this bromine water must be thrown
away, and a fresh quantity used. The bottle containing the
bromine water should be kept away from the direct light of the
sun, and care should be taken that no organic matter fall into
the bottle, such as grease, chips of cork, &c. These enter into
new combinations with the bromine, and lead to error as to the
amount in solution.

Daguerre himself introduced some very considerable improve-
ments in the process of iodizing. He avoided the use of metal
strips, and gave some curious experiments on the action of
edges, grooves, &c., in determining the deposition of vapouj'.
M. Daguerre stated that, but for the difficulty of fixing them,
the bands might be very much reduced in size ; for it is suffi-
cient for them to produce their effect that there be a solution
of continuity between them ; and this is proved by the fact that
nearly the same result is obtained by engraving at the -gth of an
inch from the edge of the plate a line deep enoiigh to reach the
copper. The objections to this are, that during the polishing
process the engraved line is filled witli dust, and it retains water,
which sometimes occasions stains. He then proposed, as a very
great simplification of this process, that the plate should be laid
flat in a shallow box containing two gTOoves, one to receive the
plate, and the other a board saturated with iodine. Around the
plate he places a border of either powdered starch or lime, and
the iodine descends from the board to the tablet. The starch
or lime absorbs the iodine with avidity, and thus prevents its
attacking the edges of the silver, and the vapour is diffused with
perfect evenness over it. Another advantage is, that the satu-
rated board may be used for several days in succession, "without
being renovated.

M. Seguier somewhat modified even this process. A box of
hardwood, varnished internally with giim lac, contains a lump
of soft wood, furnished with a card of cotton sprinkled with
iodine. Upon this is placed a plate covered with card-board on
each of its faces. One of these card-boards furnishes, by radia-
tion, to the metal the vapour of iodine, while the other returns
to the cotton that which it had lost. It suffices to turn the
plate from time to time, in order that the operation may go on
with equal rapidity. A plate of glass is placed upon the upper
card-board, where it is not operated on. The plate is sustained
a little above the charged cotton by frames of hardwood var-
nished with gum lac. By increasing the distance between the
cotton and the plate, or the contrary, we are enabled to suit the

TO DEVELOPE THE IMAGE FORMED ON THE PLATE. 243

arrangement to the tempei-ature of the season, and thus always
operate with facihty and promptituda M. Seguier also states,
that a single scouring with tripoli, moistened with acidulated
water, is sufficient to cleanse the plates thoroughly, thus doing
away with the tedious process of scoui'ing with oil, and after-
wards the operation of heating the tablet over a spiiit lamp.
M. Soliel has proposed the use of the chloride of silver to de-
termine the time required to produce a good impression on the
iodized plate in the cameiu. His method is to fix at the bottom
of a tube, blackened within, a piece of cai"d, on which cliloride
of silver, mixed with gum or dextrine, is spread. The tube thus
disposed is tiu-ned towards the object of which we wish to take
the image, and the time that the chloride of silver takes to be-
come of a grayish slate colour, Avill be the time required for the
radiations in the camera to produce a good eflect on the iodated
silver.

These remarks have been introduced as supplementary to the
generally approved modes, as they are suggestive in themselves
of still frnther improvements.

Section IY. — To Develope the Image formed on the Plate.

The plate, prepared by one of the methods directed, has been
placed in the camera, and the image impressed upon it — atten-
tion being given to the points especially alluded to in the chap-
ter " General Remarks on the iise of the Camera Obscura."

The image is developed on the daguerreotype plate, as has been
already described, by the use of mercimal vapour. In the ori-
ginal process (Fig. 10), one form of a mercurial vapour-box is
given ; and Fig. 63, d, represents another.

It matters little in what manner the plate is placed in the
mercurial bath ; the mercmy should be volatilized very slowly,
and the image allowed to have its fiill development before it is
removed from the box. Cai-e should be taken that the operator
avoids as much as possible contact with the mercurial vapour,
since continued exposure to its influence might lead to serious
inconvenience.

M. Claudet has adopted a most admirable arrangement in this
respect, as is shown in the woodcut on the following page.

Fig. GO represents a small dark chamber fixed outside the
apartment in which the operations are carried on, but opening
iuto it by means of sliding glass doors. On either side of the
chamber are placed pieces of yellow glass, through which, on

244

PRACTICE OF PHOTOGRAPHY.

opening the shutters bj which thej are covered, a sufficient
quantity of light is admitted to serve any useful pm^pose, with-
out in any way interfering with the sensitive suifaces of the
plates. Within this chamber are placed two mercuiy boxes, each

containing a small quantity of that metal. One of these is shown
in section. Each box is placed over a water bath, .supplied by
means of a pipe with water from a cistern above, and a small
sand bath is placed between the mercmy box and the water
bath. By means of the gas-bui'ner beneath the box, the water
is heated, the mercury volatilized slowly and deposited on the
plates, which are fixed on the grooves shown on the sides of
the box in section. The windows being closed, any mercurial
vapoui's which may escape fi-om the box may pass out into the
air through proper ventilators, and the operator is thus protected
from the injui'ious efiects of the merciu'j'.

fixing the daguerreotype image. 245

Section Y. — Fixing the Daguerreotype Image.

It has already been stated that the solution of hyposulphite
of soda is the most effective agent for removing all the unchanged
iodide of silver, after the application of, and the development
of the image by, the mercurial vapour.

This being effected, greater permanence is given by the appli-
cation of a solution of gold.

The process as described by JM. Fizea'u, to whom we are in-
debted for its introduction, is as follows : —

" Dissolve eight gi^ains of chloride of gold in sixteen ounces
of water, and thii-ty-two grains of hyposulphite of soda in foiu'
ounces of water : pour the solution of gold into that of the soda,
a little by Kttle, agitating between each addition. The mixture,
at first slightly yellow, becomes afterwards perfectly limpid.
This liquid now contains a double hyposulphite of soda and gold.
'' To use this salt of gold, the sxuface of the plate should be per-
fectly free from any foreign substance, especially dust ; conse-
quently it ought to he washed with some precautions which might
be neglected if it was to be finished l)y the ordinaiy mode of washing.
" The following manner generally succeeds the best : the plate
being yet iodized, and perfectly free from grease on its two sur-
faces and sides, should have some drops of alcohol poured on the
iodized siu-face ; when the alcohol has wetted all the siu-face,
plunge the plate into a basin of water, and after that into a solu-
tion of hy^Dosulj^hite of soda.

" This solution ought to be changed for each experiment, and
to consist of about one part of the salt to fifteen of* the water :
the rest of the vrashing/'is done in the ordinaiy way, only taking
care that the water should be as free as possible from dust.

"The use of the alcohol is simply to make the
water adhere perfectly aU over the sm-face of the
plate, and prevent it from quitting the sides at
each separate immersion, which would infallibly
produce stains.

When a pictm-e has been washed, with these

precautions, the treatment -ndth the salt of gold is

very simple. It is sufiicient to place the plate on

Qj^ ■ ' 2^^ a support, fig. 61, or fig. 63, g, and pour upon its

'^'J^Iinsil^"' sm-face a sufficient quantity of the salt of gold

61. that it may be entirely covered, and heat it with

a strong spirit-lamp ; the pictm-e will be seen to

brighten, and become in a minute or two of great force. When

this effect is produced, the liquid should be pom-ed off and the

plate washed and chied.

246

PRACTICE OF PHOTOGRAPHY.

" In tliis operation the silver is dissolved, and the gold pre-
cipitated upon the silver and merciuy, but wdtli very different
results : in effect, the sUver, which, by its reflection, forms the
shades of the picture, is in some way darkened by the thin film
of gold which covers it, from which results a strengthening of
all the dark jDarts. The mercury, on the contrary, wliich, in the
state of an infinite number of small globules, forms the lights, is
augTnented in its solidity and brightness by its union with the
gold, from which results a great degree of permanency, and a
remarkable inci-ease in the lights of the picture."

The plates are then washed by means of an arrangement of
this order. The apparatus represented in fig. 62 may be employed,
a is a vessel sufl3.ciently large to take the
plate, and not more than half an inch
Avide : this is filled with distilled water,
which is heated by means of a spirit-lamp ;
6 is a stand supporting the trough, and c
a holder for the plate. After the plate
has been immersed for a few minutes,
it is to be dra^Ti out slowly, and by
blowing on it the water is I'emoved, and
the warm metal rapidly dried. Such are
the principal processes which have been
go adopted in the daguerreotype manipu-

lation. Other modes for giving perma-
nency to the dagueiTean image have been adoj^ted, but none of
them have been so thoroughly successful as those.

It appears advantageous to quote a few of the modified forms
of proceeding for fixing these pictures, when obtained, which
have from time to time been recommended.

M. Preschot, in a letter to M. Arago, remarks —
" In one of the sittings of last month you mentioned a process
for fixing photogenic images on metal. Knomng, as I do, the
interest you take in the beautiful discoveiy of the daguerreotype,
I hope you will excuse the liberty I take in troulaling you with
results which I obtained in experiments made a few months ago.
" Photogenic images, obtained by M. Daguerre's process, may
be fixed by treating them with a solution of hydi-osLdphite of
ammonia. For this purpose, a concentrated solution of this
fluid is mixed yvith three or fom* volumes of pui*e water, wliich
is poiu-ed into a flat vessel, in sufiicient qviantity that the jjlate
may be steeped in it horizontally, and just covered with the
fluid. When, by the action of the fluid, the tints of the di'awing
are sufiiciently changed, which occurs in less than a minute, the
plate is to be withch-awn, and put into a flat vessel containing

FIXING THE DAGUERREOTYPE IMAGE. 247

water : it is afterwards taken out and di'ied. By this process
the polished parts of the metal are tinged gray by the sulphui-et,
and the amalgamated parts are not attacked, or, at least, but
very little. The tints may be varied by the concentration of
the fluid or the duration of the immersion ; however, too long
an action turns the lights yellow. PhotogTaphic images, treated
in this manner, bear iiibbing with the finger wdthout losing any
of their details. "

M. Choiselat proposed a plan which "has been rarely acted
upon, but which is well deser\Tng of attention.

Chloride, and particularly iodide of silver, dissolved in hypo-
suljihite of soda, may be advantageously employed for fixing the
images of the daguerreotyj)e. Steeped in these solutions, they
are, imder the electro- chemical influence exerted by the copper
on the dissolved silver, coated with a film of silver, and thus
become ineflfaceable.

Instead of the hyposulphite, a mixture of iodide or bromide
of potassium, holding the silver salt in solution, may be em-
ployed.

The iodide of silver best adapted for this operation is said to
be that which is obtained by treating, "\vith the aid of heat, a
plate of this metal by the iodide precipitated from alcohol by
water, aftei'wards dissolving the iodide formed and adhering to
the plate in the hyposulphite.

Dr. Berres, of Vienna, assisted by Mr. F. Kratochwila, has
succeeded by another process, beai-ing some analogy to that of
M. Fizeau, in fixing the daguerreotype designs. He takes the
photograph produced in the usual manner by the process of
Daguerre, holds it for a few minutes over a moderately warmed
nitric acid vapom-, and then lays it in nitric acid of 13" or 14"
Reaumur (61^" or 63^^ Falu-enheit), in which a considerable
quantity of copper or silver, or both together, has been j^treviously
dissolved. Shortly after having been placed therein, a precipi-
tate of metal is formed, and can be changed to any degree of
intensity. The photogTaphic picture coated with metal is now
removed, washed in. water, cleaned, and dried ; it is then polished
with chalk or magnesia, and a dry soft cloth or leather, after
which the coating will become clean, clear, and transpai-ent, so
that the pictiu-e, with all its details, can again be seen.

This proceeding is of a most imcertain character — since, with
every precaution, the acid solution frequently " bites" into the
plate, producing an etching.

248 PRACTICE OF PHOTOGRAPHY.

Section VI. — Simplification of the Daguerreotype.

The following experiments for tlie simplification of the daguer-
reotype processes, were made by me many years since ; I have
therefore retained such portions of the original paper as do
not appear to be entirely uninstructive.

Procure a well-plenished copper plate of the required size,
and polish it, first with pumice-stone and water, than with snake-
stone, and bring it up to a mirror surface with either rotten-
stone or jewellers' rouge. Plates can be piu-chased in a high
state of preparation from the engravers. Having prepared the
copjDer plate well, rub it mth salt and water, and then with
some silvering powder. No kiud answers better than that used by
clock-makers to silver dial-plates. It is composed of one part
of well-washed chloride of silver, five parts of cream of tartar,
and four parts of table salt. This powder must be kept in a
dai'k vessel, and in a diy 'place. For a plate six inches by five
as much of this comjrtosition as can be taken up on a shilling is
sufiicient. It is to be laid in the centre of the copper, and the
fingers being wetted, to be quickly rubbed over every part of
the plate, adding occasionally a little damp salt. The copper
being covered with the silvering, it is to be speedily well washed
in water, in which a little soda is dissolved, and as soon as the
surface is of a fine silvery whiteness it is to be di-ied with a very
clean warm cloth. In this state the plates may be kept for use.
The fii'st process is to expose the jjlate to the heat of a spirit
flame, until the silvered sm-face becomes of a well-defined golden-
yellow colour : then when the lAate is cold, take a piece of
cotton, dipped in veiy dilute nitric acid, and rub lightly over
it until the white hue is restored, and diy it with very soft clean
cloths. The plate is then immersed in a weak solution of the
iodide of potassiiun, in which a small portion of iodine is dissolved.
The silver is thus converted, over its sm-face, into an iodide of
silver ; and in this state it is exposed to the solar rays, which
blacken it. When diy it is to be again polished, either with
dilute acid or a solution of carbonate of soda, and afterwards with
diy cotton, and the smallest possible portion of prepared chalk ;
by this means a sm-face of the highest polish is produced. The
rationale of this process is, in the first 23lace, the heat applied
di'ives ofi" any adhering acid, and efiects more perfect union
between the copper and silver, so as to enable it to bear the
subsequent processes. The fii'st yellow sui-face appears to be an
oxide of silver, with, possibly, a minute qviantity of copper in
combination, which being removed leaves a surface chemically
pure. Copper plates may also be yqyj beautifully silvered by

SIMPLIFICATION OF THE DAGUERREOTYPE. 249

galvanic agency, by which we are enabled to increase the thick-
ness of the silver to any extent, and the necessity for the heating
process is removed, the silver being absolutely pure. The best
and simplest mode is to divide an earthenware vessel with a
diaphragm of membrane or paper : one side should be filled with
a very dilute solution of sulphuric acid, and the other with either
a solution of ferroprussiate of potash, or chloride of sodium,
saturated with chloride of silver. The copper plate, varnished
on one side, is united, by means of a copper wire, with a plate of
zinc. The zinc plate being immersed in the acid, and the copper
in the salt, a weak electric current is generated, which precipi-
tates the silver in a very uniform manner over the entire sm'face.

At a very early stage of my inquiries I foimd that the influence
of all the rays, exceptiug the yellow, was to loosen the adhesion
of the iodized surface, with the imder layer of unaffected sUver.
When this changed film was removed by rubbing, the silver be-
neath always exhibited the most perfect lustre, and I have hence
invai'iably adopted this mode of polishing my dagiierreotype
plates. The requii^ed surface is tlius produced with one-third the
labour, and a very great savdng of time ; besides which, the silver
is in a much more susceptible state for receiving the vapour of
the iodine. The plate being thus prepared, we proceed in the
manner before directed.

It is somewhat singular, that on the first notice of Daguerre's
pictm-es, long before the publication of his process, when I
learnt that they were on " hard polished tablets," I entertained
the idea that plates of copper thus silvered were oxidized, and
then acted on by iodine. I applied the iodine both in solution
and vapoiu- ; but, of coui-se, as the mercury was not used, I failed
to efiect any perfect pictures. Tt is, however, worthy of i-emark,
that on one occasion, having placed a piece of silvered copper in
a trough containing a weak solution of iodine, with some leaves
of hemlock superimposed, these being kept close by means of a
piece of glass over all the exposed portions, the silver was
completely removed, and the copper abraded to a considerable
extent, while beneath the leaves the silver was scarcely affected.
I thus procured a very beautiful etching, the figm-es being in
high relief This was frequently repeated with success ; but
other inqrdi'ies having di-a\\Ti off my attention, the process has
been long neglected, although I am convinced it is capable of
being turned to much useful account.

In November, 1839, I pursued a series of experiments with
bromine, but no veiy definite advantage was obtained. Some
cui"ious effects which I noticed at that time are worthy of notice.
I copy the remarks made in my memox'andum-book at the time.

250 PKACTICE OF PHOTOGRAPHY.

4. Exposed a plate to the vapour of bromine : it assumed a
leaden-gray colovir, which, blackened by light veiy readily. Ex-
posed this to mercury without much impro-sdng the effect or
altering the lights. Upon immersing this plate in a solution of
the mimate of soda, the parts unacted on by Kght became a jet
black, whilst the paints on which light had acted were dissolved
off, lea\'ing a clean coating of silver. The effect was most
decisive — a black picture on a white ground.

8. Allowed three plates to assume — the first a straw-yellow,
the second a steel-blue, and the third a dull blue, and examined
their sensitiveness ; the plate which had arrived at the dull blue
colour appeared to be the most sensitive.

These experiments, which were then pursued with a view to
produce more permanent pictm-es — to fix the mercuiy, or to
engrave the plate — were, however, abandoned, and have not yet
been resumed, although it is desii-able that some one should
tm-n his attention to this point. On one occasion, after ha\dng
prepared a picture according to the process prescribed by
Daguerre, T placed it, without removing the iodine, in a vessel of
chlorine ; the pictvire was obliterated, and very speedily blackened.
On expo.sing tliis black plate to light, it almost in.stantaneously
whitened. This is mentioned to show the extent of ciu'ious
subjects which jDhotography opens out for examination.

The apparatus for the daguerreotype sho-wTi in the vignette
may be enumerated with advantage.

a. Is the camera obscura, T\^th the screen upon which the
image is seen, and by which the focus is adjusted, partly raised ;
and when this is accui-ately determined a screw is shown by
which it is secured.

b. Silver plate and edges for the same.

c c. Are bromide and iodine boxes of walnut, enclosing each
a stout porcelain pan : each pan is furnished with an air-tight
glass cover. On the upper edge of each box is a groove for
holding the plate. On withdrawing the glass cover of the iodine
pan, the plate is exposed to its action, and the coloiu' produced
is observed by holding a sheet of white paper in such a position
that its reflection may be seen on the plate, which enables the
operator to judge of the progi'ess of the operation. When the
plate has obtained the requii"ed colour, the glass cover is pushed
in, so as to cover the iodine pan, and the cover over the bi'omine
pan is withdi-awn. The plate is now removed from the iodine
box and placed over the bromine box, and the colour observed as
before. When the plate has received the proper amount of
bromine, which is perceptible by the colour, the cover of the
bromine pan is pushed in, and the plate is again jjlaced over the

SliTPLIFICATIOX OF THE DAGUERREOTYPE.

251

iodine pan for a few seconds, until the ultimate colour required
is produced, and it is then ready for removal to the camera.

d. Improved mercuiy box, of walnut, with sliding legs, ii-on
cistern, glass windows for ins]:)ecting the development of the

picture, mounted with thermometer for ascertaining the tempera-
ture of the mercuiy.

e e. Are plate holdei-s, with clamp for securing the same.

f. Is a box for holding the daguerreotype plates.

(J. A levelling stand, used in the fixing process, see page 245.

h. A flat peculiar dish for washing, see fig. 1, page 13.

i. Is a hand-biiff.

The pictures being completed, they are mounted in morocco
or ornamented cases, such as are shown below.

CHAPTER VII.

THE COLLODION PROCESS.

"With the advance of this beautiful art, there appears to be a
progressively increasing desire to produce more artistic results ;
and improvements are constantly being introduced.

Collodion, as the film upon which to spread the photographic
agents, beyond all other preparations, offers, in its exceeding
sensibility, beavity of details in the finished pictures, and ease of
operating, so many decided advantages, that a separate chapter
has been devoted to its consideration.

' Collodion is a peculiar preparation, formed by dissolving gun-
cotton in ether containing a little alcohol. It is a very mucila-
ginous solution of a volatile character, the ether evaporating and
leaving a film of the utmost transparency behind. It is not all
kinds of gun-cotton which dissolve equally well in ether.v Ac-
cording to my experience the most easily soluble is prepared by
soaking good cotton in a saturated solution of nitrate of potash
for some time ; it is then, in a moist state, plimged into sulphiu'ic
acid with which but a small quantity of nitric acid has been
mixed : after remaining in the acid for about a minute, it is well
washed with water until no trace of an acid taste is discovered,
and then dried at a temperatui'e but very slightly elevated above
that of the apartment. ^^

Mr. Archer, to whom, with Mr. Fry, we are mainly indebted
for the introduction of this preparation as a photographic agent,
gives the following as his processes for j^reparing gun-cotton : —

" There jire two receipts for making g-un-cotton, from either of
which a good dissolving cotton may be obtained. Several others
have been described, but I should only be confusing the subject
to attempt to give the whole ; and it would be foreign to the
limited piu'pose of this work to do so. The results, however,
vary so much with the strength and proportion of the acids
used, as to render it extremely difficult to name any one in
particular which would entirely succeed under all circumstances.
In all cases it is more easy to prepare a cotton which will
explode readily, and yet not he at all soluble, than one which will
entii'ely dissolve in rectified sulphuric ether.

THE COLLODIOX PROCESS. 253

Take of cliy niti-ate of potash in powder, 40 parts

Sulphm'ic acid 60 ,,

Cotton 2 „

" The nitre, sulphuric acid, and cotton, are weighed in the
above proportions, and placed near at hand within reach of the
operator, to prevent delay in mixing when the operator has
commenced. Then pom* the propoi-tion of sulphiiric acid into
the powdered nitre, stii'iing them well together for a few
seconds with a strong glass rod. Immediately the two ai-e
mixed add the cotton, having previously pulled out the fibres,
and mix them well together with two glass rods, in order that
the whole of the cotton may come in contact with the nitric
acid vapour, which is being i-apidly generated from the mixture.
This action mtist be continued for about two minutes ; then
quickly remove the cotton mth the adhering nitre and sulphuric
acid from the basin, with the glass rods, and plunge it into a
large quantity of water ; it is to be well washed in repeated
changes of water until all the acid and nitre are washed away.
The cotton is then collected togethei-, and first pressed between
the hands to ch'ain ofi" the water, and then still fui-ther dried by
pressui'e in a cloth ; the fibres of cotton can now be carefully
separated, and himg up wdth pins to the edge of a shelf, or any
other convenient place, to diy. There is no necessity to use
artificial heat, as the small quantity requisite for a few oiinces
of solution can easily be di'ied without it.

'' The next receipt is by certain proportions of nitric and
sulphuric acids : —

Take 1 oz. by measure of nitric acid, specific gi-avity 1-450
1 oz. „ sulphuric ditto ordinary

80 gi's. by weight of cotton.

" The fibres of cotton must be well separated, as in the pre-
ceding mode. The two acids are first mixed, and the requisite
proportion of cotton added as quickly as possible, and weU
stii-red with two glass rods for not more than fifteen seconds :
the gun-cotton is removed from the acids, and plunged into
water to vmdergo the same washings, &c., as in the former
receipt.

" It will be seen that the cotton is not exposed to the action
of the mixed acids, in this last mode, longer than is necessary to
satvirate the cotton ; should the action be continued further, the
solubiUty of the cotton is entirely lost.

" Water must not be spared in washing the cotton, for not a
trace of acid should be left ; the collodion would be injiu'ed by
any remaining."

254 PRACTICE OF PHOTOGRAPHY.

The substance lignine — wliicli is tlie tiiie wood of every variety
of plantj and has the composition C 38, H 24, O 20 — is capable
of being converted into a material having an analogous constitu-
tion to true gun-cotton. With strong nitric acid lignine com-
bines directly, and forms a substance called xyloidine. This may
be prepared by immersing a piece of paper in strong nitric acid,
and then washing it well in piu'e water ; it thus assumes the
feel and toughness of parchment, and is so combustible as to
serve for tinder. The composition of xyloidine is expressed by
C 12, K 2, H 8, O 18. Starch dissolves by digestion in strong
nitric acid, and on adding water xyloidine is precipitated. There
are several other materials of which it may be prepared, and
nearly all these substances are soluble in ether, forming collodion.

Gun cotton is a compound of lignine with nitric acid — 100
parts of cottou producing 170 parts of gun-cotton.

To Prepare the Collodion. — Thirty grains of g-im-cotton pre-
pared as described should be taken and placed in 18 fluid ounces
of rectified sulphiu-ic ether, and then 2 ounces of alcohol should
be added ; making thus an imperial pint of the solution. The
cotton, if properly made, will dissolve almost entirely ; any small
fibres which may be floating about should be allowed to deposit,
and the clear sokition poured oflf previously to the process of
iodizing it. Mr. Delamotte adds a few drops of ammonia to the
collodion, which may possibly prevent an acid reaction, which
sometimes takes place.

To Iodize the Collodion. — Mr. Ai-cher's method is as follows ;
and I believe no better course can be piu'sued.

Prepare a saturated solution of iodide of potassium in alcohol,
say 1 oz., and add to it as much iodide of silver as it will take
up. Or to 1 oz. of alcohol add an excess both of iodide of po-
tassium and iodide of silver ; after a day or two, and with repeated
shaking at intervals to facilitate the operation, a satui-ated solu-
tion of the two salts will be obtained, and if this is filtered ofi"
into another bottle, it will always be found ready for use. The
first bottle can be kept as a stock bottle, to obtain a still further
supply by replenishing it with alcohol, and adding now and then
small additional quantities of the two salts. The iodide of sil-
ver can be readily obtained by precipitation. For instance, take
1 oz. of solution of nitrate of silver used in the process, 30 grs.
of nitrate of silver to 1 oz. of water, and add to it sufiicieut of a
solution of iodide of potassium in water as wiU throw down the
whole of the nitrate of silver as an iodide. When this precipi-
tated iodide of silver has settled, which it very readily does, the
liquid above must be poured off", and fresh water added, repeat-
ing this washing several times. The iodide of silver after this

THE COLLODION PEOCESS. 255

is dried, and then put into a bottle with a small quantity of
alcohol, just sufficient to keep it moistened. The quantity of
the solution of iodide of silver which can be added to 1 oz. of
collodion, must depend upon the quantity of alcohol in the col-
lodion. Iodized collodion is liable to decomposition, and iodine
is liberated. This is prevented by putting a little piece of pure
metallic silver in the collodion, which preserves it colovu'less and
of a constant strength. The collodion process now resolves itself
into

1st. Cleaning the Glass Plate, — One of the most successful
general manipulators in the oixlinary forms of the collodion pro-
cess is Mr. Home, to whom I am indebted for much important
information on this process. A variety of substances, such as
tripoli, nitric acid, spiiits of wine, (fee, have been recommended
for cleaning the glass : but all these Mr. Home thinks are quite
superfluous; the only articles actually necessaiy being a clean
cloth or two, and a wash leather that has been well and tho-
roughly rinsed tlii'ough several changes of clean water, to deprive
it as much as pos.sible of the di'essing which a new one contains,
and a httle liquid ammonia, not strong, but the ordinary liquor
ammonue of the shops. If this is not at hand, a little caustic
potash or soda will answer as well, the pm-poi-t of it being to
remove any greasy matter attached to the sui-face, as glass is
frequently mai-ked \nih. soap; and although it might appear at
first sight that clean water must thoroughly remove this article,
the operator will be certain of spoiling many of his pictui-es if he
depend upon water alone.

The plan ]Mr. Home recommends is as follows : — Poui- upon
the plate a few drops of ammonia, rub it well over both surfaces,
and thoroughly rinse through two waters, allowing the water to
flow over the plate either by poiu-iug from a vessel or holding
under a tap; now, with a clean cloth wipe perfectly diy, and
finally well mb Avith a leather. Simple as this may appear,
there is much more in it than will be at first imagined, for un-
less the glass is free fr-om stains it is quite impossible to l)e suc-
cessful. The plate may be washed perfectly clean, but the sur-
face not thoroughly di-ied. Then, again, some hands are very
warm, and if the plate is allowed to rest too much upon any one
part, or held too long in the fingers at any one particular spot,
that will become warmer than the siuTounding part, from the
glass being a bad conductor of heat. The cloth and leather
should therefore be sufficiently large, that the plate ma\' be as
it were insulated as much as possible from the hands, that no
unnecessary lieat shall be applied. At the same time the em-
ployment of a waim cloth is very useful, for the heat is then

256 PRACTICE OF PHOTOGRAPHY.

equally diffused over the plate, and, wh.at is very essential, tlie
surface perfectly and quickly di-ied.

Coating the Plate. — It lias ali-eady been pointed out liow
necessaiy it is to handle the plate as little as possible in cleaning ;
we therefore suppose the operator to have the plate in a clean dry
leather, from which it is taken to receive the collodio-iodide of
silver. The plate must be held hj the left hand perfectly hori-
zontal, and then with the right a sufficient quantity of iodized
collodio should be poiu*ed into the centre, so as to diffuse itself
equally over the su.rface. This should be done coolly and steadily,
allowing it to flow to each corner in succession, taking cai-e that
the edges are all well covered. Then gently tilt the plate, that
the superfluous fluid may return to the bottle fi'om the opposite
corner to that by which the plate is held. At this moment the
plate should be brought into a vertical position, when the
diagonal lines caused by the fluid runniug to the corner vnll fall
one into the other and give a clear flat surface. To do this neatly
and effectually, some little practice is necessaiy, as in most tilings,
but the operator shoidd by no means huny the operation, but do
it systematically and quietly, at the same time not being longer
over it than is actually necessary, for collodion being an ethereal
compound evaporates very rapidly. Many operators waste their
collodion by imagining it is necessary to perform this operation
in great haste ; but such is not the case, for an even coating can
seldom be obtained if the fluid is poured on and off again too
i-apidly ; it is better to do it steadily, and submit to a small loss
from evaporation. If the collodion becomes too thick, thin it
with the addition of a little fresh and good ether.

Exciting the Plate. — Previous to the last operation it is neces-
sary to have the liath ready, which is made as follows : —

Nitrate of silver 30 grains.

Distilled water 1 ounce.

Dissolve and filter.

The quantity of this fluid necessary to be made must depend
upon the form of trough to be used, whether horizontal or verti-
cal, and also upon the size of j^late. The kind used by Mr. Home
is the vertical, though many stdl jirefer the former, and attach
a piece of Indian rubber to the back of the plate as a handle
whilst applying the collodion, and to keep the fingers from the
solution whilst dipping in the bath. "With the vertical troughs
a glass dipper is provided, upon wluch the plate rests, preventing
the necessity of any handle or the fingers going into the hquid.
If, however, the glass used is a Little larger than is required, this

THE COLLODIOX PROCESS. 2o I

is not necessary. Having then obtained one or other of these
two, and filtered the liquid, previously fi-ee fi'oni any particles
of dust, (fee, the plate is to be immersed steadily and icithout
hesitation, for if a paivse shoiild be made at any paii, a line is
sure to be formed, which will print in a subsequent part of the
process.

The plate being immei'sed in the solution, must be kept there
a sufficient time for the liquid to act freely upon the surface,
particularly if a negative picture is to be obtained. As a. general
rule, it vAll take about two minutes, hut this will vary imth the
temperature of the air at the time of operating, and the condition
of the collodion. In cold weather, or indeed anything below SO'*
Fakrenheit, the bach should be placed in a warm sittiation, or
a proper decomposition is not obtained under a veiy long time.
Above 60' the plate will be certain to have obtained its maxi-
mum of sensibility by two mintites' immersion, but below this
temperature it is better to give it a little extra time.

To facilitate the action, let the temperature be what it may,
the plate mtist be lifted out of the liquid two or three times,
which also assists in getting rid of the ether from the surface,
for without this is thoroughly done a uniform coating cannot
be obtained; but on no account should it be removed until the
plate lias been immersed about half a minute, as marks are apt to
be produced if removed sooner.

Having obtained the desired coating, the plate is then ex-
tremely sensitive, and, therefore, we presume the operator has
taken eveiy precaution to exclude ordinary day-light.

The room must be closed against any portion of day -light, and
candle alone employed, placed at a distance from the operator to
give the requisite light. Yellow glass, which has been recom-
mended for glazing the operating room, does not furnish suffi-
cient protection from the chemically active rays.

The plate thus rendered sensitive must then be lifted from
the solution and held over the trough, that as much liquid as
possible may drain off previous to its being placed in the frame
of the camera, and the more efiectually this is done the better ;
at the same time it must not be allowed to dry.

The question is often asked, how soon after coating the plate
with iodized collodion should it be immersed in the nitrate
bath? Now, this is a difficult question to answer. We have
said the time of immersion is dependent upon the tempera-
ture and quality of the collodion ; so likeA\"ise must we be
governed as to time before immersion. To make collodio-iodide
or xylo-iodide, for, chemically speaking, there is no difierence

258 PRACTICE OF PHOTOGRAPHY.

in the two, it is necessary that the ether should contain a
certain quantity of alcohol, or the cliiFerent ai"ticles are not
soluble : therefore, if we take a fresh bottle, and coat the plate
from this, it contains its full dose of ether, and wdth the
thermometer ranging between 60° and 70° the evaporation will
be very rapid, and consequently a tough film soon formed. If,
on the other hand, we are using a solution which has been in use
some time, and many plates, perhaps, coated, the proportion of
alcohol is much greater, and not being of so volatile a natiu'e, it
will necessarily take a longer time to acquii-e the requisite fii'm-
ness for immersion. If, for instance, after coating a plate, we
find on immersion it does not colour freely, we have then reason
to suppose the plate has not been immersed sufliciently quick ;
but if, on the other hand, we find the film very tender, and upon
(hying it cracks, then we have reason to know that plates pre-
pai-ed fr-om that bottle must not be immersed qmte so soon.
The larger the proportion oj' alcohol tlie more sensitive ivill be the
2)lates, and the quicker and inore even ivill he the action of the
bath; but a longer period may be allowed for the sensitive film
to hai'den before immersion.

The next question also often asked is, how long must be the
exposiu'e in camera? a question more difficult to answer than
the last, without knowing something of the character of the lens
and the intensity of sunshine. Practice alone can determine,
combined with close obsei-vation of those parts which shotJd be
the shadows of a pictiu-e. If, for instance, in developing we find
those parts less exposed to the light than others developing im-
mediately the solution is applied, then we have reason to suppose
the exposui-e has been too long ; but if, on the contraiy, they
develope veiy slowly, we have proof the time allowed has not
been sufficient to produce the necessaiy action. In a good pic-
tui'e we should see first the whites of a di'ess appear, then the
forehead, after which we shall find, if the light has been pretty
equally diffused, the whole of the face, and then the dress.

The following remarks, by the Count de MontLzon, are of
value —

To Iodize Collodion. — I have tried many methods of iodizing
collodion. Those which have given the most successful results
are the following : —

1st. In 1 oz. of collodion put a little iodide of sUver and about
3 or 4 graius of iodide of potassium, and then shake it well up.
The collodion becomes veiy tiu-bid, but on being left for some
hoiu's it gradually clears up, begiuuing at the bottom. When it
is quite clear, pour off the liquid into another bottle.

THE COLLODIOX PROCESS. 259

2d. To 1 oz. of collodion add 2 grains of iodide of ammonium.
This ■will give veiy beautiful gradation in the half-tones, but not
so vigorous a picture as the first.

3d. In 8 di-achms of pure alcohol dissolve perfectly 8 grains of
iodide of ammonium or iodide of potassium, and i grain of iodide
of silver; then add 24 drachms of collodion. The iodide of silver
ought to be fi'eshly made, or the residting negative vrill be of
inferior quality. The iodide of ammonium too ought to be
newly made. This collodion is one of the most sensitive, but
the half-tones produced by it are inferior.

4th. In 8 drachms of alcohol dissolve 8 grains of iodide of
potassium, 4 of iodide of ammoniiun, and ^ grain of iodide of
silver; then add 24 drachms of collodion. This forms a very
sensitive medium.

5th. In 2^ oz. of coUodion, 5 drachms of alcohol and 5 minims
of liquid ammonia, dissolve 14 gi-ains of iodide of ammonium.
This forms a very good collodion, veiy sensitive and coloiu'less.

6th. In 2 di-achms of alcohol dissolve 6 grains of iodide of
potassium, and add 6 drachms of coUodion.

We now come to the

Method of Operating. — I employ nothing but water to clean
the glass plate with, using plenty of it, and iiibbing the glass
with the hand till the water flows freely over the surface. It
must be well chied and nibbed clean with a linen cloth wliich
has been well washed without the use of soap. When the col-
lodion comes away from the glass, it is almost always in conse-
quence of the existence of grease or dirt, or of a little moisture
upon the siu-face.

Pour the coUodion upon the glass in the usual way, and
almost immediately immerse it in the bath of nitrate of sUver,
30 grains to the ounce of water, lifting it in and out of the solu-
tion to allow the ether to escape. When it assumes a bluish
opal hue, it is ready for use. By adding a Uttle alcohol to the
solution, one part of alcohol to ten parts of water, and one part
of nitrate of silver, the collodion is more speedily rendered sen-
sitive, and the image produced is more vigorous.

It seems of some importance to immerse the glass in the
nitrate bath, and to place it in the slide in the same direction as
that in which the coUocHon was poured off the glass plate.

After the appearance of the opal hue, if the bath be an old
one, the plate may be left in it for some time without injmy ;
but if the bath be new, it must not be left longer than is neces-
sary to excite, or the rdtrate would attack the iodide of silver.

To obviate this, it is weU in making a new bath to add 1
grain of iodide of sUver to each ounce of the nitnite solution.

360 PRACTICE OF PHOTOGRAPHY.

It is unnecessary to filter the bath, as it is often altered in its
nature by passing thi-ough paper containing injimous chemical
constituents. A little blotting-paper di'awn over the surface
will remove any particles of dust that may be floating upon it.

If the bath contain alcohol, it should, when not in use, be
kept in a stoppered bottle.

The Development of Image. — To effect this the plate must
be taken again into the room, and with care removed from the
slide to the levelling stand.

It wiU be well also to caution the operator respecting the re-'
moval of the plate. Glass, as before observed, is a bad conductor
of heat ; therefore, if in taking it out we allow it to rest on the
fingers at any one spot too long, that portion will be warmed
through to the face, and as this is not done until the developing
solution is ready to go over, the action will be more energetic
at those parts than at others, and consequently destroy the
evenness of the picture. We should, therefore, handle the jjlate
with cai'e, as if it already possessed too much heat to be com-
fortable to the fingers, and that we must therefore get it on the
stand as soon as j)ossible.

Having then got it there, we must next cover the face with
the developing solution.

This should be made as follows : —

Pyrogallic acid 5 grains.

Distilled water 10 oz.

Glacial acetic acid . . . . 40 minims.
Dissolve and filter.

Ml'. Delamotte employs

Pyrogallic acid 9 grains.

Glacial acetic acid .... 2 drachms.
Distilled water 3 ounces.

Now, in developing a plate, the quantity of liquid taken mvist
be in proportion to its size. A plate measuring 5 inches by 4
will require half an ounce ; less may be used, but it is at the risk
of stains ; therefore we would recommend that half an ounce of
the above be measured out into a per/ectli/ clean measure, and
to tliis from 8 to 12 drops of a 50 grain solution of nitrate of
silver added.

Pour this quickly over the surface, taking care not to hold the
measure too high, and not to pom- all at one spot, but having
taken the measure properly in the fingers, begin at one end, and
carry the hand forward ; immediately blow ujdou the face of the
plate, which has the efi'ect not only of difiusing it over the sur-

THE COLLODION PEOCESS. 261

face, but causes the solution to combine more equally witli the
clamp surface of the plate : it also has the effect of keeping any
deposit that may form in motion, which, if allowed to settle,
causes the picture to come out mottled. A piece of white paper
may now be held under the plate, to observe the development of
the pictiu'e; if the light of the room is adapted for viewing it in
this manner, well; if not, a light must be held below, but in either
case arrangements should be made to view the plate easily whilst
under this operation, a successfid result depending so much upon
obtaining sufficient development ■without caiTying it too far.

As soon as the necessary development has been obtained, the
liquor must be poured off, and the siu'face washed with a little
water, which is easily done by holding the plate over a dish and
pouring water on it, taking cai-e, both in this and a subseqixent
pai't of the process, to hold the plate horizontally, and not ver-
tically, so as to prevent the coatmg being torn by the force and
weight of the water. ,

Protosulphate of iron, which I first introduced as a photogra-
phic agent in 1840, may be employed instead of the pyi-ogallic
acid with much advantage. The beautifid collodion portraits
obtained by Mr. Tunny of Edinbui'gh, are all developed by the
ii'on salt. The following are the best proportions : —

Protosulphate of iron .... 1 oz.

Acetic acid 12 minims.

Distilled water 1 pint.

This is to be used in the same manner as the former solutions.

Fixings of Image. — This is simply the removal of iodine
from the siu'face of the plate, and is effected by pouring over it,
after the water, a solution of hyposulphite of soda, made of the
strength of 4 oz. to a pint of water. At this point daylight may
be admitted into the room; and, indeed, we cannot judge well
of its removal without it. We then see, by tilting the plate to
and fro, the iodide gradually dissolve away, and the different
parts left more or less transparent, according to the action of
light upon them.

It then only remains to thorovighly wash away every trace of
the hyposulphite of soda, for, should any of the salt be left,
it gradually destroys the picture. The plate should, therefore,
either be immersed with great care in a vessel of clean water,
or, what is better, water poiu'ed gently and carefully over the
siu-face.

After this it must be placed upright to dry, or held before afire.

We have now carried the operator carefully thi-ough every
stage of the process, from the cleaning of plate to the fixing of

262 PRACTICE OF PHOTOGRAPHY.

image ; but our remarks have reference to collodio-iodide alone ;
that is gun-cotton dissolved in ether, charged wdth an iodide of
sUver. "We cannot, however, consider our task finished ^\^thout
mentioning the addition of gutta-percha to the collodion. This
valuable discovery was made by Mr. P. W. Fry, to which gentle-
man belongs some of the most iinporcant steps made in the
aii;.

The sensibility of the plates appears to be more materially in-
creased by the addition of the gutta-percha ; indeed, pictures by
superposition may be obtained with absolute instantaneity, and
in the camera oljscvira in less than a second of time.

The plan of proceeding to obtain this extreme sensibility, as
recommended by Mr. Fry, is to obtain a thick and strongly
charged collodio-iodide, and to two parts of this add one of a
saturated ethereal solution of gutta-percha, allowing it to stand
a day or two to clear itself, previous to being used.

The plate is ihen coated iu the usual manner. As the ether
evaporates a peculiar white film comes over, at which time it is
ready for immei*sion in the bath. This must be conducted as
previously described, and, from its extreme sensibility, with, if
possible, greater precaution than before.

For the development of negative pictures, Mr. Fry recom-
mends the pyi'Ogallic solution, rather stronger than that pre-
viously given, about one gi-aiu to the ounce, "s\dth the addition
of an extra portion of acetic acid, and the plate re-dipped in the
nitrate hath, in preference to adding silver solution to the pp'o-
gallic acid.

In fixing the image after development, it is necessary to keep
the hyposidphite on longer than "ndth the ordinary collodion, as
the iodide is held -svdth greater tenacity. In other respects the
method of jwoceeding is precisely the same.

Mr. Thomas has modified the collodion process as follows : —

1 st. To prepare the glass. — Roughen the s\u*face of the edge about
one-sixteenth of an inch all roimd A^ith coarse emery paper ; tliis
prevents contraction of the film, and enables you to pump upon
it, if necessary, Avithoiit any fear of coming off.

2d. To clean the glass if nev/. — Make a mixtiu'e of spirits of
wine and liquor ammonia equal parts, render it as thick as cream
with tripoli ; with a piece of cotton wool, kept for this purpose,
rub a small quantity over that side scratched as described, wash
well under a tap of water, and wipe diy with a piece of old
linen washed A\dthout soap, and kept scrupulously clean for this
purpose.

N.B. To clean a glass after having tised it, when not var-
nished, wash off the collodion film with water, and chy as above.

THE COLLODION PROCESS. 263

Always Avipe tlie glass jiist before use, and breathe upon it ; if
clean, the moisture evaporates evenly.

3d. Pour into the centre as much collodion as the glass will
hold, and poiu' off at that corner diagonal to the one by which
the glass is held : prevent the formation of lines, by altering
quickly the position of the glass before the film dries ; with
observation and practice dexterity is easily acquii-ed.

l:th. As soon as the collodion ceases to nm, plimge the pre-
pai'ed glass, without stopping, into the following bath : —

Into a 20-oz. stoppered bottle put

Nitrate of silver 1 ounce.

DistHled water IG ounces.

Dissolve.

Iodide of potassium .... 5 grains.

Distilled water 1 di-achm.

Dissolve.

On mixing these two solutions a precipitate of iodide of sUver
is formed. Place the bottle containing this mixture in a sauce-
pan of hot water, keep it on the hob for about twelve hoiu"s.
shake it occasionally, now and then remo^-ing the stopper. The
bath is now perfectly satm-ated ■n-ith iodide of silver ; when
cold, filter through white filtering paper, and add.

Alcohol 2 drachms.

Sulphui-ic aether ... 1 di-achm.

A convenient way of satiu'ating the nitrate of silver bath
Avith iodide of silver, is to di&solve the 1 oz. of nitrate of sil-
ver in 2 oz. of the water, add the solution of iodide of potas-
sium to this strong solution ; the precipitate thus foiTiied is by
shaking entii-ely dissolved; now add the remaining 14 oz. of
water, when the iodide of silver is again thi'own down, but in
such a finely divided state as to render the complete saturation
of the bath more perfect, obviating the necessity of frequent
shaking. After half an hour, add the alcohol and ether, and filter.

oth. Allow the pi-ej^ared glass to remain in this bath eight or
ten minutes : just before taking it out move it up and down
three or four times; drain it, but not too closely; when in the
frame place upon the back a piece of common blotting-paper to
absorb moisture ; the sooner it is placed in the camei-a the better.

6th. The time of exposure can only be ascertained by pi'actice,
no rules can be laid down, and I am unacquainted with any
royal road but that of experience, leading to constant success in
this most important section of photogi"aphy.

7th. The plate being taken from the camera and placed upon

2G4 PRACTICE OF PHOTOGRAPHY.

a levelled stand, develope immediately the latent image with the
following solution : —

Pyrogallic acid 3 grains.

Distilled water 1 ounce.

Acetic acid (glacial) ... 1 drachm.
Mis.

Take one part of this solution and two parts of distilled water
for use. The pyrogallic solution made with proper acetic acid
and of the above strength, may be kept for a month or more in
a cool place away 6'om the light.

8th. When the image is sufficiently intense, wash freely with
common filtered water; then pour on a saturated solution of
hyposulphite of soda, which should immediately remove the
iodide of silver, wash again well with water ; allow as miich as
the plate will hold to soak in for at least half an hour, to re-
move aU ti-aces of hyposidphite of soda ; lastly, stand up to dry,
and if required, varnish with amber varnish.

Having, by the foregoing means, obtained and fixed a negative
photographic image on glass, and which is capable of producing
positives upon paper by the ordinary photographic means, it is
as well, previous to obtaining these, to render the tender film of
collodion less liable to injury.

This can be accomplished by means of a varnish, of which
there are different kinds that may be used.

By far the best kind of varnish which can be employed is one
for which we are indebted to Dr. Diamond. This is made by
powdering some amber and putting it into clxloroform ; by the
application of a gentle heat under pressure, with care, a perfect
solution takes place. This varnish flows readily over the plate,
and dries in a few minutes, leaving a beautifully transparent
hard glaze upon the picture.

It was shown by Mr. Home in the early days of coUodion,
that the negative images could be converted into positive ones
by mixing with the pyrogallic solution a very small quantity of
nitric acid ; but it has since been sho^vn by Mr. Fry, and others,
that a better result may be obtained by the use of proto-sulphate
and proto-nitrate of iron.

The former salt is readily obtained, and in a very pure form.
It should be used as follows : —

Proto-sulphate of iron . . . . 10 grains.-

DistiUed water 1 oz.

Nitric acid ....... 2 drops.

To develope the image pour the above over the plate, taking care
not to caiTy the development too far.

THE COLLODION PROCESS. 265

The proto-nitrate may be obtained by double decomposition,
as recommended by Dr. Diamond : 600 gi-ains of proto-sulphate
of iron are dissolved in one ounce of water, and tbe same quantity
of nitrate of baryta in six ounces of water ; tliese being mixed
together, proto-nitrate of iron and sulphate of baiyta are formed
by double decomposition ; also by dissolving sulphuret of iron
in dilute nitric acid, as recommended by jNIr. Ellis, who proceeds
as follows : —

To one ounce of nitric acid and seven of water, add a small
quantity of sulphuret of iron broken into fragments. Place the
vessel aside, that the sulphnretted hydi-ogen may escape, and the
acid become saturated with ii'on. Pom- oif the liquid, and filter.
Then boil ux a florence flask, to get rid of the sulphm-, and again
filter, when a dark green liquid will be obtained, which is the
proto-nitrate of iron. This should be kept in well-stopped
bottles, and protected from the aii- as much as possible, to prevent
its changing into a pernitrate, in which state it is quite useless
as a photogi-aphic agent.

To develope the picture mix one part of the above proto-
nitrate with three of water, and apply it to the plate in the
ordinary way, when a most beautiful clear image can be obtained.

The negative image being developed, a mixtui'e of pyi-o-gallic
and hypo-sulphite of soda, which has imdergone partial decom-
position, is potired over the plate, and then it is gently warmed.
Upon this the darkened parts are rendered brilliantly white by
the formation of metallic silver. This picture being backed
up with black velvet assumes the air of a fine daguerreotype,
without any of the disadvantages arising from the reflection of
light from the polished silver siu-face. For this beautiful residt
photogi-aphy is indebted to Dr. Diamond, who is still pm-suing
the subject with much zeal. We have also seen similar efiects
produced by Mr. Fry and Mr. Berger, by the use of the proto-
sulphate of iron solution and pyi-ogallic acid. The image is first
developed by the iron and the solution poured ofi"; immediately
another of pyrogallic acid is poured on, and the efiect is produced.

The pictm-es are fixed with the hyposulphite in the usual
method.

A peculiar whitening process was introduced by Mr. Archer,
which is as follows : —

The pictiu'e being thoroughly washed in plenty of water, after
fixing with hypo-sidphite of soda, is treated in the following
manner : —

Prepare a saturated solution of bi-chloride of mercury in mu-
riatic acid. Add one part of this solution to six of water. Pour
a small qiiantity of it over the pictiu'e at one corner, and allow

266 PRACTICE OF PHOTOGRAPHY.

it to inin evenly over tlie glass. It will be found immediately to
deepen the tones of the pictm-e considerably, and the positive
image will almost disappear ; presently, a peculiar whitening will
come over it, and in a short time a beautifully delicate white
picture will be brought out.

The negative character of the drawing will be entirely de-
stroyed, the white positive alone remaining. This picture, after
being well washed and dried, can be varnished and preserved as
a positive ; but nevertheless, even after this bleaching, it can be
changed into a deep-toned negative, many shades darker than it
was oi'iginally, by immersing it, after a thorough wasliing, in a
weak solution of hyjjo-sulphite of soda, or a weak solution of
ammonia. The white pictm^e will vanish, and a black negative
will be the result.

It is very singular that the pictiu"e can be alternately changed
from a white positive to a black negative many times in succes-
sion, and very often with improvement.

Thus, by the above process, a most perfect white positive
or a deep black negative is produced, quite distinct from each
other.

In the first part of this after-process, it will be observed that
the effect of this bi-chloride of merciuy solution is to deepen the
shades of the picture, and this peculiarity can be made available
to strengthen a faint image, by taking the precaution of using
the sokition weaker, in order that the first change may be com-
pleted before the whitening effect comes on.

The progress of the change can be stopped at this point by
the simple appliciation of water.

The author first pointed out the remarkable action of corrosive
sublimate, in his paper, published by the Royal Society, on the
Daguerreotyjie process on paper.

M. Adolphe Mai-tin has published some remarks on the col-
lodion in the Comptes Rendus of 5th July, 1852.

The collodion he employs is made of —

30 gi'ains of cotton.
750 grains of nitrate of potash.
1500 gi-ains of sulphuric acid.

This is well washed and dissolved in 10 volmnes of ether and 1
volume of alcohol : by this, 15 gr'ains of gun-cotton are dissolved
in 1860 grains of ether, and 930 grains of alcohol : add then to
this collodion, 15 gx'ains of nitrate of silver transformed into
iodide, and dissolved in 20 grains of alcohol by means of an
alkaline iodide. M. Adolphe Martin prefers iodide of ammonium.
The plate is next plunged into a bath of 1 part distilled water,

THE COLLODION PROCESS, 267

1 x^gth nitrate of silver, and l^^jth nitric acid. The image is
developed by proto-snli^hate of iron, and he effects the change
from negative to positive by a bath of double cyanide of silver
and potash, consisting of about 2 quarts of water, in which are
dissolved 375 grains of cyanide of potassium, and 60 grains of
nitrate of silver. The pictures thus produced are remarkable
for their intense whiteness.

We must allow jMr. Archer to give his own description of the
very ingeniously constracted camei'a, which he has devised for
the collodion process. The practice of the collodion process in
the open air is a matter of some difficulty ; some ingenious
cameras have been devised for this piu'pose ; but thei-e are few
better contrivances than that introduced by Mr. Archer, and
modified by Messrs. Griffin & Co.

Description of the Camera for the Collodion. — '• I will proceed
to give a general description of the camera I have constnicted,
premising that it admits of being made as a very light folding
camera, if thought necessary.

"It is a wooden box, 18 inches long, 12 inches wide, and 12 '
inches deep, and is capable of taking a picture 1 inches square.
Externally it may be thus described : — In front it has a sliding
door, with a circular opening in it, to admit the lens : this
sliding door enables the operator to lower, or raise, the lens,
and consequently the image fonned by it, on the ground glass,
as the view may require. The two sides of the camera have
openings cut in them, into which sleeves of India nibber cloth
are fixed, to admit the hands of the operator, and are ftu-nished
with India rubber bands at the lower ends, which press against
the wi'ists, and prevent the admission of light.

" The back of the camera has a hinged door fitted at its
upper part -sv-ith an opening of just sufficient size for the eyes,
and shaped so as to fit close to the face. A black cloth is tied
round this end of the camera, to prevent any ray of light
peneti-ating at this opening. In the top of the camei-a, neai'
the front, is inserted a piece of yellow glass, to admit a small
quantity of yellow light, and is closed with a hinged door, to
regulate the quantity of Hght reqviired.

" The interior of the box is fiii-nished with a sliding frame,
to support the ground glass or the bath and the prepared plate ;
and it has a stop, by means of which any focus from 3 inches to
15 inches can easily be obtained.

" The bottom of the camera is furnished with a gutta percha
tray, about 1 inch deep, to hold the washings, &c., when the
camera is in operation.

" Also, the bottom of the camera at the back has an opening

268 PKACTICE OF PHOTOGRAPHY.

cut in it, extending nearly the whole width of the camera, and
as far in as the edge of the gTitta percha tray.

" This opening is intended to admit, when the camera is in
use, a light wooden case containing the glass bath, focusing
frame, stock of glass, and paper required in the process.

" There are various other little contrivances which I have
not specified ; such as a drawer for the pictures, a shelf for
bottles, &G.

This form of camera will admit of the following manipu-
lation : — Having placed it upon a stand pointing to the object
to be taken, the hinged door at the back is opened, and the bath
is three parts filled with the solution of nitrate of silver ; a
plate of glass is then taken from the cell, and cleaned if •
necessary.

"The collodion is poured on in the manner previously de-
scribed; when the film has set a little it is immersed in the
nitrate of silver bath, and the lid of the bath is closed down
upon it. The next step is to obtain the focus with the ground
glass : this can be done whilst the collodion is becoming iodized.

"After adjusting the sliding frame to the proper focal distance,
the camera must be closed, and the rest of the process conducted
by passing the hands through the sleeves, and placing the eyes
close to the aperture in the back of the camera, and drawing the
black cloth over the front of the head.

" By the aid of the yellow light admitted from the top, the
operator can carry on the rest of the process. The plate is now
ready for the action of light, and is taken from the bath ; or the
bath itself, with the plate in it, is placed in the sliding frame
The refracted image is at once thrown upon the sensitive plate.
After the reqviisite exposure, the plate is taken from the bath,
and the picture is developed with the solution previously
described. The progress of this operation can be seen by aid of
the yellow light, keeping the eyes close to the aperture behind.

"When, from experience, the pictm-e is sufiiciently brought
out, a little water is poured on the glass to wash off the deve-
loping solution, and the cfrawing is partially fixed by the appli-
cation of a small quantity of a solution of common salt.

"The cfrawing may now be removed from the camera without
fear of being injured by light, and the remainder of the opera-
tions can be conducted outside the camera.

"If the film is sufficiently strong to bear removal from the
glass, the following procedure is adopted : — The plate of glass
is placed horizontally upon the back lid of the camera, which
is hrmg so as to form a temporaiy table, and the film is loosened
from the edge of the glass with a flat strip of glass ; a sheet of

THE COLLODION PROCESS. 269

damp paper is then placed flat on the drawing, and rather within
its upper edge ; the film is turned over the edge of the paper,
and a glass rod is placed just witliin the edge. The sheet of
paper with the collodion in contact with it is now raised from
the glass, and rolled ujj on the glass rod. When the drawing
is entirely enclosed in the paper, the rod is removed, and the
delicate film thus encased is put away into its proper receptacle,
to be finally fixed and mounted at leisiu-e.

''The drawing thus rolled up can be preserved for months
without injmy, provided it is kept slightly damp ; and if each
drawing is enclosed in another sheet of papei*, its preservation
is stdl fm'ther secured.

" The advantages of a camera of this kind may be thus enu-
merated.

'•' It allows the preparation on the spot of the most sensitive
surfaces ; their immediate use whilst the sensibility is at its
maximum ; the ready development of the image, and after
fixing.

" All these operations being carried on consecutively, the
operator can, after the first trial, see what residts the progress
of his laboiirs is likely to produce.

" It gives him the power of shading ofi" any portions of the
view during the action of the light, by holding in front of the
prepared plate and near the lens a movable screen, or any flat
piece of wood, as the case may requii'e ; thereby preventing the
too rapid action and consequent solarization of the distant por-
tions of the scene. The spire of a church, for instance, pointing
upwards into a bi-ight sky, often requires this precaution to
prevent its being entirely lost. Other instances of this efiect will
readily suggest themselves to those at all acquainted -sN-ith the art.

" The camera can be made, with slight modifications, applicable
to any other process on paper or glass, and of coiu'se obviates
the necessity of any kind of portable tent." — Archer, F.S.,
Manual of the Collodion Photographic Process.

The following figures represent JNIr. Archer's camera, as con-
structed and improved by Mr. Griffin : —

The figure 67 is a section of the camera, and 68 its external form,
which, with a view to portability, is constnxcted so as to serve
as a packing-case for the entire apparatus represented by figs.
67 to 73. a is the sliding door that supports the lens h. c c are
the side openings fitted with cloth sleeves to admit the operator's
arms, c? is a hinged door at the back of the camera, which can
be supported like a table by the hook e. f is the opening for
looking into the camera during an operation. This opening is
closed, when necessary, by the door g, which can be opened by

270

PRACTICE OF PHOTOGRAPHY.

the hand passed into the camera through the sleeves c. The
yellow glass window which admits light into the camera during

an operation is under the door h. i is the sliding feme for
holdiiig the focusing glass, or the frame with the prepared glass,

either of which is fastened to the sliding frame by the check k.
The fi-ame slides along the rod I, I, and can be fixed at the proper
focus by means of the step m. n is the gutta percha washing

THE COLLODIOX PROCESS.

271

tray, o is an opening in the bottom of the camera near the
door, to admit the well p, and which is closed, when the well is
removed, by the door q. The well is divided into two cells, one
of which contains the focusing glass, and the other the glass
trough, each in a frame adapted to the sliding frame i. On each
side of the sHdiug door that supports the lens a, there is, within
the camera, a small hinged table r, supported by a bracket s.
These two tables sei'\'e to support the bottles that contain the
solutions necessary to be applied to the- glass plate after its ex-
posure to the lens.

Figs. 69 and 70 represent two cases, containing the various
insti-uments and chemical preparations required for the coUodion

process, a, fig. 69, is a grooved cell for a series of glass plates.
6 is a receptacle for the lens of the camera, c contains a spii'it
lamp ; d, a paii- of glass measures ; e, a porcelain pestle and

mortar. The door / encloses a space containing a funnel with
filter papers, and silks and leathei-s for cleaning the plates, g con-
tains a small retort stand, a porcelain capsule, and a box with
scales and weights.

272

PRACTICE OF PHOTOGRAPHY.

The case, fig. 70, Ls di^^.ded into two compartments. One
side, a, contains twelve stoppered glass bottles, with the various
chemical preparations required by the operator. The other side,
which can be closed by the door h, contains a supply of photo-
gi'aphic paper, both for negative and positive pictures.

Fig. 71 is the glass trough for holding the nitra.te of silver
solution.

71. 72.

Fig. 72 is a fi-ame for fixing in the camera such plates of glass
as do not require to be exposed to the lens while stUl in the
glass trough.

Fig. 73 is a pressure fi-ame for the preparation of positive from
negative photographs.

Fig. 74 is a section of this
frame.

Both of these frames, figs. 72
and 73, are so contrived as to
be sidtable for plates of many
different sizes. In the frame
represented by fig. 72, the bars
a and h are both movable, to
permit the fixing of the plate
in the camera directly opposite
the centre of the lens. In the
frame represented by fig. 73,
the bar a alone is movable, and is fastened by screws that move
in the slits h, h.

The whole of these boxes and frames can be conveniently
packed id the camera. The box, fig. 68, is passed lq by the
side-door ; the well, p, and all the other cases and frames, by the
door d; and the camera, thus loaded for transport, is put into a
strong leather case.

73.

THE COLLODION PROCESS. 273

It has been proposed to use the bromide, instead of iodide of
silver, in collodion.

The most successful operator -with the bromodized collodion
appears to have been M. Laborde, TS'ho communicated the fol-
lowing as the results of his pi-actice to La Lumiere, French
photographic joiuTial : —

" I have studied the action of bromides by themselves and in
combination vdth. collodion. My choice naturally fell upon
those soluble in alcohol or ether, and I have tried the bromides
of iron, nickel, cadmium, zinc, and mercury. The bromides of
iron, nickel, and cadmiimi yielded the best restdts, and among
these I give the preference to the bromide of cadmium. I have
found it to possess so many advantages, that I have been several
times tempted to banish all iodides ti'om my preparations.
\5 gi-ains of bromide of cadmium, added to 1^ ounce (by weight)
of solution of collodion, gives a liquid which may be used at once,
and which has been kept for about five months, up to the pre-
sent time, without perceptible change. Sidphate of iron, or
pyrogallic acid, are used for developing j gallic acid produces but
a veiy middling effect ; almost all the details of the image appeal"
at once, the effect of the weakest radiations becomes sensible;
but the extreme tints of the proof are not siitficiently different
to allow of being printed 6-om with success. The following pro-
cess is made use of to give them an actual value in this respect.

" By adding a weak proportion of iodide of potassium to the
bromide of cadmium the sensibility is increased, and we obtain
at the same time a gi'eater difference between the extreme tints
of the proof; the negatives are therefore superior. The follow-
ing are the proportions usually employed : —

Bromide of cadmium . . . . 12 grains.
Iodide of potassium .... 0-.3 gi-ain.
Collodion I5 ounce.

" At first the iodide of potassium tinges the collodion with yel-
lowish-red, but the bromide of cadmium by degrees removes this
tint, and the solution becomes coloiu-less."

Mr. W. Ci'ookes has lately commenced the investigation of
these phenomena -^-ith a zeal and accuracy from wliich we may
expect some important results. The following communication,
made to the Photographic Society of London, will be read with
interest : —

" I have for some time past been working with bromized collo-
dion, and as. from my expierience, it seems likely to become an
agent of great value, perhaps the following accoimt of some ex-
periments ■vs'ith it may prove of interest to any photographer.

274: PRACTICE OF PHOTOGRAPHY.

who lias tlie time and means at his disposal to investigate the
subject more fully in its practical application.

"To prepare the collodion I proceed as follows : — Mix together
equal bulks of sulphuric acid, specific gravity 1 '80, and nitric acid,
specific gravity 1 "50 ; stir well with a glass-rod, and then, while still
warm, immerse as many pieces of good Swedish filtering-paper
as the vessel will conveniently hold. Allow them to remain
together for one houi' ; then poixr the liquid away, and wash the
paper until free from the slightest ti'ace of acid, and allow it to
dry in a warm room.

" Take 8 drachms of the piu-est washed ether and ^ drachm
of spirits of wine, 60° above proof and dissolve in this 6 grains
of the above-prepared paper. This collodion may be bromized
in the following manner : — In a small bottle place about 2 grains
of crystallized nitrate of silver and about 10 grains of pure
bromide of ammoniimi; pour on this 2 drachms of spirit 60°
above proof, and allow them to remain together for some hours,
shaking the mixtiu'e several times. One drachm and a half of
the supernatant liquid are to be added to every ounce of the
previously prepared collodion. Thus bromized, it will remain
perfectly colourless and good for a long time.

" I excite the j^late in a 30-grain silver bath, wliich has been
previously satm-ated with bromide of silver ; about two minutes'
stay in this bath is generally sufficient, though a little longer
time does not injure it. The film of bromide of silver is a pale
orange by transmitted and blvie by reflected light, and is very
transj^areut. For developing, I prefer protonitrate of ii^on,
being more accustomed to it, but have no doubt that in other
hands pyrogallic acid woidd answer equally well. The above pro-
jDortions of paper, alcohol, and bromizing compound may be
varied within certain limits without much inflviencing the result.
I have given the proportions which I am most in the habit of
using, but would recommend that the experimentalist shoidd
ascertain for himself whether a slight departure from the above
proportions would give a collodion of the consistency and strength
of that with which he is most accustomed to manipulate.

" The chief advantages it seems to possess over the ordinary
iodized collodion, besides its great sensitiveness, are the follow-
ing. In a landscape the required opacity of the more strongly
illiuninated parts (the sky, for instance) is not lost by over-
exposiu'e; vegetation is also more easily copied. Its superior
sensitiveness to colom-ed light is, however, most strikingly showTi
when coloured glass or sulj)hate of quinine (as suggested by Sir
John Herschel) is employed to absorb the strongly acting in-
visible rays. To prove this, I arranged several flowers and

THE COLLODION PKOCESS. 275

plants with a view to obtain a great contrast of colom-, light,
and shade. The best pictm-e I coixld obtain of them on iodized
collodion was, as I had anticipated, wanting in half-tint, very
few of the colours producing an adequate impression. When,
however, bromized collodion was used imder the same cii'cum-
stances, but with the interposition of the bath of sulphate of
quinine, eveiy pai-t came out with nearly the same gradation
and depth of hght and shade as existed in nature : this picture
on bromized collodion behind the quinine bath requii'ed 40
minutes ; on iodized, without the quinine solution, 4 minutes •
but when I attempted to take a photogi-aph on iodized collodion,
v/ith the quinine bath interposed, I fovmd that with the light I
was working in, t)ie plate would not keep a sufficient length of
time to enable me to obtain an image.

"The advantages of using bromine in the place of iodine, for all
objects having a green or vellow coloiu-,have been already pointed
out."

CHAPTER VIII.

THE USE OF ALBUilEX OX GLASS PLATES, &C.

Section I. — ALBrjiEX ox Glass.

In the Technologist for 1848, M. Niepce de Saint Victor pub-
lished his mode of applying albumen to glass plates. M. Blan-
quart Everard followed ; and successively albumen, gelatine,
seram, and other animal substances, have been recommended
for application on glass : but few of them have been found to
answer so perfectly as albumen applied according to the direc-
tions of M. Le Gray.

He recommends that the whites of fresh eggs, equal to about
five fluid ounces, be mixed with not more than 100 grains of
iodide of potassiiyn, and about twenty grains of the bromide,
and half that quantity of common salt.

He then directs you to beat this mixture in a large dish ■n-ith
a wooden fork, until it forms a thick white froth ; to let it repose
all night, and the next day to decant the viscous liquid which
has deposited, and use it for the preparation of your glasses.

For this purpose take thin glass, or, what is much better,
ground glass, on ichich the adherence is more per/ect ; cut it the
size of your camera frame, and gi-ind the edges.

The success of the proof is, in a great measure, due to the
evenness of the coat of albumen.

To obtain this, place one of your glasses horizontally, the
unpoHshed side above (if you use ground glass, which I tliink
preferable), and then 2)oiu' on it an abundant quantity of the
albumen. Take a inile of glass, veiy straight, upon the ends of
which have been fastened two bands of stout paper steeped in
white wax : hold this with the fingers in such a manner that
they will overlap the sides of the glass plate about one-eighth
of an inch. You then di'aw the rule over the glass with one
sweep, so as to take oflf the excess of albumen. The object of
the sKp of paper is to keep the glass rxde from the siu'face of
the plate, and insui-e a thin but even coating of the albuminous
mixture.

Thus, in making the paper band more or less thick, you vary

THE USE OF ALBUMEN OX GLASS PLATES. 1 1 t

the tMckness of tlie coating. Or you may arrive at the same
i-esult by pasting two naiTow bands of paper on the sides of
the plate, and passing simply the inile do^\'n. I prefer the fii-st
means, because, with the second, one is almost sure to soil the
glass in sticking on the paper.

You must never go the second time over the glass with the
i-ule, or you will make air bubbles. When thus prepared, per-
mit the plate to diy spontaneously, keejiing it in a horizontal
position and free from dust, ^^^len the coat of albumen is well
dried, submit your glasses to the temperature of 160° to 180°
Fahrenheit ; this you may do either before a quick fixe, or by
shutting them up in an ii'on saucepan well tinned, with a cover ;
you then place the saucepan in a bath of boiling water ; the
action of the heat hardens the albumen ; it becomes perfectly
insoluble, and ready to receive the aceto-nitrate of silver.

The glass thus prepared may be kept for any length of time.
I prepare the first coat also by satiu'ating the former mixtm-e
with gallic acid, which gives it more consistency and gi-eater
sensitiveness.

When you -wdsh to make a proof (Ijy using the preparation
moist), you pkmge the glass thus prepared in a bath of aceto-
nitrate of silver, described in the second ojieration of the
negative paper. This operation is very delicate, because the
least stoppage in its immei'sion in the bath will operate on the
sensitive coating, and cause irregularities which nothing can
remedy.

To obtain this instantaneous and regular immersion, I make
a box with glass sides, a trifle larger than the plate, and about
half an inch vdde, ^vith wooden gi-ooves, similar to those in the
Daguerreotype plate box : into this I poiu* the aceto-nitrate,
and let the prepared glass fall into it with a single movement,
leaving it to soak fom- or five minutes in the bath ; then remove
it, wash well ^vith distilled water, and expose it in the camera
while moist. The time will vary from two to thii-ty minutes,
or nearly double that time if the glass is diy.

When you wish to oj^erate "s\-ith the glasses dry instead of
moist, it is proper to dip them in a bath of gallic acid a quarter of
an hour after they are taken out of the aceto-nitrate bath ; then
well wash them with distilled water, and chy them as dii-ected.

When you take the plate out of the camera, you develope the
image in the same way as the negative on paper, by putting it
into a bath of satiu-ated gallic acid : when it is well developed,
fix it by the same method indicated for the jxaper.

To obtain a positive proof, it is sufficient to apply on the
negative proof a sheet of common positive paper, or, better

278 PRACTICE OF PHOTOGRAPHY.

still, a sheet of the positive albuminized paper, which is pre-
■viously described.

You then put it in the pressure frame, placing above it a piece
of black cloth pasted on one side of a thick sheet of glass ;
then shut the frame, giving to the proof a slight pressure ;
after which, expose it to the light. In order to follow its
action you may just raise it by one corner of the glass, to judge
of the tint which the image takes : when you think it sviffi-
ciently exposed, take it out of the frame, and fix it the same as
the positive paper.

Niepce de Saint Victor has recently published a process in
wliich he employs starch instead of albiimen on the glass plates.
The main features of this process are as follows : — About 70
grains of starch are rubbed down with the same quantity of
distilled water, and then mixed Tvith three or foiu* ounces more
water ; to this is added 5^ gi'ains of iodide of potassium dis-
solved in a very small quantity of water, and the whole is boiled
until the starch is properly dissolved. "With this the glass plates
are carefidly covered, and then placed to dry on a perfectly
horizontal table. When thoroughly dried, the aceto-nitrate of
silver is applied by wetting a piece of paper, placing this on the
starch, and over it another piece of paper wetted with distilled
water. This mode of preparation furnishes, it is said, tablets of
great sensibility ; but the starch is liable to bi-eak oflT from the
glass, and there is much difficulty in spreading it uniformly in
the first instance.

Section II. — Mr. Malone's Process.

Some very ingenious experiments have been made by Mr.
Malone, from whose communication the following remarks are
quoted : —

" To the white of an egg its own bulk of water is to be added;
the mixtiu-e, beaten with a fork, is then strained through a
piece of linen cloth, and preserved for use in a glass stoppered
bottle ; then a piece of plate glass, cleaned with a solution of
caustic potash, or any other alkali, is to be washed with water
and di-ied with a cloth. When the glass is about to be used,
breathe on it, and rub its sm-face with clean new blotting paper ;
then, to remove the dust and fibres which remain, use cotton-
wool, or a piece of new linen. Unless this latter, and, indeed,
eveiy other precaution, is taken to prevent the presence of dust,
the pictui-e will be fall of spots, produced by a greater absorp-

THE USE OF ALBOIEX OX GLASS PLATES. 279

tion of iodine (in a subsequent process) in those tlian in the
sm-roimcling parts.

"On tlie clear glass pour the albumen, inclining the plate
fi'om side to side until it is covered ; allow the excess to iim
ofl" at one end of the comers, keeping the plate inclined, but
neai'ly vertical. As soon as the albumen ceases to drop rapidly,
breathe on or warm the lower half of the plate ; the warmth
and moisture of the breath "w-ill soon cause it to part with more
of its albumen, which has now become more fluid : of course,
care miLst be taken to warm only the lower half. Wiping the
edges constantly hastens the operation. Until this plan was
adopted, the coatings were seldom uniform ; the upper half of
the plate retained less than the lower. When no more albumen
mns do^\Ti, dry the plate by a lamp, or by a common fire, if the
dust that it is inclined to impart be avoided.

" The next operation is to iodize the plate. Dilxite pure
iodine "svith dry white sand in a mortar, using about eqiial parts
of each ; put this mixture into a square vessel, and place over
it the albuminized plate, previoiisly heated to about 100° Fah.
As soon as the film has become yellow in coloiu", resembling
beautifully stained glass, remove the plate into a room lighted
by a candle, or through any yellow transparent substance, yel-
low calico for instance, and plunge it vertically and rapidly into
a deep narrow vessel containing a solution of one hundi-ed gi-ains
of nitrate of silver to fifty minims of glacial acetic acid, diluted
with five ounces of distilled water. Allow it to remain until
the transparent yellow tint disappears, to be succeeded by a
milky-looking film of iodide of silver. Washing with distilled
water leaves the plate ready for the camera.

" It may be here noted that the plate is heated in iodizing for
the purpose of accelerating the absorption of the iodine : an ex-
posure to the vapoiu- for ten minutes, with a few seconds' im-
mersion in the silver solution, has been found to be sufiicient."

Hydi'ochloric acid, chlorine or bromine, may be used with
the iodine to give increased sensibility to the plate.

The plate is removed fi^-om the camera, and we poiu' over it
a saturated solution of gallic acid. A negative Calotjqie image
is the result. At this point previoiis experimentalists have
stopped. We have gone fiu-ther, and find that by pouring upon
the surface of the reddish brown negative image, diiring its
development, a strong solution of nitrate of silver, a remarkable
effect is produced. The brown image deepens in intensity imtil
it becomes black. Another change commences — the image
begins to grow lighter ; and finally, by perfectly natiu'al magic,
black is converted into white, presenting the curious phenomenon

280 PRACTICE OF PHOTOGRAPHY,

of the cliange of a Talbotype negative into apparently a positive
Daguerreotype, the positive still retaining its negative properties
■when viewed by transmitted light.

To fix the picture, a solution of one part of hyposulphite of
soda in sixteen parts of water is poured upon the plate, and left
for several minutes, until the iodide of silver has been dissolved.
Washing in water completes the process.

" The phenomenon of the DaguerreotyiDe," says Mr. Malone,
" is in this case produced by very opposite agency, no mercury
being present ; metallic silver here producing the lights, while in
the Daguerreotype it produces the shadows of the pictiire. We
at first hesitated about assigning a cause for the dull white
granular deposit which forms the image, judging it to be due
simply to molecular arrangement. Later experiments, however,
have given iis continuous films of bright metallic silver, and we
find the dull deposit becomes brilliant and metallic when bur-
nished. It should be observed that the positive image we speak
of is on glass, strictly analogous to the Daguerreotype. It is
positive when viewed at any angle but that which enables it to
reflect the light of the ray. This is one of its characteristics.
It must not be confounded with the continuous film image which
is seen properly only at one angle ; the angle at which the other
ceases to exist. It is also curious to observe the details of the
image, absent when the plate is viewed negatively by transmitted
light, appear when viewed positively by reflected light."

Section III. — M. Martin's Process.

Mx". Mayall published a form of process, employed by M. Mar-
tin, which differs in no essential particular from those already
described ; but as involving some rdceties of manipulation, on
which, the writer says, depends the perfection of his finished
pictures, it is thought advisable to quote it.

" First. The albumen of a fresh egg must be beaten into a
snow-like mass with a bimch of quills, dropping into it ten drops
of a saturated solution of iodide of potassium ; allow it to stand
six hours in a j^lace free from dust, and moderately warm, — say
60°.

" Second. A piece of hand-plate glass, eight inches by six, with
the edges ground smooth, must be cleaned as follows : with a
piece of cotton wool rub over both sides ^svith concentrated nitric
acid, then rinse well with water, and dry. Stick a wafer on that
side which I wUl now call the back, to maik it j pounce ujDon

THE USE OF ALBUMEX ON GLASS PLATES. 281

the face a moderate quantity of fine tripoli, moistened with a
few th'ops of a concentrated solution of cai'bonate of potash ;
then with a piece of cotton wool rub the surface briskly in circles
for about five minutes ; then with chy tripoli ; then with clean
cotton to clear away all the dusty particles.

" Tldi'd. To the centre of the back stick a gutta percha ball,
as a handle : strain the prepared albujnen through clean linen ;
poxu' it gently into the centre of the cleaned side of the glass,
keep it moving until the surface is entii-ely covered, mn it into
the corners, and finally pour ofi" any excess at the four corners ;
disengage the gutta j^ercha handle, and place the glass on another
slab, that has been levelled l)y a spirit level, in a place perfectly
free fi-om dust, and modei-ately warm. I will call this my iodo-
alhuminized glass ; it will keep for any length of time, and may
be prepared in daylight.

" Fourth. To excite (a yellow shaded light only being used),
dissolve 50 grains of niti-ate of silver in 1 ounce of distilled water
and 120 grains of strong acetic acid ; pour the whole of this so-
lution into a cuvette, or shallow porcelain dish, a little lai'ger
than the glass plate ; place one end of the iodo-albviminized
glass in the solution ; with a piece of quill support the upper
end of the glass, and let it fall suddenly on to the solution,
lifting it up and down for ten seconds ; take it out and place it,
face upwards, in another dish, half filled with distilled water ;
allow the water to pass over the suiiace twice ; take out the
glass, rear it up to dry ; it is ready for the camera, and wdll
keejj in tliis state ten days, — of course, shut up from daylight,
in a moderately warm place, bvit never moist. The solution
may be filtered into a black bottle, and will do again by now
and then adding a few drops of acetic acid, and keeping it in the
dark. Expose in the camera fi-om four- to ten minutes, accord-
ing to the amount of light and the aperture of the lens. Sup-
pose I say a lens of thi-ee inches diameter, sixteen in focus for
parallel rays, a one inch diaphragm placed thi-ee inches in front
of the lens (one of Ross's photogi-aphic lenses is just the thing),
the exposui'e woidd be in good light about five minutes.

" Fifth. Develope as follows. Place the glass, face upwards,
on a stand with adjusting sci-ews to make it level ; pom- a con-
centrated solution of gallic acid over the siu'face ; the image ■wall
be fi'om half an hoiu- to two houi-s in coming out. It is best to
apply a gentle heat, not more than 10° above the temperature
of the room, it being 60°. Should the image still be feeble,
jjour off the gaUic acid, rinse the proof -with water, and pour on
to it equal quantities of aceto-nitrate of silver and gallic acid
reduced one-half with water. The image will now quickly de-

282 PRACTICE OF PHOTOGRAPHY.

velope ; arrest it in four or five minutes, -wash, it well in three
waters, and fix with hyposulphite of soda as follows : —

" Sixth. Three di-aclims of hyposulphite of soda to one ounce
of water. Allow the proof to remaiu in this solution until all
the yellow iodide disappears, wash it well, rear up to dry, and it
is finished.

" Success is sure to attend any one practising this method,
provided the eggs are fresh and the glass is dean : if the glass is
not clean, or the eggs are stale, the albumen will split ofi" in fixing.

" Caution. — Wash all the vessels, as soon as done with, "with
nitric acid, and then with water. Every precaution shoidd be
used to avoid dust. The albumen of a duck's egg is more sensi-
tive than that of a hen's ; and ii-om an experiment of to-day, I am
almost certain that of a goose is more sensitive than either." —
Athenceum, No. 1220.

Section IV. — Miscella:nt:ous Modified Processes.

Several other preparations have been employed, -n^th variable
success, and recommended for procm-ing an absorbent film upon
glass plates — amongst others, the sei-um of milk has been used
by M. Blanqiiart Everai'd ; others combine with the albumen
or gelatiue, grape sugar and honey ; the object of these being
to quicken the process, which they appear to do in vu^tue of
their power of precijiitating the metals from theii' solutions.

Blanquart Everard has lately communicated the following to
the Paris Academy of Sciences, as an instantaneous process : —
"Fluoride of potassium, added to iodide of potassium, in the
preparation of the negative proof, produces instantaneous images
on exposm-e in the camera. To assiu'e myself of the extreme
sensibility of the fluoride, I have made some experiments on the
slowest preparations employed in photogi-aphy — that of plates
of glass covered with albumen and iodide, requiring exposiu'e of
at least sixty times longer than the same preparation on paper.
On adding the fluoride to the albumen and iodide, and substi-
tuting for the washing of the glass in distilled water after treat-
ment with the aceto-nitrate of silver, washing in fluoride of po-
tassium the image immediately on exposm-e in the camei-a ob-
scura, I have indeed obtained this result (but imder conditions
less powerfiil in their action) without the addition of the fluoride
to the albumen, and by the immersion only of the glass plate
in a bath of fluoride after its passage thi-ough the aceto-nitrate
of silver. This projierty of the fluorides is calciUated to give
very valuable results, and -will j^i'obably cause, in this branch of

THE rSE OF ALBUilEX OX GLASS PLATES. 283

photographic art, a change equally as radical as that effected by
the use of bromine on the iodized silver plates of Daguerre."
A process published in the author's Researches on Liyht, in
1844, and named the Fluorotype, sufficiently establishes my
claim to priority in the use of the fluorides.

Messrs. Ross and Thompson, of Edinburgh, hare been emi-
nently successfid operators •with the albumen process. Many
of theii- pictiu'es, which are of large size, exhibiting more artistic
effect than is jDroduced by any other jjhotogi-aphei's. Some of
the positives produced are veiy fine. At the meeting of the
British Association in that city, these gentlemen exhibited some
positive images on glass plates : these were backed up with
plaster of Paris, for the purpose of exalting the effects, which
were exceedingly delicate and beautiftd.

Messi-s. Langenheim, of Philadelphia, have, however, recently
introduced into this coxmtry specimens, which they tenn Hyalo-
types. These are positive picttu-es, copied on glass from nega-
tives, obtained upon the same material. Theii- pecidiarity is
the adaptation of them for magic-lantern slidei-s. The process
by which they are prodiiced is not pul)lished, but judging from
the effects obtained, the probabiHty is that a very slight vai-iation
only from the processes described has been made. The idea is
an exceedingly hapjiy one, as by magnifying those images which
are of the utmost delicacy and the strictest fidehty, perfect re-
flexes of nature are obtained.

There can be no doubt but other means of coating glass with
sensitive materials may be employed. Certainly the use of
albumen is a ready method, but this medivun appears to interfei-e
with the sensibility which it is so desii-al>le to obtain. As stated,
by using combinations of iodine and fluorine salts, there is no
doubt but the sensibility may be most materially improved, and
we fijid many of the continental photographei*s using honey and
grape sugar with much advantage.

I would, however, venture to suggest that films of silver pre-
cipitated fi'om the solution of the nitrate by gi"ape sugar,
aldehyde, or gim-cotton dissolved in caustic alkali, upon which
any change could be aftei-wards produced, appear to promise
many important advantages.

Section Y. — Positive Photographs from Etchings on Glass

Plates.

A very easy method of producing any number of positive
photographs from an original design is in the power of every

284 PRACTICE OF PHOTOGRAPHY.

one having some sliglit ai-tistic talent. The merit of having
suggested the process I am about to describe has been claimed
by Messrs. Havell and Wellmore, and also by Mr. Talbot ;
indeed, there appears no reason to doubt the originality of either
of these gentlemen, Mr. Havell ha^'ing prosecuted his experiment
in ignorance of the fact that Mr. Talbot had used the same
means to diversify his photographic specimens. Mr. Talbot
proposes that a plate of warmed glass be evenly covered with a
common etching ground, and blackened by the smoke of a candle.
The design is then to be made, by carefully removing from the
glass all those parts which should represent the lines and shadows,
and shading out the middle tints. It will be evident that the
light passing through the uncovered parts of the glass, and being
obstructed by the covered portions, will impress on the white
photographic papers a correct picture, having the appearance of
a spirited ink di'awing.

Mr. Havell's method was to place a thin plate of glass on the
subject to be copied, upon which the high lights were painted
with a mixture of white lead and copal varnish, the proportion
of varnish being increased for the darker shading of the picture.
The next day Mr. Havell removed, with the point of a pen- knife,
the white ground, to represent the dark etched lines of the original.
A sheet of prepared paper having been placed behind the glass,
and thus exposed to light, a tolerable impression was produced j
the half tints had, however, absorbed too much of the violet rays,
an imperfection which was remedied by painting the parts over
with black on the other side of the glass ; if allowed to remain
too long exposed to the sun's rays, the middle tints became too
dark, and destroyed the effect of the sketch. Another method
employed by Mr. Havell was to spread a groim^d composed of
white lead, sugar of lead, and copal varnish, over a plate of glass,
and having transfeiTed a pencil cb-awing in the usual manner, to
work it out "with the etching point.

Various modifications of these processes have been introduced
by different artists, and they evidently admit of many veiy
beautiful applications. When the etching is executed by an
engraver, the photograph has all the finish of a delicate copper-
plate engraving. The only thing which detracts from this me-
thod of photography is, that the great merit of self-acting power
is abandoned.

Etchings upon collodion plates are now employed for printing
from ; and several works, to be illustrated in this way, ai'e now
in progress for publication in Scotland.

CHAPTER IX.

OJf THE PRODUCTION OF POSITIVE PHOTOGRAPHS BY THE USE OF
THE SALTS OF lODINi:.

A VERT short time after the publication of Mr. Talbot's processe.s,
which I anxiously repeated with various modifications, I dis-
covered a singular property in the iodide of potassium {hyclrio-
date of potash) of again whitening the paper darkened by ex-
posui'e, and also, that the bleaching process was very much
accelerated by the influence of light. Early in the year 1839,
Lassaigne, Mr. Talbot, Sii- John Herschel, and Dr. Fyfe appear
to have fallen on the same discovery.

As this process, giving by one operation pictures with their
lights coiTect is of much interest, I gave it for a very con-
siderable time my undivided attention. The most extraordinary
character of these salts is, that a very slight difference in the
strength of the solutions, in the composition of the photographic
paper, or in the character of the incident light, produces totallv
opposite effects ; in one case the paper is rapidly whitened, in the
other a deep blackness is produced almost as rapidly. Sometimes
these opposing actions are in equilibrium, and then the paper
continues for a long time perfectly insensible.

I am inclined to hope these researches have reduced to cer-
tainty their somewhat inconstant effects, and rendered this
method of producing photographs one of the most easy, as it is
the most beautiful. That the various positions I wish to estab-
lish may be completely understood, and to insure the same re-
sults in other hands, it will be necessary to enter into a somewhat
detailed account of the various kinds of paper used, and to give
tolerably full directions for successfully using them, either in the
camera, or for dx-amngs by application, — to examine attentively
the effects of different organic and inorganic prepai-ations on the
paper, and to analyze the influence of the diflerent rays upon it.
See also Part I., Chapter VI., Section VIL, page %'2.

28 G PRACTICE OF PHOTOGRAPHY.

These particulars will be copied chiefly from my paper, "On
the Use of the Hych-iodic Salts as Photogi-aphic Agents," pub-
lished in the London and Edinburgh Philosophical Magazine for
September and October 1840, to which will be added the results
of my experiments since that time.

The variable texture of the finest kinds of paper occasioning
irregularities of imbibition, is a constant source of annoyance,
deforming the di*a. wings -wdth dark patches, which are very difficult
to remove : consequently my first endeavoiu-s were directed to
the formation of a sm"face on which the photographic prepara-
tions might be spread with perfect vmiformity.

A variety of sizes were used with very uncertain residts.
Neax'ly all the animal glutens appear to possess a colorific j^ro-
perty, wliich may render them available in many of the negative
processes ; but they all seem to protect the darkened silver from
the action of the solutions of the iodides. The giims are acted on
by the nitrate of silver, and browned, independent of light, which
browning considerably mars the efiect of the finished picture.
It is a singular fact, that the tragacanth and acacia gums render
the pictiu'es pi'oduced much less pei-manent. I therefore found
it necessary for general practice to abandon the use of all sizes,
except such as enter into the composition of the paper in the
manufactiu'e. It occvirred to me that it might be possible to
saturate the paper with a metallic solution, which should be of
itself entirely uninfluenced by light, on which the silver coating
might be spread without suffering any material chemical change.
The results beiag curious, and illustrative of some of the pecu-
liarities of the process, it will be interesting to study a few of
them.

Sulphate and Muriate of Iron These salts, when used in

small proportions, appeared to overcome many of the fii-st diffi-
culties, but all the dramugs on papers thus prepared faded out
in the dark. If, after these photographs have faded entirely out,
they are soaked for a short time in a solution of the ferrocyanate
of potash, and then are exposed to the light, the pictm^e is re-
vived, but with reversed lights and shadows.

Acetate and Nitrate of Lead.— These salts have been much
used by Sir John Herschel, both in the negative and positive
processes, and, it ajipears, Avith considei'able success. I found a
tolerably good result when I used a saturated solution ; but
papers thus pi-epared required a stronger light than other kinds.
When I used weaker solutions, the drawings were covered with
black patches. On these a little fm-ther explanation is requu-ed.
When the strong solution has been used, the iodine which has
not been expended in forming the iodide of silver — which form

ON THE PRODUCTION OF POSITIVE PHOTOGRAPHS. 287

the liglits of the picture — goes to form the iocli Je of lead. This
iodide is soluble ia boiling water, aud is easily removed from the
paper. When the weaker solution of lead has been used, instead
of the formation of an iodide, the hydriodate exerts one of its
peculiar functions in producing an oxide of the metal.

Muriate and Nitrate of Copper. — These salts, in any quantities,
render the action of the iodides very quick ; and, when used
in moderate proportions, they appeared to promise at first much
assistance in quickening the process. I, have obtained, with
papers into the preparation of which nitrate of copper has
entered, perfect camera views in ten minutes ; but experience
has proved their inapplicability, the edges of the parfs in shadow
Ijeing destroyed by chemical action.

Chlorides of Gold and Platinum act shnilarly to each other.
They remain inactive until the pictui'e is formed ; then a rapid
oxidation of these metals takes place, and all the bright parts of
the j)ictiu'e are darkened.

An extensive variety of preparations, metallic and non-metal-
lic, was used with like effects, and I am convinced that the only
plan of obtaining a pei-fectly equal svu-face, without impairing
the sensitiveness of the paper, is careful manipulation with the
ordinaiy muriates and silver solutions.

By attention to the following dii-ections, which are simple in
theii' character, but arrived at by a long series of inquiries, any
one may prepare photographic papei-s on which the bleaching
solutions shall act with perfect uniformity.

The kind of paper on which the sUver is spreatl is an object of
much importance. A paper kno^vn to stationers as satin post,
double-glazed, bearing the mark of J. Whatman, Tiu-key MUl, is
decidedly superior to every other kind I have tiied. The dark
specks which abound in some sorts of paper must be avoided,
and the spots made by flies very carefully guarded against.
These are of small consequence dming the darkening process, but
when the bleaching wash is applied, they form centres of chemi-
cal action, and the bleaching process goes on around them, inde-
pendently of light, deforming the cb-awing with small rings,
which are continually extending theii' diameters.

The Solution of Silver. — Take of cxystallized nitrate of silver
120 grains, distilled water 12 fluid ch-achms; when the salt is
dissolved, add of alcohol 4 fluid ch-achms, which rendei's the
solution opaque ; after a few hoiu's, a minute quantity of a dark
powder, which ajipeai's to be an oxide of silver, is deposited, and
must be sepai-ated by the filtei'. The addition of the alcohol to
the solution was adopted from an observation I made of its in-
fluence in retarding the chemical action, which goes ou in the

288 PEACTICE OF, PHOTOGRAPHY.

shade, of the hydriodates on the salt of silver. Its use is, there-
fore, to make the action depend more on luminous influence,
than would be the case without it.

Nitric Ether. — Sweet spirits of nitre not only checks the
bleaching process in the shade, but acts with the iodine salts to
exalt the oxidation of the silver, or increase the blackness of it.
In copying lace or any fine linear object, it is a very valuable
agent; but it is useless for any other pui-poses, as all the faintly-
lighted parts are of the same tint.

Hydrochloric Ether, used as the solvent of the silver, and
applied without any saline wash, has a similar property to the
nitric ether ; but as it is readily acted on by faint light, it is of
greater value. However, papers prepared with it must be used
within twenty-fovir hoiu's, as they quickly lose their sensitive-
ness, and soon become nearly useless.

To fix with any degree of certainty the strength of the solu-
tion of the salts which will in all cases produce the best effects,
appears impossible ; eveiy variety of light to which the paper has
been exposed to darken, reqvdiing a solution of diSerent specific
gravity.

In darkening these papers, the directions given at page 83
should be strictly followed.

Any' of the following preparations may be employed as the
bleaching agents.

Iodides of Potassium and Sodium. — The former of these salts
being more easily procured than any other, is the one generally
employed. The strength of the solution of these salts best
adapted for the general kinds of paper is tliu-ty grains to an
ounce of water. The following results will exhibit the difierent
energies manifested by these solutions at several strengths, as
tried on the same paper by the same light : —

120 graiQS of the salt to an ounce of water took ) , f, . ,

to whiten the paper

^..r

QlUUt

do.

do. •

■• do.

do.

.do.

do.

10 do. do. to do. 12 do.

The other salts correspoud nearly with these in their action;
a certain point of dilution being necessary with all.

Iodide of Ammonia, if used on unsized paper, has some ad-

PN THE PRODUCTION OF POSITIVE PHOTOGRAPHS. • 289

vantage as to quickness over the salts either of potash or of soda.
This preparation is, however, so readily decomposed, that the
sLze of the paper occasions a libemtion of iodine, and the conse-
quent foi-mation of brown-spots.

Iodide of Iron.— This metallic iodide acts with i-apidity on the
darkened paper ; but even in the shade its chemical energy is
too great, destroying the shai-pness of outline, and impairing the
middle tints of the drawing. It also renders the paper veiy
yellow.

Iodide of Mang'anese answers remarkably well when it can be
prociu-ed absolutely free of iron. When the manganesic solution
contains iron, even in the smallest quantities, light and dark
spots are formed over the pictui-e, which give it a curious speckled
appearance.

Iodide of Baritun possesses advantages over eveiy other simple
iodine solution, both as regards quickness of action and the
sharpness of outline. A solution may, however, be made still
superior to it, by combining a portion of iron with it. Forty
gi-ains of the hydriodate of baryta being dissolved in one ounce
of distilled water, five grains of very piu'e sulphate of iron
should be added to it, and allowed to dissolve slowly. Sulphate
of baryta is precipitated, which should be separated by filtration,
when the solution is composed of iodide of barium and iron.
By now adding a drop or two of diluted sulphuric acid, more
baryta is precipitated, and a portion of hydriodic acid set free.
The solution must be allowed to stand until it is clear, and then
carefully decanted off fi-om the secUment, as filtering paper de-
composes the acid, and free iodine is liberated. By this means
we procure a photographic solution of a very active character.
It should be prepared in small quantities, as it suffers decomposi-
tion \inder the influences of .the atmosphere and light.

Hydriodic Acid, if used on paper which wiU not decompose
it, acts veiy readily on the darkened silver. A portion of this
acid, free La any of the solutions, most materially quickens the
action. From the barytic sokition it is always easy to set
free the required portion, by precipitating the barytes by sul-
phuric acid. As the iodide of bariiim is rarely kept by the retail
chemist, it may be useful to give an easy method of preparing
the solution of the required strength.

Put into a Florence flask one ounce of iodine, and cover it
with one fluid ounce and a half of distilled water ; to this add
half a drachm of phosphorus cuff into smal pieces ; apply a very
gentle heat until they imite, and the liquid becomes colourless ;
then add another fluid ounce and a half of water. It is now a so-
lution of hydriodic acid and phosphoric acid. By adding carbon-

290 PRACTICE OF PHOTOGRAPHY.

ate of barytes to it, a pliosphate of baiytes is formed, wliich,
being insoluble, falls to tbe bottom, whilst tbe soluble iodide of
barium remains dissolved. Make up the quantity of the solu-
tions to nine ounces with distilled water, and carefully preserve
it in a green glass stoppered bottle.

For di-awings by application, less care is required than for the
camera obscura. With a very soft flat brush apply the solution
on both sides of the prepared paper, until it appears equally ab-
sorbed; place it in close contact Avith the object to be copied,
and expose it to sunshine. The exposure should continue until
the parts of the paper exposed to uninterrupted light, which first
change to a pale yellow, are seen to brown a Little. The obser-
vance of this simple rule will be found of very gi-eat advantage
in practice. Immersion for a short time in soft water removes
the broAvn hue, and renders the bright j)arts of the pictiu-e
clearer than they would be otherwise.

Engravings to be copied by this process — which they are
most beautifully — should be soaked in water and superimposed
on the photographic papers, quite wet. If the jjaper is intended
to be used in the camera, it is best to soak it in the bleaching
solution until a slight change is apparent, from chemical action
on the silver : it is then to be stretched on a slight fi\ame of
w-ood, which is made to fit the camera, and not allowed to touch
in any part but at the edges ; placed in the dark chamber of the
camera at the proper focus, and pointed to the object of which a
copy is requii-ed, which with good sunshine is eSected in about
twenty minutes, varying of course with the degree of sensibility
manilested by the jjaper. If the wetted paper is placed upon
any porous body, it will be found, owing to the capillary com-
munication established between difierent points, that the solu-
tion is removed from some parts to others, and different states
of sensitiveness induced. Another advantage of the frame is,
the paper being by the moistiu'e rendered semi-transparent, the
Light penetrates and acts to a greater depth ; thus cutting out
fine lines which would otherwise be lost. However, if the
camera is large, there is an objection to the frame; the solution
is apt to gather into drops, and act intensely on small spots, to
the injury of the general effect. When using a large sheet, the
safest course is to sj)read it out when wetted upon a piece of
very clean ivet glass, great care being taken that the j^aper and
glass are in close contact. The pictm'e is not formed so quickly
when the glass is used, as when the paper is extended on a frame,
owing to the evaporation being slightly retarded. The addi-
tional time requii-ed — about one-sixth longer — is, however, in
most cases, of little consequence.

ON THE PRODUCTION OF POSITIVE PHOTOGRAPHS. 291

The picture being formed bj tbe influence of sunshine, it is
required, to render it unchangeable by any fiui.her action of the
luminous fluid, not only that the salt of iodine be entu-ely re-
moved from the paper, but that the iodide of silver which is
formed be also dissolved out of the di'awing.

By vreR washing the drawing in wann water, the iodide of
potassium is removed, and the pictures thus prepared have been
stated to be permanent j and if they are. kept in a portfolio, and
only occasionally exposed, they are really so : for I shall show
jDresently, that tney have the property of being restored in the
dark to the state in tohich they zvere jjrior to the destructive action
of light. A drawing which I executed in June, 1839, which has
often been exposed for days successively to the action of sunshine,
and has altogether been very little cared for, continues to this
date (December, 1853) as perfect as at fii-st. These photogTaphs
vnl\ not, however, bear long-continued exposure without injiuy
— about three months in summer, or six weeks in winter, being
sufficient to destroy them. As this gradual decay involves some
very cm'ious and interesting chemical phenomena, I shall make
no excuse for dwelling on the .suljject a little.

The di'awing fades first in the dark parts, and as they are per-
ceived to lose their definedness, the lights are seen to darken, im-
til at last the contrast between light and shadow is very weak.

If a dark paper is washed \n.th. an iodide in solution, and ex-
posed to sunshine, it is fii'st bleached, becoming yellow ; then the
light again darkens it. If, when quite dry, it is carefully kept
from the light, it will be found in a few days to be again restored
to its original yellow colour, which may be again darkened by
exposiu-e, and the yellow colom- be again restored in the dark.
The sensitiveness to the influence of light diminishes after each
exposiu'e, but I have not been enabled to arrive at the point at
which this entfrely ceases. If a dark paper, bleached by an
iodide and light, be again darkened, and then jilaced in a
bottle of water, the yellow is much more quickly restored, and
bubbles of gas will escape freely, which will be found to be
oxygen. By enclosing pieces of iodized paper in a tube to
darken, we discover, as might have been expected, some hydro-
gen is set free. If the paper is then well (fried, and carefrilly
shut up in a warm dry tube, it remains dark ; moisten the tube
or the papei", and the yellowness is speedily restored.

Take a photogi-aph thus formed, and pla-ce it in a vessel of
water: in fxfew clays it wiU fade out, and bubbles of oxygen mil
gather around the sides. If the water is examined there vnW be
found no ti-ace of either silver or iodine. Thus it is evident the
action has been confined to the paper.

292 PRACTICE OP PHOTOGRAPHY.

We see that the iodide of silver has the power of separating
hydrogen from its combinations. I cannot regard this singvJar
salt of silver as a definite compoviud : it appears to me to com-
bine ^vith iodine in uncertain proportions. In the process of
darkening, the liberation of hydrogen is certain; but I have not
in any one instance been enabled to detect free iodine : of course
it must exist, either in the darkened surface, or in combination
with the unaffected under layer : possibly this may be the iodide
of silver, with iodine in simple mixture, which, when light acts
no longer on the preparation, is liberated, combines with the
hydrogen of that portion of moisture which the hygrometric
nature of the paper is sure to furnish, and as an liydriodate again
attacks the darkened surface, restoring thus the iodide of silver.
This is strikingly illustrative of the feding of the photograph.

The picture is formed of iodide of silver in its light parts, and
oxide of silver in its shadows. As the yellow salt darkens under
the influence of light, it pai-ts with its iodine, which immediately
attacks the dark oxide, and gradually converts it into an iodide.
The modus oi^erandi of the restoration which takes place in the
dark is not quite so apparent. It is possible that the active
agent being quiescent, the play of affinities comes undistiu-bed
into operation; that the dark j^arts of the picture absorb oxy-
gen from the atmosphere, and restore to the lighter portions the
iodine it has before robbed them of A series of experiments
on the iodide of silver in its pure state will still more strikingly
exhibit this very remarkable peculiarity.

Precipitate with any iodide, silver, from its nitrate in solution,
and expose the vessel containing it, liquid and all, to sunshine;
the exposed surfaces of the iodide will blacken : remove the
vessel into the dark, and, afier- a jew liours, all the blackness "will
have disappeared. We may thus continually restore and remove
the blackness at pleasure. If we wash and then well dry the
precij)itate, it blackens with difficulty, and if kept quite diy it
continues dark ; but moisten it, and the yellow is restored after
a little time. In a watch-glass, or any capsule, place a little
solution of silver ; in another, some solution of any iodine salt ;
connect the two with a filament of cotton, and make up an elec-
tric circuit "with a piece of platina "wire : expose this little ar-
rangement to the light, and it will be seen, in a veiy short time,
that iodine is liberated in one vessel, and the yellow iodide of
silver formed in the other, which blackens as quickly as it is
formed.

Place a similar arrangement in the dark ; iodine is slowly
liberated. iVo iodide of silver is formed, but around the "wire
a beautiful ciystallization of metallic silver. Seal a piece of

ON THE PRODUCTION OF POSITIVE PHOTOGRAPHS. 293

platina wire into two small glass tubes ; these, when filled, the

one with iodide of potassium in solution, and the other with a

solution of the nitrate of silver, reverse into two

watch-glasses, containing the same solutions ;

the glasses being connected with a piece of

cotton. An exposui^e diu-ing a few hoiu-s to

daylight will occasion the solution of the iodine

salt in the tube to become quite bro^\Ti with

liberated iodine : a small portion of the iodide

of silver will form along the cotton, and at the

end dipping in the salt of silver. During the

night the liquid will become again coloui-less 75

and transparent, and the dark salt along the

cotton will resume its native yellow hue.

From this it is evident that absolute permanence will not be
given to these photographs until we succeed in remo\'ing from
the paper all the iodide of silver formed. The hyposulphites
dissolve iodide of silver; therefore it might have been expected,
a 2>riori, they would have been successful on these cLrawings. If
they are washed over with the hyposulphite of soda, and then
quickly rinsed in plenty of cold water, the drawing is improved,
but no better fixed than with cold water alone. If we persevere
in using the hyposulphite, the iodide is darkened by combining
with a portion of sulphur, and the lights become of a dingy
yellow, which is not at all pleasant.

No plan of fixing will be found more efficacious with this
variety of photograjihic di-awings, than soaking them for some
hours in cold water, and then well washing them in hot water.

It often happens that a picture, when taken from the camera,
is less distinct than could be desired : it should not, however, be
rejected on that account. All the details exist, although not
visible. In many cases the soaking is suificient to call them into
sight ; if they cannot be so evoked, a wash of weak ammonia or
muriatic acid seldom fails to bring them up. Care, howevei",
must be taken not to use these preparations too strong, and the
picture must be washed on the instant, to remove the acid or
alkali.

One very sing-ular property of these photographs is, that when
first prepared, and after the w^ashing, they are not fixed or other-
wise ; but when exposed to sunsliine, they change in their dark
parts from a red to a black. This peculiarity will be found by
experiment to be entirely dependent on the influence of the red
rays, or that poi-tion of the sunbeam which appears to have the
greatest heating power : hence regarded as the seat of greatest
calorific power.

29i PRACTICE OF PHOTOGRAPHY.

I have before mentioned the peculiar state of equilibrium in
which the paper is when wetted -with the hydriodate, and that a
slight difference in the incident light will either bleach or
blacken the same sheet. If fovu- glasses, or coloured fliiid.s, be
prepared, which admit respectively the blue, green, yellow, and
red rays, and we place them over a paper washed with some
solution of an iodide having an engraving superposed, it ^^-ill be
bleached under the influence of the blue light, and a perfect
jiicture produced ; while, under the rays transmitted by the
green glass, the drawing will be a negative one, the paper having
assumed, in the parts which represent the lights, a very defined
blackness. The yellow light, if piu'e, will produce the same effect,
and the red light not only induces a like change, but occasions
the dark pai-ts of the engraving to be represented in strong
lights: this last pecidiarity is dependent on the heating i^ays,
and oj)ens a wide field for inquuy. My point now, however, is
only to show that the darkening of the finished photogi'aph is
occasioned by the least refrangible rays of light; whereas its,
preparation is effected by the most refrangible.

I know not of any other process which shows, in a way at
once so decided and beautiful, the wonderful constitution of every
sunbeam which reaches us. Yet this is but one of numerous
results of an analogous character, produced by these opposite
powers, necessary to the constitution of that solar beam, which
is poured over the eai-th, and effects those various changes which
give to it diversified beauty, and renders it conducive to the
well-being of animated creatiu-es.

Before quitting this branch of the art, it will be interesting to
examine the modifications which have been introduced by some
continental inquii-ei's.

M. Lassaigue, who has claimed priority in the use of the
iodide of potassium, satiu-ated his paper with a sub-chloride of
silver, which was allowed to assume a violet-bro^vn coloxu", and
it was then imjjregnated with the iodidated solution.

M. Bayard simply allowed ordinary letter jiajDcr, prepared
according to Mr. Talbot's method, to blacken by light. He
then steeped it for some seconds in a solution of iodide of
potassiiim, and laying it on a slate he placed it in the camera.

M. Verignon introduced a somewhat more complicated pro-
cess. His dii^ections are — ^White paj^er should fii'st be washed
with water acidulated by hydi'ochloric (muriatic) acid ; then,
after being well diied, steeped in the following solution : — "Water
foiu'teen parts, with one part of a compound formed of two
parts of muriate of ammonia, two parts of bromide of sodium,
and one of chloride of strontium. The paper cbied again is

ON THE PEODUCTION OF POSITIVE PHOTOGKAPHS. 295

passed into a veiy weak sohition of niti-ate of silver. There is
thus fomied, by double decomposition, a chloride and bromide of
silver, which is made to tmii black by exposing the paper to the
light for about half an horn-. To use this paper, it is steeped in
a very weak solution of the iodide of sodium, and placed, quite
wet, into the camera obscura, at the proper focus. In fine
weather, M. Verignon states, the efiect is produced in twelve
mi unites. I have, however, never produced a good picture by
this process in less than thirty minutes. A great objection to
this mode of preparation is the very rapid deterioi-ation of the
paper: every day it will become less and less sensitive to light,
and at the end of a fortnight it is useless.

The papei's recommended for use in the former pages have the
advantage of keeping well, jirovided ordinaiy care is taken with
them. It is necessary to exclude them from the Hght, to keep
them very dry, and, as much as possible, they should be pro-
tected fi-om the action of the air. I have kept papei-s, prepared
with the muriate of ammonia, baryta, and strontia, for twelve
months, and have foimd them but very little impaired.

Dr. Schaf haeutl allows jDaper prepared in the way mentioned
at a former page to darken in a bright sun-light. It is then
macei'ated for at least half an horn', in a liquid prepared by
mixing one part of the already described acid nitrate of mercury,
vrith. nine or ten pai-ts of alcohol. A bright lemon yellow
precipitate of basic hyponitrate of the protoxide of qviicksilver
falls, and the clear liquor is preseiwed for use. The macerated
paper is removed fi'om the alcoholic solution, and quickly di-awn
over the surface of diluted muriatic acid (one part strong acid to
seven or ten of water), then quickly washed in water, and
slightly and carefidly dried at a heat not exceeding 212° of Fahr.
The paper is now ready for being bleached by the rays of the
Sim ; and in order to fix the ch-awing, nothing more is required
than to steep the pajoer a few minutes in alcohol, which dissolves
the fi-ee bichloride of mercmy. I must confess, however, that
in my hands the process has not been so successful as it is
described to have been by the author.

It is perhaps necessaiy to remark, that we cannot multiply
designs from an original bleached photograph. The yellow
coloiu- of the paper is of itself fatal to transfers, and independently
of this, the wet hychiodic sohition would immediately destroy
any superposed photogi-aph.

\Ye have seen in a former chapter that the white photographic
papei-s are darkened by the blue, indigo, and violet rays. On
the dark papers washed with the iodine salts in solution, the
bleaching is effected most energetically by the violet rays : it

>96

PR.A.CTICE OF PHOTOGRAPHY.

proceeds "w-itli lessening intensity to the blue, while aU the rays
below the yellow have a darkening iuiluence on the paper. This
effect will be best illustrated by figiu'e 76, in which is shown —
somevsrhat exaggerated for the sake of distinctness — the very
remai'kable action which takes place : cleai-ly establishing the fact
first noticed by "VTollaston, that the two extremities of the
spectrum have different powei*s.

The remarkable manner in which the point of greatest inten-
sity is shifted from the blue to the violet, when papers have
but a very slight difference in their composition or mode of
preparation, is an extremely ciu-ious point of philosophical

Visible
Spectrum.

Impressed
Spec'trnm.

Violet. .

Indigo .
Blue....

Green. .
Yellow.
Orange.
Ked....

inqmiy. It vnll be evident from what has been said, that it is
necessary the focus of the \iolet rays shoidd be always chosen
in using these papers in the camera.

CHAPTER X.

GE^TERAL REMAEKS ON THE USE OF THE CAilERA OBSCURA. THE

PHOTOGRAPHIC PE>TAGRAPH.

These remarks "will apply vritli equal force to all the processes
by wkich -views of external objects can be obtained; but tbey
have more especial reference to those highly sensitive ones, the
Daguerreotype, the Talbotype, and the Collodion processes.

It has ah-eady been stated
that a single achromatic
lens, producing a large
image, should be employed
for motionless obj ects, "where
time is not of consequence.
For a building, a statue, or
the like, it is not of much
consequence whether one
minute or ten may be con-
sumed ia the operation of obtaining its impression. With the
human figure and animals the case is very diflFerent : the utmost
concentration of the solar radiations is therefore required to
insure rapidity of action. This is effected by the double combi-
nation of lenses, which are usually moimted
and adjusted as shown in the above fig-
ure, 77. In fig. 78, the mounting of the
single lens arrangement is shown.

It is often of the utmost importance, to
obtain definition of the objects, that aU
extraneous rays should be cut ofi"; this
is efiected by means of a diaphragm of
stops, which can be obtained to fit any
lens. With this adjustment any sized aperture can be obtained.

Section I. — Buildings, Statues, Landscapes, and Foliage.

The great defect ia nearly all the photographic pictiires which
are obtained is the extreme conti-ast between the high lights and

298 PRACTICE OF PHOTOGRAPHY.

the shadows, and in many an entii'e absence of the middle tones
of the pictiu'e.

In the very beautiful productions of Mr. Buckle, of Peter-
borough, which were displayed in the Great Exhibition, there
was a very remarkable degree of fine definition, united with a
beautiful blending of the respective parts which constituted the
picture. There was no glaring contrast between the lights.
Those parts which were the most brilliantly ilkiminated were
softened into the middle tones of the picture, and those again faded
gradually into the deep shadows. In the works of M. Martin
and M. Flacheron, whose processes I have given, the same har-
monizing of lights and shadows was generally foimd to exist.

The usual mistake with amateui-s is that of selecting bright
sunshine as the pex'iod for operating. It is thought, when a
cathedral, for example, is brilliantly lighted up by svmshine, is
the time for obtaining a photographic copy of it. A little i-eflec-
tion will convince the operator that this is the case only under
particular conditions.

"When the projecting parts of the building are flooded with
sunshine, they cast the deepest possible shadows ; consequently,
in the photogi-aphic picture the pi-ominent points would appear
brilliantly white, and the shadows intensely dark.

It will be understood that I refer always to the positive, or
completed picture.

A clear blue sky, reflecting its light upon a similar structure,
produces less prominent illumination of the bold ornamental
parts, and gives more light to those pai-ts on which the shadows
are cast. A photograph taken imder such conditions of light
and shade will l)e far more beautiful than the spotted productions
which ordinarily result from the practice of operating when the
sun is shining brightly on the object.

In the same manner, when the sim shines brightly on the
leaves of trees, a very large quantity of light is reflected from
their surfaces, the other parts appearing by contrast in almost
absolute shadow. Hence, nearly all photographic views of
forest scenery have moi'e the appearance of scenes which have
been sprinkled with snow than foliage glowing with sunshine.

An artist studies in his productions the most efiective dis-
position of the lights and shadows, and it is by the harmonious
disposition of these that he succeeds in gi\dng a peculiar charm
to his productions. Nearly all photographic pictures, although
they have the merit of strict truthfulness, appear to want this
great beauty of art. This has mainly arisen from the circumstance
that intense illumination has been sought for under the idea of
producing the sharpest picture ; and it is true that thus we do

PORTRAITS FROM THE LIFE.

299

obtain a very perfect definition of outline. Many productions
are remarkable for tbis, and, indeed, reproduce witb unnaturiil
exactness all tbe minute details of tbe objects copied; wbereas
the human eye never sees this extraordinary sharpness of out-
line in nature ; upon the edges of every object there are fringes
of light which soften off theii* outlines, and subdue the general
tone of objects, blending all harmoniously. Perhaps there is
more than ordinary difficulty in producing this in a representa-
tion of nature which is effected by means of a lens. The artist
may, however, do much : all times, even of bright illumination,
are not fitted for producing a picturesque photogTaph. Natm-e
should therefore be looked at with an artist's eye, and the happy
moment chosen when the arrangements of light and shade give
the most pictiu-esque effects, and when these are in a condition
to be correctly rejiroduced according to the laws by which actinic
influences are regulated.

Section II. — Portraits from the Life.

It is important for the production of a correct likeness that
as small an ai^erture as possible should be used. By doing this
thei-e is great loss of Hght, and consequently the necessarily pro-
longed time must be compensated for by greatly increased
sensibility in the plates.

It is also important that arrangements should be made to
cut off from the lens all light proceeding from extraneous objects:
this is best effected by the modes adopted by M. Claudet.

The camera is placed, as shoT\Ti in section. Fig. 79, within an
arrangement of ciu'tains which, as wiU be seen in the vignette
heading to this part, page 175, is capable of adjustment, so as

300

PRACTICE OF PHOTOGRAPHY.

to have any required opening in front of the camera. The whole
of this screen being mounted on rollers is easily moved ; there-
fore the o2)erator has it in his power to adjust the opening, and
to shut off all adventitiuos radiations, thus securing the effec-
tiveness of the rays proceeding directly from the sitter, or the
object to be copied.

The sitter should be placed in the easiest
possible jjosition compatible with the arrange-
ment of the body as nearly as is possible in a
vertical plane. This is necessary, as the parts which
are nearest the glass suffer a very considerable
degree of distortion and enlargement. Of course
great steadiness is required on the part of the
sitter during the few seconds he submits to the
operation of the photogi-apher. It is usual to
support the head by a rest fastened to the back of
the seat, as shown in fig. 80 ; but where the person
can maintain a steady position without this, the
result is generally the most satisfactory, the "rest "
not unfrequently giving an air of stiffness to the
sitter. In a great number of 2:)ortraits a dark and
unnatural shade is thrown under the eyes : this
arises from the employment of a "top light." The
light falling vertically produces the shadow of the
brow over the eye, and gives a sombre character
to the face. This is objectionable also, as beiug annoying to
the sitter, who assumes in consequence a somewhat painful
expression.

Those who have attended to the analyses of the spectmm,
included in the second division of our subject, have become
aware that the radiations from all coloured objects are not
alike. A long description would not render this so apparent
as a single illustration. The frontispiece represents, therefore,
a group of flowers, including the three j^rimary colours, and a
few comjDOund tints; an exact copy of a photograph taken from
such a group is placed beneath it, and the result is shown.
Hence it is of the utmost importance, particiUarly to ladies, that
they should be dfrected to avoid in their dresses, when about to
sit for their portraits, such colours as would produce darks for
lights, and the contrary.

PHOTOGRAPHIC PEXTAGEAPH.

301

Section III. — Photographic Pentagraph.

In a letter to Sir Jolin Herschel, which was published in the
Athencewm, ]Mr. Stewart directed attention to a means by which
photographs could be readilv enlarged. The plan had been in
operation amongst photographic artists for some time, but it was
not usually adopted, and Mr. Stewart's Letter certainly brought
it into general use. The folloAving is Mr. Stewai't's description
of the requii'ed apparatus : —

" One of my friends here, Mr. HeUmann, has lighted on an
ingenious method of taking fi-om glass negatives positive impres-
sions of different dimensions, and with all the delicate minute-
ness which the negative may possess. This discovery is likely,
I think, to extend the resom-ces and the application of photo-
graphy, — and with some modifications, which I will explain, to
increase the power of reproduction to an almost unlimited
amoimt. The plan is as follows : — The negative to be reproduced
is placed in a slider at one end («) of a camera or other box,
constiiicted to exclude the light throughout. The surface pre-
pared for the reception of the positive — whether albumen, col-
lodion, or paper — is placed in another slider, as usual, at the op-
posite extremity (c) of the box, and intennediately between the
two extremities (at h) is placed a lens. The negative at a is
presented to the light of the sky, care being taken that no rays
enter the box but those traversing the pai^tly transparent nega-
tive. These rays are received and directed by the lens at h upon
the sensitive surface at c, and the impression of the negative is
there produced with a rapidity proportioned to the light admit-
ted, and the sensibility of the surface presented. By varj^ng
the distances between a and c, and c and h, any dimension re-
quired may be given to the positive impression. Thus, from a
medium-sized negative, I have obtained negatives foiu- times
larger than the orisinal,

and other impressions re- "" * —

duced thirty times, capa-
ble of figuring on a watch-
glass, brooch, or ring.

"a, c. Slides to hold the
frames containing the ne-
cjative and the receiving
surface for the positive.

" h. Frame, with thelens.

SI.

" e. Dotted lines, indicating the top of the box or frame.
''f;f. Bottom board, or foundation of the same.

302 PRACTICE OF PHOTOGRAPHY.

" Undoubtedly one of the most interesting and important ad-
vantages gained hj this sinqole arrangement is, the power of
varying the dimensions of a picture or portrait. Collodion giving
results of almost microscopic minuteness, such negatives bear en-
larging considerably without any very percej)tible deterioration
in that respect. Indeed, as regards portraits, there is a gain in-
stead of a loss ; the power of obtaining good and pleasing like-
nesses appears to me decidedly increased, the facility of subse-
quent enlargement permitting them to be taken sufficiently
small, at a sufficient distance (and therefore with greater rapidity
and certainty), to avoid all the focal distortion so much com-
plained of; while the due enlargement of a portrait taken on
glass has the eflfect, moreover, of depriving it of that hardness of
outline so objectionable in a collodion portrait, giving it more
artistic effect, and this without quitting the perfect focal point,
as has been suggested.

" But there are many other advantages obtained by this process
For copying by engraving, &c., the exact dimension requii-ed of
any picture may at once be given to be copied from.

"A very small photographic appai-atus can thus be employed,
when a large one might be inconvenient or impracticable, the
power of reproducing on a larger scale being always in reserve.
Independent of this power of varying the size, positives so taken
of the same dimension as the negative, reproduce, as will be
readily undei'stood, much more completely the finer and more
delicate details of the negatives, than positives taken by any other
process that I am acquainted Avith.

"The negative also may be reversed in its position at a, so as to
produce upon glass a positive to be seen either upon or under the
glass. And while the I'apidity and facility of printing are the
same as in the case of positives taken on paper prepared with
the iodide of silver, the negatives, those on glass particidarly,
being so easily iujin-ed, are much better preserved, all actual con-
tact with the positive being avoided. For the same reason, by
this process positive impressions can be obtained not only upon
wet paper, &c., but also upon hard inflexible substances, such as
porcelain, ivoiy, glass, &c. ; and upon this last, the positives
being transparent, are appHcable to the stereoscope, magic
lantern, &c.

" By adopting the following arrangement, this process may be
used lai'gely to increase the power and speed of reproduction
with little loss of eifect. From a positive thus obtained, say on
collodion, several hundred negatives may be produced either on
paper or on albimiinized glass. If on the latter, and the dimen-
sion of the original negative is presented, the loss in minuteness

PHOTOGRAPHIC PENTAGRAPH. 303

of detail and harmony is almost imperceptible, and even wlien
considerably enlarged, is so trifling as in the majority of cases to
pi'ove no objection in comparison with the advantage gained in
size, while in not a few cases, as already stated, the pictm^e ac-
tually gains by an augmentation of size. Thus, by the simiil-
taneous action, if necessary, of some hundreds of negatives, many
thousand impressions of the same pictm-e may be produced in
the course of a day.

"I cannot but think, therefore, that this simple but ingenious
discovery ^yill prove a valuable addition to oiu" stock of photo-
graphic manipulatory processes. It happily turns to account
and utilizes one of the chief excellencies of collodion — that ex-
treme minuteness of detail which, from its excess, becomes almost
a defect at times — toning it down by increase of size till the
harshness is much diminished, and landscapes, always more or
less un pleasing on collodion from that cause, are rendered some-
what less dry and cmde.

"A very Httle practice wdll suffice to show the operator the
quality of glass negatives — I mean as to vigour and development
— best adapted for repi'oducing positives by this method. He
wnll also find that a gi'eat power of correction is obtained, by
which overdone parts in the negative can be reduced and others
brought up. Indeed, in consequence of this and other advan-
tages, I have little doubt that this process will be very generally
adopted in poi-trait-taking.

" Should yoiu' old idea of preserving pubHc records in a concen-
trated form on microscopic negatives ever be adopted, the imme-
diate positive reprodiiction on an enlarged readable scale, without
the possibility of injury to the plate, will be of ser^'ice."

CHAPTER XI.

THE STEREOSCOPE.

From the interest which this very interesting optical instm-
ment has excited, and the very intimate relation which exists
between it and photography, since it is ordy practical to produce
images suited for the instrument by the agency of the camera
obscura, it is thought advisable to devote a short chapter to
some notice of it. It is not intended that any examination of
the phenomena of vision, or of the application of the stereoscope
to the explanation of single vision with a pair of eyes, shall be
attempted ; these questions would be somewhat out of place in
the present manual, and would occupy too large a space if pro-
perly dealt with.

The stereoscope is before the world : a simple description,
therefore, of the forms under which it may be constructed, and
a sufficient explanation of its principles, is all that can here
with proj^riety find a place. The name is compounded from two
Greek words, signifying solid, and / see, and adopted from the
fact that two pictures on a plane surface will, when adjusted
in the instrument, resolve themselves into one image, and that
image will acquire an apparently distinct solidity, being repre-
sented as an object having three dimensions, — length, breadth,
and thickness.

"The theory" — of single vision with a pair of eyes — says
Mr. Wheatstone, in his valuable Memoir " On some remarkable
and hitherto unobserved Phenomena of Binocular Vision," —
" The theory which has obtained the greatest currency is that
which assumes that an object is seen single because its pictures
fall on corresponding points of the two retinte ; that is, on points
which are similarly situated with respect to the two centres, both
in distance and position. This theory supposes that the pictures
projected on the retinre are exactly similar to each other, corre-
sponding points of the two pictures falling on corresponding points
of the two retinae." Leonardo da Vinci, in his Treatise on
Painting, has some remarks on the peculiarity of vision, which
bear in a singular manner on the phenomena of the stereoscope,

THE STEREOSCOPE.

305

to tlie effect, tliat a painting, though conducted with the greatest
art and finish to the last perfection, both with regard to its con-
tours, its lights, its shadows, and its colours, can never show a
relievo equal to that of natiu'al objects, unless these be viewed
at a distance, and with a single eye ; for if an object, as an orange,
be viewed by a single eye, all objects in that space behind it,
which we may suppose to be included in its shadow, are Ln\Tsible
to that eye ; but open the other eye without moving the head,
and a portion of these becomes visible : those only are hid from,
sight which are included in the space covered by the two shadows
formed by two candles supposed to be placed in the positions of
the eyes. The hidden sr)ace is so much the shorter according to
the smallness of the object, and its proximity to the eyes. Upon
this Mr. Wheatstone remarks : — " Had Leonardo da Vinci taken,
instead of a sjjhere, a less simple figure for the purpose of his
illustrations — a cube for instaape — he would not only have per-
ceived that the object obsciu-ed fi-om each eye a different part of
tlie more distant field of view, but the fact would also have been
forced upon his attention, — that the object itself presented a
different ajjpearance to each eye."

If any of my readers will be at the trouble to look at a
simple solid form, keeping the head perfectly steady, with a single
eye, and make an outline drawing of the image as seen — say, first
with the left eye, and then with the right eye — it will be foimd
that two dissimilar forms Avill be obtained analogous to those
represented in the following diagi-am: —

82.

By a little effort, it is easy, by squinting, to resolve these two
figures into one, when it will be foimd that an apparently solid
image is formed from these dissimilar oxitlines of a solid.

The stereoscope of Professor Wheatstone is arranged to pro-
duce this in a more effective manner. The instrument, fig. 83,
consists of two plane min-ors, so adjusted that their backs form

306

PRACTICE OF PHOTOGRAPHY.

an angle of ninety degrees with, eacii otlier. These mirrors are
fixed by their common edge upon an horizontal board, in such a

manner that, upon bringing it close to the face, each eye sees the
image reflected from the two ends of the instrument in a difier-
ent miiTor ; at each end of the board are panels in which the
drawings are placed. The two|»eflected images coincide at the
intersection of the optic axes, and form an image of the same
apparent magnitude as each of the component pictures. This in-
strument is called the reflecting stereoscope ; and as it will admit
of being made of any size, so as to allow of the introduction of
large pictm-es, it oSers many advantages. Mr. Wheatstone sug-
gested in his memoir, already quoted, the use of an instrument
constructed with prisms, which is analogous to the beautifully
poi-table lenticular stereoscope of Sir David Brewster, described
by him in the Philosophical Magazine.

This instrument consists of two semi-len-ses, placed at such a
distance that each eye views the picture or drawing opposite to
it, through the margin of the semi-lens, or through parts of it
equidistant from the margin. A lens, a a, being cut in two

halves, these are fixed into a frame l and R, and adjusted to such
distances that the centres of the semi-lenses correspond with the
pupd of the eyes. The distance of the centre of one pupil from
the other is at an average 2| inches, and to this the semi-lenses
may be adjusted ; but if the instrument is provided with the
means of effectiag a little change in this respect, it will often be
found to be of considerable advantage.

THE STEREOSCOPE.

307

" When we thus view," says Sir David Brewster, " two dis-
similar drawings of a solid object, as it is seen by each, eye
separately, we are actually looking through two prisms, which
produce a second image of each di-awing, and when these second
images unite, or coalesce, we see the solid image which they
represent. But in order that the two images may coalesce,
without any effort or strain on the part of the eye, it is neces-
sary that the distance of the similar pai-ts of the two drawings
be equal to tivice the sepai'ation produced by the prism. For
this piirpose measure the distance at which the semi-lenses give
the most distinct view of the di-awings ; and having ascertained,
by using one eye, the amount of the refraction produced at that
distance, or the quantity by which the image of one of the
drawings is displaced, place the drawings at a distance equal to
twice that quantity; that is, place the drawings so that the
average distance of similar parts in each is equal to twice that
quantity. If this is not correctly done, the eye of the observer
will correct the error by making the images coalesce "without
being sensible that it is making any such effort. When the
dissimilar drawings are thus united, the solid will appear stand-
ing, as it were, in relief, between the two plane representations."

The lenticular instrument, fitted for use, is shown in figure 85 ;
it consists of a frame of wood or metal; the two semi-lenses are
fixed in brass tubes, which are capable of being adjusted to ac-

308

PRACTICE OF PHOTOGRAPHY.

commodate tlie differences of sight in different individuals. At
the bottom of the box, as seen through the opening, are placed
the two stereoscopic pictui-es, which may consist either of dia-
grams, similar to those already represented, or of images taken
by the daguerreotype, talbotype, or coUodion processes. These
photographic processes enable us to obtain such copies of exter-
nal natiu-e as are i*equired to prodixce the magical results with
which the stereoscope renders us familiar. It is required to take
two pictures of a single object, at such a difference of angle as
will produce the solidity which is evident in ordinary binocular
vision, as the result of vie\\'ing two dissimilar images, under
certain conditions, on a plane surface.

The two accompanying figures represent a bust as viewed by
each of the two eyes singly. If the experiment is tried upon a

bust or statue, it will be found that one eye will see surfaces
which are invisible to the other. Thus in these examples it
will be quite apparent, upon examination, that the line of the
cheek is more distant from the line of the nose in one than in
the other image, and that a similar inequality exists in several
other parts. By a little practice, any reader may, by squinting,
resolve these two images into one, and thus produce the stereo-
scopic effect. Now the object is to place the camera in the
position of the eyes, and thus obtain the representation of two
images, as viewed by each eye separately. This may be effected
with a single camera, by adjusting it at a certain measured dis-
tance from the object to be copied, and having obtained one
picture, move it round about twenty degrees, and take the

---<i7c

THE STEKEOSCOPE. 309

second image, a may represent the object to be copied, b being
the distance at which the camera c is placed, which is, say 100

i> feet from the point a ;

the picture fi'om this
\ point being taken, the

"":-^\S^^ cameiTi is moved roxmd,
'; still preserving the
\ same distance from the
object by means of a
cord attached to the
camera obsciu-a., or by
/ measuring the space.

5.' Two cameras with lenses

^7 of the same focal length

may be employed, and are indeed employed, by M. Claud et and
others, for the piu-poses of obtaining stereoscopic portraits, and
it is foxrnd that with lenses of the same focus, the figui-es are
sufficiently exact for all practical purposes, and produce the
most perfect stereoscopic pictiu-es. Sir Da^dd Brewster con-
tends that it is not practicable to obtain sufficient exactness by
either of these methods. He therefore proposes the use of a
binocular camei^a, which he thus describes : — "In order to
obtain photographic pictiu'es mathematically exact, we must
construct a binocidar camera, which will take the pictures
simidtaneously, and of the same size ; that is, a camera with
two lenses of the same apertiu'e and focal length, placed at the
same distance as the two eyes. As it is impossible to grind
and polish two lenses, whether single or achromatic, of exactly
the same focal lengths, even if we had the very same glass for
each, I propose to bisect the lenses, and construct the instru-
ment with semi-lenses, which will give us pictures of precisely
the same size and definition. These lenses should be placed
with theii" diameters of bisection parallel to one another, and at
a distance of 2^ inches, which is the average distance of the
eyes in man ; and when fixed in a box of sufficient size, will
form a Ijinocular camera, which will give us, at the same instant,
with the same lights and shadows, and of the same size, such
dissimilar pictures of statues, buildings, landscapes, and living
objects, as will reproduce them in relief in the stereoscope."

RULES FOR TAKIXG STEREOSCOPIC VIEWS OF LA2rt>SCAPES.

If the nearest object be distant 50 feet, arrange the two
cameras perfectly horizontally, and parallel to each other, plac-
ing them 2 feet apart, measured from the centre of each lens.

310 PRACTICE OF PHOTOGRAPHY.

Let the distance between them (measured in this way), be
always dii-ectly proportionate to the distance of the nearest
object, which must be determined by the tape; thus, when the
object is t^^^ice the above distance off (100 feet), place the cameras
4 feet apart; for 150 feet, 6 feet apart, a'li.d so on.

For trees it does not seem veiy important that perfect hori-
zontal! ty of the cameras should be preserved. If they are
aiTanged horizontally, it is only possible, in very peculiar situa-
tions, to avoid making the foreground disagreeably and dispro-
portionately large.

IMi'. Fenton does not keep the cameras parallel in taking
landscapes; but, observing Prof Wheatstone's inile, as above,
respecting distances, inclines them so that the same object may
occupy as nearly as possible the centre of each ground-glass plate.

Extraordinary relief of distant objects may be obtained bj"
arranging the cameras with special reference to them. Thus,
views taken across the Thames, by placing the cameras 12 feet
apart, produce an astonishing effect. In such -vT^ews, of course,
no near objects must be admitted.

Horizontal position of the cameras is absolutely necessary
when buildings are taken; otherwise, when viewed in the ste-
reoscope i-ilaced horizontally, they appear to be falling over.

There appears but one objection to the binocular camera of
Sii' David Brewster, and that is one arising from the circum-
stance of employing really the veiy worst poi'tion of the lens ;
i.e., the two sides. This, however, in practice is not foiind to
be of any disadvantage ; the images are sufficiently perfect,
although not so absolutely correct as those formed by the centre
of the lens, and they are certain of being resolvable into a dis-
tinct image of thi-ee dimensions.

The following notes on the stereoscope were communicated by
Professor Wheatstone to the Photographic Society : —

" The most perfect and generally usefid form of the stereoscope
is that with reflecting mirrors described in my earliest memou*
' On Binocular Vision,' published in the Philosoj)hical Transac-
tions for 1838. Pictiires of any size may be placed in it, at the
proper point of sight, 'svith the proper convergence of the optic
axeg, and it admits of every requ.isite adjustment to make the
pair of binocular pictures coincide correctly.

"I have described in my second memoir a portable stereoscope
which folds into a small compass, and which is well suited for
pictiu'es not exceeding six inches by foiu*. I have since con-
structed an instrument, very convenient for caiTying about,
which is adapted to exhibit joictures of the largest dimensions
u.sually taken, as well as smaller ones, and which may be made

THE STEREOSCOPE. 311

use of either for mounted or unmounted pictm-es. When closed
it occupies a space of 9 inclies in length, 5 in breadth, and 4^ in
height j when expanded the instrument is 2 feet in length, 1 foot
in height, and 9 inches in depth. The base and sides consist of
jointed bars on the principle of the lazy-tongs ; the two miixors
fold together back to back, and, by means of a hinge on their
support, fall into a groove on the base fitted to receive them.
On the top of each of the expanding sides a clip 9 inches in length
receives the picture (which there is no need to mount on card-
board), and holds it by the pressm-e of a suitably disposed sj^riug ;
and a similar but detached spring clip is applied to the lower end
of the picture, in order to keep it flat and in a vertical position.

"The pictures being fixed in the clips, so that theii- reflected
images shall appear single and coincide in all theii' parts, the
accui'ate adjustment to the sight of diflferent persons is efifected
by sliding to and fro the pillar which supports the min-ors; the
optic axes being caused to converge more as the mirrors are
moved towai'ds the eyes, and vice versa. As the height of the
sides is variable through every degree, the pictures are easily
adjusted to the same level by pressing on the side which is
highest. The length of the base being also variable, the pic-
tui-es, if it be required, may be placed at different equal distances
from the min-ors. If the pictures are not straight with respect
to the sheets of paper on which they are placed, one end may
be brought lower than the other merely by di-awing down that
end so that it shall not enter the clip so far as the other.

" The instiixment is furnished ^^dth a pair of ordinary spectacle
lenses, No. 24. If the pictures were so placed that their reflected
images coincided when the optic axes made an angle of IS'',
corresponding to the distance of 12 inches, no lenses would be
requisite, as the distance of the binocular image, the conver-
gence of the optic axes, and the adaj^tation of the eyes to distinct
vision, woidd have their cvistomary con-espondence. But, for
reasons I have elsewhere stated, a much better effect is produced,
and the objects appear larger and more distant, when the pictiires
are so placed that, to cause their* most distant corresponding
points to coincide, the optic axes are parallel, or nearly so ; in
this case, however, in order to see the objects distinctly, the
rays proceeding from them must be rendered less convergent,
and for this purpose lenses are necessary.

"The lenses are movable in a vertical dii-ection, in order that
they may be fixed at the proper point of sight ; the effect of a
stereoscopic pictiu-e gi-eatly depends on its being thus viewed,
though it is a circumstance which is very generally disregarded."

With the single camera, taking the precaution named, with

312 PRACTICE OF PHOTOGRAPHY.

two lenses of the same focal length, or with the semi-lenses,
stereoscopic pictures may be obtained without difficulty.

The magic result of the resolution of two plain pictures
into one, possessing to the eye the most positive solidity, is so
striking when witnessed for the first time, that it appears to be
a deception of the senses. Even when fully accustomed to the
phenomena of the stereoscope, there is an indescribable charm
in the beautiful pictiu-es, that they are gazed at again and again
with increasing admiration. Living forms appear to stand out
in all the roundness of life ; and where colours have been judi-
ciously applied to the daguerreotype or calotype portrait, it is
not possible to conceive a more perfect realization of the human
form than that which stands forth, prominently, from the back
ground of the stereoscopic picture. Statues, in like manner, are
almost realized again in their miniatm*e representations. Archi-
tectural piles are seen in all that exactness of proportion and
gradation of distance, which is, in their minute reproduction,
singularly interesting ; and in landscapes, the stereoscope gives
us a reformation of every image in apparently the most perfect
solidity and truth of distance. In the stereoscope we have at
once an instrument which enables us to study many of the
phenomena of vision, and to reproduce loved and beautiful
objects, or interesting scenes, tlu-ough the agency of those rays
by which they were illuminated, in that strange perfection which,
in its mimicry of visible external natiu'e, almost baffles the
examination of hrmian sense.

CHAPTEE XII.

Photographic Exgraveng.

^Ir. Fox Talbot has, since the publication of the former edi-
tion of this treatise, published some accoxmt of his experiment
to etch on steel the impression produced by the solar rays. He
thus describes his process : —

" The first thing to be done is to select a good steel plate, and
to immerse it for a mini;te or two in a vessel containing vinegar,
mixed -ttith a little .svdjjhuric acid. The object of this is to di-
minish the too great polish of the surface ; for other^vise, the
photographic preparation ■would not adhere "well to the surface
of the steel, but would peel off. The plate is then to be "well
"washed and dried. Then, take some isinglass and dissolve it in
hot "water. The solution should be strong enough to coagidate,
"when cold, into a fii-m jellv. This solution of isinglass or gelatine
should be strained "while hot through a linen cloth to piuify it.
To this must be added about half as much of a satiu^ted solution
of bichi'omate of potash in "water, and they should be "well stirred
together. When cold, this mixtiu-e coagidates into a jelly,
"which has veiy much the appearance of orange jelly. The me-
thod of using it is to liquefy it by gentle heat, and to pour a
quantity upon the centre of the steel plate. Then take a glass
rod, hold it horizontally, and spread the liquid uniformly over
the plate. Then incline the plate, and poiu' off the superfluous
gelatine. Let the steel plate be placed upon a stand, and kept quite
horizontal, that the liquid may not run to one side of the plate.
Then place a spiiit lamp beneath the plate, and "warm it gently
tiU the gelatine is quite di-ied up. When dry, the film of gela-
tine ought to be bright yeUo"w and very uniform. If clouded
bands appear upon the siu-face, it is a sign that there is too Little
gelatine in proportion to the bichromate, which must therefore
be corrected. The steel plate now coated "v\-ith gelatine, is ready
to receive a photographic image of any object. First, let us sup-
pose the object is one capable of being applied closely to the sur-
face of the plate ; for instance, let it be a piece of black lace or
the leaf of a plant. Place the object upon the plate in a photo-
graphic copying-frame, and screw them into close contact. Place

314 PRACTICE OF PHOTOGBAPHY.

this frame in the direct light of the sun for a short time, vary-
ing from half a minute to five minutes. Let it then be removed
and the plate taken out, and it will be fovmd impressed with a
yellow image of the object upon a ground of a brown coloui-, as
might be expected from the well-known photographic property
of the bichromate. The plate is then to be placed in a vessel of
cold water for a minute or two, which dissolves out all the bi-
chromate and most of the gelatine also from the photographic
image, i. e., from those parts of the plate which have not been
exposed to the smi, being protected by the object ; while, on the
contrary, it dissolves little or none of the gelatine film which
has been fnlly exposed to the sun's rays. The consequence of
which is, that instead of a yellow image, we have now a white
one, but still upon a ground of bro^mi. The plate is then re-
moved from the water into a vessel of alcohol for a minute, and
it is then taken out and placed upright on its edge in a warm
place, where in the course of a few minutes it becomes entirely
dried. Tliis terminates the photographic part of the operation.
If the plate is carefully examined while in this state, it appears
coated with gelatine of a yellowish brown coloiu", and impressed
with a white photogra^iliic image, which is often eminently
beautiful, owing to the circumstance of its being raised above
the level of the plate by the action of the water. Thus, for in-
stance, the image of a piece of black lace looks like a real piece
of very delicate white lace of similar pattern, closely adheiing to,
but plainly raised above, the brown and polished surface of the
plate, which serves to display it veiy beautifully. At other
times the white image of an object offers a varying display of
light when examined by the light of a single candle, which indi-
cates a peculiar molecular arrangement in the particles of gela-
tine. These photographic images are often so beautiful that the
operator feels almost reluctant to destroy them by continuing
the process for engraving the plate.

" In order to explain how such an engi-aving is possible, it is,
in the first place, to be observed that the photographic image
differs from the rest of the plate, not only in coloiu', but, what
is of much more importance, in the thickness of the film of gela-
tine which covers it. The coating of gelatine on the rest of the
plate is, comparatively sj)eaking, a thick one, but that which
originally covered the image has beenmostly removed by the action
of the water, a small portion, however, almost always remaining.
It therefore naturally happens that when an etching liquid is
poured on to the plate, it first penetrates through the thin gela-
tine covering the image, and etches the steel plate beneath.
But the next moment it penetrates likewise through the thicker

PHOTOGRAPHIC ENGRAVING. 315

coating of gelatine, and thus spoils the resiJt by etching the
"whole of the plate. Nitric acid, for instance, does this, and
therefore cannot be employed for the purpose. Since the other
chemical liquids wliich are capable of etching steel have a certain
analogy to nitric acid in their corrosive jiroperties, they also for
the most jDart are found to fail in the same manner.

"This was a difficvilty. But after some researches I found a
liquid which etches steel perfectly well, and at the same time is
fi*ee from the inconvenient property of jjenetrating the gelatine
film. This liquid is the bichloride of platinum. In order, how-
ever, to use it successfully, it must be mixed -with a cei-tain
quantity of water, neither more nor less (I mean, to any material
extent), otherwise its action becomes irregular. The best way
is to make a perfectly saturated solution, and then to add to it
one-foiu-th of its bxilk of water. Then correcting this by a few
trials, a solution of proper strength is finally obtained. Sup-
posing, then, that we have prepared such a solution, the opera-
tion of etching the plate is performed as follows : — The plate is
laid on a table, and a small quantity of the bichloride being
poiu'ed upon it, it is to be rapidly difiiised and spread over the
whole plate with a camel-hair brush. Not much liquid is poured
on, because its opacity would prevent the operator from distin-
giiishing the efiect produced by it on the metal. For this reason,
it is hardly necessary to make a wall of wax round the plate ; that
is, if the portions to be etched are confined to the central jjart of
the plate, and do not approach very near to the edge. The
efiect of the liquid upon the plate is not at first visible, since it
disengages no gas ; but after the lapse of a minvite or two, the
white photographic image begins to darken, and soon becomes
black in every part. "When this change is complete, the image
often looks veiy beautiful, though quite altered from what it
was before. The operator should carefully watch the image until
he thinks that it is finished, or not likely to be fiu'ther improved
or developed by continixing the process any longer. He then
inclines the plate gently, and pom-s ofi" the liquid by one corner
of the plate. The plate is then dried with blotting-paper, and
then a stream of salt water, which is better than fi-esh water for
this purpose, is poured over the plate, which removes all traces
of the etching liquid. The plate is then nibbed with a wet
sponge or linen cloth, which in a short time detaches and re-
moves the film of gelatine, and discloses the etching that has
been effected. When the object is not of a nature to be applied
directly to the surface of the plate, the most obvious method of
proceeding is, of course, to place the prepared plate in the focus
of a camera, and to direct the camera to the object. But in

316 PRACTICE OF PHOTOGRAPHY.

consequence of the low degree of sensitiveness of bichromate of
potash, this would take, generally speaking, too long a time to
accomplish. The better way in practice, therefore, is to take a
negative photograph of the object on paper with a camera, and
from this to obtain a positive copy either on glass or paper,
which should be very uniform in texture, and moderately trans-
parent. Then this positive copy is placed on the plate in a pho-
tographic copying-frame, and being placed for a few minutes in
the sun, it impresses the plate with a photograpliic image ; which
image, etched as above described, and printed off upon paper,
will finally give a positive representation of the object. If the
object depicted upon the plate by the sun's I'ays is broad and
uniform, for instance, the opaque leaf of a plant, then, of course,
the etching is uniform also. When this is printed off, it pro-
duces an effect which is not always satisfactory. I will there-
fore now explain a moflification of the process which destroys
this uniformity, and which in many cases produces a great im-
provement in the general effect.

" For this pui'pose I must remai-k, in the first place, that if a
piece of black gauze or crape is the object selected for represen-
tation, it produces an engraving of itself Avhich is marvellously
accurate. But when two folds of the gauze are laid across each
other obliquely, then the resulting engraving requires a lens in
order to separate from each other and distinguish clearly the
lines belonging to the two portions of the gauze. Now, if this
engraving is printed off, the result offers to an eye at a moderate
distance the appearance of a uniform shading. Now, I avail
myself of this circumstance to modify my original process as fol-
lows : — suppose the object to be the opaque leaf of a plant, of
irregular outline, first, I cover the prepared plate with two ob-
lique folds of black crape or gauze, and place it in the sunshine
for two or three minutes. The effect of this is to cover the plate
with a complicated image of lines passing in all directions. Then
the leaf is substituted for the crape, and the plate is replaced in
the sunshine for two or three minutes more. The leaf being
then removed from the plate, it will be seen that the sun has obli-
terated all the lines that were visible on the parts of the plate
exterior to the leaf, converting all those parts to a uniform
brown. But the image of the leaf itself is still covered -with a
network of innumei-able lines. Now, let this be etched in the
way already described, and let the resxilting etching be printed
off. The result is an engraving of the leaf, which, when beheld
by the eye at a certain distance, appears uniformly shaded, biit
when examined closely, is found to be covered with lines very
much resembling those produced by an engraver's tool, so much

PHOTOGRAPHIC EXGRAVIXG. 317

SO tliat even a practical engraver woT;ild probably be deceived by
the apjDearance. This crape ari'angement I call a photograjjMc
veil ; and as I think it likely that the idea ^411 prove useful, I
will make a few more remarks upon it. It is cleai' that an
arrangement composed of two thicknesses of ordinary crape or
gauze is but a nide attempt at a photogi-aphic veil. To realize
the practical utility that may result from the idea, supposing it
to be bonie out by fui-ther experience, it would be pi'oper to
fabricate a much finer material, and to employ five or six thick-
nesses of it, or else to cover a sheet of glass in any convenient
manner with an innumerable quantity of fine Hnes, or else with
dots and specks, which must be opaque and distinct from each
other. The result of practically emjDloying such a method, sup-
posing always that it answers in practice, as I think it probably
•will, would be an etching apparently uniform, but really con-
sisting of separate small portions, in consequence of which it
would hold the ink much better, and other obvious advantages
would also be obtained. Another mode of accompKshing the
same object is to cover the plate originally M-ith an aqua -tint
groirud. But then a fresh one would be requii-ed for every plate,
whereas a single veil would serve for any nimiber of plates in
succession. Experience alone can decide between these difierent
methods. "When the etching is finished, the plate should be
very soon coated with wax to protect it. A few houi's' exjDosure
to the atmospheric air nxsts and destroys the etchings when
newly made, although it does not do so afterwards. The oxida-
tion only attacks the lines of the etching, the rest of the jilate
sustaining no injiuy, if the air is tolerably diy."

By the process of M. Niepce {Heliograjyhy) described in the
historical sections, some etchings on metal plates were obtained.
Recently his nejihew, M. Kiepce de St. Victor, has been retui-n-
ing to this process with some success — ^being assisted in the
etching process by M. Lemaitre. These gentlemen have thus
described their process : —

"The steel to be operated on having been freed from grease by
whitening, M. Lemaitre pours upon the polished siu-face water
acidulated with hydrochloric acid, in the proportions of 1 part of
acid to 20 parts of water : this is what he does in aquaforiis en-
gi-aving before applying the varnish ; by this means the varnish is
made to adhere perfectly to the metal. The plate must then be
well washed A^dth piu-e water and diied. Kext, by means of a
roll of cloth covered with leather, he spreads upon the polished
surface the bitumen of Jud?ea dissolved in essential oil of lavender,
submits the vami-sh thus applied to a moderate heat, and when
dry, preserves the j^late from the action of light or damp.

318 PRACTICE OF PHOTOGRAPHY.

" On a plate thus prepared, I apply the face of a dii-ect (or posi-
tive) photogi-aphic proof upon albiuninized glass or waxed paper,
and expose it to the light diu'iug a length of time, varying ac-
cording to the nature of the proof to be reproduced, and the in-
tensity of the light ; in any case the operation is never very
long ; for a proof may be obtained in a quarter of an hour in the
sun, and in one hoiu' in diffuse dayhght. Indeed it Ls necessary
to avoid prolonging the exposui'e too much, for in such case
the image becomes visible before the operation of the solvent,
which is a signi that the jiroof is a failm-e, because the solvent
will no longer produce any effect.

"I use, as solvent, 3 parts of rectified oil of naphtha and 1 part
of benzine (prepared by Colas). These proportions have usually
given me good results ; but they may be varied, in proportion
to the thickness of the layer of varnish and the time of exposure
to light, for the moi'e benzine there is the more solvent the
action. The essential oils produce the same effect as benzine,
that is to say, they remove the portions of varnish which have
been preserved from the action of light. I have disco veiled that
ether acts in the opposite way.

" In order quickly to arrest the action and remove the solvent,
I throw water upon the plate so as to form a sheet, and I thus
remove the whole of the solvent. I then dry the drops of water
which remain upon the plate, and the heliograpliic operations
are terminated.

" It now remains to speak of the operations of the engraver.
M. Lemaitre has undertaken to describe them.

" Note of M. Lemaitre.

" Composition of the mordant or biting liquid :

Nitric acid 36° 1 part (by volume).

Distilled water 36'' 8 „ „

Alcohol 36^ 2 „

" The action of the nitric acid, diluted with water and alcoholized
in these proportions, commences immediately the mordant is
poured upon the steel plate, prepared in the manner just de-
scribed, while the same quantities of nitric acid and water, with-
out alcohol, have the inconvenience of remaining inert for at
least two minutes after contact ; I leave the strong mordant only
a very short time upon the plate, I remove it, wash and well
dry the varnish and the engraving, so as to be able to continue
and bite more deeply into the metal without injuring the helio-
graphic laj^er. For that purpose I use resin reduced to very
fine powder ; placed at the bottom of a box prepared for this

PHOTOGRAPHIC ENGRAVING. 319

piirpose, I agitate it by means of a pair of bellows, so as to form
a sort of cloud of dust which is allowed to settle upon the plate,
as is the practice in aqua-tint engraving. The plate is then
heated ; the resin forms a network all over the engraving, and
it consolidates the varnish, wliich is then capable of resisting for
a long time the corrosive action of the mordant (nitric acid
diluted with water, without addition of alcohol). It forms on
the blacks a fine grain, which receives the printing ink, and
enables us to obtain good and numerous impressions after the
varnish and the resin have been removed by the aid of heated
greasy substances and essential oils.

"The residt of all these operations is, that, without the help of
the bui"in, we may reproduce and engrave on steel all photogi-aphic
impressions on glass or paper, and without employing the camera
obscura."

Processes analogous to these have been employed to obtain
impressions upon lithogi'aphic stones, which, having been treated
in the usual manner, are rendered capable of producing any
number of impressions.

APPEXDIX,

The following coiTespondence, important in the histoiy of pLo-
tography, appeared in the Times newspaper of AugustlS, 1852 : —

THE PHOTOGRAPHIC PATEXT EIGHT.

We have been requested to publish the following corre-
spondence between the Presidents of the Royal Society and the
Royal Academy, and the patentee of the ai-t of photogTaphy upon
paper, with the \dew of definitively settling a question of con-
siderable interest to aitists and amateiu-s of photography in
general : —

No. 1.

London, July, 1852.

Dear Sir, — In addressing to you this letter, we believe that
we speak the sentiments of many persons eminent for their love
of science and art.

The art of photogi'aphy upon paper, of which you are the

[ inventoi', has arrived at such a degree of perfection that it

i must soon become of national importance ; and we are anxious

I that, as the art itself originated in England, it should also

receive its further perfection and development in this coimtry.

At present, however, although England continues to take the

lead in some branches of the art, yet in othei-s the French are

unquestionably making more rapid progress than we are.

322 APPKNDIX.

It is very desii-able tliat we slioxild not be left behind by the
nations of the Continent in the improvement and development
of a piu'ely British invention ; and as yon are the possessor of
a patent right in this invention, which will continxie for some
years, and which may, perhaps, be renewed, we beg to call yonr
attention to the subject, and to inquu-e whether it may not be
possible for you, by making some alteration in the exercise
of yoiu' patent rights, to obviate most of the difficulties which
now appear to hinder the f)rogress of the art in England. Many
of the finest applications of the invention will, probably, require
the co-operation of men of science and skilfid artists. But it is
evident that the more freely they can use the resoiu'ces of the
art, the more probable it is that their efforts will be attended
with eminent success.

As we feel no doubt that some such judicious alteration would
give great satisfaction, and be the means of rapidly improving
this beautifid art, we beg to make this friendly communication
to you, in the full confidence that you will receive it in the same
spirit — the improvement of art and science being oiu- common
object.

ROSSE.

C. L. EASTLAKE.

To H. F. Talbot, Esq., F.R.S., &c.,
Lacock Abbey, Wilts.

No. 2.

Lacock Abbey, July 30.

My Deae Lord Rosse, — I have had the honour of receiving
a letter from yourself and Sir C. Eastlake respecting my
photographic invention, to which I have now the pleasure of
replying. ^

Ever since the Great Exhibition, I ha^'c felt that a new era
has commenced for photography, as it has for so many other
usefiil arts and inventions. Thousands of persons have now
become acquainted with the art, and, from having seen such
beautiful specimens of it produced both in England and France,
have natm-ally felt a wish to practise it themselves. A variety
of new applications of it have been imagined, and doubtless
many more remain to be discovered.

APPENDIX. 323

I am unable myself to piirsiie all these niimerons branches of
the invention in a manner that can even attempt to do justice
to them, and moreover, I believe it to be no longer necessaiy,
for the art has now taken a firm root both in England and
France, and may safely be left to take its natural development.
I am as desirous as any one of the lovei-s of science and art,
whose wishes you have kindly undei'taken to represent, that our
country shoidd contiaue to take the lead in'this newly-discovered
branch of the fine arts ; and, after much consideration, I think
that the best thing I can do, and the most likely to stimulate to
fiu'ther improvements iv photography, will be to invite the
emulation and competition of oiu- artists and amateurs, by re-
laxing the patent right which I possess in this invention. I
therefore beg to reply to your kind letter by offering the patent
(with the exception of the single point hereafter mentioned), as
a ti'ee present to the public, together with my other patents for
improvements in the same art, one of which has been very
recently gi-anted to me, and has still thirteen years unexpired.
The exception to which I refer, and which I am desii'ous of still
keeping in the hands of my own licensees, is the application of
the invention to taking photographic portraits for sale to the
public. This is a branch of the art which must necessarily be
in comparatively few hands, because it requii-es a house to be
built or altered on purpose, having an apartment lighted by a
skylight, &c., otherwise the portraits cannot be taken indoors,
generally speaking, without great difficulty.

With this exception, then, I present my invention to the
covmtry, and trust that it may realize our hopes of its future
utility.

Beheve me to remain, my dear Lord Eosse,

Your obliged and faithful servant,

H. F. TALBOT

The Earl of Eosse, Connaught Place, London.

324

APPENDIX.

COREESPONDENCE OF ENGLISH AND FEENCH WEIGHTS
AND MEASUEES.

English Weights.

English Grains. French Grammes.

2-6803 — Troy .

- 15438-

1000- = 1 Kilogramme

1 Pound Avoirdupois

7000-

453-4

1 _ Troy .

5760-

373-096

] Ounce Troy

480- •

31-091

1 — Avoirdupois

437-5

28-338

1 Drachm Apothecaries

60-

3-8864

1 — Avoirdupois

27-344

1-7711

1 Imperial grain .

0-065

15-438

1- =1 Gramme

1-5438

0-1 = 1 Decigramme

•1544

0-01 = 1 Centigramme

•0154

0-001= 1 Milligramme

English Imperial Meji

lSDRES.

French LrrRES.

1 Gallon .

. 4-5455

1 Pint = 20 fluid ounces

. 0-5682

1 Decigallon =16 fluid ounces . . 0'4545

Fluid ounce, being the bulk of an
avoirdupois ounce of VFater at 62°
Fahr

Fluid drachm

Septem, being the bulk of 7 grains of

To'ooo

water at 62° Fahr. or the THKnn of

a gallon
0-22 Gallon
1-76 Pint .
2-20 Decigallon .
2-2 Septems
15-4 Grains of water at 62° Fahi-.

0-2841
0-0355

0-000455

1-000

0-001

1 Centimetre Cube

INDEX.

Acetate of lead, photographic use of,

116.
Achromaticity, 168.
Actinic focus, 137.
Actinism, term proposed. 111.
Albumen in Talbot's instantaneous

process, 31.
Albumen processes, 95.

on paper, 223.

on glass plates, 276.

Alcohol extracts, colooring matter of

flowers, 62.
Alkahes, stimulating effects of, 67.
Ammonia-citrate of iron, 53.
Animoriia-nitrate of silver, 193.
Amphitype, the term, 33.

of Sir J. Herschel, 59.

Apparatus required, 183.
Appendix, 391.
Arago on chemical rays, 142.
Archer on the collodion process, 252.

on whiting coUodion image, 265.

Archer's collodion camera, 268.
Asphaltum used in hehography, 12.

Bayard on positive photographs, 295.

Becquerel modifies Ponton's process, 71 .

on rays of spectrum, 133.

Becquerel's coloured photographs, 160.

Berard on magnetizing power of violet
ray, 5.

on chemical spectrum, 9.

Berres on fixing daguerreotypes, 247.

Berzelius on peculiar conditions of
salts of gold, 9.

on precipitates of gold, 55.

Bingham on chromatype, 72.

on calotype process, 212.

Binocular vision, Wheatstone on, 305.

Biot, M., on the calotype, 21.

Bitumen of Judea used by Xiepce, 12.

Bleaching power of iodine salts, 285.

Bockman, action of light on phospho-
rus, observes, 6.

Botanical specimens, to arrange, 185
Brewster on the spectrum, 108.

on the stereoscope, 308.

Bromide of sUver and mercurial vapour,

81.
of silver, 118.

of potassium, use of, 119.

of iodine, to prepare, 238.

Bromine box, 238.
Bromidized collodion, 273.
Brook's photographic registration, 140.
Buffing daguerreotype plates, 235.
Buildings, &c., to copy, 298.

Calotype, the, 20.

improvements in the, 27.

modern improvements in,

206.

process on gelatine and albu-
men, 222.

on wax-paper, 227.

Camera obscura for daguerreotype, 39.

varieties of, 186.

folding, 191.

for coUodion, 267.

general remarks on,

298.

Catalysotype, the, 75.

Change on silver salts examined, 116.

Channing modifies calotype, 213.

Charles, M., said to have produced
photographs, 5.

Chemical spectrum, 110.

China clay used in paper, 180.

Chlonde of sUver observed to blacken, 4.

of silver, 115.

of silver, to form, 194.

Chlorine, combination with hydro-
gen, 9.

Chromatic aberration, 167.

Chromatype, the, 71.

Chrysotype, the, 54.

Claudet and Goddard improve the
daguerreotype, 99,

326

INDEX.

// Claudel on the daguerreotype, 132.

.'-^ 's mercurial chamber, 243.

Collodion process, 252.

to prepare, 254.

process, to iodize, 254.

introduced in photography,

95.

Colours destroyed by comjilementary

ray, 65.
■ of positive photographs by one

process, 84.

■ , question of producing them

natui'ally, 158.

, actinic, explained, 171.

of plate when sensitive, 237.

Complementary rays destroy cohnirs 65.

Cooper's method of washing paper, 195.

Copper, plated, used by Niepce, 14.

Corchorus japonica, colouring matter
of, 64.

Corrosive sublimate, action of on pho-
tographs, 87.

Cotton, gun, for collodion, 252.

Crookes on the use of bromine, 273.

Cundell on the calotype, 206.

Cj'anotype, the, 48.

Daguerre on Niepce's process, 15.

, history of his discovery, 34.

■ observes variations in photo-
graphic action, 113.
• improves iodizing

process,
242.

Daguerreotype, the, 34.

, improvements in, 43.

applied to paper, 85.

— , theory of the, 124.

■ , the improved, 230.

• simplified, 248.

Davy, Sir H., on change in puce-co-
loured oxide of lead, 5 .

• , Sir H., on Wedgwood's experi-
ments, 7.

, Dr., on change in solution of

coiTOsive sublimate, 8.

Delamotte on developing collodion
image, 259.

Desmortiers on discoloration of Prus-
sian blue, 6.

Developing calotype picture, 209.

. daguerrean image, 243.

Diactinic lens explained, 171.

Diamond, Dr., his practice on collo-
dion, 264.

Dippel, animal oil of, used by Niepce, 15.

Discoloration of papers, 179.

Double salts of iron in cyanotype, 53.

Draper, Dr., on daguerreotype pro-
traits, 96.

— -, Dr., on radiant images, 152.

Draper's experiments with chlorine, 9.

experiments on solar spec-
trum, 113.

Dynactinometer, the, described, 146.

Eggs, white of, photographic use, 225.

Energiatype original of ferrotype, 73.

Englefield, Sir H., on heating power
of solar rays, 5.

»- on phosphorescence, 9.

Engravi\ig by M. Niepce's process, 14.

■ , photographic, 314.

Everard Blanquart's calotype, 213.

BlanqPh't's albuminized pa-
per, 226. ,,.. M

Exciting collodjoai^late, 256.

Experiments on salts of silver, 115.

on iodide of silver, 292.

Exposure in the camera of calotype
paper, 209.

Ferro-tartrate of silver, 58.
FeiTocyanide of potassumi, photo-
graphic use of, 79.
Ferrotype, the, 73.
Fixing the daguerreotyj^e picture, 43.
■ the photographic picture, 198.

the calotj-pe picture, 210.

collodion image, 261.

Fizeau's bromine water, 237.
Fizeau fixes with chloride of gold, 245.
Flacheron's caloty]3e process, 215.
Flowers, colouring matter of, 61.
Fluoride of silver, 120.
FluorotyiDe, the, 80.

Focimeter, the, described, 145.
Focus of lenses for photography, 137
Forces, antagonistic, in sunbeam, 112.
Fresnel on chemical action of rays, 142.
Fry on the collodion process, 252.
Fulminate of silver, 121.
Fyfe, Dr., on phosphate of silver, 121.

Gallo-nitrate of silver in calotype, 208.

Gaudin, M., on daguerreotype, 134.

Gay-Lussac and Thenard's investiga-
tions of chlorine, 9.

Gelatine on paper, 222.

Glass plates, cleaning of, 255.

, albuminized, 276.

Goddard, Wm., first introduces use of
bromine, 47.

327

Goddard on bromide of iodine, 239.
Gold and silver, difference between,

according to the alchemists, 4.

, employed in chrj'sotype, 55.

, salts of, used as photographic

agents, 89.
Gray, Le, on fixing photographs, 202.

, processes on glass, 276.

, |albumen process on paper,

224.
Green glass, use of. 109.
Griffin's modification of Archer's

camera, 269.

Harrup refutes Eumford's experi-
ments, 5.

Havell's etchings on glass, 284.

Heat and chemical action considered,
111.

Heating powers of the prismatic rays,
109.

Heliochromes of Niepce, 161.

Heliograph of Mr. Jordan, 141.

Heliography discovered by Niepce, 11.

, the process so called, 12.

Herschel on fixing photographs, 205.

, Sir W., on heating power

of solar rays, 5.

, Sir J., on the prismatic rays,

108.

on the photographic camera,

189.

on bromide of silver, 221.

Herschel's photogi-ajAic processes, 48.

chrysotype, 54.

History of the discoveries in photogra-
phy, 3.

of photography, general sum-
mary, 100.

Horn, silver observed to blacken, 4.

Home, Mr., on collodion, 255.

Hungarian liquid, 237.

Hyalot;y-pes, 283.

Hydriodic acid, its bleaching power,
290.

Hydrogen and chlorine, 9.

Hyposulphite of soda used in washing,
47.

of soda, 201.

Indices of refraction for chromatic

rays, 167.
Instantaneous process of Talbot, 31.
Instrument, meteorological, registered

by photography, 139.

Instruments used in daguerreotype,

251.
Iodide of iron used by Talbot, 32.

of silver, 118.

of silver, to form, 197.

of silver, reasons for rejecting

it, 221.

Iodides, various bleaching powers of,

288.
Iodized gelatine paper, 223.
Iodizing daguerreotype plate, 37.

collodion, 254.

Iron, iodide of, used by Dr. Woods, 32.

- sulphate, used by Robert Hunt
32.

- , ammonia citrate of, 53.

- , protosulphate of, photographic
use of, 75.

- , syrup of iodide of, used by Dr.
Woods, 76.

- salts used in collodion, 265.

Jordan, Mr., first registers instruments
by photography, 139.

on iodizing paj)er, 213.

Jugglers of India, statement respect-
ing, 4.

Knight's dynactinometer, 149.

Laborde on bromodized coUodion, 273.
Landscapes, peculiarity of, on Niepce's

plates, 14.
Langenheim's pictures on glass, 283.
Lassaigneon positive photographs, 295.
Latent light, Moser on, 151.
Lavender, oil of, used by Daguerre, 15.
Lemaitre on engraving, 319.
Lenses, on, 163.

, forms of, 165.

Lenticular stereoscope, 308.

Light and chemical action considered,
111.

, refraction of, 163.

Lithographic impressions by photo-
graphy, 319.

Lxmiinous power of the solar raj's, 109.

Magnetic power of violet ray, 5.
JNIaloue's pictures on porcelain ta-
blets, 29.

albumen process, 278.

Martin, Adolphe, on collodion process,

266.
Martin's calotype process, 214.
— process on glass plates, 281.

328

INDEX.

Materials, fixing, employed, 200.
Media, coloured, employed by MeUoni,

109.
Melloni on separating light and heat,

109.
Mercurial vapour and bromide of

silver, 81.

chamber, 243.

Mercury applied to daguerreotype, 41.

, photographic properties of,

57.

Metals impressed with juxtaposed
images, 153.

Miscellaneous processes, 69.

Moigno Abbe on M. Charles' pro-
cess, 5.

Montizon, Coimt de, on the collodion
process, 258.

Morichini on magnetism induced by
solar rays, 5.

Moser, M., on the formation of the
daguerrean images, 151.

Muller's calotype process, 217.

Negative photographs, 183.

wax-paper, to prepare, 229.

Newton, Isaac, Sir, on the spectrum,

108.

Niepce and Daguerre, 34.

, de St. Victor, applies albumen,

95.

, de St. Victor's, coloured photo-
graphs 160.

Niepce 's process of engraving, 818.

production of pictures by

light, 10.

Nitrate ofsilver, to spread on paper, 193.

ofsilver, 114.

Nomenclature, new, proposed, 171.

Operations of preparing dagueiTeotype

plate, 35.
Organic salts of silver, 122.

Papaver rheum, colouring matter of, 63.

orientale, 65.

Paper, Ilerschel's experiments on, 177.

, photogenic, of Fox Talbot, 19.

, qualities of, 176.

, selection of, 175.

■ — — , calotype, 206.
Pentagraph, photographic, 302.
Petit, iafluence of hght on crystal-
lization, 4.

Philosopher's stone, 4.
Phosphate of silver, 120.

Photogenic drawings, 18.
Photographic lens, conditions of a, 166.
processes of Herschel,

48.

177.

properties of mercury,
, peculiarities of paper.

Photographometer of Claudet, 136.

, the, described, 143.

Photographs, positive, by one process,

82.

on glass plates, 92.

in natural colours, 159.

— , positive, by iodine, 285.

Photography, practice of, 157.
Pictures on porcelain tablets, 28.
Plate, daguerreotj'pe, to polish, 235.

, to give sensitive surfaceto, 241.

Ponton's, Mr., process, 69.
Poppy, red, 68.

Porcelain tablets, pictures on, 28.
Portraits from the life, to take, 300.
Portraiture by daguerreotype, 96.
Positive photographs from etchings,
283.

photographs by one operation,

285.

photographs, 183.

Potash, bichromate of, 69.
Potassium, ferroeyanide of, as a fixing
agent, 200.

, bromide of, as a fixing agent,

201.
Preparation of gun cotton, 252,
Preschot on fixing daguerreotypes, 246.
Pressure frames, 185.
Processes modified, 282.
Process of daguerreotype, 34.
Proto-nitrate of mercury used, 57.
Prussiate of potash, formation of in
cyanotype, 51.

Rays, exciting, 133.

• , continuating, 133.

, passage of, through lenses, 165.

Reflecting camera obscura, 99.

steresocope, 307.

Refractive powers, 164.
Registration, photographic, 139.
Regnault's calotype process, 219.
Reproduction of pictures, spontaneous,

60.
Researches, early, on the chemical

action of the solar rays, 3.
Ritteron the invisible chemical rays, 5.

INDEX.

329

Ronald's pliotographic registration,
140.

Eoss and Thomson's albumen pictnres,
283.

Rides for stereoscopic landscapes, 310.

Eumford, Count, on the chemical pro-
perties attributed to light, 4.

Sage, M. B. G., on change in realgar, 8.
Salt as a fixing agent, 200.
Schafhaeutl on positive photographs,

296.
Scheele's researches on light, 4.
Seebeck experiments on chlorine, 9.

produces colour on chloride

of silver, 9.

Seguier on iodizing, 242.

Senebier on chemical action of violet

ray, 4.
Senicio splendens, colouring matter of,

62.

photographic use of

juice, 6S.

Sensibility of daguerreotvpe plate,

133.
Sensitive papers, to prepare, 193.

coating, to give to a plate, 236.

dagueiTeotype mixtures, 236.

Shaw, Mr. G., on daguerreotv'pe, 125.
Silver salts on glass plates, 92.

, bromide of, 118.

• ■ , iodide of, 118.

Silver-nitrate, 193 ; Ammonia, nitrate,

194 ; Chloride, 194 ; Iodide, 197.

Smee on ferrosesquicyanuret of potas-
sium, 49.

Soda, hyposulphite, 201.

Solar agency producing chemical
change, 108.

Solvent used in heliography, 12.

Somerville, Mrs., on magnetizingpower
of violet ray, 5.

Sparaxis tricolor var, 67.

Spectnun, theoretical, 112.

di\-ided by Becquerel, 133.

bleached by iodine, 297.

Stands, levelling, 192.

Stereoscope, the, 305.

Stewart's calotype process, 218.

Studies, photographic, recommendation
for, 198.

Sublimate, corrosive, as a fixing agent,
205.

Sulphocyanate of potash, 52.

Sulphuretted hydrogen used to pre-
pare paper, 87.

Sun-drawing, note on the term, 11.

Talbot's, Henry Fox, photogenic draw-
ings, IS.

description of the calotype, 25.

instantaneous process, 31.

process of photographic en-
graving, 314.

Theomography, 151.
Thomas', Mr., collodion process, 263.
Towson, Mr., on portraiture, 96.
Transparent bodies, refraction of light

by, 167.
Tunny, Mr., developes with iron salts,

261.

Unsized and sized papers, 179.

Valicour's bromide of iodine, 238.
Varnished paper for photography, 30.
Verignon on positive photographs, 295.
Viola odorata, 67.
Violet ray, magnetic power of, 5.
Vogel, M., action of light on fat, 8.

, Do. on corrosive subhmate, 8.

Wax-paper of M. le Grey, 95.

■ process, 227.

, to render sensitive, 229.

"Wedgwood's method of copying paint-
ings on glass, 6.

Wheatstone on the stereoscope, 305. j

Wollaston, Dr., on the decomposition
of gum-guaiacum, 5.

Wood, action of chlorine and iodine
on, 90.

Wood's, Dr., on catalysotype, 75.

Woolcott's American mixture, 240.

mirror camera, 99.

Yellow glasses, action of, 135.
Young, Dr., experiment of, 8.

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Kavy's A)rniiilturnl Chemistry, .... 3
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lllu8triitiiin«of Zoolo^iy. -.-.-. ^
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Arts and Manufactures.

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K*.-r!eBliisti»'al Misror.v. - . _ . . .
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niM.ls'» Karly cliristianity, . . . . .

nl^^lr>• of (;reti-e. .......

History of *ireei-e anil Macedonia, . . .

History of tlie Jews. ......

!iist<ir}' of I onie. .......

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Coleridge on .\ietlio*l. .--..,
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History of tJreek Literature. - - . - -

fJreek and Itoman Philosophy,

Roman Literature. - - . . -

Iniportance of I itorature to *'en of I'usiness.
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HtcMldart's I'niversal oraiumar, - . - .
W liately's l.<.i.-ie. . . ....

lilieturic. ...---.

Miscellaneous.

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Salgairns's Cookery. ...... 3

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Clal^w Ciiiveisitv AlUllu. ... . . 3

IJrimns Popular l.iliiary. T

llorat i Opera illustrated edition, .... 5

.Mamajie Otfering

Miti'hisun': Siottish Song. . . . . . G

Morrison's Hook- Keeping. - .... 6

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Poison's I.-ivv of Nations. ..... 7

I'ope's Poetical Works and Translations. . . 7

Senior's Poliucol J^cuuoujy, . - . . . g

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t yrlopa-di.i of neligious Aiietiloie. . . -

ryi'loi»Jt?dia of Ileligious I'iograpliy. . . -
Cyi-lt'picilia of Heligious Uenomutations,
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Coinnieiitary on tite Fplte.^ians.

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Airv's Con'rihutions to Encvclopcedia Metropolitana, 2

Hiwk of Nature 2

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Kii

of As

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4 Mathematics . . . . «

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C I the .Meilical Sciences. ... 6

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Hi>tory.

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E. RICH, ESQ.

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CHARLES KNIGHT, ESQ.
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