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1- 



TEXT-BOOKS OF SCIENCE 



ADAPTED FOR THE USB OF 



ARTISANS AND STUDENTS IN PUBLIC AND SCIENCE SCHOOLS 



PHOTOGRAPHY 



LONDON : PRINTED BY 

SPOTTISWOODE AND CO., NEW-STREET SQUARE 

AND PARLIAMENT STREET 



A TREATISE ON 



PHOTOGRAPHY 



BY 



W. DE WIVELESLIE ABNEY, F.R.S. 

CAPTAIN IN THE CORPS OF ROYAL ENGINEERS, AND LATE INSTRUCTOR 

IN CHEMISTRY AND PHOTOGRAPHY AT THE SCHOOL 

OF MILITARY ENGINEERING, CHATHAM 






11, • «^ 



\ 



. FEB ^ 7ft . 



^ 



^Ui r\K' 



\^P 



LONDON 
LONGMANS, GREEN AND 

1878 



CO. 



PREFACE. 



The aim of this book is chiefly to give a rational 
explanation of some of the different phenomena to 
be met with in photography ; and with this to give 
sufficient practical instruction to enable the student 
to produce a landscape picture which shall be techni- 
cally good, and at the same time to be of use to him 
if he make photography an aid to research. 

In regard to the theories which the author has 
enunciated, it is believed that experimental evidence 
completely justifies their adoption. Though rapid 
advance has been made of late years in rule of thumb 
photography, yet the progress has been but slow in 
the science of it since the days when Herschel, 
Draper, Becquerel, and others instituted their re- 
searches ; and if this book can be but the means of 
enlisting a few earnest workers in investigating some 
of the remaining problems which still require solu- 
tion, another aim of the author will be accomplished. 

In a measure supplemental to this treatise, as far 



vi Preface, 

as practical work is concerned, is * Instruction in Pho- 
tography/ by the same author. At one time it was 
hoped that the two might be combined, but after 
much consideration it appeared impossible to compress 
the necessary matter into the pages to which each 
volume of this series is limited. 

This will explain why some — what may be called 
commercial — applications of photography have not 
been noticed in the present work, as it was thought 
more advisable that the available space should rather 
be devoted to the more theoretical aspect of the 
subject 

The author has to record his thanks to Mr. Dall- 
meyer, and to Mr. H. P. Robinson, for the criticism 
they passed on the proof sheets of chapters xxix, and 
xxxi. respectively ; and also to Mr. J. Traill Taylor 
for the permission to use copies of some of the 
diagrams of lenses in chapter xxix. which illustrated 
his article in the 'British Journal Photographic 
Almanac,' 1870. 

Nor can the author omit to acknowledge the 

great obligation he is under to Mr. C. W. Merrifield, 

F.R.S., the editor of a part of this sel-ies, for the 

valuable help and advice which was so cordially 

given whilst this work was passing through the press. 

South Kensington Museum: 
January 1878. 



CONTENTS. 



CHAPTER I. 
HISTORICAL SKETCH OF THE DISCOVERY AND PROGRESS 



OF PHOTOGRAPHY. 



Scheele and Ritter*s Experiments 
Wedgwood and Niepce 
Daguerre's Discovery 
Talbbt's Photogenic Drawing 
Positive and N^;ative Pictures 
The Calotype Process 
The Collodion Process 
Processes with Gelatine, &c. 



PAGE 

I 

2 

3 

4 

4 

5 

5 
6 



CHAPTER II. 

EXPERIMENTS WITH LIGHT. 

The Prismatic Spectrum .- . 6 

Light, Heat, and Actinic Rays 8 

Limits of the Spectrum 9 

Work caused by the Absorption of Rays lo 

CHAPTER in. 
THEORY OF SENSITIVE COMPOUNDS. 



Arrangement of Molecules 

Chemical Effects not an absolute measure of Work expended 



II 

12 



viii Contents, 

PAf;K 
Comparison of Photographic Compounds with Explosives . ♦ 1 3 

Oscillations of a Molecule 14 

the co-existence of Longer Waves with Shorter Waves . .15 
The Visible and Invisible Photographic Image . . . .18 

Methods of Development 19 

Fixing the' Photographic Image 21 



CHAPTER IV. 
THE ACiflON OF LIGHT ON VARIOUS COMPOUNDS. 

Action of Light on Silver Chloride 21 

Action of Light on Silver Iodide 23 

Absorbents of the Liberated Atoms 25 

Action of Light on Silver Bromide .26 

Action of Light on Organic Compounds of Silver . -27 

Action of Light on Ferric and Uranic Compounds . . .29 

Action of Light on Chromium Compounds 31 

Action of Light on Asphaltum, Dyes, and a Mixture of Chlorine 

and Hydrogen 33 

Light causing a Combination between Sulphur and Antimoniuretted 

Hydrogen .34 



CHAPTER V. 

ON THE SUPPORT AND THE SUBSTRATUM. 

Essentials of a Support 35 

The Veliicle Holding the Sensitive Compounds on the Support . 36 



CHAPTER VI. 
THE DAGUERREOTYPE. 

Preparation of the Sensitive Surface , 39 

Development of the Image 40 

Strengthening the Image by Gold 41 

Reptoductions of Daguerreotypes 41 



Contents, ix 

CHAPTER VII. 
CX)LLODION. 

PAGB 

Action of Sulphuric Acid on Organic Matter . . . .42 
Action of Anhydrous Nitric Acid on Cotton . . . '43 

Preparation of Pyroxyline 45 

The Solvents of P3rroxyline ....... 49 

Formula for Plain Collodion 50 

Order of Sensitiveness of Sensitive Silver Compounds prepared 

from different Metallic Compounds . . . . '52 

Formulae for Iodized Collodion • 53 

Testing Plain Collodion • • 54 



CHAPTER VIII. 

PREPARATIONS NECESSARY FOR WORKING THE WET 

PROCESS. 

Cleaning the Plate 55 

The Sensitizing Bath . .58 

Distilling Water 59 

Formulae for the Sensitizing Bath . . . .61 
Keeping the Bath in Order 62 



CHAPTER IX. 

ON THE DEVELOPMENT OF THE PHOTOGRAPHIC IMAGE. 

Theoretical Considerations of Development . . . .63 

On Different Strengths of Developers '65 

On Viscid developing Solutions 66 

Formulae for Developers 67 

CHAPTER X. 

GIVING INTENSITY TO AND FIXING THE IMAGE. 

Different Methods of giving Intensity 70 

Formulae for Intensifiers 71 



X Contents, 

PACiF. 

Action of the Solvents in Fixing the Image . . . -74 

Formulae for Fixing Solutions . . . . . . '75 

Varnishing the Collodion Film 76 



CHAPTER XI. 

MANIPULATIONS IN WET-PLATE PHOTOGRAPHY. 

Cleaning the Plate 77 

Coating the Plate with Collodion ...... 78 

Sensitizing • • 79 

Development 80 

Intensifying the Image 81 

Fixing the Image 83 

Varnishing the Negative 84 

CHAPTER XII. 
DEFECTS IN NEGATIVES. 

Defects due to the Chemical Processes 85 

Defects due to the Lens 86 

Irradiation 87 

CHAPTER XIII. 
POSITIVE PICTURES BY THE WET PROCESS. 

Formula for Sensitizing Bath 89 

Formulae for Developer and Collodion 90 

CHAPTER XIV. 
DRY PLATE PROCESSES WITH THE BATH. 

Substrata 9i 

Collodion 93 

Sensitizing 93 

Applying the Preservative 94 

Drying the Film 95 

Development . 9" 



Contents. xi 

PAGE 

Experiments with Alkaline Developers 98 

Comparison of the Methods of Development . . . .102 

Formulae for Developers 10 

Intensifying the Image 105 



CHAPTER XV. 

DESCRIPTION OF THE GUM GALLIC AND 
ALBUMEN-BEER PROCESSES. 

Details of the Gum Gallic Process . . . . . .106 

Albumen-beer Process 109 

CHAPTER XVI. 
EMULSION PROCESSES. 

Experiments with Silver Bromide Emulsion . . . .112 
Canon Beechey's Unwashed-Emulsion Process . . . • '^S 

CHAPTER XVII. 
EMULSION PROCESSES. 

Washed-Emulsions 119 

Preparation of Emulsion 119 

Transferring Films to Gelatinized Paper . . . . .124 

CHAPTER XVIII. 
THE GELATINO-BROMIDE PROCESS. 

Preparation of Plates 125 

Development 126 

CHAPTER XIX. 

CALOTYPE PROCESS. 

The Preparation of the Iodized Paper 128 

Sensitizing 139 



xii Contents, 

PACK 

Developing and Fixing the Image 131 

Waxing the Paper 132 

Process of Le Gray 132 

CHAPTER XX. 
SILVER PRINTING. 

Experiments with Sensitized Papers 133 

Theory of Toning 13^ 

Fixing the Print 142 

Eliminating Sodium Hyposulphite 143 

CHAPTER XXI. 

MANIPULATIONS IN SILVER PRINTIN(;. 

Preliminary Preparation of the Paper 1 44 

The Sensitizing Bath 145 

Printing the Picture 146 

Toning the Print 14S 

Fixing the Print 149 

Washing the Print 1 50 

Testing for Sodium Hyposulphite . . . . • '5' 

Defects in Prints 152 

CHAPTER XXII. 
COLLODIO-CHLORIDE PROCESS. 

Formula for Collodio-Chloride 152 

Paper for Collodio-Ohloride . . . . . . '153 

CHAPTER XXIII. 

PRINTING WITH IRON AND URANIUM COMPOUNDS. ' 

Printing Processes with Salts of Iron 155 

Chrysotype 156 

The Platinum Printing Process 157 

Salmon and Garnier's Process .158 

Poitevin's Process with Ferric Chloride and Tartaric Acid . .158 

Printing Process with Uranium Salts 159 



Contents. xiii 

CHAPTER XXIV. 
PRINTING WITH CHROMIUM SALTS. 

PA(iE 

Swan's Process i6i 

J. R. Johnson's and Sawyer's Improvements . . . .163 

Preparation of Tissue 166 

Sensitizing the Tissue . .168 

Developing the Print 170 

CHAPTER XXV. 

MISCELLANEOUS PRINTING PROCESSES WITH 
CHROMIUM SALTS. 

Willis's Aniline Process 172 

The Powder Process 173 

Woodburytype 174 

CHAPTER XXVI. 

PHOTO-LITHOGRAPHIC TRANSFERS. 

Requisites in the Paper and Ink 1 76 

Formula for Preparing the Paper 177 

Inking the Printed Picture and Developing the Transfer . . 1 78 

CHAPTER XXVII. 

PHOTO-ENGRAVING AND RELIEF PROCESS?:S. 

Niepce's Process .181 

Ehrard's Process . . . . . . . . • . 183 

Woodbury's Photo-Engraving Process 184 

CHAPTER XXVIII. 
PHOTO-COLLOTYPE PROCESS. 

General Principles of the Photo-collotype Processes . . .186 

Albert's Process . . ^ 187 

Process by Capt. Waterhouse 189 



XlV 



Contents, ' 



CHAPTER XXIX. 



THE LENS. 



Laws of Refraction . 

Dispersion and its Correction 

Spherical Aberration 

Use of the Diaphragm or Stop 

Use of the Swingback 

Distortion of the Image . 

The Optical Centre . 

Portrait Lenses 

Landscape Lenses . 

Further Uses of the Stop . 

Illuminating Power of the Lens 

Definition of the Focus of an Object 



PAGE 
191 

196 
197 
198 
199 
202 
203 
206 
207 
208 



CHAPTER XXX. 



APPARATUS. 



Cameras . 

Hare's Changing Box 
Warnerke's Roller Slide 
The Jont^ Camera . 
The Camera Stand . 
Howard's Dark Tent 
Rouch's Dark Tent . 
The Dark Room 



211 
216 
217 
218 
220 
222 
224 
226 



CHAPTER XXXI. 

ON THE PICTURE. 

Principles to be followed in Choosing a Point of View . . 227 
Examples illustrating Landscape Photography . . . .229 

Examples illustrating Groups 240 

Epitome of Simple Rules to be followed in Landscape Com- 
position 242 

Placing the Camera 243 

Judicious Use of the Swingback 244 

Architectural Pictures ........ 245 

Exposure .......... 246 



Contents. xv 

CHAPTER XXXII. 
ACTINOMETRY. 

PAGE 

Draper's Experiments 247 

Bunsen and Roscoe's Silver Chloride Actinometer . . . 249 

Roscoe's Actinometer 250 

Reading the Tints of the Actinometer 255 

Discussion of the Tnith of Light and Shade in Photographs . 256 

Ordinary Actinometers , . 262 



CHAPTER XXXIII. 
PHOTO-SPECTROSCOPY. 

HerschelFs Experiments 263 

Arrangement for a Photo-Spectroscopic Apparatus . . . 265 
Heliostat 271 



CHAPTER XXXIV. 

SENSITIVENESS OF DIFFERENT SALTS. 

Discussion of the Influence of the Spectrum on Sensitive Silver 

Salts 272 

Reversing Action of the Rays of Low Refrangibility . . 274 

Photographs in Natural Colours . . . . . .276 

Photographs of the Thermal end of the Spectrum . .278 



CHAPTER XXXV. 

SOLAR, STELLAR, AND METALLIC SPECTRA. 

Lockyer's Photo-Spectroscopic Apparatus 280 

Method of Elxamining Metallic Vapours 282 

Star Spectra 285 

Miller's Researches in Absorption Spectra .... 286 

The Thermal Prismatic Spectrum 287 



xvi Contents, 

CHAPTER XXXVI. 
CELESTIAL PHOTOGRAPHY. 

PAGE 

Solar Photography with a Fixed Telescope .... 288 

The Photo-Heliograph 292 

Apparatus for Solar Photography 296 

Solar Eclipse Photographs 299 

Lunar Photography 300 

Stellar Photography 304 

CHAPTER XXXVII. 
PHOTOGRAPHY WITH THE MICROSCOPE. 

Apparatus required 305 

Monochromatic Light 308 

CHAPTER XXXVIII. 
MISCELLANEOUS APPLICATIONS OF PHOTOGRAPHY. 

Photographic Registration of Meteorological Instruments . . 312 

Photography applied to Military Service 313 

Photography and Crime 314 



APPENDIX. 

Table of Strength of Alcohol 315 

Nitric Acid 316 

Sulphuric Acid . . . . .317 

of Elements 318 

Comparison of the Metrical with the Common Measures . .31^ 



»» >» »» 

»» »» »» 



INDEX 32 



I 



A TREATISE 

ON 

PHOTOGRAPHY. 



CHAPTER I. 

HISTORICAL SKETCH OF THE DISCOVERY AND PROGRESS 

OF PHOTOGRAPHY. 

To the alchemists of the sixteenth century belongs the 
honour of having first noticed the change that took place 
in silver chloride (known to them as * Luna comua') by ex- 
posure to light, but they regarded the darkening as a species 
of transmutation of metals, and it remained for Scheele, the 
Swedish chemist, in 1777, to investigate the properties of 
this compound, though his researches led at the time to 
no practical end. Scheele found, when he exposed silver 
chloride to the action of light beneath water, that in the 
fluid was dissolved a substance which, on the application 
of silver nitrate, gave once more silver chloride, and that, 
after applying ammonia to the blackened body, an insoluble 
residue of metallic silver remained behind. These were the 
only facts elicited at the time, and a delay of more than half 
a century occurred before they were put to really good pur- 
pose. In 1 80 1 Ritter, of Jena, repeated the experiments 
of Scheele, and discovered that the chloride darkened rapidly 
in those rays of the spectrum which lie beyond the extreme 
violet To him also is due the announcement that the red 

B 



^ Historical Sketch. 

fays have the property of undoing the work effected by the 
violet, though he attributed the effect to the wrong cause. 

In 1802 Thomas Wedgwood read a paper before the 
Royal Institution, entitled * An account of a Method of 
Copying Paintings on Glass ^ and of making Profiles by the 
agency of Light upon Nitrate of Silver.' 

With these experiments of Wedgwood's Sir Humphry 
Davy was associated, and in their record we find it stated that 
muriate of silver was more readily acted upon by light than 
the nitrate, and that white leather used as a basis gave better 
images than paper. Images obtained by the solar micro- 
scope were impressed without ariy serious difficulty, but no 
means was discovered of rendering them anything but tran- 
sitory when exposed to daylight. For Charles, a French- 
man, has beefii claimed the credit of employing at an earlier 
date the same method of obtaining black profiles by the 
action of light, but there seems to be no authentic proof 
extant that this claim should be allowed. Dr. Wollaston, 
in 1803, discovered that gum guaiacum, when exposed to 
the action of the blue rays of light, became changed in 
colour, and that on exposing those altered portions to the 
red rays, the original tint was restored. 

In 1814 Photography was to receive a new votary in 
the person of Joseph Nic^phore de Ni^pce. Leaving 
the salts of silver, he devoted himself to the study of the 
action of light on resins. After several years of research, 
he at length completed the process known as heliography, 
which consisted in the production of a picture in bitumen 
on a polished metal plate. The discovery he made in regard 
to this resin was that, after insolation, it became insoluble 
in its ordinary solvents. An exposure of many hours in a 
camera obscura was necessary to produce the required effect; 
hence, as may be imagined, the views taken by this means 

^ A mistake often occurs in the reading of this sentence. Wedg- 
wood did not make the copies on glass, but copied paintings which 
were drawn on glass. 



Daguerre's Discovery. 3 

were wanting [in vigour, owing to the shifting direction of 
the sunlight, and, as we shall see later on in this work, 
from other causes, were of necessity deficient in delicate 
lights and shades. In 1827 Ni^pce came over to England, 
with the intention of drawing the attention of the Royal 
Society to his discovery, but his process being secret, his 
communication was not received, and he returned to France. 
In 1824 Daguerre, a French painter, began a series of ex- 
periments in the same direction, and in 1829 he and Niepce 
entered into a partnership, and presumably it was the 
knowledge of the latter*s method of working which gave the 
former the idea of the daguerreotype. Niepce had employed 
silver plates covered with asphaltum, which, after expo- 
sure and application of the solvent, left the metal bare in 
parts. The image thus formed was brown ; the shadows 
being represented by the metallic surface. In order to 
produce a proper effect, it was necessary that the parts 
covered by the bitumen should be whitened and the bare 
parts darkened. After various experiments, he applied iodine 
to the picture, subsequently removing the bitumen. It is to 
be presumed that Daguerre noticed the action that the light 
produced on those portions of the plate which had been 
converted into iodide. At any rate to Daguerre belongs the 
glory of the discovery that an image could be produced on a 
silvered plate which had been exposed to the vapour of iodine, 
though it was by fortuitous circumstances that he hit on the 
method of developing an invisible image. 

In January 1839 the discovery of the daguerreotype pro- 
cess was first announced, and in August of the same year 
the details of production were given to the world, Daguerre 
and Niepce the younger (the successor of Nicephore), obtain- 
ing a pension from the Government of France. Whilst 
Daguerre was working in France, we find that one of our 
own countrymen. Fox Talbot, had been experimenting in 
another direction. Bearing in mind the work of Scheele 
and Wedgwood, he devoted himself to the production of 

B 2 



4 Historical Sketch. 

drawings, &c, on silver chloride, and in January 1839 he 
read a paper before the Royal Society on ' Photogenic 
Drawings.' His method of procedure was somewhat as 
follows ; Writing paper was coated with a solution of 
common salt, and after drying was brushed over with silver 
nitrate; by this means silver chloride was obtained, with 
a slight excess of the nitrate, in which condition it proved 
excessively sensitive to light. Various bodies, such as lace 
and ferns, were laid on this paper, and a reversed facsimile 
of them in black and white was produced, and he iixed the 







impressions by solutions of bromides and chlorides. When 
such a reversed facsimile was placed over similarly-prepared 
paper, and the light allowed to act through it, the result was 
the formation of a facsimile, only this time not reversed in 
shades. 

These two prints were respectively named the negative 
and the positive (fig. 1 and fig. 2.) 

Comparing this process with the former, we see what 
an immense advantage Talbot's process had over the 
daguerreotype. With Talbot's any number of copies of a 
subject could be cheaply produced, whilst with the latter 



Calotype. 5 

one positive was the sole result, irnless expensive electro- 
chemical means were resorted to. 

The Rev. J. B. Reade was also an ardent experimentalist 
in this process, and to him is to be ascribed the discovery 
of the accelerating power of gallic acid, in the presence of 
silver nitrate, for the production of an image, and also for 
the devdopment of the invisible image by the same agency. 

From this discovery, together with that of Daguerre's, 
Fox Talbot reasoned out the calotype process, which he 
patented in 1841. By it an invisible image is formed on 
silver iodide on paper, and developed by gallic acid. In 
this process of Talbot's a negative image was formed, while 
by the first process the positive pictures were produced ; 
and it should be remarked that the same method of pro- 
ducing silver prints obtains to the present day with scarcely 
any alteration. 

Sir John Kerschell had drawn attention to the possi- 
bility of producing photographic pictures on glass, and in 
1843 had actually printed, in a camera obscura on silver 
chloride, deposited on such a plate, a picture of his 40-foot 
telescope. Ni^pce de St Victor made a further great advance 
when he succeeded in holding the sensitive salts of silver on 
glass by using albumen as a vehicle, but to Le Gray must 
be accorded the. credit of suggesting collodion as suitable for 
retaining them in situ. Scott Archer, with whom was asso- 
ciated Dr. Hugh Diamond, in 185 1, however, introduced the 
collodion process in the practical form in which it exists 
to-day, and it may safely be said, that, with the exception of 
the daguerreotype process, no more important discovery in 
photography has been made. 

In 1839 Mungo Ponton published the fact that potas- 
sium dichromate, when applied to paper and dried, altered 
in composition when exposed to the influence of light. 
This annoimcement caused much investigation into the 
subject, and it was subsequently discovered that not only 
was the chromate altered in composition, but that the sizing 



6 Experiments with Light, 

of the paper was oxidised. Gelatine, gum, starch, albumen, 
were all found to become insoluble when exposed in 
contact with it; and Poitevin utilised this fact in the pro- 
duction of pictures in powdered carbon by a process 
analogous to that subsequently to be described in these 
pages. Swan, Johnson, Woodbury, and others, have more 
recently extended its application by the production of images 
formed in gelatine, coloured with pigments; whilst a still 
wider field has been opened by Albert, Edwardcs, and others, 
in the production by mechanical means of prints in printers* 
ink from a gelatine image, founded on the fact that oxidised 
gelatine is incapable of absorbing water. 



CHAPTER II. 

EXPERIMKNTS WITH LIGHT. 

Before entering into the theory of photography, it will be 
convenient to enter briefly into some of the phenomena of 
light; for it is with this form of energy that the photo- 
grapher has to deal It will be as well first to try one prac- 
tical experiment with light, in order to clear up certain 
difficulties which may present themselves. 

Darken a room of some 12 or 14 feet in length by means 
of shutters (light wooden frames covered with opaque paper 
will answer), and in one fix a small plate of glass, a, which 
has previously been covered with tinfoil, and on which, with 
a sharp knife, has been cut a straight line laying bare the 
glass ; on a table place a glass prism, b, the centre of which 
is at the same height as that of the slit, and have a screen, 
covered with white paper, c, and a lens, e, of about 12 inches 
focus, ready at hand; outside the window arrange a mirror, d 
— an ordinary looking-glass will answer — in such a position 
as to reject the sunlight on to the slit; interpose the lens, k. 



8 Experiments with Light, 

about 6 inches from a, in such a manner as to cause the 
beam to fall upon the prism, b. The floating dust in the 
room will immediately show that the original beam of white 
light has been split up into a series of coloured rays, 
and a position for the screen may then be found which 
will cause the top and bottom edge of the spectrum (as this 
glorious band of colour is termed) to be sharply defined; and 
if the cut in the tinfoil be fine enough, a series of dark linesi 
will traverse it vertically. With these, however, we have no- 
thing to do at present. Now experiments, the results of 
which have formed the groundwork of mathematical reason- 
ing on the theory of light, have conclusively proved that light 
as light is merely a sensation. Permeating all known space 
is assumed to be an imponderable and elastic fluid known 
as ether, and in it a luminous or heat source is able to 
generate a series of ripples or waves, flowing unbrokenly 
and continuously from it. What the prime form of these 
undulations may be we cannot tell. They may be, and 
most probably are, compounded of an almost infinite number 
of different undulations, when ordinary * white light is the 
impression given to the eye, and each of these series of 
undulations vary in length from crest to crest. Those of 
certain lengths are able to affect the nerves which line the 
retina of the eye; whilst some of these are able to affect 
other nerves lying in pur bodies, producing the sensation 
of heat ; others again, though incapable of producing the sen- 
sation of light or heat, exhibit themselves by their effect on 
certain compounds, causing chemical combination or decom- 
position. Of those waves whose impact on the eye produces 
the sensation oflight the shortest is about 600 millionths of a 
millimetre, whilst the longest is about 350 millimetres. The 
former give the sensation of a violet colour, the latter 
of a brilliant red. Examining the spectrum thrown on the 
screen, the intermediate colours of blue, green, yellow, and 
orange are seen, and the wave-lengths producing their effects 

' ucnoi polaiised. 



Waves* 9 

bn the eye lie intermediate between the limits given above. 
There is uncertainty as to the lower limit to which the heat- 
producing rays are refracted, but probably to a length equal 
to that of the visible spectrum, whilst the range in length of 
the chemically active waves other than those situated in the 
visible portion of the spectrum, and which He beyond the 
violet (being called the ultra-violet or fluorescent rays), 
is, if anything, still more uncertain. It will be evident on 
reflection that it must be accidental that, between certain 
limits, the waves should be capable of producing a sensa- 
tion of light or of heat. The exact upper limit of the thermal 
spectrum is unknown, but from theory it must be co-termi- 
nous with the chemically active rays, as will be seen further 
on, the inferior limit of the capacity of any waves to pro- 
duce decomposition is as yet unascertained. All those 
series of waves which effect decomposition in any compound 
are called actinic rays, and, as will be seen, the range of 
these vary for every ordinary photographic compound. 

It may help us in a right comprehension of our subject 
if reference is made here to one quahty of these undulations. 
The interstellar ether in which these waves ripple is assumed 
to permeate every body, solid, liquid, and gaseous ; and it 
depends upon the disposition of the ultimate molecules of 
the body whether it is opaque or transparent to any of the 
visible or dark rays of light. It must be borne in mind that 
the molecules of every substance are presumably in a state 
of vibration, the extent and velocity of which depend partly 
upon the temperature, and partly upon the nature, of the 
substance, and that this must ever be so unless the purely 
theoretical condition of absolute cold be arrived at. Sup- 
posing, then, we have a glass, which with white light falling 
on it allows only the transmission of red light, and we look 
through it at the spectrum formed by white light, we should 
find that it cuts off* the whole of the colours excepting the 
red, obliterating them mox^ or less perfecAy, X\vaX*\^, Vs^Xfcc^- 
nic^J language, it absorbs them. Now, accoxdki!^ \.o ^ 



10 Experiments with Light, 

ideas of the conservation of energy, this absorption must 
indicate the performance of some kind of work. It may be 
that it causes the already vibrating molecules of the glass to 
take up and swing in some complicated manner with those 
rays particularly absorbed, and thus to cause a rise in tem- 
perature in the body, so small indeed, perhaps, as to be 
indistinguishable, owing to the rapid cooling due to radiation; 
or it may be that work is performed in effecting chemical 
decomposition, for even glass is thus affected by light. The 
rays which simply pass through the glass produce no effect 
on it — their energy is unimpaired. 

It should also be noted that where light is not entirely 
absorbed, but is only reduced in intensity, even then 
also work must be performed by it ; for the intensity of 
any coloured or white light is dependent on the extent, or 
amplitude, as it is termed, of the wave or waves ; and any 
diminution of the amplitude indicates that a portion of its 
available energy has been exhausted, and that therefore a 
transference of the portion so expended must have been 
made to the body through which it passed. This exchange 
or transference of energy is an important subject in all 
photographic matters ; it explains many of the phenomena 
in photography which often present a great difficulty to the 
beginner or to the rule-of-thumb photographer, whilst it is 
all-important in the right understanding of the revelations 
which are made by the spectroscope. 

It may then be laid down as an unalterable law, that 
where there is absorption of light {whether of dark or visible 
rays) by any body, work of some description must have been per- 
formed in that body. An account of the valuable experi- 
mental research of Joule on the mechanical value of light 
is given at length in the * Philosophic Magazine ' for 1843, ^^^ 
is deserving of special study. 



Theory of Sensitive Compounds, 1 1 



CHAPTER HI. 

THEORY OF SENSITIVE COMPOUNDS. 

Every particle of matter may be considered to be made up 
of molecules, each molecule consisting of constituent atoms. 
Thus a particle (and when we say particle we mean to con- 
vey the idea of the smallest visible quantity of matter) of 
silver iodide is composed of molecules of a like definite 
composition, the components being— two atoms of iodine 
and two of silver, or multiples of these numbers. The 
physical aspect of matter often conveys to the mind an idea 
of a certain kind of arrangement in the molecules ; as does 
the analysis of a compound, if not of the absolute arrange- 
ment of the atoms, at all events of the arrangements 
which they cannot take. Oxalic acid, for instance, we 
know is composed of carbon, hydrogen, and oxygen, having 
the formula Cj Hj O^, or the exact equivalents of water, 
Hj O, carbon monoxide C O, and carbon dioxide C O2, 
yet the compound is totally different in its physical cha- 
racters and chemical reactions from any of these. From 
this we can argue that the atoms of its molecules must be 
separated in such a manner that 
the oxygen molecules cannot seize f«g. 4. 

upon the hydrogen to form water, 
or on carbon to form carbon mo- 
noxide or dioxide. When the 
atoms are so arranged as to be 
incapable of forming a molecule 
of a simpler type, they occupy a 

position of excessively stable equilibrium, and it would be 
necessary to expend a large amount of work to separate 
them. On the other hand, where the atoms of the mole- 
cule are so grouped that by rearrangemetiX Xive^ Tsva^j lorwv 
perhaps wore than one molecule, eacYi ot ^YaOa. \xv^ \ifc 




12 Theory of Sensitive Compounds, 

of less complex character, it often happens that all the 
atoms are in state of stable, though verging on indifferent, 
equilibrium. We may take as an illustration of this state 
of equilibrium the frustum of a pyramid standing base 
uppermost, on a narrow section parallel to the base. It 
is apparent that the work expended in order to cause the 
frustum to find a new position of more stable equilibrium 
(or, in other words, to fall on to one of its sides), may be 
made as small as we please by diminishing the area of the 
section on which it stands. Whilst falling, the body can do 
a certain amount of work, which will be quite independent 
of the amount of work expended to cause its fall. So with 

• the atoms of a molecule which are in this state of almost 
indifferent equilibrium ; a very small amount of work need 
be expended in order to cause them to take up more stable 
positions ; but the kinetic energy they may possess whilst 
passing to this new state, need be no measure of the work 
performed upon them. A measurement of the work per- 
formed by their re-arrangement would principally tell what 
amount of work had been expended in some chemical pro- 
cess, in order to place them in that state bordering on 
indifferent equilibrium. It is possible, however, under cer- 
tain circumstances, to compare two or more energies with 
one another, by comparing the effects they produce on such 
molecules. Extending our previous illustration, supposing 
we had a row of such frusta of pyramids, and that it was 
found that one pellet of a number (all being of equal weight) 
when striking one frustum with a certain velocity, was able 
to cause it to fall, and also that in every case the accuracy 
of aim was undoubted, and that in falling one frustum did 
not strike its neighbour ; then at any interval after the 
commencement of a bombardment the amount of work 
expended in projecting the pellets could be compared by 
simply counting the number of frusta which had fallen. 
It is in a manner akin to this that the comparative 

values of the intensity oi \ho%Q rays which produce chemical 



Effect of Vibrations, 13 

decomposition in sensitive compounds are found. The 
molecules of the compound answer to the frusta, and the 
pellets to the number or amplitude of the waves impinging 
on them. The method of estimating the number of mole- 
cules altered in composition, is by noting the colour or the 
attractive power on other matter which they possess. In our 
illustration we assumed that one frustum never interfered 
with another during its fall, and, so far as the compounds, • 
which are photographically sensitive, are concerned, this is a 
correct assumption, for the alteration in one molecule does 
not cause an alteration in the neighbouring one. In other 
sensitive compounds this may not be the case. It is frequently 
the case that the rearrangement of the atoms of a molecule 
calls into play such a large amount of kinetic energy (it may 
be in the form of heat), that the neighbouring atoms are 
caused to rearrange themselves, and so on. In this case the 
destruction of the original form of the molecules may be so 
rapid, and the potential energy converted into kinetic may be 
so large, that we may have a compound which is an explo- 
sive. With these latter compounds the energy existing in 
a vibration is often sufficient to cause explosion. The 
vibration may be that of the longer waves produced in the 
medium we have already discussed, or may be those pro- 
duced in the atmospheric or other gases. Thus, radiant 
heat may cause it, as also sound. It has been experi- 
mentally proved that many explosives are particularly sensi- 
tive to vibrations of a definite wave length ; thus, the 
vibration to which nitro-glycerine is most sensitive is not 
the best with which to cause the explosion of gun-cotton. 
It has also been asserted that the atoms of the molecules 
of iodide of nitrogen can be caused to be dissociated by the at- 
mospheric waves which are due to sound of a particular pitch. 
In order to understand more readily how it is that the 
molecules of such bodies may be disturbed by waves of a. 
certain length, it must be recollected that t\\ey 3ite vcv ^. 'sXaX^ 
oi agitation. In solids the paths they describe ax^ \vKv\\.^^n 



14 



TJteory of Sensitive Compounds. 



Fig. 5. 



f^ 



though the excursions they take will be the greater, the 
higher the temperature of the body ; and from analogy it 
may be assumed that the agitation is really a definite oscilla- 
tion, though the paths described may be very complex. 
Now, ordinary white light, as has already been pointed out 
in the last chapter, most probably consists of an almost 
infinite series of undulations of varying length, traversing a 
medium, and it is quite conceivable that the molecules of a 
body, whose oscillations synchronise with one of these series 
of ethereal waves, may have their paths altered in form, and 

their amplitude increased to such a degree, 
that a rearrangement of the atoms must en- 
sue. In order to illustrate the effect of one 
oscillation upon another, the late Professor 
Rankine employed the following contrivance. 
A is a lath to which is suspended a leaden 
bob, B, some six or seven pounds in weight ; 
c is a string attached to b, by which is sus- 
pended a wooden bob, d. The whole is 
caused to oscillate on an axis placed at x. 
When the length of the string is such as to 
cause the heavy and the light pendulum to 
sychronise accurately, a slight horizontal dis- 
placement of B will cause the length of 
amplitude of the oscillations of d to increase 
to such an extent that the latter will pass 
the semicircle and tumble. When the syn- 
chronism is only nearly perfect, the amplitude of d will at 
first increase, gradually stopping the oscillation of b, when 
it will diminish, and finally come to rest and bring b into 
oscillation once more, and so on. If we take the swing 
D as the type of the oscillation molecule, and that of b as 
the oscillation of the ethereal medium, it will be seen how 
perfect and nearly perfect synchronism will increase the 
oscillation of the molecule. The same illustration applies 
to a part of the theory of explosives, whether caused to 




QP 



Waves of Varying Length. 15 

explode by the energy of radiant heat, or by that of 
atmospheric or gaseous waves. This is in accordance 
with what we have abeady advanced : it is only those 
waves which are entirely or partially absorbed, and whose 
amplitude is consequently annihilated or reduced, which can 
do work on a body : therefore, in choosing any particular ray 
of light with which to cause this class of decomposition in a 
compound, it is a sine qud non that it must be absorbed ; 
in addition to which, some atoms must be less loosely 
bound to the molecules than are others. It is found prac- 
tically that the bodies employed for photographic purposes 
are affected principally by the waves of short length, and that 
as a rule those of greater length are inoperative ; and here 
we come to a great distinction existing between the re- 
arrangement of the atoms of the molecules, in explosives and 
in photographic compounds. The short wave lengths do 
not affect the former, though the longer ones, which we 
call radiant heat, can do so. Now the e?iergy transmitted 
from a hot and luminous body by the medium lies 
principally in those waves which are capable of pro- 
ducing what we call heat (in fact the energy can only 
properly be estimated by ascertaining the heating effect due 
to the radiations) and as the heat produced in a body by 
the waves of lengths such as 450 millionths of a milli- 
metre is insignificant, and when they are of a length of 200 
millionths of a millimetre is absolutely immeasurable, it 
it is evident that the energy expended on the production 
of these last wave-lengths is small ; at the same time it 
happens that their production, as a rule^ necessitates the 
existence of those of greater length. Thus, a platinum wire 
inserted in an electrical circuit may be heated, and yet only 
radiate dark rays ; by increasing the current it may become 
cherry-coloured, and a spectroscopic examination will 
demonstrate that only red rays are emitted, whilst at the 
same time it may be shown that the inteiv^AtY ol >2cv^ ^-ax^^w 
rays is increased. By further increasing xVie cxvrc^xvX., Nicv& 



i6 



TJuory of Sensitive Compounds. 



yellow, green, blue, violet, and ultra-violet rays may in sue* 
ccKHion be caused to radiate from the wire; all the first 
emitted rays increasing in intensity. The accompanying 
diagram shows the increase of the red, orange, blue, and 
ultra-violet rays produced by the expenditure of work in 
a steam-engine driving a Gramme's magneto-electric machine. 



Fio. d. 



fipMW 




H OPI t C 



c n 



Th« ordlnAtc* of the curve dhow the ioteniity of the different ray» as com- 
pftred with ihoM emitted from an ordinary wax candle of known weight 
and rate of burning. 

In order, therefore, to displace the molecules of small 
stability of the photographic compound which are in equili- 
brium, it is as a rule necessary to produce waves of great 
length as well as waves of short length, and this may mean 
the existence of a great heat energy at the primary source 
of radiation though not necessarily at a reflecting surface. 
Now the usual result of the displacement of an atom from 
H'/jai we may caU the sensitive molei^uU is to form a fresh 



MoUadar Vibratum. 17 

sffSd body, and conseqnentljr the potential energy of the 
molecule is small, also the nmnber of these molecules acted 
upon in a given time is smaU in comparison with the total ; 
hence the kinetic energy (which may take the fonn of heat) 
that may be generated by the chemical decomposition 
and recombination falls far short of that required to produce 
even red light, much less waves of still shorter length. We 
thus see that although one molecule of an explosive ^v sf^ 
after its potential energy has become kinetic, can cause 
vibrations of such a character as to effect a disruption of the 
neighbouring molecules, yet a similar disturbance produced in 
a molecule of a photographic compound is not capable of caus- 
mg an extension of the action beyond the molecule itself, and 
tiiat it requires a renewed action of the disturbing force to do 
it At first sight this seems unfortunate, but when we con- 
sider what would happen were such an event possible, it is 
tipparent that the production of a photographic image in 
such a case would be impossible. 

In a succeeding chapter it will be found that a molecule 
of chloride of silver responds principally to the swing of the 
waves in the blue {)art of the spectrum, and that it undergoes 
a change, owing to the throwing off of one of its consti- 
tuent atoms; yet the same body may, by the aid of an 
artifice, be fiised by the dark rays of heat, which are 
comparatively of great wave-length, and though it in itself 
becomes luminous, emitting the very same rays that, when 
falling on it, can cause one of its atoms to be shaken off, 
yet it remains unaltered. In this last case the vibrations of 
the molecules are not of the definite character needed to 
cause the change. A small force, applied at definite in- 
tervals, may cause a body to attain a great amplitude of 
vibratiorL A boy may cause a violent oscillation of a church 
bell if he time his pulls at the rope properly, and the 
accumulated energy may be such that it may drag the ringer 
up, though the work he may have executed aV^aiOti^vi^^^^ 
the rope may be very smalL On the othex Y^aivA.^ ^'^ rvmi^x 

c 



1 8 Sensitive Compounds. 

mAy expend the name anunint of tnergy at the wrong time, 
and the effect on the bell will be insignificant The ex- 
periment given at p. 14^ fig. 5, illustrates this effect. 

A* l>efore stated^ the number of molecules affected in a 
short interval of time by lijg^t may be so small that their 
change in atomic comj/osition may be invisible to the eye, or 
in physiml appearame may be of a similar nature to the com- 
pound from which they are derived, in which case even a pro- 
longed exf>osure to the actinic rays would j>roduce no visible 
effect When the sensitive compound is formed in a thin layer 
held in situ on some substratum, such as paper, glass, &c., 
the lij^t reflected and radiating from an object after passing 
through a lens may be caused to fall upon its surfaice and 
form an image. When the rays arc of such a nature as to 
cause the equilibrium of the constituent molecules to be 
disturbed, the change will take place only m such parts 
of the thin lamina as are illuminated ; and thus an invisible 
image formed by the shaken compound may be impressed if 
the time of exi>osure be short, or the change produced be 
such as not to be within the scope of our vision. Otherwise 
upon long exposure a visible image may be produced, the 
resulting compound being different in appearance from the 
original. 

As the point is of great importance, we must again 
direct attention to the fact, that the two images are exactly 
alike in chemical composition, one differing from the other 
solely in the number of molecules altered. Fortunately, 
methods exist of rendering visible to the eye what is ordi- 
narily and primarily invisible, and this operation is termed 
the development of the image, ITie invisible image is 
frequently termed latent, an appellation which, though 
convenient, is yet open to some criticism. We will now 
discuss the various ways in which development may be 
effected. 

ist Method, — The new compound may possess an attrac- 
t/vc force. If a rod or wire of zinc be placed in a solu- 



TIu Effects of L ight 1 9 

tion of lead acetate, chemical operations immediately com- 
mence. The outside particles of the zinc enter into com- 
bination with the acetic acid of the lead acetate, and par- 
ticles of lead are deposited upon the rod in their stead. 
As the action continues the lead further reduced is, by a 
certain well-ascertained law, attracted to the lead already 
deposited. Spangles of the metal in a crystalline form at- 
tach themselves to the rod and then to one another, until 
what is known as a lead tree results, just as a magnet will a 
string of nails suspended from one of its poles. In a similar 
way a silver tree may be formed from a solution of its salts, 
provided the reduction be slow. So the action of light on cer- 
tain sensitive compounds, especially amongst which may be 
mentioned those of silver, is to cause the formation of a body 
which is capable of attracting the metal (of which it is itself a 
salt), when slowly deposited from a solution. This first de- 
posit is capable of attracting still more of the metal, and 
thus an image is completely built This action is more fully 
treated at p. 64. 

2nd Method, — The altered compound may be able to 
effect a reduction to a metallic state of a metal from a 
solution of its salt, which the original compound may be 
incapable of doing. In this case the metal would be natu- 
rally precipitated on the altered compound, and the attrac- 
tive force of the fireshly-deposited metal would determine 
the attraction of any other that might be caused by extra- 
neous causes to deposit itself. In this method, as in the last, 
it is evident that the minutest portion of the altered com- 
pound is able to effect a building up of the image. See p. 159. 

2,rd Method, — ^The image may be formed by the partial 
reduction, to a more elementary state, of the altered com- 
pounds, when treated with certain solutions, which reduction 
in the original compound was impracticable ; also in this 
reduced state it may exercise the same attractive force as 
above We shall have an example of this m i!)ik2X\xv& ^t- 
yeiopznent See p. gj, 

C2 



20 Sensitive Comptmnds. 

4th Mttfwd. — The altered compotmd may be ca^palAe of 
iormm^ a colotrrerl boflywhen treated with metalik or ether 
<$oiutio>n!!^. In thi^ ca.<ie it is manifest that the image must be 
due sKolely to the ammmt of the sensitive salt originally 
a)t€rfcrl in compojwtion, and its vigour must consequently 
derf/end iif^on the time the light has acted Of this method 
f/f derveloptnentwe shall have examples in the more sensitive 
ferric salw. 

5/^ Mdh0d. — The attractive force of an altered mole- 
cule may be utilised by causing met2i]hc or other vapour to 
t>0ftdetjse upon it in preference^ to the neighbouring mole- 
cules which may not have been changed by light This 
first condensation may determine the following condensa- 
tion. Of this we have an example in the development of 
the daguerrcotyi>c plate, 

(M Method. — The alteration in the compound may be 
shown by its incapacity to absorb moisture, 

1th Method, — The new compound may l>e incapable of 
entering into sc^ution^ though the original compound may 
t;c readily soluble. 

The chemical agents which are utilised in order to allow 
the development of the latent image to take place will be 
discussed as each method is brought under particular con- 
sideration. It is to be remarked that these agents are 
technically called developers, a term which, critically speak- 
ing, is a misnomer, as in the majority of cases the part they 
play is a secondary one, and one which they fill whether 
applied to development or not The term is convenient, 
however, and will be adopted in this work, though the student 
muMt in hiH own mind make the reservation indicated when 
coming acrosH the term. 

Intensifying an image already developed or visible is a 

term applied to a proce»» whereby the image is (1) rendered 

more visible to the eye, or (2) rendered more absorbent of, 

and therefore less transparent to, some particular kind of 

^/ght^ be it white, blue, red, yellow, &c. Both of these results 



The Action of Light 2 1 

can be obtained by following the methods indicated for 
developing the image. Fuller information regarding the 
necessary procedure will be given as various processes are 
described. 

Fixing an image is rather a vague term. It is intended 
to express that the image due to the first exposure and sub- 
sequent development shall be so treated as to undergo no 
change, leading to obliteration. This is usually effected by 
clearing the image of all that portion of the sensitive com- 
pound which has not been acted upon by light, and thus of 
rendering it incapable of being obliterated by fresh exposure 
or appearing indistinct If the sensitive compound were 
absolutely colourless, and the action of light were to leave 
the new compound colourless, the developed image would 
need no clearing or fixing ; but, since all the sensitive com- 
pounds are either coloured themselves, or are converted by 
light into others possessing colour, there is evidently no 
safety, except in their entire removal 



CHAPTER IV. 

THE ACTION OF LIGHT ON VARIOUS COMPOUNDS. 

The action of light on various substances must have been 
a matter of remark from the earliest times. The tanning 
of the skin, the fading of colours, must all have been 
noted long before an attempt was made to ascertain the 
cause of such alteration. However, as we pointed out in 
the historical sketch, silver chloride was the first substance 
whose behaviour was philosophically examined ; and we 
propose to study the principal^ silver compounds before 
proceeding to other sensitive bodies. Scheele, as we 
have seen, found that chlorine was given oft dwTvxv^ ^-x^^- 
sura G'om the chloride, and that aftet UeatocckeoX. ^1 ^^ 



2 2 Tlie A ction of L ighU 

blackened body with ammonia, metallic silver was left 
behind. There is not much need to carry the investigation 
further than Scheele, only the conclusion that he accepted, 
viz. that metallic silver was separated at the time of ex- 
posure, should be viewed with much doubt, particularly 
when it is found that the darkening action of the chloride 
takes place even when immersed in the strongest nitric acid. 
The accepted theory seems to be that exposure to light 
reduces any chloride to the state of subchloride, thus : 

Silver Chloride =» Silver Subchloride + Chlorine 
Ag^Clj « AgjCl + CI 

When the same compound is moistened the reaction appears 
to be different, as chlorine decomposes the water with 
which it is in contact,, forming hydrochloric acid (HCl) 
whilst the other atom of hydrogen and the oxygen atom in 
the molecule of water combine with another atom of chlorine 
to form hypochlorous acid (HCIO). If, instead of exposing 
the silver chloride in a dry state or in the presence of 
moisture, it is exposed in presence of free silver nitrate, 
fresh silver chloride is formed, and this same compound of 
chlorine and oxygen liberated ; and it is found generally 
that the darkening takes place much more rapidly when any 
body which will take up the chlorine is in contact with it. 
Thus, stannous chloride will cause more rapid darkening, 
from the readiness with which it absorbs chlorine. The 
student would do well to repeat the experiments of Scheele 
and those subsequently indicated, in order to convince him- 
self that these reactions really occur. The easiest method 
of procuring pure silver chloride is to precipitate it from 
a solution of silver nitrate by an excess of pure hydro- 
chloric acid, and to wash it thoroughly by decantation, re- 
peating the washing to such a point that the supernatant 
water shall no longer show acidity when tested with blue litmus 
paper. This method of procedure prevents the possibility 
of contamination by the organic matter of filter paper. The 



Silver Chloride and Iodide. 23 

silver chloride^ if required in a dry state, should be dried 
in the dark over a water-bath, in a watch-glass or porcelain 
capsule. A test-tube is a convenient vessel in which to give 
the exposure to the light, and the subsequent washings are 
conveniently carried out by simple agitation and pouring off 
the liquid A note-book^ should invariably be at hand in 
which to describe the phenomena that may be obser\'ed in 
these or any other photographic experiments, and it will 
be found convenient to attach consecutive numbers to each. 
It must ever be remembered that there is no experiment 
properly carried out, with a set object in view, which is 
not worthy of record. The most trivial deviation from the 
expected results of an experiment often causes some new 
line of thought to be taken up, and may suggest important 
investigations. 

The next silver salt that requires a careful study is the 
iodide ; and it is owing to certain peculiarities in its beha- 
viour when exposed to light, that so much difficulty has arisen 
in defining the true changes that take place in it. 

Silver iodide may be produced in two or more 
ways. The most common is by treating a silver nitrate 
solution with a soluble iodide, such as ammonium. If the 
former be in excess, even in minute proportions, after most 
careful washing, it will be found that the compound darkens 
slightly on exposure to light, whilst if the latter be in excess, 
there is no apparent change in colour. To explain this last 
phenomenon, it must be remembered that the bonds of at- 
traction between the iodine and the silver atoms of an iodide 
of silver molecule are much less readily broken than those 
of the constituent atoms of the chloride or bromide ; for we 
find that there is greater amount of heat generated in the 
formation of the former than in the two latter, and that, 
therefore, the separation of an atom of iodine from the mole- 
cule of silver iodide (Ag2l2) is much less readily effected than 
is that of the chlorine from the chloride or bTom\tvei\cyK\\Jev^ 
bromide y and, when we come to consider the ettecX. ol ^^ 



24 ^^ Action of Light. 

ImiOfX of Ikdit on sadi mokciles, ve shall readily under- 
hand the dimcultr diat exists in causing it to diange. The 
^Mk% which will break up the dilohdes or boHnides are 
ifiJ^ilfioeDt to produce any alteration when nothing but pure 
lodMe is presenL 

When, however, there is an excess, however sli^t, of 
silver nitrate, the conditions are quite altered ; for then 
there is a compotmd at hand which is ready to seize any 
iodine which may be brought near it. Thus, when the silver 
nitrate is present, the molecule of iodide is at once changed 
in chemical composition, and a subiodide is formed in a 
similar way to the formation of subchloride firom the 
chloride. 

Silver Iodide » Silver Subiodide -i- Iodine 
AgjI, = AgjI + L 

It may here be remarked that in one respect iodine is 
unlike chlorine in behaviour; it is incapable of forming 
hypoiodous acid (HIO), though chlorine, as already pointed 
out, forms HCIO ; hence there is some difficulty in ascertain- 
ing theoretically the exact reaction which takes place between 
the liberated iodine and the silver nitrate which is neces- 
sarily present to produce the change. 

A simple experiment, however, which it is well to repeat, 
throws light upon it Take washed silver iodide, and place 
it in a test-tube containing in solution silver nitrate which has 
previously been thoroughly boiled in order to expel any air 
which it may contain. If an air-pump or an exhausting 
syringe be at hand, the boiling may be dispensed with, and 
the same end attained by creating a vacuum in the tube. 
Now expose to light ; in a short time bubbles of gas will be 
found collecting in the solid iodide, and with care these may 
be collected, and on testing by the ordinary means will be 
found to contain oxygen. From this we may suppose that 
the liberated iodine decomposes the water in contact with it 
(as docs chlorine), and produces hydroiodic acid (HI) and 



Absorbents, 25 

The former combines with the surrounding silver nitrate, 
and we have a total reaction, as follows : — 

Silver Silver . -i v ♦ Silver ^ Silver Iodide 

Iodide"*" Nitrate "*" ^^^^^^ " Subiodide "*" ^^XS^^ + (newly formed) "*" 

Ag^j + AgNO, + H,0 = Ag,I + O + Agl + 

Nitric Acid 

HNO, 

If any iodine absorbent be placed in contact with washed 
silver iodide, prepared with an excess of soluble iodide, the 
reaction that takes place is apparently more simple, the 
iodine atom combining directly with such a body. It may 
thus be stated as a law that in order to produce a change by 
the action of light in silver iodide^ some body must be present 
which can absorb iodine,^ 

There are one or two suggestive experiments which 
may impress this on the mind. The first is to silver a 
glass plate as if for a mirror, and then to expose it to 
the action of iodine vapour (as in the daguerreotype 
process) to such a degree, that the whole of the extremely 
thin film of metal is converted into iodide. On exposing 
such a plate to sunhght no change is visible, nor can 
one be brought to the cognisance of the senses by bring- 
ing developing agents in contact with it. If the film be 
not wholly converted into iodide, this result will not occur, 
as the metallic silver is an iodine absorbent Another ex- 
periment, which is very conclusive, is as follows : Prepare 
a film of silver iodide, as in the wet process, and im- 
merse it in potassium iodide solution till any excess of 
silver nitrate is converted into silver iodide, and wash 
thoroughly for an hour, and dry. Next take a small square 
piece of silver leaf and apply it to one portion of the iodide 
siurface, brushing it well down, in order that real contact may 
be obtained. To another small portion apply a solution of 
tannin in alcohol, and after drying expose the plate to the 

> This law seems to have been first emphalkaW^ wi>Mi'd'k\sA \s^ 
Vqgei, though a. claim has been made by Poitevm. 



r 



26 The Action of Light. 

light On developing, as indicated at p. 103, a dailcen- 
ing action will be apparent after a short interval of time on 
those portions of the plate treated with the silver leaf and 
the tannin. 

The action will be most intense in the latter, as might 
naturally be expected, the whole thickness of the iodide 
being in the one case brought in contact with the absorbent, 
whilst only those particles which form the surface are 
brought in contact with it in the latter. The experiment is 
more tellmg if the plate be exposed behind a negative, with 
the uncoated side next the image. 

If instead of the silver leaf a thin silvered plate be pressed 
into firm contact with a sensitive collodion film, prepared as 
p,<, y above, it will be found that even a fair 

exposure is sufficient to cause the for- 
mation of an image on both, which, 
though unrecognisable to the senses, 
is yet capable of being developed by 
the proper methods. This experiment 
thus serves to show conclusively that 
iodine is liberated by the impact of 
light ; for, were the change one merely of molecular arrange- 
ment (as many enquirers have held to be the case), no image 
could be formed on the metal plate, the possibility of de- 
veloping an jmage on it being dependent on the presence of 
silver iodide (see the daguerreotype process). 

I'he beluviour of silver bromide is similar to that of 
silver chloride ; hypobromous acid being formed under 
similar circumstances to those in which hypochlorous acid is 
produced. The chloride and bromide are both soluble in 
ammonia (which is an important point when dry plate pro- 
cesses are considered), whilst the iodide is not. It may be 
here recorded that with the chloride and bromide, as with 
the iodide, the presence of silver nitrate increases sensitive* 
ness to a very high degree. 

BcMcs the salts already mentioned there are other in. 




f 



Organic Salts of Silver. 27 

organic compounds of silver, such as the fluoride, phos- 
phate, silicate, which are altered by the action of light, but 
these are comparatively unimportant. There are, however, 
certain organic compounds formed, the action of light upon 
which can only be briefly noted here, though a fuller de- 
scription of the phenomena will be given in a subsequent 
chapter. 

When organic matter is brought into contact with a 
soluble salt of silver, a definite compoimd is often formed, 
and the effect of impact of light upon this is somewhat com- 
plex to trace. Thus, if we form an albuminate of silver 
by bringing a solution of silver nitrate in contact with one 
of albumen, and expose it to light whether there is an excess 
or defect of the silver salt present, a darkening of the com- 
pound results. 

The blackened compound is not a true silver oxide, 
though chemical considerations lead us to infer that the 
colouration is dependent on the formation of silver oxide, 
in combination with organic matter. The same results are 
obtained if gelatine or other kindred body is substituted for 
albumen. It will be as well if the student exi>erimen tally 
compare the effect of light on an organic silver salt with 
that on silver chloride, as both are employed in the silver 
printing process. 

The following experiments will naturally suggest them- 
selves. Take sodium chloride and dissolve in water and 
add an excess of silver nitrate to it, by which we have 
precipitated silver chloride formed; also take the same 
solution and allow the sodium chloride to be in excess. 
Carefully spread the moist chloride on pieces of glass, and 
expose to light. Both will readily darken, more especially 
the former, which will gradually assume an inky black 
tint, whilst the latter remains a pale violet. From what 
ias already been said, the cause of this phenomenon will 
be apparent, the chlorine liberated in the ^ist c^s»^ \% 
rapidly absorbed^ whilst in the second it is laeiA^ \\f^ vsx 



28 The Action of Light, 

solution, clinging as it were to the silver subchloride, and 
ready to reduce it back to the same state as before. If 
to the silver chloride, in which the sodium chloride is in 
excess, we now add a little stannous chloride which is 
ready to absorb chlorine, the blackening will proceed as 
rapidly in the one case as in the other. Now treat all these 
residues with nitric acid, and they will all be found to 
remain imattacked by it, but instantly yield to a strong 
solution of sodium hj^osulphite, leaving metallic silver in 
small quantities behind. Next precipitate albumen in ex- 
cess, or otherwise of silver, and expose to light ; the darken- 
ing will proceed more rapidly and to a greater depth in the 
one case than in the other. Treated with ammoni* but 
little alteration is visible, but on applying nitric acid, the oxide 
at once disappears. If, however, it be treated with sodium 
hyposulphite, it will remain nearly unaltered in appearance. 
Next treat the undarkened albuminate of silver with hypo- 
sulphite, and it will dissolve, leaving a milkiness in the solu- 
tion ; on further adding ammonia to the solution, however, 
this will disappear entirely. If both the darkened bodies are 
treated, after the sodium hyposulphite has been applied, with a 
solution of hydrogen sulphide (HgS), the former will blacken 
from the formation of silver sulphide, the latter will bleach 
from the formation of a new organic compound; the bearing 
of this experiment will be seen when we consider the fading 
of silver prints. 

Again, to a similarly treated precipitate of chloride and al- 
buminate add potassium cyanide; the one ■*i'ill be but slightly 
acted on, whilst the other will be speedily attacked. In deter- 
mining the fixing agent to employ in silver printing, this point 
has to be taken into consideration. If experiments with other 
organic bodies be carried on in a similar manner, it will be 
found that the same phenomena will be observed; the dis- 
tinction between the nature of the reduced organic com- 
pound will be seen in the different colours they assume. 
From these simple experiments, iTaeii, vf ^ kain, that the 



Uranium and Iron Salts. 29 

darkening action of silver chloride is aided by the presence 
of a chlorine absorbent; that the subchloride thus formed is 
unaltered by nitric acid (in fact the darkening action takes 
place as rapidly in the presence of nitric acid with silver 
nitrate as if the latter be alone in excess); that the subchloride 
is split up by sodium hyposulphite into metalUc silver and 
silver chloride, the latter being destroyed by it as shown at 
p. 74. That in organic matter which forms a compound with 
silver nitrate, when acted upon by light, the silver is reduced 
to a state of organic oxide, and that the presence of an excess 
of silver nitrate is not absolutely necessary ; that the dark- 
ened compound is unaffected by sodiiun hj^osulphite ; that 
potassium cyanide is a solvent of this oxide, and not of the 
metallic image formed from the sub-chloride. 

The next metallic salts to which we shall refer, in regard 
to their behaviour when exposed to light, are those of iron 
and uranium. Their reactions are almost precisely similar. 
To Sir John Herschel we owe most of our knowledge of 
the iron compounds ; whilst to Nidpce de St. Victor is 
probably due the discovery of the particular properties 
of uranium. If we brush over a piece of paper a neutral 
solution of ferric chloride, and, after allowing it to dr}-, 
expose it to light, the yellow colour imparted to it will 
be found gradually to disappear, leaving the surface appa- 
rently bleached. If, now, we allow a solution of potassium 
ferri-cyanide to flow on to the exposed paper, it will be found 
that a deep blue colouration is immediately produced, whilst 
if applied to the imexposed paper no such phenomenon 
would be observed. From chemical experiment we know 
that, in order to produce the blue precipitate, it is necessary 
to have in contact with the potassium ferricyanide some 
ferrous compound. Since it was a ferric compound, viz. ferric 
chloride, which was applied to the paper, we are led to con- 
clude that the action of light has been to reduce this salt 
to the state of ferrous chloride. 

By shnihr experiment we become coiwmeed. xSc^^V ^^ 



30 The AcHoft of Light, 

action of light on all ferric salts, under certain conditions^ is 
to reduce them to the ferrous state. It may be remarked 
that in order to produce the requisite reduction, the presence 
of organic matter, such as the size of the paper, with some of 
these iron salts seems a necessity ; if this be absent, the action 
is very slow. And, again, the organic compound should be 
of such a nature that it is ready to combine with the atoms 
thrown off, in the same way as that already indicated for 
silver iodide. There are a variety of bodies which will 
combine with these atoms ; but unfortunately, as a rule, 
they have a greater affinity for the atoms than has the iron 
compound with which they are only loosely combined. The 
organic matters with which they will combine without being 
torn away from the iron are rather slow absorbents, and 
therefore generally the sensitiveness is not great. For, as 
with the silver iodide, the sensitiveness depends chiefly on 
the readiness of the neighbouring matter to absorb what is 
thrown off. 

In order, then, for iron salts to become as sensitive to 
light as silver salts, some body must be found which, per se, 
will not reduce them to the ferrous state or decompose them, 
yet which, when the atom is liberated, will seize it with 
greater facility than any body with which we are as yet ac- 
quainted. As a rule, the development of these pictures 
is carried out by either method 2 or 4 (p. 19), the details of 
which will be given in the section on printing with these 
salts. Since these compounds are comparatively but little 
sensitive to light, they are chiefly used for obtaining positive 
prints; an exposure in the camera to produce a developable 
image would have to be very prolonged. 

The same experiments carried out with regard to the 
uranium compounds give identical results. The uranic 
compounds are reduced to uranous, and the methods of 
development are similar. 

To the same class of metals belongs vanadium, the 
interesting compounds of which were mvesl\gaX.ed \i^ Pxo- 
/essor Roscoe. The reactions are simi\ai to l\ve a\>oN^. 



On Chromium Salts. 31 

The last metallic compounds to which we shall refer at 
length are those of chromimn combined with the alkalis. 

The salts found most sensitive to light are the dichro- 
mates, though the chromates are also, to a certain extent, 
capable of being acted upon. Mungo Ponton first indicated 
the principle which governs their employment If a solu- 
tion of a dichromate, such as that of potassium, be brushed 
over paper, and be allowed to dry, and be then exposed 
to light beneath an engraving, it will be found that in those 
portions corresponding to the white paper the orange colour 
will gradually assume a delicate brown tint, whilst on the 
parts shaded by the lines the salt remains unchanged. 
The eye then at once tells that some chemical change has 
taken place in the chromium compound. Chemists are 
accustomed to employ the dichromate to convert a ferrous 
salt into a ferric, and by having it in a solution of known 
strength, and ascertaining when the reaction is complete, the 
amount of iron in the ferrous solution can be estimated 
quantitatively. Thus we have, say, the amount of ferrous 
chloride to test quantitatively : the amount is calculated by 
applying the following equation : 

Ferrous Potassium Hydrochloric _ Ferric Potassium 
Chloride **" Dichromate * Acid " Chloride "*" Chloride "*" 
6 FeClj + KgCrP, + 14 HClg = 3 FegClg + 2 KCl + 

Chromic Chloride + Water 
Cr^Cle + 7 H2O 

It will be seen that the potassium dichromate readily parts 
with its oxygen and potassium, and becomes converted into 
a pure chromium compound. The change induced by the 
light is analogous to this, there being every reason to believe 
that the following equation is a type of the reaction that 
occurs — 

Oiiganic Matter + Potassium Dichrdhiate = Potassium Hydroxide + 
CjpHj+jOg + KgCrgO, = 2KHO T 

Chromic Oxide + Organic Matter 
Cr,0, + C,H^O,+, 



32 The Action of Light. 

An analogous reaction of a chromium salt in . the 
presence of an organic compound, without the impact of 
light, is found in chromium trioxide. If alcohol be dropped 
on these dry crystals, oxygen is evolved so rapidly that the 
spirit is ignited by the energy of the act of combination. 
Now the dichromate contains less oxygen than the acid 
(H2Cr04) formed by the trioxide (CrOa), hence the evo- 
lution of oxygen from it is likely to be less easily effected 
by organic matter than from jthe latter. The swing caused 
by the waves of light is sufficient to effect the change, that is 
indicated by the equation above. It will be noticeable that 
not only is the chromium compound altered in composition, 
but that also the organic matter is deprived of hydrogen ; 
and it is the fact of this deprivation, or change in organic 
matter, that renders the dichromates valuable for photogra- 
phic purposes. It will be found, after experiment, that the 
dichromatised paper prepared as above is nearly insensitive 
when moist, and that the image can be formed most readily 
when it is dry. The reason of this is probably that when 
dry the organic matter and dichromate form a real compound, 
which is, however, readily split up on remoistening. If, 
however, the contact be long continued, an alteration in the 
position of the atoms of the molecules probably commences. 
This might account for the insolubility of old carbon 
tissue, and it may be presumed that the change which is 
rapidly effected by light is much more slowly accomplished 
by the long contact even in the dark. 

The development of pictures taken on ordinary sized 
paper is usually effected by method 4 (p. 20), and will 
be noticed when treating of the aniline process. When, 
however, the paper is coated with a layer of organic matter, 
such as gelatine or albumen, the development of the picture 
may be effected by methods 6 or 7. Colloidal bodies 
available for photographic purposes, when oxidised, are 
changed in })hysical as well as in chemical properties. 
ist, they cannot after oxidation be dissolved by water, either 



On Organic Bodies. 33 

hot or cold, though before oxidation they may be easily 
soluble. 2nd, they will not absorb water, and consequently 
will not increase in bulk, if the impact of light be pro- 
longed. These modes of development will be entered into 
fully when treating of the carbon and collotype printing 
processes. 

It is scarcely necessary to refer to the salts of other 
metals ; they are mostly too insensitive to the action of 
light even for contact printing. Robert Hunt, in his excel- 
lent 'Researches on Light,' has entered fully into the 
phenomena observable with most of these compounds, and 
the student should study that work for further information. 

Of organic bodies there are a variety which respond to 
the chemical vibrations. First and foremost, as being of prac- 
tical utility, is the substance known as asphaltum, or bitumen 
of Judaea. It is the substance which was first employed by 
Ni^pce for practical photography, and it still retains its 
place amongst useful photographic compounds. It is readily 
soluble in a variety of menstrua, such as benzole, chloro- 
form, and turpentine. After exposure to light, it loses its 
excessive solubility, and it is not only possible, but practi- 
cable, to dissolve away from a thin layer of it all those por- 
tions which have not been acted upon by light. For certain 
photo-engraving and relief-printing processes it is still em- 
ployed, on account of its resistance to the action of acids (see 
p. 182). It seems that during exposure it becomes oxidised 
to a certain extent. 

Amongst other sensitive organic compounds may be 
named the extracts of flowers and leaves and certain dyes. 
These are more interesting than useful, and it is sufficient 
to mention them. 

Among gaseous bodies which are sensitive to light we 
may name chlorine, when exposed in the presence of hydro- 
gen. If in a dimly lighted room the proper combining 
volumes of these two gases be luixed in a glass bulb, o\ oxJcv^x 
convenient holder, and then exposed to tbe dk^cX. t^^% oV 

D 



34 TJte Actum of Light 

ih« ftun^ or other strong wurceodi^i whidi emits Ae shorter 
w^atre-leogths^ it will be found that they combine with explo- 
sive yUActtct to iorm an equal volume of hydrochloric acid« 
in diffused <laylight the combination takes place much 
more quietly^ and attempts have been made to utilise this 
a<;tion to measure the ^ actinism ' of any light to which it may 
be exj>oscd (see ^ actim/metry '), The affinity of chlorine for 
hydrogen is so great that it causes a decomposition of the 
water in an aqueous solution, when exposed to the light, 
though it has no power to do so if kept in the dark. 

ll)e latestdiscovery of lightcausinga combination between 
a gas and a solid is due to Mr, Francis Jones, of Manchester, 
He found that in sunlight, if sulphur was brought in contact 
with antimoniurette^l hydrogen or stiliine, the orange sulphide 
of antimony was formed ITie equation representing the re- 
action is as follows: — 

AmiiiKmttiretted Hydrogen + Sulphtir 
» Antimtmy Sulphide + Hydrtfgcn Sulphide 

2 SbH, -f 6 S - Sb^, + 3 11^, 

With arseniuretted hydrogen (As H3) a like reaction 
takes pla^;e,^ 

After this l>rief rhutn'e of the sensitive compounds the 
student will at on<;e distinguish the advantage to be gained 
by the employment of the simpler salts of silver for obtaining 
images in the camera. It is these alone which are susceptible 
of rapid develoj^ment by exercising an attractive force when 
the altered molecules are few in number; whilst all the other 
compounds require a large number of particles to be 
changed in order that the image may be made visible at all. 
With the iron salts per se the development by attraction may 
be rcsorte<l to ; but it will be found on experiment that the 
ttttra<itive forr^; is so small that it does not nearly equal that 
of the silver r,om(>ounds. Hence we may assert that, for pro- 
ducing developable images in the camera, the chief portion 
of the sensitive salt muHt consist of one of these silver salts, 

' Am Mng #«ib>tituttd for Sb in the above equation. 



On the. Support and Substratum, 35 

and that other metallic salts can best be utilised for obtaining 
impressions by long exposure, and are therefore chiefly 
adapted for obtaining positive proofs from negatives. 



CHAPTER V. 

ON THE SUPPORT AND SUBSTRATUM. 

In judging of the kind ot support on which to receive 
an image, whether it be developed or formed by the con- 
tinued action of light, it must be considered for what pur- 
pose the image is to be employed. If it is tp be em- 
ployed as a screen, or a negative from which to form a 
picture complementary to it in photographic density and 
position, then evidently the more transparent the support 
is, the better it will be for this particular purpose. As a 
rule, it is only images impressed in a camera which are 
employed as * negatives,' and as these may be said to be 
invariably taken upon the sensitive salts of silver, which are 
easily acted upon chemically by extraneous matter, it is 
evident that a substance should be employed which is unaf- 
fected by them and by the agents which cause the develop- 
ment of the image. In addition to this quality, a certain 
amount of rigidity in the support is convenient, though not 
essential, as the operations involved are of such a character 
as to cause this to be a desideratum. Evidently glass answers 
the object most thoroughly. Less fully does paper when 
waxed answer these requirements ; for with it there is trans- 
lucency and not transparency, and none of the other 
qualities. A support of collodion and india-rubber com- 
bined, such as recently has been advocated by Warnerke, 
answers to the first two requirements, and, since the opera- 
tions involved in his method of working do iioX. "xvfcc^'5»'^\\.^\ft. 
rigidity, it is a suitable one. 

D2 



36 On the Substratum, 

To give a correct idea of an image by reflected light, that 
is, to look at it as a picture is looked at, it should appear 
as highly coloured or as black as possible when contrasted 
with the ground on which it rests. Formed by develop- 
ment (after exposure in the camera), it possesses always 
more or less density, the density approximately varying 
inversely as the intensity of the reflected actinic light which 
has acted on it If the developed image be of a 
dark colour, the proper effect of light and shade will be 
reversed. To give a correct representation, the deposit 
must be transparently white, and the support dark-coloured 
or black. In the daguerreotype process the support is a 
metal plate, which is by contrast dark when compared with 
the mercurially-developed image. In the collodion pro- 
cess, if the image can be made to appear white by reflected 
light, the support may be of glass, if it be backed with 
some dark-coloured substance, such as black velvet or var- 
nish, or it may be of metal darkened with some substance 
that is unaffected by the chemical agents employed (an 
example of this we have in the ferrotj^e plates). On 
the other hand, if the picture be produced by a subse- 
quent operation from a negative, the image should be 
as transparently dark as possible, and the ground white. 
In this case the support (unless other considerations 
forbid it) may be white paper, opal glass, or any other white 
medium. It is also worthy of notice that in order to pro- 
duce a proper representation of light and shade the ground 
should always be in contact with the image. An example 
of a certain false gradation given in an image, in which this 
important rule is neglected, is to be found in the old collodion 
positives on glass. 

Heretofore nothing has been said regarding the vehicle 

employed for holding the sensitive compounds in situ on 

the support, and this requires a detailed consideration. It 

need scarcely be said that some sort of vehicle is generally 

necessary. The fact that most of the compounds employed 



On the Support. 37 

for photographic purposes are solids, and whether formed by 
precipitation from or evaporation of a solution are in a pul- 
verulent state, at once clearly demonstrates the necessity. 
We must distinguish between two cases. 

1. In the case in which the image is formed by develop- 
ment, it is essential that the developing agent should 
cause no chemical change or discolouration in it any more 
than in the support Now the sensitive compounds may 
be formed in the support itself. Thus a solution of potas- 
sium bromide brushed on to paper, and then followed 
by a similar application of silver nitrate, will cause the forma- 
tion of silver bromide in the paper. The paper will in 
this case act as a support and vehicle too. An invisible 
impressed image can be developed on it, but it would be 
found that it is liable to stains due to the organic matter of 
the sizing. It is also only translucent and not trans- 
parent. And it is therefore, as a rule, unadvisable to use it 
for holding the sensitive compounds in situ when * nega- 
tives' are required. Again, if the support be not at the 
same -time a medium, it is essential that the latter should 
adhere to the former during the operations of development 

2. When the image is to be formed entirely by the direct 
action of light, the above conditions are not a necessity. 
Then paper will be suitable for a medium, as there is nothing 
extraneous to the sensitive compound to act upon it At 
the same time there is nothing to forbid the employment of 
all other vehicles which are not acted upon by the sensitive 
compound. In the production of camera pictures collodion 
is almost exclusively^ employed; and it is particularly suitable 
for holding precipitable silver compounds in position, as it is 
a ready solvent of most of the bodies which it is necessary 
to use, and the precipitation can be effected in the viscous 
collodion itself. The silver compounds are thus formed 
in a finer state of division than they would be if precipitated 
from an aqueous solution. This point is mosl\Ta^otya.xsJ^\a\ 

t For exception see the ^elatino-broxnldc process, v ^^ 



38 The Daguerreotype. 

light impacts upon surfaces : and as the surfaces of similar 
particles increase as the square of the diameter, whilst their 
masses increase as the cube, it is evident the smaller the 
particles are the larger will be the available area for the 
same quantity. Collodion is a transparent, semi-viscous 
fluid, made by dissolving pyroxyline (gun-cotton) in a 
solution of ether and alcohol, and is for ordinary purposes 
totally unacted upon by the sensitising solution of silver, 
though, when specially prepared, it is believed that an organic 
compound of silver is formed. Other advantages of collodion 
are to be found in the property it has of setting in a gelatinous 
form, previous to its final desiccation, and also that the sol- 
vents teed evaporise rapidly at ordinary temperatures, leav- 
ing the* safts,. which are to form the precipitate, if not in 
solution, yet with their individual particles in such an ex- 
tremely minute state of division as to be undistinguishable 
to the eye. 

Gelatine and albumen are equally good solvents of the 
salts alluded to, but they have the drawback that they take 
long to set, and dry very slowly, and at the same time they 
form definite compounds with silver. 



CHAPTER VI. 

THE DAGUERREOTYPE. 

Under the head of silver processes, the first that would 
naturally occupy the attention is that due to Daguerre. It 
is even at the present date adopted for some kinds of work; 
for instance it was employed by the French expeditions, sent 
to observe the transit of Venus in December 1874. 

The daguerreotype process consists, as already stated, of 

the formation of a sensitive surface of silver iodide, or silver 

yod/de and bromide, on a silvered plate,by rfteaxi^ol\5cv^4vttcl 



Manipulations, 39 

action of iodine, or iodine and bromine. One of the most 
difficult (and difficult only because the greatest cleanliness in 
every detail is required) parts of the whole process is tlie pre- 
paration of the silvered surfece before its coming in contact 
with the halogen. The plates are usually copper, on which a 
film of metallic silver is deposited by the electro-plating pro- 
cess, and when they leave the silvering solution have, as a rule, 
a frosted appearance. After being cut to the proper size, and 
the comers clipped of about ^th of an inch for convenience' 
sake, they are ready for polishing. A plate in this state 
may be placed on a flat table, and four thin strips of wood 
nailed round it to prevent it slipping. To the surface 
is then applied tripoli powder in alcohol with Canton 
flannel, and worked about to such an extent that it is per- 
fecdy free firom scratches, and is fairly smooth. The next 
operation consists in polishing it. This is effected by means of 
a buffi That which the writer has found effective is made 
by enclosing a wooden ball, of the size of a small apple, in a 
skin of felt and then of cotton wool Over this is stretched 
a piece of the finest Chamois leather. On to die surface of 
the silver is then scattered a small quantity of jeweller's 
rouge, and the buff" is caused to travel over the plate from 
end to end and side to side alternately till it becomes of the 
highest polish. This polishing should take place almost im- 
mediately before the sensitising operation is commenced, 
otherwise there is a liability of the „ 

surface attracting impurities from 
the atmosphere. To sensitise the 
plate two sensitising boxes are 
required An illustration of that 
employed when daguerreotype 

was commonly practised for portrait work will give an idea 
of the sort of contrivance required. On the bottom of 
the box c is placed iodine in powder ; a is a piece of card- 
board, which fits into grooves as shown, b ^ ax^ >S^^ ^m^- 



40 The Daguerreotype. 

ports on which the silvered plate is to rest.* The iodine 
will volatilise at ordinary temperatures, and condense on 
the surface of the cardboard next to it. When a plate 
is to be sensitised the cardboard is reversed, and the iodine 
volatilises from the top surfece on to the silver plate. The 
plate gradually receives a thin coating of iodide, passing 
through various stages of colour. When a ruddy colour is 
reached, it is placed in a similar box to that already described 
(omitting the cardboard), at the bottom of which is a mix- 
ture of bromine and calcium hydrate. The bromine attacks 
the surface, and with the iodide forms silver bromo-iodide. 
When the surface assumes a steel-gray or violet colour the 
plate is removed, and once more placed in the iodine box 
for a third of the time originally necessary. In this state 
the plate is exceedingly sensitive, and is ready for exposure 
in the camera. The exposure may be made at once, or it 
need not take place for several hours; Claudet, in fact, found 
that, by keeping, the sensitiveness increased. The time 
necessary to impress an invisible but developable image is 
very short, a few seconds being all that is necessary. Practice 
alone can tell the exact time required, but it is soon learned 
approximately after a few trials. The development is ac- 
complished by exposing the impressed surface to the vapour 
of mercury. A cast-iron tray, with wooden sides and lid, 
is convenient : it may form a box similar to that shown 
fpr the iodising operation. At the bottom is placed a thin 
layer of mercury, the temperature of which is raised to about 
1 50® Fahr, The plate is placed in the box, face downwards, 
on the supports, and the development is allowed to proceed, 
the process being watched as it progresses by inspecting it 
from time to time in a non-actinic light If the exposure be 
right the image will be brilliant, if under-exposed it will be 
weak ; whilst if over-exposed it will be covered with a veil 
of mercury, 

' When sma)hr sixes are to be used they may be held in frames 
io the inacr Irmaes of a camera sUde. 



Intensification of the Image, 41 

The development, it will be remarked, is due to the 
attraction of the subiodide for the metallic mercury vapour, 
and to no other cause. In order to fix the image the plate 
is immersed in a 10 per cent solution of sodium hypo- 
sulphite. After a few seconds the unaltered iodide AgjI^ 
and the Agl of the subiodide (Agal) are dissolved away, and 
the image is left as a white amalgam of mercury aiM 
silver on a darker coloured background. After a thorough 
washing in distilled water the picture is permanent, but its 
appearance may be improved by toning it ; i.e. intensifying 
it with gold to darken the silver, and render the amalgam 
still purer in colour. This is accomplished by pouring over 
it, in such a quantity as just not to run over the edges, 

1. Gold trichloride • , . • 'i gramme. 
Distilled water . • , . , 50 cc. 

2. Sodium hyposulphite . • . *4 gramme. 
Distilled water . . . . 50 cc. 

The two solutions are well mixed together, and, after 
flowing them on the plate, a spirit-lamp is moved about 
beneath its bottom surface, until the toning action com- 
mences. The more rapid the deposition of the gold, the 
more satisfactory the image. When complete, the plate 
must be well washed in a dish of cold water, and finally 
rinsed with distilled water. Drying is best accomplished 
by gende heat, applied first at one end, and gradually moved 
down. Any large drops of water should be absorbed by 
blotting-paper. 

Daguerreotypes may be reproduced by electrotypy, if 
the plate be immersed almost immediately after toning in 
the copper solution. The ordinary electrotyping process 
answers every purpose : for the details, reference must 
be made to books treating specially of the subject. The 
fact is mentioned here, as it shows that the image, after 
all these operations, is in relief though naturally to a 
very limited extent, yet still sufficiently to caM^s^^ Njcsfc 
reflected yight to give all the necessary gtadaXvotv^ ol \vi^ 



42 The Collodion Process, 

and shade. Sir W. Grove also introduced a method of 
etching daguerreotype plates by means of the battery. He 
immersed the plate in a solution of hydrochloric acid two 
parts, and water one part, and opposed by a platinum 
plate placed at '2 inches from it. When the current was 
generated by a couple of Grove's cells, an oxy-chloride 
of silver was formed, and after thirty seconds the plate 
was found to be sufficiently bitten. The oxy-chloride 
was removed, and for fine work was found of sufficient 
depth to allow it to be printed from with printer's ink in the 
printing-press. This process has not come much into 
vogue, as it is one which is too delicate for ordinary opera- 
tions, and the silvered copper is expensive in comparison 
with the other metajs employed for the purpose. The most 
recent development of photo-engraving and the production 
of reliefs are described in a subsequent chapter. 



CHAPTER VII. 

COLLODION. 



Pyroxyline is prepared by acting upon cotton, paper, or 
other kindred substances with a mixture of nitric and 
sulphuric acids. For an example of the process we may take 
cotton, which has a definite formula of CeHioOs- Sulphuric 
acid has the property of absorbing water from any organic 
substance with which it is in contact; for instance a drop of 
oil of vitriol on cloth or paper rapidly chars it, owing to the 
destruction of the constituent atoms, through its affinity 
for water. Thus, if we take the cotton itself, it will be seen 
that each molecule contains 6 equivalents of carbon, and 
just sufficient hydrogen and oxygen to form 5 molecules 
of water ; the oil of vitriol is thus capable of splitting up 
the molecule of cotton, appropriating the ^^ mcAftcules of 



The Chemistry of Generation, 43 

water, and leaving the carbon behind.^ Another good ex- 
ample of the abstraction of an equivalent of water from 
a molecule is in that of ethyl alcohol, or spirits of wine. 
If this be distilled over in the presence of concentrated 
sulphuric acid, we have ether * as the product. When the 
acid is diluted with water, its destructive power is hmited, 
though as the water evaporates from it the power returns. 

Now the strongest nitric acid which is usually obtainable 
contains a large proportion of water. Thus nitric acid, if 
1*457 at 60^ F. contains only 84 per cent, of HNO3, hence 
it is that when this is mixed with sulphuric acid, the water is 
abstracted from it, and the true nitric acid (HNO3) is left to 
act on any body with which it is brought in contact- This 
is undiluted, and is capable of acting on cotton in a some- 
what peculiar way. It abstracts either 2 or 3 atoms of hy- 
drogen (according to the strength of the acids employed, 
and the temperature), and replaces them by 2 or 3 molecules 
of nitrogen tetroxide (NO2) with the formation of water. 
The formula stands thus : — 

Cotton + Nitric acid + Sulphuric acid + "^.f-!"^ combined with the 

^ nitric and sulphunc acids 

CeH,o04 + 2 HNO, + H,S04 + Aq. 

Water formed by the decomposi- 
■■ P)nroxyline + Sulphuric acid + tion of the cotton, and that 

combined with the acids 

= CeH8(NOj)A+ H2SO4 + 2H20 + Aq. 

Or, 

Cotton + Nitric acid + Sulphuric acid + Water = Gun Cotton + 
CeH,oOa+ 3HNO, + H2SO4 + Aq »CeH,(N02),0.+ 

Sulphuric acid + Water + Water. 
H2SO4 +3H2O+ Aq. 

' It is for this reason that if the most dilute sulphuric acid be spilt 
on the clothes, or passed through a filter-paper, and be allowed to 
dry, a charring takes place. In the first case neutralisation with an 
alkali, or in the second very thorough washing, will prevent the 
disaster. 

* Hence the name sulphuric ether. The following equation exhibits 
the reaction. 

Ethyl alcohoi^ Ether + Water abstracted by the sulpWric. ^c\^ 



44 Collodion Processes, 

The first being the gun-cotton, as used for collodion, 
and the second being the well-known explosive com- 
pound. It will be noticed that the sulphuric acid remains 
unaltered in composition, its sole function being to ab- 
sorb the water formed by the operation. The fact of the 
existence of the tetroxide of nitrogen in the altered cotton 
can be demonstrated in its combustion in an exhausted 
glass vessel by the red fumes which tinge the gaseous pro- 
ducts. 

The same reaction as the above can be obtained by em- 
ploying potassium nitrate (KNO3) instead of the nitric acid, 
though in this case a portion of the sulphuric acid becomes 
converted into potassium sulphate. Thus : — 

Potassium nitrate + Sulphuric acid »= Potassium sulphate + Nitric acid. 
KNO, + H2SO4 = KH(S04) + HNO,. 

The above equations represent the reaction that theo- 
retically takes place when cotton is treated with nitric and 
sulphuric acid in the above proportions, but there are other 
points to be attended to in practice. 

The proportion of the acids to each other materially 
affects the properties of the p)n*oxyline. Sulphuric acid 
parchmentises paper when it is immersed in it or floated in 
it, that is, renders it tough and of close texture. The chemi- 
cal effect produced by the sulphuric acid is hardly known, but 
if prolonged, it is known that the paper is dissolved. Parch- 
njentised paper treated with nitro-sulphuric acid has different 
qualities to that in which the parchmentising is omitted. With 
the former a tough collodion results, though it is more pow- 
dery. An excess of sulphuric acid beyond that necessary to 
produce the reaction shown in the equations acts in a similar 
way to treating the cotton first with tlie acid, for it partially 
parchmentises the cotton previous to its conversion into 
pyroxyline, and as this is beneficial to a collodion, when not 
carried beyond proper limits, an excess of this acid is always 
emjoJojed. 

The amount of dilution of the adds m\3a. \«^.\&\ ^l^o 



Effect of the Dilution and the Temperature of Acids. 45 

largely modifies the resulting compound. When little 
or no water is added, the pyroxyline gives an unevenly 
flowing collodion which is strongly contractile when 
drying. When a large proportion of water is added, the 
collodion is limpid, flows readily, and is apt to give a matt 
appearance on drying. By increasing still further the amount 
of water, the cotton when immersed will entirely dissolve 
in the acids. Evidently then a mean between no water 
and the amount necessary to produce dissolution should 
be employed. 

The efiect of the temperature of the acids on the cotton 
is also marked by the behaviour of the resulting pyroxy- 
line, as the effect of heat is to aid chemical change. Pyroxy- 
line made at low temperatures forms a collodion that is 
always glutinous and difficult to flow over a plate, whilst the 
higher the temperature the more easily will it flow. It 
should be remarked that the same effect is produced by the 
addition of more or less water to the acids. It is, there- 
fore, possible by diminishing the amount of water and 
increasing the temperature to obtain the same amount of 
fluidity in a collodion as would be gained by the full amount 
of water at a lower temperature. 

With the above facts before us we can evidently manu- 
facture various qualities of pyroxyline which may be suitable 
for different purposes. With the wet process, where a so- 
lution of silver nitrate comes in contact with the soluble 
iodides, &c., dissolved in the collodion, its conditions should 
be ; — Firstly, that it should be fairly porous ; and, secondly, 
that it should be fairly tough. This is effected by adding 
a moderate proportion of water to the mixed acids, and by 
immersing the cotton in it at a medium temperature. 

The following are the proportions which Hardwich (who 

was the first to thoroughly investigate the manufacture of 

pyroxyline fit for collodion) states should be observed : — 

Sulphuric acid, sp. gr. 1-842 at 15° C. . «>oo cc. 

Nitric acid, sp. gr, i -456 .... \(j6"6 ccl. 
^^^^^ , . l^VI ^^- 



46 Collodion Processes, 

The nitric acid and water are first poured into a strong 
glazed porcelain dish, and well mixed, the sulphuric acid is 
added- last, the liquid being kept well stirred as it is poured 
in. The temperature will generally rise to 75° or 85° (if to 
the latter, it may be suspected that the acids are too dilute), 
and it must then be allowed to cool gradually to 65**. A 
dozen balls of cotton wool,^ weighing about i^ grammes each, 
having been prepared, should be immersed separately in 
the fluid, and after thorough soaking (assisted by a glass or 
porcelain spatula, fig. 9), be allowed to remain at the bot^ 
tom of the vessel. The immersion should take place rapidly, 
otherwise decomposition takes place, and this, when once 
commenced, will cause the temperature to rise rapidly, and 
the whole of the cotton will be dissolved with the evolution 
of nitrous fumes. The balls must be left in the acid from ten 
minutes to a quarter of an hour, and they are then presum- 
ably in a state ready for washing. The longer the immer- 
sion, the more likely are they to become insoluble in ether 
and alcohol, approaching more nearly the state of explosive 
gun-cotton. They are next raised by the spatula, the excess ^ 
of acid as far as possible squeezed out of them against the 
side of the vessel, and then they are dashed into a vessel 
holding a large quantity of water. All traces of the acids 
are eliminated by washing in frequent changes of water, or, 
better still, in running water. To test when this is complete, 
a piece of blue litmus-paper should be pressed against the 
wet cotton, and if after two minutes it remains unaltered it 
may be assumed that the washing is complete. The pyroxy^ 
line should now tear easily, and not be readily separable 
into the original balls, and should weigh about 30 grammes. 
If the original fibre be easily distinguishable, the temperature 
probably fell during the operation, or sufficient water was 

* The cotton should have previously been well steeped in soda and 
water, and be then thoroughly washed and completely dried. 

2 If the precaution be not taken of squeezing out the acids, there 
is a great probability of a solution of a portion of the cotton taking 
place. 



Manufacture of Pyroxyline, 47 

not added. If the weight fall much below that indicated, 
the water was probably a little in excess, or the temperature 
was too great. It cannot be too strongly impressed upon 
the student that the strength of acids is all-important, and 
if the amount of water present with them be above that 
indicated, that so much water must be deducted from that 
given in the formula. A specific gravity bottle is a very 
convenient means of ascertaining the strength of the solu- 
tion, for, the specific gravity once known, the amount of true 
acid present can be found from the tables given in the ap- 
pendix Other methods for ascertaining the specific gravity 
will be found in most works on Chemistry. 

The next formula for preparing pyroxyline of the same 
character is given without comment, as the above remarks 
apply to it. 

Sulphuric acid, 1*842 . « , . 170 cc. 
Dried potassium nitrate * . . . no grammes 

Water 28*3 cc. 

Best dried cotton wool . , .4 grammes. 

Hardwich states that the chances of failure with this 
process are very slight if the potassium nitrate be not too 
much contaminated with potassium chloride. In the above 
operations a thermometer is absolutely necessary. It should 
not be mounted in wood, but should be graduated on the 
stem itself. It may be supported in a clamp, as shown in 
the figure. For dry processes the foregoing formulae give 
pyroxyline, which some consider as too tough and homy, 
and some hold, though the ^vriter does not, that this is 
especially the case for processes where the sensitive salt 
of silver is formed in the collodion itself (see chap. xvi.). A 
modification in the proportions of acid and water can be 
made to suit those who prefer a more limpid collodion. It 
has also been found by some workers that in the latter 
process the presence of a little nitro-glucose is a desideratum. 

' The potassium nitrate should be dried at a temperature of about 
120**. Placing it in an air-bath is the most coTwemenl tosJiJasA «i'l 
obtaining' the temperature. 



48 Manufacture of Pyroxyline. 

The following method secures its formation, though, if the 
resulting pyroxyline be well washed, it is in a great measure 
eliminated. It would seem better to add the nitro-glucosc 
to the collodion, but as this has not been established from 
long experience, it has been thought better to give the pro- 
cess as published by M. Leon Wamerke, in a communi- 
cation to the Photographic Society of Great Britain. Six 
grammes of the finest cotton-wool are put into a porcelain jar, 
and 2 grammes of gelatine dissolved in the smallest quantity 




of water are added. The cotton is impregnated with the 
gelatine by pressing it with a wooden spatula, and when 
this is effected the cotton is carefully dried by the aid of 
heat. It is then ready for immersion in acids which are of 
the following strength : — 

Nilric acid, 1-45 175 cc. 

Water 68-3 cc. 

Sulphuric acid, 1-840 262 -5 cc. 

Or, 

Nitric icid, 14^ 1941 cc . 

Water 49'3 cc. 

SuJphuric acid, I -340 262-500. 



Solvents of Pyroxyline. 49 

The acids and water are mixed in the order named, and 
when a steady temperature of 70° is obtained the gelatinised 
cotton is immersed in it for twenty minutes. With some 
cotton the amount of water given above is inadmissible, as 
it immediately dissolves. The proportions of acids should 
be kept, and the water diminished to such ^ degree that the 
solvent action is reduced. After washing and drying, the 
resulting pyroxyline will be found to have lost considerably 
in weight, and it should be almost powdery in appearance, 
and readily disintegrable. It will be found highly soluble in 
a mixture of ether and alcohol, and as much as 2 per cent, 
may be required to give a sufficient body to the collodion. 

Hitherto cotton has alone been mentioned as capable of 
forming pyroxyline ; but it may be stated that every analo- 
gous substance may be similarly treated. Thus linen and 
paper are amenable to the above treatment, and for some 
purposes they give superior results ; for instance, Whatman's 
drawing paper has been found by Wamerke to give better 
results than the gelatinised cotton in the last process. Being 
already sized with gelatine, there is no need for the preli- 
minary treatment pointed out. 

The action of the solvents employed in the collodion on 
the pyroxyline deserves a passing remark, as many modifi- 
cations in the resulting film can be caused by judiciously 
varying their proportions. The specific gravity of the 
alcohol employed should invariably be ascertained, as the 
condition of the sensitiveness of the plate depends much 
upon its strength. With a collodion made at a low tempera- 
ture, the presence of a certain percentage of water is advis- 
able, as its horny nature is thereby modified, and a certain 
degree of porosity obtained. A specific gravity of '820 is 
in this case admissible. With the pyroxyline such as that 
obtained by the last formula, the water should be a minimum, 
as it is already porous, and the presence of water is apt 
to make it reticulated and rotten. The specific gca.v\t^ vcv 
this case should rarei/ be over -812. An excess ol i)LcOcv.c\ 

£ 



so Collodion Processes. 

also tends to give porosity, and therefore sensitiveness; but if 
the addition be carried to an extreme, the very porosity dimi- 
nishes sensitiveness, as the sensitive salts formed in the film 
coagulate into too large particles. Alcohol also diminishes the 
rapidity of setting. Ether, on the other hand, tends to close 
the pores of the film, as is demonstrated by coating a plate 
made with an excess of it, when it will be found that a con- 
traction takes place, causing the film to leave the edges of 
the plate, or to split on drying. The ether employed should 
be as pure as possible (this is not insisted on by manufac- 
turers of collodion) as otherwise it is apt to liberate the 
halogen from the dissolved salts, giving rise to an alkaline 
reaction which is one cause of rottenness in the film, and an 
apparent want of body in collodion.^ 

The following are collodions for different processes. 

For the wet process : — 

No. I. Pyroxyline, Hardwick's formula 
Alcohol, '820 
Ether, 725 

No. 2. Pyroxyline, Hardwick's formula 
Alcohol, '820 
Ether, 725 

No. I is most suitable for cold, and No. 2 for warm weather. 



12 to 14 grammes 
450 cc 
550 cc. 

12 to 14 grammes 
500 cc. 
500 cc. 



For dry processes with the bath : — 

No. 3. Pyroxyline, first formula 
Pyroxyline, last formula 
Alcohol, -813 or -814. 
Ether 725 
Water 



10 to 12 grammes 
4 prammes 
5<x> cc. 
500 cc. 
Quant, suff. 



The water is shown in No. 3 to remind the student it puts a 
power into his hand of modifying the collodion in structure 
by its addition. It frequently happens that No., i or 2 

* The student would do well to try the experiment of adding a 
small quantity of caustic potash to a phial of collodion, and noting the 
action that takes place. 



On the Salts dissolved in Collodion, 51 

formula may also be improved for dry processes by attend- 
ing to the amount of water present. 

The next point to be determined is the amount of 
bromide and iodide to be dissolved in the collodion, and to 
determine their proportions it will be well to enter into 
detail as to their behaviour when converted into the silver 
compounds and exposed to the light. Iodide of silver in a 
film is capable of forming a dense image with a short ex- 
posure, but the gradations in density are often wanting when 
the light is extremely bright ; added to which, if organic 
matter be present with it even such as is to be found in 
many collodions, the picture is apt to be veiled and wanting 
in vigour. 

Bromide of silver, on the other hand, is especially 
adapted for those collodions which have an organic re- 
action. It has usually been accepted that the iodide is the 
more sensitive of the two salts, but recent investigations 
tend to show that, with suitable development, the bromide 
has the advantage, when developed by Method 3 (p. 19). It 
does not yield such a dense image, but the detail in the 
denser parts is always present. The failing of the bromide 
consists rather in its comparative insensitiveness to very 
faint light as found in deep shadows. A bromo- iodide of 
silver, however, combines the advantages of the bromide 
with that of the iodide ; for the wet process and certain of 
the dry processes, it possesses every essential quality for the 
production of a good picture. The proportions of bromine 
and iodine in combination vary considerably, from i part 
of the former to 10 parts of the latter (which is just suffi- 
cient to secure cleanliness and freedom from veil \^ith all 
ordinary preparations of collodion and bath) to 3 parts 
to 2. The latter proportion is never employed except in 
dry-plate processes. The iodide is usually fixed at about 
from 6 to 10 grammes per litre. The sensitiveness of the 
surface in all cases depends on the mode of development 

employed. Thus for a wet plate, Vogel has found that 

£ 2 



tifc imnmtian' u oninit q- )mmmtt ^umd "h? tdiiuit ^^C) i 
n jwarc: tiK ircaresc :*jianvcaitaai» wnisr wttr. the ikSkst- 
iiTH tiKtioii- r. s- limimssitAi ~o "nt: nniuler- ntiwurrsion. ii 
nsLV ixinv \K sua luu: ne irrjpiimuir n jrrnmiiu^ "xr wiii^c 

ti*'^ jmrxsi. r'lii Bfetai v:nr viu«::r -m; ouun: xmi: hpcramie 
im: vimhmist vnitn' rar')uui:'=*i ntu ~ji^ :'jil«Ah«-mv, -aeix'es to 

1irt:t. Qutft^ 'TSRairiny TOTimtritii -rjj> ?cao;»i inar "she 3ttf%ft)s 
IT. ?5:nir.nica! efecr ixni t!:=c^cv ix:iibioi»i '!?^OTw«in nno 

^Ak v:^ <2jffioKsace iiL rreir beharteor wa«*5t iivx^T^i on a 
youV^. W:±i. rac litrer he will icd Jt rr^xrN- T!v^win^ fluid; 
wida tx^ former ooe wisich is naorti ^-.uiiaous^ jmd d^flicult 

h aiy> appears that the diaereni meulUc salts in solu- 
ti//o ^^u)!« difierent d^rees of sensiiix-eness in a film. 
'i^m Uax f<*een investigated by Wamerke, who places 
i\mn in the following order for imparting sensitiveness and 

OH**r of «enwtiveness . . . Zn Cd Na Fe NH^ K U 
0»/W ofintcnMty of image . Zn U NH, Cd Na K Fe 

Tlifl alkaline iodides are those which are most prone to 
dej^HHH^^ under the action of ether, particularly if it be 

ncc, for a collodion to keep long, it is neces- 

rest form be employed. As before shown, 

e is decomposed, the alkali decomposes the 

«ring it very fluid and defective in setting 




Formula for Salted Collodion. 5 j 

qualities, whilst the iodine itself increases the density of the 
image, probably by the formation of a silver iodate. 

In bromised collodion it is very rare for bromine to be 
set free, and in bromo-iodised collodion the dark colour 
obtained by long keeping is invariably due to the iodine 
liberated, for uncomhinedhxomvci^ will always displace iodine. 
To increase the density of a developed image it is always 
advisable to add a littie tincture of iodine to a collodion. 

In choosing the iodide or bromide of any particular 
metal for iodising or bromising a collodion, it must be 
remembered that it is the amount of iodine and bromine 
that are the essentials, and not the metal. Hence, 4 grains 
of ammonium iodide and 4 grains of cadmium iodide mean a 
totally different quantity of iodine. A table of these quan- 
tities will be found in the appendix. 

The following formulae will be found to give collodion 
suitable for the ordinary wet process : — 

No. I. Ammonium iodide . . , , ,7 grammes 
Cadmium bromide . . . . .4 grammes 
Plain collodion * . . , , , I litre. 

No. 4. Ammonium iodide . . , . .8 grammes 
Cadmium bromide , , . . • 2 '5 grammes 
Plain collodion «»..•! litre. 

No. 3. Cadmium iodide . . , . « 9 grammes 
Cadmium bromide . , . « , 4 grammes 
Plain collodion ..... I litre. 

Nos. I and 2 are speedily ripe enough for use; with a 
little alcoholic tincture of iodine added they may be em- 
ployed immediately. 

No. 3 requires keeping, as at first it will not flow freely. 
A sample of collodion such as No. 3 has been kept two 
years without deterioration, the precaution being taken to 
keep it in the dark and in a cool place. 

* It must be borne in mind that the collodion may be made by i, 
2, or 3 formula, and the pyroxyline may be of the varying types shown 
at p. 50. Formula No. 2 is that usually to be recommended. 



54 Collodion Processes, 

No. 2 is suitable for dry-plate work and for interiors, 
but as a staple article No. i is recommended. 

For a simple iodised collodion the following formula 
may be adopted: — 

No. 4. Ammonium iodide . •' . • 8 grammes 

Plain collodion • * • . I litre 

Or, 

No. 5. Cadmium iodide • • • . 10 grammes 
Plain collodion .... I litre 

No. 4 should be used immediately after making, whilst 
No. 5 will keep almost indefinitely. 

The next formula is for a simple bromised collodion : — 

No. 6. Zinc bromide 16 grammes 

Plain collodion .... I litre 

For all the above iodides and bromide substitution may 
be made with others, and it by no means follows that those 
chosen as examples will prove the most sensitive, though 
experience has shown they give good results. It is cus- 
tomary in preparing plain collodion to omit half of the 
alcohol, and to employ that half as a solvent for the haloid 
salts. This is convenient but not absolutely necessary. It is a 
good plan to make a note of the. date of the manufacture of 
the collodion, as also of its iodising; useful information is 
often given by such memoranda. 

Testing Plain Collodions, 

Plain collodion should be tested before iodising, and the 
following tests may be applied, recollecting that a film that 
may not be suitable for the bath process may still be suitable 
for an emulsion process, and vice versa. 

Coat a plate (in the manner described at p. 79), and 
ascertain if when dry the film dry dead white, opalescent, or 
transparent. If the first, it is unsuitable for any process; if 
the second, it may be employed for emulsion work; whilst 
if the third, it may be suitable for any process. 

Coat a plate, and, after tlie collodion has set, mark if it 



Cleaning the Glass Plate. 55 

is powdery to the touch, or if on applying the finger it 
comes away in strips. If the former, it may be good for dry- 
plate work; if the latter, for both dry plates or the wet 
process. 

Coat another plate, and, after setting, wash the film 
under the tap till all the solvents are washed out, and note 
if it take an even film of water or if it repels it at parts. If 
the latter it is too horny to use in the bath processes ; a 
little potassium carbonate may improve it. 

Note if the collodion flows freely, viscously, or lumpily. 
Too limpid a collodion will fail to give density; too viscous 
a collodion is unsuitable for any but small plates, whilst a 
lumpy collodion will give irregular images. The flowing 
qualities of a collodion arising from the pyroxyline may 
often be corrected by altering the proportions of ether and 
alcohol. 

If the film be reticulated, having marks like a crape pattern 
on it, the solvents may not be sufficiently anhydrous, or the 
pyroxyline may be in fault, as before stated. 

The collodion should also be tested after iodising: ; the 
defects will be noticed when treating of the defects in 
negatives produced by the various processes. 



CHAPTER VIII. 

CLEANING THE GLASS PLATE. 



The plate, before being taken into use, should be most 
carefully cleansed from dirt of any description. The success 
of a photographer may be said to depend in a great measure 
on the effectual manner in which he completes this opera- 
tion. The dirt that is to be looked for on a glass plate is 
that due to the manufacture, that due to subsequent ex- 
posure to the atmosphere and to the hands of the packers,. 



56 CoUodioH Processes. 

* • 

and sometimes that due to the chemical compounds with 
which it may have been in contact Ordinary plates are 
sometimes found to be gritty on what should be the polished 
surface, and the application of acid may dissolve the grits 
away. Hence it is a goo<l plan to treat all new plates 
with a solution of dilute nitric acid (lo parts of water to i 
of acid). This \i-ill not rid them of mechanical dirt, such 
as dust or grease. The presence of dust is readily ac- 
counted for, but the origin of the greasy matter is far more 
difficult to imderstand. If a plate that is thoroughly cleaned 
be put away in a plate box for a few days, and be then exa- 
mined by breathing on it, it will be found that it shows signs 
of repelling the aqueous vapour from the breath in certain 
parts, and that a subsequent cleaning of the plate is neces- 
sary to render it fit for use. This phenomenon can be 
accounted for on the supposition that organic matter of a 
fatty nature is to be foimd in the atmosphere, and when we 
remember that the lungs expire not only carbon dioxide, 
but also various organic matters, we should expect that in 
an inhabited house this latter might condense on some 
dry cool surface. The danger of using plates on which 
this deposit exists will be apparent by a simple experiment. - 
Rub a warm finger or hand over the plate, coat with collo- 
dion, sensitise, but do not expose to light ; then apply the 
developing solution and watch the result. It will be found 
that where the contact has been made, a reduction of metallic 
silver will take place, and as development proceeds a dark 
stain will be produced. Imagine a similarly treated plate, 
prepared as before, exposed in the camera and developed \ 
a dark deposit will take place both where the hand has 
touched and also where the invisible image has been im- ■ 
pressed. It may be said that all animal organic matter has 
the property of causing a tendency for metallic silver to be 
reduced from the solutions of its salts. A similar remark 
applies to the mercury compounds which sometimes get in- 
visibly reduced in the surface of the glass. The composi- 



Detergents, $7 

tion of dust is of a most varied nature, and not unfrequently 
consists of ferric oxide, sodium chloride, and other earthy 
constituents. The reduction of silver nitrate in the presence 
of some of these would be certain. 

Alkalis have the property of converting greasy into 
saponaceous matter, and spirits of wine will dissolve both 
soap and grease ; hence both are employed as detergents. 
Mechanical dirt requires friction to remove it, and this should 
be just sufficient for the purpose, yet not enough to injure 
the surface of the glass. Such bodies we have in tripoli 
powder and rouge. The former is recommended on account 
of its being less gritty than the latter. The most common 
cleaning solution is made as follows : — 

Spirits of wine 50 cc. 

Tripoli powder : — Quantity sufficient to make a thin 

cream 

Ammonium hydrate. . . . . i cc. 

Mr. Warren De la Rue for his astronomical photography 
employed a solution of potassium dichromate and sulphuric 
acid. This is doubtless a most effective detergent, but the 
use of sulphuric acid is open to objection on account of the 
damage it may do to the dress or hands. 

The writer has heard of a process of cleaning recom- 
mended, in which it was proposed to employ potassium 
cyanide, followed by nitric acid. The student is earnestly 
recommended not to attempt this plan, as it is poisonous 
and highly dangerous (see p. 74). 

Boiling the glass plate in caustic soda or potash has also 
been proposed. This is apt to injure the surface of the 
plate, owing to the slight solubility of vitreous matter in 
solutions of the caustic alkalis. Perhaps no more effective 
method for securing a clean plate can be adopted than 
by first treating the plate with a cold solution of caustic 
potash, rubbing it well in with a rag, and then immersing it 
in dilute nitric acid and washing under the tap. A final 
thorough rinse in distilled water, and a rapid drying in a 



58 



Collodion Processes, 



water-oven, will leave the plate in as clean a state as can be 
desired. 

SENSITISING BATH. 

The sensitising solution, that is, the solution in which 
the collodion containing the soluble iodides or bromides, or 
both, are immersed in order to form the iodide bromide, or 
bromo-iodide of silver, may be said to be invariably made 
of silver nitrate dissolved in water. The purity of both con- 
stituents is of the highest importance, as any extraneous 
matter may be fatal to obtaining good results in develop- 
ment. Distilled water is naturally the purest form of water 
that can be obtained, but even this is sometimes contami- 
nated with organic matter in solution, which is apt to 
react upon the sensitive salt. The manner in which am- 

FlG. lO. 




monia is carried over with the aqueous vapour is well known 
to any chemist, and in a similar way hydrogen sulphide can 
be carried over. The latter contamination is most hurtful to 
sensitiveness, and the former might cause fog. It may be use- 
ful to point out the best mode of distilling water in a small 
way, in order to obtain absolute purity. 

A glass retort is always clean, and dirt can be more 



Distilled Water. 59 

readily seen than if it be of metal. The form known as 
Liebig*s condenser is therefore recommended instead of the 
ordinary still. The water should be placed to the level of the 
flask A shown in the diagram, and a little (say a gramme 
to a litre) caustic potash should be dissolved in it. This 
will free the water of any ammoniacal compounds when 
warmed. The distillation takes place through the glass 
tube, by round which is placed a glass jacket, Cy containing 
water. Cold water is allowed to enter the jacket by the 
tube, dy and the heated water is carried off by ^ ; an universal 
clamp, B, is useful for holding the condensing apparatus in 
position. 

The first 50 cc. of each litre distilled should be rejected, 
and the distillation should not be continued beyond that 
point where 100 cc. are left in the retort. The distillate 
may then be considered to be pure enough for photographic 
purposes. If an ordinary worm still be employed, care 
should be taken that the worm is clean, free from dust, and 
not of lead. The water should be distilled over as before, 
the first and last portions being rejected. If distilled water 
cannot be obtained for making up the solution, spring water, 
if not impregnated with sulphates, will generally answer. 
Failing these, river water, and lastly rain water, after twice 
filtering through charcoal, must be resorted to. At first it 
may seem strange to place rain water last on the list, but 
it should be remembered that it is almost invariably col- 
lected from the roofs of houses, and is consequently 
sure to be contaminated with organic matter, and also 
inorganic matter. Rain water, if it could be collected 
directly as it falls, would save the necessity for using 
distilled water. A method of purifying ordinary water for 
bath purposes is as follows. Boil and filter it, add a little 
barium nitrate to it, and see if it turns milky. If such be 
the case, add a small fiirther quantity, together with a few 
crystals of silver nitrate to each litre of water, and place in 
the sunlight. After a few hours' exposure, the organic^ 



6o CoUodioii Processes. 

matter and sulphates will be at the bottom of the con- 
taining vessel, and the supernatant n^ter may be decanted, 
syphoned, or filtered off. An excess of barium nitrate is 
not hurtful to the solution, for, as wiU be seen at p. 62, its 
addition is recommended. 

Nitrate of silver should be pure. The uncrystallised 
will be found sufficiently free from nitric acid to be available 
for forming a bath solution needing no doctoring. It is 
sometimes adulterated ; if any suspicion of this arise, a cer- 
tain known quantity of the crj^stals should be dissolved 
up in water, and the amount of silver nitrate really pre- 
sent calculated by any of the methods usually adopted. 
Silver nitrate is readily soluble in its own weight of water, 
but this strength would be quite unsuitable for a sensitising 
solution for two reasons : first, silver iodide is soluble to 
a certain extent in silver nitrate solution. The stronger the 
latter, the greater the amount of iodide dissolved. A varia- 
tion in temperature also affects the quantity capable of 
being held in solution. Now, even supposing that at the 
temperature at which the bath was formed immersion of an 
iodised plate took place, the heat evolved in the act of 
combination between the soluble iodide and the silver 
nitrate to form the sensitive compound would be suffi- 
cient to cause the iodide in the film to be partially dis- 
solved out. Secondly, the formation of the iodide would 
be so rapid that there would be a coarseness in the 
particles unsuitable for rapidity. Sutton has demonstrated 
that where any iodide is in the solution, 10 per cent, is as 
great a strength as can well be managed, whilst a 5 per cent, 
solution is the limit in the other direction. When bromides 
alone are employed, the strength may be 15 per cent., as the 
silver bromide is almost insoluble in silver nitrate solution. 

Inj|Dg;>aring a bath it is generally saturated with silver 

io^l'^^Ktoevent the silver nitrate dissolving away portions 

^^ ^HF^ surface. Some skilled photographers, how- 

m ^Khe . saturation to take place from the film 



The Silver Nitrate Bath. 6 1 

itself, a method which is recommened to the student, if he 
excercise ordinary care in working his plate. The degree 
of acidity of the bath depends much on the iodising or 
bromising of the collodion. To secure the greatest degree 
of sensitiveness, if iodide alone be present the solution 
should only be feintly acid, with bromo-iodide it should 
be distinctly acid, whilst with bromides alone it should be 
very acid. The rationale of the different degrees of acidity 
is as yet not known accurately, more investigation into the 
subject being required ; but it may be presumed that it is in 
a measure dependent on the behaviour of the silver bromide 
and iodide when exposed in the presence of silver nitrate. 

The following formula for the silver-bath solution is a 
standard one where iodide or bromo-iodide of silver is the 
sensitive salt to be produced : — 

Recrystallised silver nitrate • « 80 grammes 
Water ...... i litre 

Potassium iodide • . . « '25 gramme 

The silver salt should be dissolved in a quarter of the water, 
and the potassium iodide added to it after solution in the 
least possible quantity of water. After shaking (which will 
cause a partial solution of the silver iodide first formed), the 
remaining water should be added, when a further emulsion 
of iodide will appear. When filtered out, the bath solution 
will be ready for use, supposing proper acidity to be 
attained. 

An excess of acidity may be corrected by the addition of 
a few drops of a sodium carbonate solution. When a per- 
manent precipitate is obtained, the requisite acidity should 
be given after filtering by adding a few drops of a 5 per 
cent solution of nitric acid. Some photographers have 
recommended the employment of acetic acid instead of 
nitric acid, but the writer has never found any benefit re- 
sulting from it — in fact the reverse ; for although acetic acid 
added to silver nitrate will not at first form silver acetate, 
yet as the solution becomes contaminated by working 



62 Collodio9i Processes. 

there is danger of compounds forming, which will combine 
with it, and finally cause decomposition between the new 
compound and the silver salt. 

As the bath solution gets worked, that is, has many 
plates immersed in it, the original purity becomes im- 
paired by the accession of ether, alcohol, and various 
nitrates from the collodion* besides any extraneous matter 
that may accidentally be carried in. After a time the 
vigour and cleanliness of the developed image will be 
found to diminish, and the strength, &c., of the bath has to 
be attended to. Gently warming it will get rid of the ether, 
and evaporating it to half its bulk will get rid of most of the 
alcohol. If organic matter be present, exposure of the bath 
(after neutralisation of the free acid with sodium carbonate) 
will cause metallic silver to be precipitated, and itself to be 
oxidised by the Hberated molecule of nitric acid, thus ren- 
dering it innocuous. 

With certain collodions acetic acid will find its way into 
the bath, and the best method of eliminating the silver 
acetates which will probably have been formed is to evapo- 
rate the bath to dryness and add some strong nitric acid. 
This will liberate the acetic acid, which may be driven off by 
a further application of heat. None of these modes of treat- 
ment will eliminate all the impurities, for all the foreign 
nitrates (except ammonium) remain almost unchanged, even 
by prolonged fusion ; nothing remains but to precipitate the 
silver as chloride, or in the metallic state. If a film, after 
withdrawal from the bath, presents an appearance as if fine 
particles of the sensitive salt had been sprinkled over it, the 
solution is * over-iodized ' ; that is, it is super-saturated with 
silver iodide. The disturbance made by the immersion of 
the plate probably causes the deposit. Diluting to double 
its bulk, next filtering, and then making up the solution to 
proper strength, will be a cure, or, as some photographers 
aver, the addition of 2 per cent, of barium nitrate will answer 
the same end. 



Development, 63 



CHAPTER IX. 

DEVELOPMENT OF THE PHOTOGRAPHIC IMAGE. 

The importance of a thorough understanding of the ra- 
tionale of developing an image in the silver compounds is 
not to be over-rated, as a close study of it furnishes clues 
to apparently mysterious results, which are so often met 
with by every student in the art. The method of developing 
the Daguerrean image has been already given, and we pro- 
pose in this chapter to confine ourselves to that employed 
in what is known as wet-plate photography and dry-plate 
photography, and also that followed in the calotype and 
other kindred processes. 

It will be recollected that by method i the invisible 
image was to be made visible by the attraction exercised 
by Uie new compound formed after the impact of light on 
the original one. As already announced in chap. iv. p. 24, 
the change effected on a molecule of silver haloid is its re- 
duction to a lower type, i.e. one containing a lesser number 
of atoms. Thus Ag2l2 was reduced under certain circum- 
stances to Ag2l, the other atom of iodine being absorbed 
by some body in contact with it. A similar change was 
shown to be effected on the silver bromide and chloride. 
We may, therefore, take as a type any one of these. We 
will choose the iodide, and follow the development from 
the earliest stage, when used in the wet process. 

It has already been shown at p. 19 that the building 
up of the image is due to the well-recognised law that every 
minute freshly-formed crystal attracts every other of a similar 
nature, and that the formation of the tree is entirely due to 
this molecular attraction, and the slow reduction of the metal 
from its solution. If the metal were deposited rapidly the 



64 Collodion Processes. 

same law would still hold good, but the attraction of one 
reduced molecule on its immediate neighbour would be 
greater than that exercised by the metal adhering to the rod, 
as the probable distances in the one case would be far less 
than in the other. A particle of such a size and weight 
would therefore be built up before the metal on the rod 
could draw it sufficiently near to overcome the force of 
gravity exercised on it ; hence it would sink to the bottom 
of the containing vessel. 

If we take a solution of silver nitrate and add to it a 
solution of ferrous sulphate, we have an almost instantaneous 
reduction of metallic silver. Thus — 

Silver nitrate + Ferrous sulphate =» Silver + Ferric sulphate 
3 AgNO, + 3 FeSO, = 3 Ag + Fe, (SO,), 

+ Ferric nitrate 
+ Fe(xNO,), 

Any other oxygen-absorbing medium which is incapable of 
causing double decomposition with the silver nitrate might 
be substituted for the ferrous sulphate. By adding an acid 
to the latter the same action takes place, but much more 
slowly, the time necessary to effect the total reduction 
being dependent on the amount of acid present. Supposing 
by some means or another we are able to cause the first 
crystals of the silver to deposit themselves in certain posi- 
tions, we may be certain from analogy that the remaining 
crystals will adhere to these and build up a miniature silver 
tree. In the wet process, and also in the dry, we have 
means of causing these first particles of silver to deposit on 
the invisible image. 

This invisible image is formed of subiodide of silver 
(Ag^I). Only one of these atoms of silver is saturated; the 
other is still ready to combine with any other atom with 
which it has an affinity. Such an atom it finds in freshly- 
deposited silver. The solution of silver nitrate is already 
present in the wet process, and in the dry processes it is 



Crystalline Attraction, 65 

added to the oxygen-absorbing agent, which is employed in 
both. 

The first deposited crystals attract others, and thus an 
image is built up. It may, however, be asked how it is that 
different density of deposit is caused. The answer to this is 
that the invisible image is formed of variable quantities of the 
subiodide, approximately proportional in fact to the intensity 
of light acting on it At any spot on the sensitive surface it 
is the integral of the attractions of the different atoms lying 
close to one another that determines the amount of the first 
deposit, and the varying mass of this determines the dis- 
tribution of the subsequent deposit 

It is an axiom that the stronger the solution of the 
reducing agent the more rapid must be the deposit, and it 
may be convenient here to discuss the bearing of this. 
Suppose adjacent particles of the sensitive surface possess 
separate attractions of, say, i, 2, 3, and 4 units, caused by the 
different intensities of light acting on those parts. The 
probabilities are that the first metallic silver atom deposited 
will be drawn to the spot possessing 4 units of attraction. 
If the interval in time for the reduction of the next atoms 
exceed that necessary for placing the first atoms in sitUy 
the attraction originally equal to 4 units will become ap- 
proximately 5, and the probabilities are that the larger pro- 
]>6rtion of the next reduced atoms will be attracted by the 5 
units than by the 3 ; and by the same action the 4 units 
may attract several atoms, whilst the 3, 2, and i units may 
have attracted proportionally less. If the reduction of a 
sufficient number of atoms to saturate the whole of the 
atoms of Ag2l take place almost simultaneously, the 
probability is that the difference in the increase of attractive 
]>ower will be less marked. Thus 4 may become 5 ; 3, 4 ; 
2, 3; and 1 become 2. It may therefore be asserted that 
the position of the first deposition will determine that subse- 
quently taking place, provided the same rate of the reduction 
be maintained. From the foregoing reasoning it will be 

F 



66 Collodion Processes^ 

apparent that the stronger the developing solution the less 
marked will be the variation in density due to the different 
intensity of light acting on the various portions of the sensi- 
tive salt. 

The more viscid a liquid and the smaller the mass of the 
particle, the slower will a particle travel through the liquid. 
An application of this law has been applied to development. 
A certain amount of colloidal substances, such as gelatine, 
albumen, or these bodies acted upon by acids, is added to 
the liquid in which the oxygen-absorbing agent is dissolved. 
Though the reduction of the silver nitrate to the metallic state, 
may take place as rapidly as in a solution in which the colloidal 
body is omitted, yet the time the metallic atoms take to travel 
through the viscous solution is lengthened to such an extent 
that an appreciable time is taken to form a visible particle 
of silver. The time, therefore, taken to build up an image 
is longer than with a solution in which the colloidal substance 
is absent ; it is found that a small quantity of the colloid 
will give sufficient viscidity to cause slow deposition. 
The examination under the microscope of an image de- 
veloped in the manner indicated above will perhaps throw 
more light on the subject than any verbal description that 
can be given. It will be found that the whole of the image 
is formed of these minute crystals, varying in size according 
to the length of time which they took to deposit. The ap- 
pearance of the film when the half-tones of the negative are 
thus examined, will be as though it had been sprinkled with 
the metallic granules by means of a pepper-box ; whilst the 
parts representing deep shadows will be represented by largQ 
patches of bare collodion, with here and there a crystal lying 
embedded in the film. The student should take every oppor- 
tunity of studying the effect of different kinds of develop^ 
ment as regards the actual physical composition of the 
image ; and he may rest assured that the highest excellence 
in any negative can never be attained when the deposit is 
coarse and highly crystalline. With a 2-inch objective.it 
should appear as a stain on the film of more or less intensity. 



FannidcB for Developers. 



67 



The following are the formuls& usually employed in de- 
velopment : — 



No. I. 



Pyn^allic acid 


I gramme 


Glacial acetic acid . 


. 20 cc. 


Alcohol 


Quant, suf. 


Water .... 


. 500 cc. 



This developing solution is usually employed for simply 
iodised collodion, and is useful when great density in the 
lights is required. The iron developers of a weak and strong 
type are as follows : — 



No. 2. Ferrous sulphate 
Glacial acetic acid 
Alcohol 
Water . 



No. 3. Ferrous sulphate 
Glacial acetic acid 
Alcohol 
Water . 



10 grammes 
30 to 40 cc. 

Quant, suf. 

I litre 

100 grammes 
40 cc. 
Quant, suf. 
I litre 



These formulae give the limiting proportions of ferrous 
sulphate to water admissible, but any quantity between the 
two may be taken. For ordinary work, about 40 grammes 
is usually taken, as giving the best results. The double 
sulphate of ammonium and iron may also be substituted 
for the ferrous sulphate, and it has the advantage of re- 
maining in solution unchanged for a long period. 

The addition of copper sulphate to an extent equal to 
half the quantity of ferrous sulphate employed is also 
recommended by some operators, and it has doubtless in 
some cases a beneficial effect. 

The addition of various colloidal substances to the de- 
velopers, as already stated, may sometimes be desirable, 
particularly where great density and fine deposit are requisite. 
Perhaps the best of any is that proposed by Mr. Carey Lea: 
it is made as follows: — 30 grammes of French glue, or ge- 
latine, is softened in 50 cc. of water, to which 3^ cc. of 
sulphuric acid is added. The water is next boiled, and the 



F 2 



68 Collodion Processes, 

gelatine dissolves, and, after adding another lo cc. of water, 
the boiling is continued for a couple of hours. Five grammes 
of metallic zinc are next added, and the boiling continued 
one hour and a half longer. The solution is allowed to 
settle, and the clear liquid decanted off. To every 3 grammes 
of ferrous sulphate, i to 2 drops of this solution suffices 
to give sufficient restraint, without the addition of any acetic 
or other acid. 

Ferrous sulphate is a very unstable body, and will absorb 
oxygen from the air, and speedily attain the ferric state ; and 
as the latter salt is incapable of absorbing more oxygen, it is 
evident that the developing qualities are thus annihilated. 
It has been in effect found that ferric sulphate is a retarder, 
that is, a body which prevents the rapid deposition of the 
metallic silver from the nitrate solution. The lesson to be 
learnt from this is, that when the developer attains a red 
colour it must of necessity be slower in action than when of 
the ordinary apple-green tint. A simple experiment with a 
developer containing ferric sulphate is worthy of trial by 
the student. Take, say 3 grammes of ferrous sulphate, and 
having dissolved it in 50 cc. of water, boil with strong nitric 
acid to such an extent that the addition of a drop of the 
solution to one of potassium ferricyanide produces no blue 
precipitate. Next precipitate the iron as ferric oxide by 
ammonia, filter, wash well, and dissolve up in the least pos- 
sible quantity of sulphuric acid, taking care to leave a slight 
residue undissolved. Make up the quantity of liquid to 
10 cc, and add 2 cc. to a solution of ferrous sulphate made 
according to formula No. 3, omitting the glacial acetic acid. 
Develop a picture with it, and note the result 

Attention should be paid in all cases to the crystals of 
ferrous sulphate employed. They are frequently mixed with 
a yellowish powder, due to the decomposition of the salt In 
common specimens this often bears a considerable proportion 
to the ferrous salt itself, and must be allowed for in making 
up the solutions. The strength of the acetic acid is also 



Alcohol in the Developer. 69 

• 

important What is commonly sold as glacial is often below 
strength. Its value should be estimated as given in various 
works on chemistry. In warm weather, owing to the in- 
creased rapidity of chemical action, more acetic acid is re- 
quired to control the reduction of the silver nitrate. Hence 
these quantities shown may require modification accord- 
ing to the temperature. 

The amount of alcohol required is invariably shown as 
Equant suf' No definite quantity could be given, as it 
varies according to the amount of alcohol present in the 
bath solution. With a new bath none at all is required, 
whilst with one in which a large number of plates have 
been sensitised as much as 40 cc. to the litre may be neces- 
sary. A deficiency or excess of the alcohol is shown by the 
solution refiising to flow evenly over the surface of the sen- 
sitised collodion, and running into rivulets and tears. This is 
caused by the difference in surface tension of the fluid on the 
plate and the developer. Any body which reduces the de- 
ficiency may take the place of the alcohol. Thus a more 
viscid solution, such as that given by the gelatine retarder, 
is effective, no alcohol being required with it, even when the 
•bath is very old. Methylated alcohol * should be avoided 
as far as possible, stains and disfigurements in the developed 
image being often attributable to it. 



CHAPTER X. 

GIVING INTENSITY TO THE IMAGE. 

Any method of increasing the apparent depth or black- 
ness of the image when received by reflected light, or of 
increasing the opacity of the image to non-actinic, or to 

' Spirits of wine, sold as methylated, sometimes contains a 
certain quantity of resinous substance, in order to satisfy the excise 
requirements. 



70 Collodion Processes, 

visual rays, is termed intensifying the image, and in both 
cases the result can be brought about by the same procedure. 
The following are modes of giving intensity to the image. 

I St. We may continue the development of the image by 
method i, if we supply more free silver nitrate solution to it 
when exhausted ; and this will give us the necessary inten- 
sity. The theoretical considerations before noted need not 
be again brought before the student, neither is any special 
experiment necessary to impress them on his mind. 

2nd. We may produce opacity to actinic rays by in- 
creasing the deposit by other means. As an example of 
what is meant, we may apply to the silver image a solution 
of merciuric chloride. 

Mercuric chloride + Silver ■■ Double chloride of mercury and silver 
HgCl, + Ag = AgHgCl,. 

This at first is grey (probably due to the formation at 
first of silver subchloride), but it finally becomes a pure 
white. It will be noticed that each atom of silver attracts 
one atom of HgCla- As regards opacity without regard to 
colour, the image must evidently be more opaque. It is, 
however, as regards actinic rays much less opaque than 
when the image was of the grey due to the silver. 

An application of ammonium hydrate to it, however, 
converts it into a jet black or deep brown. 

Here we have a still further deposition on the silver atom, 
which is therefore denser, and, being black, is very opaque 
to actinic rays. 

As another example of this mode of intensification we 
may instance the effect of copper bromide on metallic silver, 
and the subsequent treatment of the deposit thus formed 
with silver nitrate.* 

Silver forming Copper _^ Silver Copper ' 
the image bromide ^ bromide sub-bromide 

Ag + CuBr, « AgBr + CuBr 

* For a detailed account see Photographic ydurnalf April 1877. 



Formula for Inteftsifiers. 71 

When treated with silver nitrate we have — 

Copper 
Silver Copper Silver _ Silver Silver nitrate in 

bromide sub-bromide nitrate bromide sub-bromide solution 
AgBr + CuBr, +2AgNo,« AgBr + AgjjBr + Cu(NO,) 



2 



It will be seen how immensely the deposit on the image is 
increased by this method. 

Lastly, intensity in an image may be secured by sub- 
stituting some other metal for the silver by chemical means. 
For example, we may apply a solution of platinum tetra- 
chloride; the silver will be converted into chloride, and the 
platinum will be deposited in its place. The silver chloride 
may be subsequently dissolved away by sodium hyposulphite 
or ammonia, or by many of its well-known solvents. 

From a study of these methods it will be apparent that 
methods 2 and 3 must each be carried out on an image 
from which everything else is removed but the metallic 
silver; method i may be employed without such removal. 

The formulae for the first method are as follows: — 

No. I. P3nr(^aHic acid .... 4 grammes 

Citric acid 4 to 8 grammes 

Water i litre 

No. 2. Ferrous sulphate . . .10 grammes 

Citric acid 20 grammes 

Water i litre 

With the latter intensifying solution detail in the shadows is 
often brought out, though absent in the development, but 
the former is the most efficacious for rapidly giving opacity 
to the image. With each of the above a few drops of a 
solution of 

Silver nitrate .... 20 grammes 
Water 500 cc. 

must be added immediately before application to the film. 
These intensifying solutions may be applied to the image 
either before or after fixing ; those which follow, however, 



72 Collodion Processes. 

require the unaltered iodides and bromides to be previously 
dissolved away. 

Iodine 'i gramme 

Potassium io.lide . . . . *2 gramme 
Water 50 cc. 

The iodine (which is held in solution by the help of the 
potassium iodide) converts a portion of the reduced me- 
tallic silver into iodide, and when continued but for a 
short time the image has a bluish-green tint', which is more 
non-actinic than if it were left in the metallic state. If 
this be not sufficient a solution of 

Potassium permanganate . . • i '5 gramme 
Water 50 cc. 

may be flooded over it. The permanganate is decomposed 
in coming in contact with the silver iodide, and insoluble 
manganic oxide is precipitated on the image. 

Another form of intensifier is made by — 

Mercuric chloride . . . . *2 gramme 
Water 750 cc. 

and, 

Potassium iodide . . . . i gramme 
Water 50 cc. 

The latter is added to the former till the red precipitate of 
mercuric iodide is on the point of becoming permanent 
This solution applied to the image converts the silver into 
double iodide of mercury and silver, which is very non-ac- 
tinic in character ; other similar methods may be adopted, 
Ipill depending on the formation of double metallic com- 
pounds. By converting the silver image into iodide by the 
application of the iodine solution, and then flooding with 
sodium sulph-antimoniate (NaQS, SbS5) commonly known 
as Schlippe's salts, a scarlet deposit is produced of 
silver sulph-antimoniate in which 2 atoms of silver replace 
the 2 atoms of sodium, the iodine combining with the 
sodium. This method of intensification is due to Carey 



ScMipp^s Salts. 73 

Lea, who described it in a paper which appeared in Feb. 
1865, in the American * Journal of Photography.' Schlippe*s 
salts are prepared by taking 

Antimony bisulpliide (finely powdered) i8 parts 
Dried sodium carbonate . . .12 parts 

Caustic soda 13 parts 

Sulphur 31^ parts 

These are ground up into a fine paste with a little water, 
and transferred to a well-closed stopped bottle, completely 
filled with water. After digestion and agitation for twenty- 
four hours, the clear liquid is filtered off, and allowed to 
evaporate spontaneously in a closed vessel over sulphuric 
acid, till lemon-coloured crystals of a regular tetrahedral 
shape are obtained. These are dissolved in water imme- 
diately before use, as the solution deposits an antimony 
compound when kept. The mother liquor may be employed 
for intensifying, but does not answer so well as the salt 
itself. The quality of the colour is dependent on the amount 
of silver converted into iodide or chloride. 

When great density is required without gradation of 
shade, the following formula is efficient when preceded by 
a saturated solution of mercuric chloride. 

The effect of this compound, as already pointed out, is 
to form a double chloride of silver and mercury, grey at first, 
but which subsequently becomes converted into a pure white 
deposit When in this state if 

Ammonium sulphide . . . 50 cc. . 
Water » i litre 

is applied, a double sulphide is formed of an intense black. 
Dilute ammonimn hydrate may also be employed, as already 
stated, in place of the sulphide. 

As regards the treatment of an image with copper bromide, 
this salt may be formed by dissolving i gramme of copper 
sulphate inio cc. of water and adding an equivalent of potas- 
sium bromide to it This solution is flowed over the plate, 
and after a whitening action on the film and thorough 
washing, a 20 per cent solution of silver nitrate is applied. 



74 Collodion Processes. 

Fixing the Image. 

As regards fixing the image, nothing need be said ex- 
cepting that the solvent used must be incapable of readily 
attacking the metallic image, and such are the solutions of 
sodium hyposulphite and potassium cyanide. It will be 
useful here to point out the mode by which this solution is 
effected. Supposing, for instance, the image be developed 
on the iodide of silver; we have on addition of sodium 
hyposulphite either 



Silver 

iodide. 

Agl 


Sodium ^ Double hyposulphite Sodium 
hyposulphite of silver and sodium iodide. 
+ NajSjO, = AgNaSp, + Nal 


or, 

Agl 


+ sNajSA - Ag,Na,3(SA) + 2 Nal 



The first silver hyposulphite is very soluble in water 
whilst the last is very insoluble ; we have, therefore, in using 
sodium hyposulphite, a danger of the formation of the 
insoluble compound — a danger not to be under-estimated in 
the matter of silver prints, when the elimination of the less 
soluble compound is a matter of great difficulty. 

With potassium cyanide the danger does not exist, for 
though silver cyanide is formed, yet it is readily soluble in a 
small excess of the potassium compound. 

Silver Potassium Doublecyanide of silver and Potassium 
iodide. cyanide. potassium iodide. 

Agl + 2 KCN = AgCNKCN + KI 

Instead of Agl in the above equations, we may substi- 
tute nearly every silver compound — thus AgNoa, AgCl, 
AgBr, AgOSiOj. Potassium cyanide, however, has the 
drawback that it is excessively poisonous, and that the 
presence of acid causes it to evolve hydrocyanic acid, a 
gas the deadly effects of which it is unnecessary to comment 
on. Another drawback to its use is the danger that exists 
of its dissolving up the finely deposited metallic silver, of 
which the half-tones of the image is composed. If used 



Varnishing. 75 

in a sufficiently weak solution, however, the solvent action 
need not be feared. All traces of the hyposulphite and 
cyanides should be removed by thorough washing, otherwise 
the transparent parts of the image might discolour, or a dis- 
integration of the film might take place through crystallisa- 
tion. 

The following solutions are those generally employed: — 

Sodium hyposulphite . . . loo grammes 
Water 500 cc. 

And, 

Potassium cyanide . • . • 30 grammes 
Water , . . , • . , 500 cc. 

Varnishing the Film, 

The collodion film being excessively delicate and easily 
torn or scratched, photographers have adopted the plan of 
covering it with a transparent film of hard resin. This is 
effected by dissolving the resin in spirits, such as alcohol, 
and flowing it over the surface. In practice it will be found 
that, in order with safety to cover the film without dissolv- 
ing or disintegrating it, the specific gravity of the methy- 
lated alcohol, with which for economy it is made, should be 
greater than that employed in the manufacture of the collo- 
dion. It may at first sight seem strange that alcohol should 
be capable of attacking the pyroxyline, but it must be re- 
membered that undiluted methyl compounds are solvents of 
it, and, unless sufficient water be present in the varnish to 
check the tendency, a disintegration at least will take place. 
It must also be remembered that the rate at which the 
solvent evaporates will cause a difference in the transparency 
of the coating. If it be allowed to evaporate spontaneously, 
the alcohol evaporates first, and leaves the water behind, and, 
as anyone will find if he drop a little varnish into water, the 
resin at once separates in minute particles, which, when so 
united together, give a translucent deposit, caused by the re- 
flections of the various surfaces. On the other hand, if heat 



^6 Collodion Processes, 

be applied, and the water be caused to disappear as rapidly or 
nearly as rapidly as the alcohol, the resin will dry transparent, 
the heat being sufficient to cause the particles to be bound one 
to another^ thus eliminating all chance of particular reflection. 

The resin should be as colourless as possible, as even 
the thin coating given to a negative picture is often sufficient 
to cut off much of the actinic light if it be of a red or yellow 
tint. As an experiment, it is only necessary to dissolve red 
Austrahan gum in spirit or water, and apply it to a portion 
of a glass plate, when it will be found that sensitive chloride 
paper darkens much less rapidly where covered with it than 
where it is bare. 

The constituents of most varnishes usually comprise 
amongst them lac and sandarac, but it is a matter of the 
greatest nicety to proportion them in such a manner that the 
film shall not split after exposure to any great variation in 
temperature. The cause of the contraction that takes place 
is not accounted for; it seems that some resins have a 
property of attracting moisture, and almost becoming hy- 
drates. This might cause an expansion of the film, whilst a 
rise of temperature might cause contraction. The whole 
blame, however, must not be laid upon the varnish, as the 
collodion film, when not free to expand and contract as it 
likes, may often produce the same effect. The following 

varnishes have been found satisfactory: — 

Unbleached lac .... 65 grammes 

Sandarac 65 grammes 

Canada balsam .... 4 grammes 
Oil of thyme or lac acetic . . 32 cc. 
Alcohol, '830 500 cc 

Or, 

Seed lac 120 grammes 

Methylated spirit .... I litre 

The lac is allowed to remain in contact with the solution 
for two or three days, with occasional shaking ; after which 
the supernatant liquid is decanted off, and thinned down 
to proper fluidity. 



Cleaning the Plate, JJ 



CHAPTER XL 

MANIPULATIONS IN WET-PLATE PHOTOGRAPHY. 

Cleaning the Plate, 

The glass plate must first be cleaned with one of the 
detergents indicated in the last chapter. If the tripoli 
pK)wder cream be employed, it should be applied with a 
small pledget of cotton wool or soft rag, taking care that both 
sides of the plate are covered with it It may either be al- 
lowed to dry on the plate, or, whilst still wet with the alcohol, 
may be wiped off witfi a soft diaper duster. In the latter case 
a little practice is required to prevent markings on the plate 
as shown by breathing on it When the tripoli powder is 
all rubbed off, a final polish should be given to both surfaces 
with a chamois leather or old silk handkerchief.^ ITie 
polishing should be such as is used in french-polishing 
a table ; not too heavy a pressure should be exercised, and 
there should be a continuous circular motion. It should 
be remembered that the effect of rubbing silk on glass is 
to generate electricity, which is sometimes a cause of non- 
adherence of the collodion to plates. The electricity in the 
glass should be allowed, or caused, to be dissipated before 
collodion is applied. There are various appliances for hold- 
ing plates during cleaning, some of which are excellent in 
their way, whilst others are toys made on principles unme- 
chanicaL They are not necessary for the size of plate with 
which an amateur is likely to work. 

If a plate be clean the moisture firom the breath will leave 
it evenly. It must be fireed firom dust before collodionising, 
by passing a badger-hair brush over its surface. 

' Before taking these into use, they should be thoroughly cleaned 
from all greasy matter by washing with soda. 



78 



Collodion Processes — Wet Plate. 



Vui. II. 




Coating the Plate with Collodion. 

The plate may be held in its centre by a pneumatic 
holder, such as that in figure ii, or at the comer by the 
fingers, if care be taken that no portion except the 
edges are touched From a half-filled 6-otmce 
bottle, or from what is known as a collodion 
pourer (fig. ii), the collodion should be care- 
fully poured upon the plate, so as to form a cir- 
cular ])Ool at the end farthest away from the 
manipulator, and gradually be allowed to cover 
the entire surface, the wave flowing from the 
right-hand to the left-hand top comer, from 
thence to the left-hand and right-hand bottom 
comers, and finally into a stock botde, whence, 
after decantation, and (if necessary) dilution 
with 2 i)arts ether to i of alcohol, it can be again employed. 
When the collodion is thus poured off, the plate will be 

in nearly a vertical position, 
and a gentle rocking motion 
should be given to it to pre- 
vent the collodion setting 
in ridges ; but the precau- 
tion should be taken not to 
grind the edges against the 
bottle, otherwise particles 
of glass may appear on 
subsequent plates. In hot 
weather the collodion does 
not take so long to set as in cold The state of the film 
can be always ascertained by cautiously touching the left- 
hand bottom comer with the finger. When no longer 
tacky, the plate is ready for immersion in the bath« The 
collodion should be filtered if necessary, or it may be 
decanted from a stock bottle by one of the ordinary syphon 
arrangements. 



Fig. 13. 




Sensitising the Plates. 



Sensitising the Plates. 

The film of coUcxiioii having set, the plate is immersed in 
the sensitising solution contained in a vertical or horizontal 
bath, the former being recom- 
mended for small plates, thoi^h 
the latter is essential for large 
sizes. A 'traveUing bath' is 
perhaps the best form of bath 
holder, as it is useful for indoor 
and also for outdoor work. It is 
of the form given in the figure. 
The top of the glass solution. 
holder B, which is held io a 
case A, is closed by a water- 
tight india-rubber top d, 
screwed down by the screws c as shoTin The ' dipper ' 
employed for carrying the plate mto the solution during 




Fiai4 



Fig. 1 



I 



the operation of sensitising may be conveniently made 
of PURE silver wire, of the accompanying shape. It is 
usually, however, made of ebonite or glass. When the plate 
is covered with the solution by a steady downward motion 
of the dipper into the bath, it is moved slightly up and down 



8o CoUodioH Processes — Wet Plate. 

in the fluid to wash off the ether from the surface of the 
film, and, when all greasy appearance has vanished, it may 
be left quietly at rest for from one to five or six minutes, 
according to the temperature and amount of bromide ' 
present The motion of the plate in the bath at first is im- 
portant ; as, if neglected, streaky negatives are apt to result, 
especially in summer weather. 

It need scarcely be said that the silver nitrate solution 
should be free from all sediment before a plate is immersed 
in it, and it should be kept in order as shown at p. 6a. 
After being very slowly withdrawn from the bath, capillary 
attraction will be exercised by the solution in the bath on that 
left on the film, and there will be but a slight quantity left on 
the plate, quite insufficient to cause the necessity of long drain- 
ing. On the other hand quick withdrawal necessitates long 
draining on a pad of blotting-paper ; the edge that occupied 
the lowest position in the dark slide should be pressed 
against it. When the surface appears free from excess of 
moisture, the plate is placed in the dark slide, taking care 
that the edge that is to occupy the top place in the camera 
is kept in the same relative position. The slide is closed 
after the back of the plate has been dried with a piece of rag 
or blotting-paper. It is here presumed that the camera 
is in position, and that the view has been focussed, follow- 
ing out the rules given in Chap. XXXII., p. 242, and that 
the exposure is given also in accordance with the remarks 
to be found on p. 246. 

Development, 

Having decided which developer is to be employed, 
making the decision after a carefiil study of the picture, and 
noting its peculiarities, the plate is removed from the dark slide, 
the same precaution of keeping uppermost the edge which 



Intensifying the Negative, 8i 

occupied the top in the camera being taken as before. If this 
were neglected the bath solution which might have accumu- 
lated at the bottom edge might flow back over the surface, 
and thus inevitably cause irregular development if nothing 
worse. The developing solution having been placed in a 
clean cup, it is swept with an even motion, without being 
aUowed to stop, over the plate, which is held by a pneumatic 
holder, or by the fingers, as in coating it with collodion. 
IJtde or none of the solution should be allowed to leave 
the film, unless it be feared that too much density will be 
given to the resulting image, in which case it is an ad- 
vantage to let it wash off a portion of silver nitrate. As 
the picture appears, the developer is caused to work round 
to every comer in succession, thus securing an evenness 
which would otherwise be wanting. An over-exposed pic- 
ture will flash out at once, and, unless the plate be imme- 
diately washed, a veil or fog will inevitably be deposited on 
the surface, caused by the too rapid reduction of the first 
particles of silver, and the consequently rapid reduction of 
the remainder. An under-exposed picture will develop very 
slowly, and will always be wanting in detail by transmitted 
light, though it may appear fully out when looked at by 
reflected light, if held over a black background such as 
the coat-sleeve. 

A properly exposed picture should develop gradually and 
evenly, and should take at least half a minute in warm 
weather to come fully out in every part. When no further 
action is manifest, the developing solution should be 
thoroughly washed away, and the next operation should be 
proceeded with. 

Intensifying the Negative, 

This operation is one in which great judgment is required 
by the manipulator. Too great an opacity will spoil the 
negative, giving a black and white picture when printed ; 

G 



82 Collodion Processes — Wet Plate. 

whilst, on the other hand, one not sufficiently opaque will 
yield a grey print, which is unsatisfactory. The opacity 
must be judged of by the colour oi the deposit as well as by 
the density, though the former need not be taken into 
account when the iron developer has been used, as the 
silver deposit caused by it is of a blackish grey. If a pyro- 
gallic acid developer be employed the colour is of a de- 
cidedly reddish tint, and proportionally non-actinic, hence 
great judgment is necessary to ensure a really good result 
When intensity is procured by using the pyrogallic solution 
the same remarks hold good, though the colour is never so 
marked as when arising from development. Whatever course 
be decided upon, it should be borne in mind that the general 
character of the finished negative will always bear an exact 
relation to that given by the primary development. Thus 
a flat-looking developed image will yield a flat-looking pic- 
ture, whilst one full of gradation will yield one similarly 
graduated. 

Should intensification be necessary, the operator must 
determine whether it would be more advantageous to conduct 
it before fixing the image, or afterwards. Should over-expo- 
sure have been given the latter will be advisable, whilst, if un- 
due exposure, it should certainly take place before fixing. The 
intensifier should be poured over the plate, and, whilst 
so remaining, a few drops of the silver nitrate solution (p. 
71) should be dropped into the cup, and then the intensifier 
poured back. The solution is again swept over the plate, 
and the required density is obtained by removal of the silver. 

It has been a point causing some discussion as to 
whether a developed picture may see light before being 
intensified. The answer to this seems simple. With an 
iodised film, which has been well washed after develop- 
ment, it may be exposed to tolerably bright light with- 
out any danger of producing a veil by the action of the 
intensifier, since silver iodide is almost insensitive to light, 
except in the presence of an lodme a\>soT\iwvl. With a 



Fixing' the Negative^ 83 

bromo-iodised film more caution is required^ though the 
writer has never found that a short exposure in a moderately 
strong light is hurtful. With a bromised film the less ex- 
posure given between the two operations the better. 

When intensifying after fixing, it is customary to flow a 
little iodine (see p. 72) over the film, then to expose it to 
light, and afterwards to use the p)n-ogallic solution. This is 
nearly useless unless a little free silver nitrate be present,, or 
all excess of iodine be washed out, any trace of which would 
render the exposure inoperative. The writer recommends a 
little bromine water instead of the iodine, for reasons which 
will be apparent on reading the chapter on emuFsions. 

In intensifying after fixing, there is a dangea: of staining 
the shadows with a reddish stain. This seems to be more 
due to a pyrogallic stain than to deposited silver, and can 
usually be got rid of by a little acetic acid diluted with an 
equal bulk of water. 

For landscape or portrait negatives it is seldom wise to 
resort to any method of intensification, except that with 
silver, as there is great risk of making the half-tones too 
opaque. The iodide of mercury formula (p^ 72) is perhaps 
the best, if anything more be necessary. 

Fixing the Negative, 

This operation calls for little remark. The plate may be 
immersed in a vertical or horizontal bath if the sodium hypo- 
sulphite solution be employed, or it may be applied by flow- 
ing it over the plate : this should always be done with the cya- 
nide solution. Attention should be paid to see that all the 
iodide, bromide, or both be dissolved away. This can be 
ascertained by reversing the plate and noting if the yellow- 
ish-green colour due to them be absent Finally the plates 
should be well washed and drained. A neat contrivance for 
holding the plates when drained is shown in fig. 16. As 
it folds up It IS suitable for field work, tYiou^ ^. ^\^\v\cv^ 

G 2 



84 Collodioft Processes — Wet Plate, 

box is usually carried, made as in the accompanying sketch, 
fig. 17. 

Fig. 16. Fig, 17. 





Varnishing the Negative. 

The plate may be allowed to dry spontaneously or by 
the aid of heat ; the latter method gives a slightly denser 
image, and therefore a negative should never be heated when 
parts of it are dried by ordinary evaporation. Before apply- 
ing the varnish the plate must be warmed (see p. 75), to 
cause the varnish to flow, and also to prevent it drying matt. 
The varnish is applied like collodion, the same procedure 
being followed exactly. When all that will has run back 
into the bottle, any excess that may have collected at the 
comer end may be removed by pressing the glass on a pad 
of blotting-paper. The plate must again be warmed. 

The sources of heat are various. In India, or in other hot 
climates it will be found that exposure to the sun's rays 
imparts sufficient warmth to the glass. In temperate climates 
the neatest way of attaining the proper temperature is by 
placing the plate in a hot-air bath, as used in chemical 
operations (see fig. 26) ; failing which, a clear fire, a Bunsen 
rose burner, or a paraffin lamp maybe brought into requisition. 
A naked spirit-lamp is dangerous without great care, and 
the solvents of the varnish, being highly inflammable, readily 
catch are from any naked flame. 



Defects in Negatives^, 85 



CHAPTER XII. 

DEFECTS IN NEGATIVES. 

A MERE mention of some of the defects that are to be met 
with in negatives will suggest a cure, whilst for others, which 
are a little recondite, explanations will be offered and reme- 
dies suggested. 

* Fog ' on a negative may be due to several causes: — 1, it 
may be due to a dirty plate ; 2, to over-exposure ; 3, to an 
alkaline bath solution ; 4, to want of acid in the developer ; 
5, to improper exposure to actinic light, either in the camera 
or dark room ; 6, to vapour in the developing room or tent. 
A minute examination of the condition of the negative and 
the state of the dark room or tentj will generally show the 
cause of the defect, which has only to be known to be 
rectified. 

A weak image may be due — i, to an unsuitable collodion, 
a weak sensitising bath ; 2, a bath charged with organic 
matter ; 3, bad lighting of the subject due to dull weather or 
a yellow light ; or, 4, an over-strong developer. 

Pin-holes in the negative may be caused by — i, dust on 
the plate ; 2, the bath being over or under iodised. 

Black specks on the picture are usually due to — i, dust 
in the camera ; 2, slide ; 3, dark room ; or, 4, dust in the 
collodioa 

Comet-like spots are almost always due to undissolved 
particles of pyroxyline in the collodion. 

Transparent spots, as distinguished from pin-holes, are 
usually due to dust in the collodion. 

A scum on the film is usually found when a plate has 
been kept for a long period out of the bath, or when a too 
strong development has been used. A pVate'wVaOcwS&XsiXi^ 



86 Collodion Processes — Wet Plate. 

kept for a long time before development should be sensitised 
(or finally dipped) in a weak bath, and only immersed in it 
sufficiently long to cause all repulsion between the surface of 
the plate and the solution to be overcome. A collodion 
containing a larger than usual proportion of bromide is also 
recommended to secure freedom from stains. 

The usual explanations given as to the cause of markings 
like watered silk, is that the collodion contains too much 
iodide, is too alcoholic, or that the pyroxylin is too strong. The 
remedies have already been indicated. Black stains at the 
comers of the plate are often caused by the bath solution 
flowing back over the sensitised surface, after having been in 
contact with the wood of the dark slide. 

Transparent markings are much more common in cold 
than in hot weather. They generally arise from unequal 
sensitising of the film in the bath, and from the developer 
refusing to flow. 

A want of sharpness in a picture may be due to in- 
accurate focussing, to a want of achromatism in the lens, or 
to the camera being shaken accidentally by the wind, or by 
the sinking of the camera legs during the exposure. If the 
lens be in fault there is no help for it but by ascertaining how 
much further backwards or forwards the ground glass of the 
camera ought to be shifted in order to get the sharpest 
result possible. This can easily be found by actual trial, 
and when noted the ground glass may be permanently 
placed in such a position relatively to the glass plate in the 
dark slide, that when the picture is visually in focus the 
position of the sensitive plate shall be chemically in focus. 

A blurring of the image can easily be accounted for ; 
though, perhaps, there has been more controversy on the 
subject than on any other photographic phenomenon. It is 
usually ascribed to geometrical reflections of the incident 
rays coming through the lens from the back surface of the 
j:lass^ and no doubt, in some cases, this is absolutely true, 
though in others it requires a mote com^\e.\.^ explanation. 



Irradiation. 



?7 



It must be borne in mind that the rays of light do not strike 
the surfece of the plate perpendicularly except at its centre. 
The accompanying diagram shows a glass plate, a a, of exag- 
gerated section, with rays of light passing through the optical 
centre, c, of the lens, b, coming from a bright line, a b. The 
ray ^ c d is perpendicular, and the ray b o, y. makes an 
angle with the perpendicular. This last ray, after passing 
through the collodion film (which for the time we may con- 
sider transparent) would be bent inwards to f, and a 

Fig. 18. 




portion would be reflected fi-om the back surface of the plate, 
and strike the thin collodion film again at g. From g, a 
portion might be reflected again, and so on. Evidently, in 
this case, a blurring might take place, but always outwards 
from the centre of the plate. If, however, the ray of light 
^ c E proceeded fi*om the extremity, ^, of the dotted line, b d, 
which may be supposed to represent a bright line of light, 
then no blurring would be apparent, as the blur from it 
would be covered by the image e m, of the bright line. 
Now in practice blurring is usually most intense when a 
dark object, such as a tree, is opposed to a bright object, 
such as the sky ; in which case we may suppose b diobQ a 
section of the sky, and a boi the tree, which we may suppose 
to be a dark Jine in section. Here the YAumii^ \^ ^n\^^\^>\^ 



88 CoUodim Processes— Wet Plate. 

not due to reflections of the incident lajrs from the glass. To 
account for it, we must look to another feature of the sensi- 
tive smface. K a sensitised fihn be examined under the 
microscope it will be found to consist of minute grains of 
silver bromide, iodide, or bromo-iodide, and each of these 
grains individually must reflect more or less light from its 
suriiEice. A beam of light, therefore, must be dispersed in every 
direction, and, as has been shown,^ the light striking at any 
point of the film is scattered and reaches the back surface 
of the plate as a disc, with intensity gradually diminishing 
from the centre. The reflection from that surface becomes 
most noticeable when the critical angle of the glass is 
reached The direction that the rays take in striking the 
particles is not of any great moment, as the diflerence in 
intensity of the reflections in any one direction is very slight 
when the angle does not difiier very largely from a right angle. 
Hence it is seen that blurring really takes place from this 
cause in all parts of a picture taken on a glass plate, but 
that it is naturally most apparent when a bright light is 
opposed to a deep shade. There is still another point in 
this particular scattering of the rays to take into account, 
and that is, the lateral scattering. Supposing the intensity 
of the light in the lateral direction to be only -^J^ of that 
in the perpendicular, the penetration into the film would still 
be considerable, and a blurring would result on this accoimt. 
In photographing fine lines close together this kind of blurr- 
ing is often most apparent, a black line being often filled up, 
or rendered too fine. It has been argued that blurring is also 
due to the lens, but a serious consideration of the matter 
will show that such an effect is hardly possible if it be toler- 
ably achromatic. 

The blurring caused by the reflection of the scattered 

rays from the plate can be destroyed by using an opaque 

body, on which the collodion shall rest, or it can be partially 

eliminated by placing a backing of some black or non- 

' See Zi?fi(i0ft, Edinburgh^ and Dublin Phil. Mag, January 1875, 



Positive Collodion Pictures. 89 

actinic colour in optical contact with the back surface. The 
lateral blurring is a more difficult enemy to face. It can be 
avoided by dyeing the film with a scarlet or yellow dye ; but 
this is at the expense of sensitiveness. It may be eliminated 
also by making the film as transparent as possible (that is, 
by reducing the particular reflection). This is only feasible 
in dry-plate processes, and then is only occasionally success- 
fill. A daguerreotype plate should be firee from all blurring, 
except the very minute amount which may be due to the 
last-named cause. 



CHAPTER XIII. 

POSITIVE PICTURES BY THE WET PROCESS. 

There is no very distinctive difference between the mani- 
pulations or the theory of the negative and positive 
processes. Any distinction between the two may be summed 
up in this : the metallic image must be of as white a nature 
as possible, whilst the background must be as dark as pos- 
sible. The image should also be as transparent as circum- 
stances will allow, for it must be remembered that all half- 
tone is dependent on this quality, and that in the half-tone 
consists the value of the picture. After a microscopic exa- 
mination of a film it will be manifest that the coarser the 
grain, the more colour should be capable of being seen 
through the image ; hence it is not amiss to have the deve- 
loper of such a nature as will produce this effect, and also 
to cause the silver deposit to assume as white a character as 
possible. 

The bath is usually made as follows : — 

Silver nitrate . . . .65 grammes 
Water i litre 

The silver iodide is added, as for the negative bath 
(p. 61). It should be slightiy acidified mlVv mtrvc. 2lC\^ 



90 Collodion Processes — Wet Plate. 

If a pyrogallic acid developer be employed, that given 
at p. 71 is perhaps as good as any. An iron developer 
nuy be advantageously made with a large proportion of 
ferrous nitrate, in order to secure the white deposit. The 
following is a formula usually adopted. 



Ferrous nitrate . 


. 7 grammes 


Ferrous sulphate . 


. 3 grammes 


Nitric acid, I '45 . 


. I 25 cc. 


Alcohol 


. Quaut su£ 


Water 


. I litre. 



Should the deposit formed by this developer be too 
granular, a little more ferrous sulphate must be added. 
The pyroxyline for the collodion should be prepared with 
weak acids, about equal parts of the sulphuric and nitric 
acids being employed, with as much water as they will 
bear without dissolving the cotton wool when the tempera- 
ture is lowered 5® below that given for preparing negative 
pyroxyline. 

The collodion should contain more ether than alcohol 
when such pyroxyline is employed. The writer has found 
that the following gives satisfactory results : — 

Ether, 725 400 cc. 

Alcohol, '812 .... 300 cc. 
Pyroxyline 10 grammes. 

The following may be added to this quantity of plain 
collodion — 

Ammonium iodide . . .7 grammes 
Cadmium bromide . . • i'5 grammes. 

As in iodising negative collodion, it may be found ad- 
visable to omit 150 cc. of alcohol from the collodion and 
to dissolve the iodide and bromide in it, and subsequently to 
make the addition when the collodion is required for use. 
A little tincture of iodine, enough to give a sherry colour 
to the collodion^ is usually necessary to secure sufficiently 



Dry Plate Processes with the Batlt^ 91 

dense pictures. The development should not be carried to 
such an extent as in the negative process. A picture on a 
glass support, when viewed by transmitted light, should, in 
fact, look under-exposed. If the support for the collodion be 
a ferrotype plate, the image may be developed very readily 
so as to give the best effect, or the black background can 
be used to assist the judgment. It need scarcely be said that 
when glass is employed the back of the plate should have 
black velvet or black varnish in contact with it. Bates's 
black varnish is recommended for backing the plate. The 
fixing solution is the cyanide solution, given at p. 75. 



CHAPTER XIV. 

DRY-PLATE PROCESSES WITH THE BATH. 

It is not proposed to enter into details of many dry-plate 
processes, as they can be ascertained by the consultation of 
various manuals. At the same time it is thought advisable 
to enter more fully than usual into the theory of the subject. 
The course usually adopted for these processes is as fol- 
lows : — 

The plate is coated with a preliminary substratum of gela- 
tine, albumen, or india-rubber, or else is given an edging with 
one of them. The collodion is then applied, and sensitising 
takes place in the usual manner. The silver nitrate solution 
is next thoroughly washed off in distilled or rain water, and 
what is known as a preservative is flowed over the surface 
of the plate. The preservative may be partially washed off, 
or it may be allowed to dry on it in undiminished strength. 
The plate is now in a state ready for exposure. 

The preliminary coating or edging of albumen is given to 
the plate in order to secure the adhesion of the collodion 
fikn. It IS found in practice, if this be om\\Xfcft.> ^'aX. '^^^ 



92 Collodion Processes — Dry Plate. 

film, on being wetted, becomes non-adherent, and floats 
oft The substratum, as it is technically called, must always 
be of such a nature as not to injure the bath solution, and, to 
guard against all risk, it is advisable that every portion of 
it should be covered with collodion. The following are 
formulae for it : — 

Albumen .... I part 

Water 50 to 100 parts 

Ammoninm hydrate — Sufficient to give it a smell of 

anunonia. 



Or 



Sheet gelatine .... 3 grammes 
Ammonium hydrate . . . 15 cc* 
Water 1500 cc. 



The gelatine should be soaked in half the quantity of 
water, and the remainder added boiling. When cool, the 
ammonium hydrate should be dropped into the fluid. This 
solution will not keep, hence it is advisable to make it up 
only just when wanted. 

When it is required to cover the entire plate with either 

of these substrata, it is usual to wet the plate with distilled 

water, and flow it over, and drain. It frequently occurs, 

however, that this method produces markings on the negative. 

A simpler and more effective plan is to cover the end of a glass 

plate (the breadth of the plate to be covered) with a piece of 

fine flannel or swan's-down calico, to moisten it with the 

fluid, and then to squeeze out all excess. This brush, known 

as Blanchard's brush, is drawn down the plate with an even 

stroke ; it gives the very finest coating possible. A washed 

and unpolished plate seems to take more kindly to the colloid 

body than if the cleaning be finished off with the silk hand 

kerchief or chamois leather. 

In lieu of either of the above solutions the following may 
be flowed over the plate like collodion, and be allowed to 
dry spontaneously : — 

India-rubber .... I gramme 
Benzine . • « • » V^^'i* 



Preparation of Dry Plates. 93 

• 

The collodion should be of such a character as to give 
great density to the developed image. It should also give a 
porous film, for it must be remembered that when the pyroxy- 
line parts with its water of hydration it becomes extremely 
impermeable to all solutions ; so much so that a homy col- 
lodion will often refuse to develop. It has been the practice 
with many practical photographers to keep the iodised col- 
lodion till it is thoroughly aged, and has a ruby tint from 
the elimination of iodine from the iodides of the alkalis, 
and the consequent combination between the alkali and the 
pyroxyline. This effect is precisely similar to that obtained 
by a modification of the ratio of the acids to the water in the 
manufacture of the pyroxyline. Collodion may also be 
rendered porous by adding water to a portion of it to such 
a degree that it gives a reticulated film, and by then adding 
the remainder unwatered. A slight opalescence of the film 
is not objectionable, and it may even dry almost matt, so 
long as the necessary disintegration of the pyroxylin is 
secured, since it is found that varnishing takes away the dead 
appearance to a great degree. 

The sensitising bath should be such as to give a good 
dense film after the plate has been immersed some time. 
The solution employed for the wet negative process is of pro- 
per strength, unless the collodion be highly bromised, in 
which case the amount of the silver nitrate may be increased 
to half as much again, or even to twice the quantity. 

After sensitising, it is usual to wash the plates to such an 
extent as to free them from all silver nitrate solution. The 
first washings are usually made in distilled or filtered water. 
Rain water is often recommended, but the operator should 
beware of it, unless it be very clean and at least be twice 
filtered. Perhaps more bad dry plates are produced by the 
use of impure washing water than by anything else. When 
the delicacy of the effect that is produced by the ethereal 
waves of light on the pure products is taken into considera- 
tion, it will be apparent that every extratveou^ io\c^ ^\xvOew 



94 Collodion Processes — Dry Plate. 

will overthrow the equilibrium of the particles should be 
avoided. Thus we might expect that hydrogen sulphide 
might cause that overthrow, and it inevitably produces 
fog. A moderate proportion of iron in the water would 
also produce like results. When the first excess of the 
silver solution is washed away the danger of the use of 
impure water diminishes rapidly, and almost any ordinarily 
pure water may be brought into requisition, but in any case 
a final rinse of distilled water is to be strongly recommended. 
Care should be taken that the surface of the plate after with- 
drawal fi-om the bath is covered by the water without stop- 
page. The first washings may be well performed in a dish, 
and an even covering of the surface can be attained after 
a few attempts. Washing in distilled water should continue 
till all repellent action due to the alcohol and ether contained 
in the bath solution disappears. 

Applying the Presen^ative. 

The preservative is usually applied by floating it on 
the surface of the film for about a minute. Care should be 
taken that its strength is not diminished by too much water 
being left on the plate. In some cases it may be applied 
by immersing the plate in a flat dish or dipping bath con- 
taining it. There are some objections to this mode of 
application however. 

It will be convenient here to discuss the ends to be 
obtained by the use of a preservative, i. It must be an 
iodine or bromine absorbent, for without this quality the 
film manifestly might be insensitive. 2. It must be capable 
of filling up the minute pores of the collodion, so that on 
re-wetting after drying it may give access to the developing 
solution. 3. It must act as a protective varnish against the 
atmospheric influences. Regarding the first point there is 
not much difficulty, as nearly every organic animal or 
vegetable compound is capaUe oi comUxvm^ with iodine. 



Drying the Film, 95 

Under the head of absorbents we may rank tannin, pyrogal- 
lo^ gallic acid, gums, gelatine, albumen, caffeine, theine, and 
other like bodies. The second requirement may be met by 
the employment of some of the above, or by the addition to 
them of sugar in various forms. The last requirement is 
more difficult to meet, and is very often neglected, as it en- 
tails that the body should not be hygroscopic. The draw- 
back to any processes, for instance, in which the preservatives 
contain gum arabic, is that moisture is attracted, and the sen- 
sitiveness of various parts of the plate is affected. No better 
varnish is known than albumen, though this has its disad- 
vantages as regards rapidity, unless the greater proportion of 
it be removed previous to dessication, or unless it itself be- 
comes a vehicle for holding the sensitive salts, as in the 
collodio-albumen process. In the writer's opinion an un- 
exceptionable preservative has yet to be found. It appears 
dubious whether it will not become advantageous to dis- 
pense with it altogether, when the balance between the 
P)nroxyline and sensitive salts is properly adjusted, as in the 
case of emulsion plates. It must then be borne in mind 
that the word * preservative ' is only employed for want of 
a better. 

Drying the Plate, 

Ordinarily speaking the film is allowed to dry spontane- 
ously, for which purpose a cupboard or box should be fitted 
up in the manner described in the various hand-books. 
Another plan that may be adopted by the student, if the 
plate be not too large, is the use of the hot-air bath, employed 
in chemical laboratories. The author has found that up to 
8^ X 6 J inches this method is useful. It is found convenient 
to allow the doors to be left open till the surface moisture 
has disappeared, after which they may be closed and the 
plates be allowed to dry at the higher temperature. Half 
a dozen plates may be dried by this means in half an hour. 



g6 Collodion Processes — Dry Plate. 

Development 

It is usual to apply a narrow edging of india-rubber 
solution round the plate by means of a piece of stick, or by 
a piece of blotting-paper held in the fingers and run round. 
A prettier piece of apparatus which is sometimes employed 
to give the edging is the following : — A camel's -hair brush, b, 
is held in position by a couple of wire loops, c c, 
Fig. 19. inserted in a stick, a, and so arranged that the 
brush may be lowered into the solution without 
wetting A. The bottom edge of the stick is 
^jB brought against the edge of the plate, so that the 
®Hi brush rests on the film, and having drawn round 
the plate, a neat edging is given. The strength 
of the india-rubber solution may be five times 
that given on p. 92. There are two methods 
of developing dry plates. The general princi- 
ple of one is the same as that given for the 
wet process, though modifications must neces- 
sarily arise from the absence of silver nitrate solution on the 
film. The other is totally different, and may be employed 
when the sensitive film contains even the smallest propor- 
tion of silver bromide, being dependent on the fact that 
certain re-agents have the power of reducing silver sub-bro- 
mide and bromide silver to the metallic state. . It is gene- 
rally admitted that iodide is unsuitable for this method of 
development, though it is kno^^n that it may be made 
amenable to it. The re-agents employed are invariably 
those compounds which have a strong affinity for oxygen. 
We have seen that amongst these is pyrogallic acid, and it 
can also be shown by direct experiment that it also has a 
strong affinity for bromine when a body is present which 
will easily part with it. Gallic acid and other derivatives 
possess similar properties, which in all cases are powerfully 
increased by the addition to them of an alkali. 

The following experiments shoM\4 b^ maA^\yj \.\\^ student 



Alkaline Development. 



97 



Fig. 20. 



that he may become acquainted with all the phenomena 
connected with this class of development. 

1. Precipitate pure bromide of silver, say 4 grammes, 
and wash thoroughly ; then place it at the bottom of a test 
tube, and cover it with a solution of pyrogallic acid (about 
•3 gramme to the 100 cc), to within a short dis- 
tance of the top. Having drawn out from \ inch 
tubing a fine funnel, let him place the end drawn 
out just above the bromide, and then pour into 
the funnel 3 or 4 drops of strong ammonia. It 
will be seen almost immediately that a black 
layer forms above the bromide, that the silver is 
reduced, and that the action continues for a 
certain time, and then stops. The blackening 
of the liquid will be found to be due to the 
alteration in the pyrogallic acid, consequent 
on the absorption by the alkali of the bromine 
abstracted from the bromide. 

2. Repeat the experiment, replacing the py- 
rogallic acid by dilute ammonia (say, -880 sp. gr. 
diluted with 10 times its bulk of water), and drop 
into the funnel a small quantity of strong pyro- 
gallic solution. The phenomena presented will 
be slightly different. A cloud will instantly form 

in the ammonia, and if the surface of the bromide has 
been protected by a small diaphragm of paper, the whole 
of the solution may be poured off, and the surface of the 
bromide will be found almost unchanged. The difference 
is caused by the solubility of silver bromide in ammonium 
hydrate ; and the portion held in solution is consequently 
more readily reduced than that remaining in the solid 
state. 

3 and 4. Repeat these two experiments, substituting 
potassium hydroxide (caustic potash) for the ammonium 
hydrate. The phenomena presented in both cas^^ -mVV \i^ 
identical; the silver bromide will be reduced itom ^^ ^<^\^ 

H 



98 Collodion Processes — Dry Plate. 

state. This is because silver bromide is insoluble in the potas- 
sium compound, 

5. The next experiment should be to dissolve a small 
quantity of silver bromide in ammonia, and then drop into 
the solution a small quantity of potassium bromide solu- 
tion. It will be found that a precipitate is immediately 
caused, which, on analysing, will be found to be silver 
bromide combined with ammonia. 

6. Repeat the foregoing experiments with silver iodide, 
first with weak solutions, and then with concentrated. It 
will be found that the concentrated will act upon the iodide, 
whilst the weaker will not The iodide not being soluble in 
ammonia, the phenomena described in experiment 2 will not 
be observable. 

Reviewing these experiments, we are led to the following 
conclusions : — that the alkaline pyrogallates have such an 
afilinity for the halogens that they are capable of breaking 
the bond existing between the silver and the bromide ; that 
the portion which is soluble in the alkali is most readily 
acted upon ; and that the addition of a soluble bromide 
diminishes the amount capable of being dissolved. Con- 
sequently, if a surface of silver bromide were treated with 
ammonium pyrogallate, we should expect that the undis- 
solved bromide was less readily reduced than that portion 
which is dissolved, and that less immediate reduction would 
take place when a soluble bromide, such as that of potas- 
sium, were present. 

In practice this is found to be true ; and there is this 
further peculiarity observed, that the sub-bromide is more 
easily reduced than the bromide. Now the invisible image 
is formed of this sub-bromide, and, if ammonia be the alkali 
employed, we have first a reduction of this sub-bromide 
to the metallic state (AggBr = 2 Ag + Br) ; a small quantity 
of the bromide is dissolved, and the metallic silver reduced 
from it is immediately precipitated on the image thus formed. 
Additional strength is also given to ftve \ma%e from the feet 



Alkaline Development, 99 

that the bromide, immediately in contact with the pri- 
marily reduced silver, is more easily reduced than that not 
so situated. It may be that freshly reduced silver is capable 
of combining with the bromide, to form fresh sub-bromide 
(Ag + AgBr = Ag2Br). At any rate, an action of some- 
what this description takes place in the film. An interest- 
ing experiment is confirmatory of this. Take an ordinary 
dry plate, such as an albumen-beer plate, and expose it 
in the camera. Coat half of it with a bromide emulsion, 
and develop it by the alkaline method. That part coated 
with the bromised collodion will be found to acquire den- 
sity. When dry, remove the film from off the glass plate 
with gelatinised paper, and also cause the adhesion of a 
similarly prepared gelatine paper to the surface primarily 
next the plate. When nearly dry, the exposed film can be 
split off from the bromised film; and on examination it will 
be found that there is an image on both films. If the sensi- 
tive salt in the collodion film exposed in the camera be 
iodide, an image may be developed, though it will be weak. 
The fact remains then that this action takes place, even 
though the films be separated by a very thin layer of albumen. 
It will also be apparent that the image will be stronger when 
developed with ammonia than with potash, for with the 
former the silver can be deposited from the solution. The 
writer has been able to intensify images from alkaline or 
neutral solutions of sodium hyposulphite and potassium 
cyanide, in which have been dissolved silver chloride, by 
this action of pyrogallic acid. The soluble bromide seems 
to play a twofold part with ammonium hydrate. It renders 
the silver bromide less soluble, and, as is known, forms a 
double bromide with that of silver. With potassium hydrate 
its only effect is to form the double bromide ; it prevents 
the solution of too large a quantity of the silver salt The 
combination between the soluble bromide and the sub- 
bromide is perhaps less certain ; a combination with a ^att 
of the hitteT may take place, and pi6ba)o\'^ ^o^^^V-^xci-s^ 

H 2 



ipo Collodion Processes — Dry Plate, 

outside it one of the atoms of silver. The mere fact that 
the metallic silver is in proximity to this new molecule pro- 
bably determines the reduction of the latter to the metallic 
state, its attraction upsetting the tottering equilibrium which 
exists for a moment after the developer has been applied. 

It should be noted that the same treatment of the bro- 
mide is effective when gallic acid is employed instead of 
pyrogallic, the power of reduction of the former being smaller 
than that of the latter. This fact proves that with a weakly 
formed invisible image the latter should invariably be used. 

In the silver bromide emulsion plates, for which this 
development is particularly adapted, it will be noticed that 
in order to obtain pictures which are free from veil or fog, 
one of three conditions is necessary — either there must be 
a little soluble bromide in the film, or else if there be an 
excess of silver nitrate there must be some free mineral acid 
or silver chloride formed by the decomposition of some 
metallic chloride present. 

These conditions are apparently conducive to the for- 
mation of bright images. It will be profitable, however, to 
consider the probable cause of this. Bromide of silver is 
usually formed by the double decomposition of silver nitrate 
and soluble bromide, such as a proportion of potassium with 
that of cadmium. Cadmium and other dyad metals may form 
two bromides, the ordinary bromide and a sub-bromide. In 
ordinary circumstances the latter compound is found in 
very small quantities, but when it comes in contact with 
silver nitrate, a sub-bromide of silver, or, otherwise, bromide 
of silver with unattached atoms of metallic silver, is formed. 
When the soluble bromide is in excess, these molecules — 
supposing them to be half molecules of silver bromide together 
with the attached atoms of silver — readily attract the excess of 
soluble bromide ; and, when these atoms of excluded silver 
lose their nascent power of attraction, they become in- 
capable of causing a reduction of the neighbouring bromide 
when a developing solution is applied, li, Vvow^ver^ these 



' Alkaline Development loi 

sub-bromide molecules remain as such, the action of the 
reducing agents is to attack these first, and the reduced 
silver exerts its power in determining the reduction of the 
neighbouring molecules; in other words, causes reduction 
where light has not acted. When there is free nitric acid 
which can act on the molecules, we have another action 
taking place. The nitric acid is capable of reducing to the 
state of bromide the small number of molecules of sub- 
bromide which may exist, by converting the loose atoms of 
silver into nitrates. Thus, AgaBr is decomposed into AgBr 
and AgNOa ; or, if the nitric acid be applied to the soluble 
bromide, such as Cd, before its contact with the silver nitrate, 
we probably have a precisely similar reaction. This explana- 
tion is founded on experiments which have lately been pub- 
lished.^ The addition of certain metallic chlorides seem to 
produce a similar result, preventing the formation of Ag2Br. 

As touching the strengths of the solutions to be employed, 
I St. The stronger the ammonia, the greater the amount 
of the silver bromide that is dissolved. If the amount dis- 
solved be in excess of that necessary to supply a gradual 
aggregation of silver on those parts on which a deposit has 
already taken place, the result must be a veil on the whole 
surface. 

2nd. The stronger the pyrogallic acid solution in the 
presence of sufficient alkali, the more rapid will be the re- 
duction of the silver bromide; hence a strong solution is 
very likely to cause fog even if sufficient soluble bromide be 
present. 

3rd. The stronger the soluble bromide solution, the less 
silver bromide is dissolved by the ammonia, but at the same 
time the effect produced by light is in a great measure can- 
celled, owing to the formation of a compound with the 
sub-bromide, which is as little acted upon by the developer 
as the bromide itself; hence we are limited as to the amount 
of bromide that should be supplied if we want an intense 

* See yffur»a/ 0/ FAotographic Society of Great Britain, V^'^l- 



102 Collodion Processes — Dry Plate, 

image, and one which is easily developable. To fix, then, a 
developer, we should start with the maximum amount of 
alkaline bromide allowable in a solution, and from that 
build up the maximum amount of ammonia and p)n:ogallic 
acid admissible. 

In comparing the two foregoing methods of development, 
it is necessary to consider the different circumstances in 
which they may be employed. The first method, it may be 
said, is invariably adopted with wet-plates, for everything that 
is conducive to success is then present There is solution of 
silver nitrate on and in the film ; the unbonded atom of silver 
in the subsalt is in a state possessing the greatest attractive 
activity, and development must take place shortly after 
exposure. If the second method were adopted, all the 
silver nitrate would have to be eliminated from the film by 
prolonged washing ; in practice it is found that the result- 
ing image is liable to be of unequal density. Again, the 
proportion of bromide to iodide in the collodion is small, 
and as the iodide is only affected by intensely strong 
alkaline developers, the chance of veiling the image through 
the reduction of the bromide unacted upon by light is in- 
creased, if sufficiently strong solutions are employed to 
cause the reduction of the sub-iodide. 

With dry processes the advantages rest with the second 
method. The development takes place hours, or even 
months, after exposure ; consequently it is quite possible that 
the free atom of silver in the sub-iodide or sub-bromide has 
partially lost its nascent energy, and that when the free sil- 
ver nitrate, together with the developing solution, is applied 
to the surface of the film, as in the first method, the inten- 
sity of its attraction is diminished. The same amount of 
attractive power may be obtained by increasing the number 
of molecules acted upon by light ; hence what in the 
wet process would be a sufficient exposure, in the dry may be 
totally inadequate. Coming to the second method, how- 
ever, the result differs. Even if the svlvti atom of the sub- 



Dry Plate Developers. 103 

bromide be partially saturated, the agents employed will 
still naturally first attack the remainder of the molecule by 
taking up tiie bromine, and liberating the other atom ot 
silver. Now this liberation takes place in close contact 
with another atom, and as the attraction varies inversely 
as the square of the distance, a much less attractive force is 
necessary in order to draw the liberated atom to its partially 
saturated neighbour. The atom once in situ attracts the 
other depositing atoms, and an image is rapidly built up. 
As a matter of fact it is found that the alkaline development 
causes a gain of 4 or 5 times in the matter of shortening 
exposure, and is therefore specially applicable to all dry 
processes, particularly where a large quantity of bromide is 
dissolved in the collodion. 

The following are formulae for the different developers : — 



1. Ferrous sulphate . 
Water 

2. Gelatine 
Glacial acetic acid 
Water 



2 grammes 
30 cc. 

4 grammes 
60 cc. 



400 cc. 

The gelatine may be dissolved in the water, and the 
glacial acetic acid added to it. This is quite as effective, 
dissolving the gelatine first in the acetic acid, and the solution 
is much more quickly made. Three parts, by measure, of No. i 
should be mixed with i part, by measure, of No. 2, and 
after filtering the developer is ready for use. It is better to 
mix them only a short time before they are required, as a 
slight precipitation takes place if they be kept long together. 
To every 4 cc. of the developer i drop of a 60 per cent, 
solution of silver nitrate should be added, and the applica- 
tion be immediately made to the plate. 

The following is a pyrogallic acid developer, which is 
used in many dry-plate processes : — 

I. 

I. P3rrogalIic acid . . . .10 grammes 
Alcohol . , . , . 60 cc. 



104 



Collodion Processes^ — Dry Plate, 



2. Silver nitrate 
Citric acid 
Water 



4 grammes 
4 grammes 
85 cc. 



When required for use, i cc. of No. i is added to ^ cc. of 
No. 2, and 30 cc. of water added. 

Of the alkaline developers the following are those usually 
employed : — 



II. 



I. Pyrogallic acid , 
Water 



2. Ammonium hydrate, 
Water 



3. Citric acid . 
Acetic acid (glacial) 
Water 

4. Silver nitrate 
Water 



880 



I gramme 
40 cc. 

1 part 
4 parts 

4 grammes 

2 cc. 
30 cc. 

I gramme 
20 cc. 



These formulae are useful when developing certain dry 
plates containing colloid substances, such as albumen or 
gelatine. The details of the development will be given in 
describing the albumen-beer process. 



III. 

I. Ammonium carbonate . 
Water .... 



Or 



Ammonium hydrate, '880 
Water. 

2. Potassium bromide 
Water. 



3. Pyrogallic acid 
Alcohol 



5 grammes 
30 cc. 

I part 
16 parts. 

I gramme 
35 cc. 

I gramme 
5 cc. 



A more recent developer, which possesses certain advan- 
tages, is as follows : — 



IV. 



I. Potassium hydrate 
Water , , , 



. I '5 gramme 
. 10 cc. 



Intensifying the Image. 105 

2. Potassium bromide ... 4 grammes 
Water 100 cc. 

3. Pyrogallic acid .... I gramme 
Water 30 cc. 

Nos. III. and IV. are known as the strong alkaline de- 
velopers. (For details as to their employment, see p. 118.) 
They are employed with a bromised film produced in the 
bath, or by the method of emulsion as will be described. 

When solutions to give increased density of deposit are 
necessary, that given at p. 71 may be adopted without 
modification. Intensity may also be given to the image by 
adopting the method given at page 73. 

Though many recommend the attainment of sufficient 
density by the application of the alkaline developer, and 
indeed recommend plates, because such opacity can be 
gained in that manner, yet it is believed that silver deposited 
by the usual intensifying process will give a greater delicacy 
of half-tone. Perhaps the reason of this may be com- 
prehended if an experiment be tried on a thin bromide 
emulsion plate. Let it be properly exposed, and developed 
solely with the alkaline developer. It will be found that 
the thickness of the film is not sufficient to give such a 
quantity of reduced silver as to render the highest lights 
nearly opaque, and if the alkaline development be continued, 
the lower lights will have only sufficient opacity when they 
are of the same depth as the former. With another 
similarly prepared and exposed plate, let a defined image 
be brought out by the alkaline method, and then intensified by 
the ordinary process. It will be found that the image is perfect 
in gradation, the highest and lowest lights being rendered in 
as good a scale of opacity as can be obtained (see Acti- 
nometry). 

The ordinary solutions, as given at p. 75, are to be 
used for fixing, a preference being given to hyposulphite, if 
the plate be subjected (subsequent to its application) to a 
borough washing; 



io6 Collodion Processes — Dry Plate. 



CHAPTER XV. 

GUM-GALLIC PROCESS. 



Of dry-plate processes, only two will be described in 
detail; descriptions of others can be found in various practical 
works on the subject. The first that will be described is 
the gum-gallic process, as introduced by Mr. R. Manners 
Gordon. His directions are given, and if carefully attended 
to will give negatives of unequalled harmony. 

To any ordinary collodion, 2 per cent, of cadmium 
bromide is added. The plate ha\'ing been given a sub- 
stratum, as shown at p. 92, is coated with this collodion 
and immersed in the sensitising bath, and allowed to remain 
in it from 7 to 10 minutes, according to the temperature. 
This length of contact with the solution is sufficient to 
allow most of the bromide to be converted into the silver 
compound. The washing should be of a thorough nature ; 
the longer the plates have to be kept, the longer it should be 
continued. The preservative made as under is next ap- 
plied, by floating it over the surface of the plate. 

I. Gum arable .... 7 grammes 



Sugar candy 
Water . 

2. Gallic acid 
Water . 



1*75 grammes 
120 cc. 

I gramme 
40 cc. 



These two solutions are mixed in the above proportions. 

No. I is best prepared by the aid of the heat of a water- 
bath. The following arrangement will be found usefiil in its 
preparation as well as in numerous other cases : — 

c is a water-bath two-thirds filled with water ; on the top 

are rings of varying diameter, fitting into one another, in 

one of which the flask a, containing the gum-water and 

sugar-csindy, is placed. A small funnel (b) is dropped into 

the neck of the flask, in the comcaX ^ax\. o^ >n\s\c1\ a portion 

of the steam condenses and T\ms\>ac^m\.o >^^ ^-a^, Ttt^ 



Filtering viscous Soluliotis. 107 

prevents too great a diminution of the liquid whilst the gum 
is in the act of dissolving. 





The mixed solutions should be Altered, but in this 
operation great difficulty is often found. The most ready 
method of effecting it is by the aid of a Bunsen water-pump ; 
by an aspirator of the usual form ; or by an exhausting syringe. 
The arrangement adopted will be seen from the accom- 
panying sketch. The pump, or other 
exhausting apparatus, &c., is attached 
to india-rubber tubing. It is preferable 
to filter the solution whilst warm ; when 
cold the pores of the filter-paper are 
rapidly filled up, and the solution re- 
fiises to pass. It may be necessary to 
fix into the funnel a platinum foil cone, 
made by cutting a piece of platinum foil 
in a circle, and cutting off a sector, as indicated in the 
annexed figure. In any case the filter-paper should be 
thin, and free from iron,' 

' Tbis may be delected by moistening it Tuith ^lyiiwAiowi v^. 
Mid letting fall on it a drop o( polassium feiro-cjanide. 




io8 Collodion Processes — Dry Plate. 

The preservative solution is floated over the plate, and, 
after remaining on about a minute, is allowed to dry. If the 
surface appear dull, it should be dried by artificial heat pre- 
vious to exposure in the camera. 

The exposure varies according to the different modes of 
development. If the gelatino-iron developer, given at p. 103, 
be employed, it should be at least 4 times that given to a wet 
plate, and if required to be kept iox a long time before being 
developed, 20 times is not too much. On the other hand, if 
the strong alkaline developer, as given on p. 104, be employed, 
the exposure may be reduced by three-fourths, that is to say, 
that a plate developed soon after exposure is as rapid as a 
wet plate, whilst if an interval elapse of six months or so 
before development, then 5 times the exposure necessary 
for a wet plate should be given. 

In order to develop the image the plate should be 
immersed in water of not less than 16° C. If the iron 
developer be employed the time of immersion should be 
two or three minutes, whilst if it be the alkaline developer 
No. III., page 104, a mere moistening is sufficient. The 
method to be adopted with the former is that already 
described. When the detail comes well out, the inten- 
sification of the image is given by the ordinary pyro- 
gallic acid solution (p. 71). These plates are excellent when 
employed in a fairly dry climate, but they are disappointing 
when much atmospheric moisture is present ; the gum softens 
and absorbs water, giving rise to spotty pictures. This 
seems to be due to a fungoid growth upon the gum, and 
there is no apparent remedy for it. 

The negatives taken by the gum-gallic process, under 
favourable circumstances, are everything that can be wished 
for, being delicate, full of detail, having the well-known 
bloom, and being fairly sensitive even with the iron de- 
veloper. In the hands of Manners Gordon it has proved 
the most trustworthy of any bath dry-plate process (except 
one) with which that eminent pViOlo^a^Vv^t Vva.s worked. 



Albumen Beer Process. 109 



ALBUMEN BEER PROCESS. 

This process has been fully described in another work by 
the writer,^ and is given as therein described. It was intro- 
duced by him for solar photography, and was employed by 
the English Transit of Venus expedition. It is, however, 
equally adapted for landscape work, and is very certain in 
its results. The collodion employed can be that described 
s-t p. 53, though for more rapid work the following is 

better : — 

Alcohol, "825 .... 450 to 350 cc. 



Ether 
Pyroxyline 
Ammonium iodide 
Cadmimn bromide 



. 350 to 450 cc. 
. 14 grammes 
. 3 '5 grammes 
. 9'0 grammes. 



The relative proportions of ether and alcohol are ad- 
justed according to the temperature in which the plates have 
to be prepared. 

With the ordinary samples of collodion the usual silver 
nitrate bath (p. 61) can be used, but with the collodion made 
as above it is advisable to use a bath containing 16 per 
cent, of silver nitrate. In both cases rapidity is increased 
by the addition of 2 per cent, of uranium nitrate. It has 
been found advantageous to dip the plates at first in the 
weaker bath, allowing them to remain in it for a couple of 
minutes, and then to transfer them to the stronger for ten 
minutes more. This mode of procedure gives very sensi- 
tive and opaque films, the greater part of the actinic rays 
being thus utilised. The sensitiveness, however, greatly 
depends upon the porosity of the film, and every effort 
should be made to attain the maximum of this quality without 
injuring the texture of the film. The addition of the largest 
practicable amoimt of water to the collodion tends to give 
this desired porosity. After sensitising the plate is slightly 
washed, and the first preservative applied, which is — 

* /ns^ruc^wn in Photography , Piper and CatVei, 



1 10 Collodion Processes— Dry Plate. 

Albumen lOO cc. * 

Water loo cc. 

Ammonium hydrate . . . I2 cc. 

This is beaten up into froth (or is mixed by pounding it 
in a mortar with silica), and, when settled, the clear liquid is 
decanted off. The solution is mixed, immediately ^ before 
use, with an equal quantity of ordinary beer or stout, and 
floated over the plate. When bottled beer is used, it is 
advisable to drive off all the carbonic acid by a gentle heat. 
The excess is drained off, and the film thoroughly washed 
under the tap for a couple of minutes, and is finally rinsed 
with a solution of plain beer, to which i per cent, of pyro- 
gallic acid has been added. 

The plate is dried in the ordinary manner. 

The. exposure with well-prepared dense plates is often 
as short as that necessary for wet plates, but great latitude is 
admissible. With 20 times the minimum exposure neces- 
sary to secure a good negative there need be no danger of 
veil. 

The development need not be effected for at least a 
month after exposure. The solutions as described in 
formula II., p. 104, are those employed. The following de- 
scription of the development is taken from the work already 
referred to. 

* The washing water before development should be of a 
temperature not less than 15° C. When the plate is washed, 
the following developer is employed : — ^To each 50 cc-. 
of No. I are added 10 drops of No. 2, and after well 
mixing with a stirring rod the solution is floated over the 
plate. 

* Almost immediately the image begins to appear, and after 
a few seconds^ interval the detail can be seen by reflected 

' Dried albumen — 5 grammes to loo cc. of water — may be substi- 
tuted for the 100 cc. of albumen. 

* This precaution is necessary, otherwise the tannin of the beer is 
precipitated by the albumen. 



Development of Albumen Beer Plates. 1 1 1 

light to gradually develop. Another 7 drops of No. 2 are 
again added to the solution, which is once more floated over 
the plate. Twenty drops of No. 3 are next poured into the 
developmg cup, and the solution from the plate poured into 
it. Again the plate is rinsed, this time by the acidified, pyro- 
gallic solution, and intensification given by the use of it with 
a few drops of No. 4. It is not advisable to allow too much 
detail to come out with the alkaline solution, but to allow a 
portion of it to be brought out by the subsequent treatment 
with pyrogallic acid and silver. The allcaline developer re- 
duces the bromide salt, and leaves the iodide to be attacked 
by the silver solution. It will be remarked that no restrainer 
such as bromide is employed ; the albumen dissolved by the 
ammonium hydrate plays the part of a retarder, but not as a 
destroyer of the latent image. When the image appears 
sufficiently dense, it is fixed by either sodium hyposulphite 
or by potassium cyanide.' 

There has been a good deal of dispute regarding the kind 
of beer to employ in the above process. Some photographers 
have recommended that all the chlorides of the beer should 
be precipitated by a preUminary dose of silver nitrate. This 
however, to the writer's mind, is a great mistake. The plate 
is at first only slightly washed, and all the chloride combines 
with the silver left in the film. After a subsequent washing 
the beer is applied again. This time the preservative is 
merely on the surface of the plate and not in the pores of 
the collodion, which have been filled up with the albumen ; 
hence the chloride in this case cannot interfere with the 
sensitiveness. Any beer will answer unless it contain very 
uncommon adulteration. It should, however, be free from 
carbonic acid, as carbonate of silver is not an easy sub- 
stance with which to work, tending to give markings and to 
cause fog on the surface of the negative. 

One point in the preparation of these plates cannot be 
too strictly attended to, viz. to keep the fingers away from all 
contact with the film during preparation. A touch, however 



i 



112 Collodion Processes — Dry Plate, 

slight, will cause a stain, and unsightly markings extending 
across the plate have been traced to the same cause. 



CHAPTER XVI. 

EMULSION PROCESSES. 



We now come to a class of dry-plate processes which 
differ from those which have been described, inasmuch as in 
them the sensitive salt of silver is held in suspension in the 
collodion. When such an emulsified collodion is poured 
upon the plate, we obtain a film capable of receiving an 
invisible impression. The emulsion is principally formed 
with silver bromide, though certain other additions are some- 
times necessary in order to ensure clearness in working. 
The silver bromide is introduced into the collodion by dis- 
solving some soluble bromide in it, and then gradually adding 
an alcoholic solution of silver nitrate, the amount of which 
may be either in defect or excess of that necessary for the 
complete conversion of the whole of the soluble bromide. 
It is found practically that silver bromide is most sensitive 
when exposed in presence of an excess of silver nitrate, but 
most prone to give veiled images, whereas if the soluble 
bromide be in excess, a developable image is formed less 
rapidly, but greater freedom from fog is secured. Some of 
the probable reasons for this may be gathered from Chap. 
XIV. In preparing an emulsion, it is rarely possible to 
hit the exact proportions which shall give neither excess nor 
defect in one or other of the emulsion-forming constituents, 
and so to secure great sensitiveness with clearness ; hence it 
is always better so to arrange the proportions that one or 
other shall be in known excess. 

The following experiments may be made with advantage, 
in order to see what will be the result of having excess of 
the soluble bromide or of silver nitrate. 



Experiments — The Emulsion Process. 113 

Prepare bromised collodion as given for the first pro- 
cess, to be described p. 115, and to 100 cc. add, in the 
first case, 6 grammes of silver nitrate ; and to another 100 cc. 
the quantities given. After leaving for twenty-four hours, 
coat plates with these two specimens respectively, exposing 
before the collodion has become dried. Note their be- 
haviour with the alkaline developer. No. Ill, p. 104. It will 
be found that with the plate in which there is excess of 
bromide there will be no developable image, whilst with 
that prepared with excess of silver nitrate there will be a 
fog dver the image. Next take plates prepared with the same 
collodions and wash thoroughly under the tap. Both now 
will give good developable pictures, but that having an 
excess of bromide will require a longer exposure to give a 
good negative. Next, take similarly prepared plates, and, 
after washing, flow over them a solution of tannin, and the 
images will be found to be more readily developable. Again, 
prepare an emulsion as before, using a soluble metallic 
chloride instead of the hydrochloric acid, and, having 
divided it into two portions, add an excess and defect of 
silver to them respectively. Prepare plates as above, and 
notice the behaviour. It will be found that with the 
slightest excess of silver there will be inevitable fog, whilst 
with the defect the behaviour will be the same as that 
given above. Perhaps the most sensitive emulsion may be 
prepared by having a slight excess of silver nitrate and 
nitric acid, omitting the chloride altogether. 

The use of silver chloride in the emulsion secures 
density ; it does not of necessity secure freedom firom fog, 
but, being more soluble in ammonia than the bromide, the 
ammonium pyrogallate readily dissolves it, and immediately 
precipitates it on the parts acted upon by light. It is 
believed that this simple explanation is capable of render- 
ing clear the use of it, as recommended by various writers. 
The following rules may be laid down : — 

I 



114 Collodion Processes — Dry Plate, 

I St. That nothing but silver bromide is necessary to give a 
good image, if the soluble bromide be in excess. 

2nd. That if there be an excess of silver nitrate, the 
emulsion must be acidified with nitric or other mineral 
acid, or be neutralised by certain metallic chlorides, to secure 
freedom from fog. 

With regard to the first part of this second rule, it will be 
remarked that the same necessity arises in the bath processes 
where much bromide is present in the collodion. It must 
also be borne in mind that if the first rule be followed, the 
density of the developed image will be strong ; whilst if the 
latter (unless chloride be present), it may be weak, unless 
some density-giving body, such as silver nitrite, glucose, 
&c., be added to the emulsion, in which case good density 
can be obtained. 

Having made an emulsion as described, it will be well to 
make another simple experiment Coat a plate with it, and 
allow it to dry. On drying, it will be found that the soluble 
salts have crystallised on the surface of the plate, rendering 
the development of an image almost impossible. Here we 
have evidence that it is necessary to remove these salts. 
There are two ways of accomplishing this, either by washing 
the plate after being coated with the collodion, or by 
washing the whole bulk of the emulsion after allowing it 
to gelatinise by evaporation of the solvents. In the last 
method the washed emulsified collodion is dried, and the 
resulting pellicle is again dissolved in ether and alcohol. 
This process is in much favour at present; it has certain 
advantages about it which cannot be gainsaid ; thus, the 
sole manipulation in getting ready a dozen dry plates is to 
coat them with the emulsion, and then allow them tP dry. 
It also has drawbacks ; one of the principal of which is the 
liability to spots on the negative, a point which is difficult to 
understand, since they probably will be entirely absent on 
phtes prepared with the same emulsion unwashed. It may 
jbe said that the present state of eicwAsioii processes is far 



Beechey's Process, 115 

more satisfectory than it was a couple of years ago. Much, 
however, still remains to be done, in order to give them 
that certainty in preparation and in resulting negatives 
which is so characteristic of some of the bath dry-plate 
processes. 

It is not proposed to enter into details of all the different 
varieties of the emulsion processes : two distinct variations 
will be given, one of which will be typical of an emulsion 
where the coated plate alone is washed, and the other of a 
washed emulsion. Both these will be of the simplest 
character, and have succeeded in the hands of the writer. 

Unwashed Emulsions. 

Canon Beechefs Process, 

The following are Canon Beechey's directions, which if 
followed will give tolerably certain results : — 

Take Cadmium bromide (dried) . . .90 grammes 
Alcohol, '805 I litre, 

and allow the mixture to stand, and then decant from it 
any quantity that may be required. To each 100 cc of it 
add I "6 cc. of strong hydrochloric acid. 

Of the above solution take . . 50 cc. 
Ether (-720) . . . . iiocc. 
Pyroxyline 2 to 2*5 grammes. 

The pyroxyline should be that prepared at high tempera- 
ture, and may contain nitro-glucose if thought advisable 
(see p. 48). It will be found necessary that it should stand 
at least a day before being used, filtering through tow only 
partially frees it from small particles of undissolved cotton. 
If much of the emulsion is likely to be required, one 
of the tall graduated glasses, as in the figure 24, will be 
found convenient, any quantity can then be syphoned or 
decanted off. 

When the collodion is clear it is leadj iox ^^\i€\>ai\s%\ 

I 2 



Collodion Processes — Dry Plate. 



Ii6 

that part which is lo be emulsified should be poured into a 
glass beaker. For every loo cc of the above, take 56 
grammes of silver nitrate and powder it carefully in an 
agate mortar, or by means of a glass stopper on a thick 
sheet of glass. Place it in a test tube, with just sufficient 
water to dissolve it, and add to it 30 cc of alcohol, -Sos. 
This alcoholic solution of silver nitrate should be added 
to the collodion (lig. 25) drop by drop, and the emulsion 




should be stirred continually whilst the additions are made ; 
finally, the test-tube should be rinsed out with another 
30 cc. of alcohol, and added to the collodion. 

After the final addition the emulsion should be veiy 
smooth and rather thick, though when poured upon a strip 
of glass it will appear of a transparent nature. After keeping 
twenty-four hours, however, it will be creamy, and appear 
of an orange colour, when a candle flame is viewed through 
small layers of the liquid. This colour is indicative of a 
proper prepajaXiQH of the emulsion, showing that the silver 
bromide is in a very minute staXe ot fimmnu. 



Beechey's Process, 117 

It is possible to cause an emulsion to have a decidedly 
bluish-green tint, in which case the particles seem to be in a 
different state of aggregation to that when the ruddy tinge 
is seen. When further improvements in the emulsion are 
made, it may be that this blue tint will be the mark of a 
more sensitive preparation. This emulsion should be used 
soon after the creamy state is attained, as otherwise it will 
again become thin, and the silver bromide will rapidly precipi- 
tate on the bottom of the bottle. 

The plate having been coated with a substratum or 
edging (see p. 92), the collodion is applied in exactly the 
same way as is the unemulsified collodion for the wet 
process. It is necessary that the emulsion should be ren- 
dered homogeneous, otherwise the film will appear granular. 
This is effected by shaking it in the bottle half an hour 
before it is applied to the plate. When the collodion is set, it is 
immersed in a dish of distilled water, or filtered rain water till 
all the repellent action between the solvents and the water 
is eliminated, and till the great excess of silver nitrate and 
the other soluble salts is washed out. It may then be passed 
through another dish of water if found necessary, and finally 
allowed to rest in a dish containing beer, to every litre of 
which 2 grammes of pyrogallic acid has been added. The 
best kind of beer is that known as sweet ale, the saccharine 
and gummy matter being more abundant in it than in that 
known as bitter ale. Any trace of silver which may remain 
in the film combines with the organic matter, and the danger 
of veil is thus reduced. The drying is conducted in the 
usual manner, care being taken not to disturb the plates till 
they are thoroughly dessicated. 

If the calculation as to the amount of silver nitrate neces- 
sary to combine with bromide and hydrochloric acid be 
made, it will be found that there is a considerable excess of 
silver nitrate in the above emulsion. The organic matter of 
the preservative is 'present to give intensity during develop- 
ment. 



Ii8 Collodion Processes — Dry Plate, 

The development is conducted by formula III. or IV. 
p. 104. With III. the following proportions : — 

No. I solution . . .4 parts 
,, 2 „ . . . 2 parts ( I part in cold weather) 
„ 3 „ . . .1 part. 

These are well mixed immediately before use, and after 
the plate has been moistened by water are flowed over 
it. If the exposure has been of right duration the image 
should immediately appear, in which case the solution should 
be flowed back into the developing cup, and the detail be 
allowed to * come up ' by the small quantity remaining in 
the film. When this is secured another part of No. 3 may 
be added, and density will gradually be attained. The 
writer prefers not to give the full density by the alkaline 
solution, but rather to gain it by the application of the 
pyrogallic acid intensifier with the silver nitrate solution 
(see p. 71). If this procedure be adopted the develop- 
ment should be stopped immediately all the detail is visible 
by reflected light, and the surface should be flooded with a 
I per cent, solution of acetic acid in water. The inten- 
sification next proceeds as in the ordinary bath dry-plate 
processes. 

With an imder-exposed picture, it the detail does not 
appear with the above proportions of the alkaline developing 
solution, a new mixture is made, nearly all of No. 2 solution 
being omitted. Unless the exposure be very much under- 
timed, this is usually efficacious. With an over-exposed 
picture the image will flash out ; the developer must at 
once be washed off, and a double amount of No. 2 added, 
or resort may at once be made to the acid intensification. 

With formula IV. (p. T05) the following proportions are 
taken : — 

No. I solution ... . .1 part 

„ 2 „ 9 parts 

,, 3 ,, 12 parts. 



Washed Emulsions, 119 

These are well mixed together and applied as if formula 
III. were in question. The image comes out rapidly, but 
at first is deficient in density ; by applying 6 parts more of 
No. 3 solution sufficient opacity of image may be gained 
without having recourse to the acid silver intensification, 
though, as before, the beauty and delicacy of the image is 
much enhanced by its employment An under-exposed pic- 
ture is developed by adding other 12 parts of No. 3 and \ part 
of No. I. It is not advisable in any circumstance to reduce 
No. 2 solution to less than 8 parts. 



CHAPTER XVII. 

WASHED EMULSIONS. 



We now come to a class of emulsions which are in great 
favour at the present time — washed emulsions. There are 
almost endless varieties of preparation, but experience seems 
to show that the simpler the formulae are kept, the more 
certain are the results. The following is a mode of pre- 
paration which has almost invariably given rapid and excel- 
lent results, and the writer strongly recommends it. 
The plain collodion is prepared as follows : — 

Ether, 730 50 cc. 

Alcohol, -820 to '830 25 cc. 

Zinc Bromide 5 gram|nes. 

Pyroxyline 2*5 to 3 '5. 

The variation in the amount of pyroxyline is given, as 
on its quality largely depends the amount which is essential. 
With ordinary pyroxyline the smaller amount will suffice, 
whereas, if it be of a short pulverulent class, the larger 
quantity will be necessary. The writer recommends the 
ordinary tough pyroxyline, prepared from ordinary cotton 
previously boiled in strong alkali, and m Xiv^ ^\.\^w^ <S^ 



1 20 Collodion Processes — Etnlulsions. 

acids given at p. 45. The zinc bromide may be dissolved 
in the alcohol, with a small amount of water in addition, if 
found necessary. To the above quantity of zinc bromide 
should be added about 30 drops of nitric acid, or i small 
drop of bromine. The reason for either of these additions 
has already been given. If the bromine be employed, care 
should be taken to estimate the quantity of silver nitrate 
with which it will combine. This may be conveniently 
executed by dropping, say 3 drops into 50 cc of water, and 
precipitating with a standard solution of silver nitrate, or 
by taking care to have an excess of silver filtering, washing 
the precipitate, and gently igniting it, in order to convert 
the silver bromate into silver bromide, and then weighing it. 
When the bromised collodion is perfectly clear from all 
floating particles, which can be secured by allowing them to 
settle, or by filtering them through cotton which has 
been previously well washed and rinsed with alcohol, it is 
ready for the addition of the silver nitrate. It is well to 
allow an excess at least of \ per cent, of the silver salt if 
great sensitiveness is required, otherwise the bromide may 
be allowed to be slightly in excess. To convert the above 
amount of zinc bromide into silver bromide would theoreti- 
cally require 7*56 grammes, but in practice it is found that 
this amount cannot be depended upon. When nitric acid 
is, used with the zinc bromide it will be found that 8*5 
grammes suffice. When the bromine is used the amoimt 
required must be subject to experiment The student may 
find it convenient to add the silver nitrate solution little by 
little till he hits the point where an excess commences, and 
then to add ^ per cent, more of silver nitrate. To ascertain 
when the excess occurs, a drop of the emulsion, from time 
to time between the additions of the silver nitrate solution, 
should be poured on to a glass plate, and a little potassium 
chromate dropt on to it ; a red colouration due to silver 
chromate shows the slightest excess. 

The silver nitrate is dissolved up as m the last process, 



Preparation of Emulsion, I2i 

and poured in as already described. It may be as well to 
note that finer-grained emulsion is sometimes made by 
keeping out half the collodion, adding the whole of the 
silver little by little, and then stirring in the other half of 
the collodion. 

In order to obtain a maximum sensitiveness, the emul- 
sion should be left for fi-om 24 hours to 60 hours, the time 
depending much on the kind of pyroxyline employed. If 
a large batch of emulsion is to be made up, it may be 
advisable to prepare 50 cc first, and at the expiration of 24 
hours, 48 hours, 60 hours, to wash it as directed below, 
and test its qualities ; and a note should be made of the 
period at which it seems most sensitive, and at the same 
time fi"ee fi"om fog. It is noteworthy that the washed emul- 
sion usually appears to possess the same qualities as the 
unwashed. If, therefore, this process of testing be con- 
sidered too tedious, the emulsion may be tested at intervals 
in the unwashed state, or, to speak more correctly, after it 
has been washed after coating the plate. When the emul- 
sion is in a proper state, it should be poured out into a flat 
dish, and be allowed to set A gentle agitation with a glass 
rod causes more surface to be exposed, and the evaporation 
consequently takes place more rapidly. In a moderately 
warm room half an hour will generally suffice to render it 
in a condition fit for the subsequent washing. This may 
be known by the glass rod separating the gelatinous mass 
in flaky pieces, which retain their shape after a minute's 
interval. 

The emulsion is next covered with distilled or pure 
water, and allowed to soak for 2 or 3 minutes, when the 
liquid is drained ofl*, and the emulsion transferred into a 
jar, and again covered with water. After stirring well, and 
allowing a time to elapse (say 15 minutes) in order that the 
water may penetrate into the interior of the mass, it is 
again drained off" and replaced by fresh water. This wash- 
ing is continued tiJJ iht wash water, titaX'^^ m^ V^^^- 



122 Collodion Processes — Emulsiofts. 

chloric acid, if an excess of silver have been employed, or 
with silver nitrate solution, if excess of bromide have been 
employed, shows only a slight opalescence. It is important 
that the washing be done rapidly, as long soaking of the 
gelatinous emulsion is greatly detrimental to securing den- 
sity of the image when alkaline development is employed. 
The reason of this lack of density seems to be due to the 
fact that some small portion of the precipitated pyroxyline 
is soluble in water, and that it is this organic substance 
which causes the silver to be reduced in the film to such a 
state of subdivision as to cause opacity. 

When the washing is complete, the emulsion pellicle is 
pressed between blotting paper in order to get rid of the 
greatest part of the water which is occluded in it, and is 
allowed to dry spontaneously or by the aid of heat. 

The first mode of drying is the safer, as it sometimes occurs 
that the heat of even a water oven, fig. 26, is sufficient to 
shrivel up the pellicle to such an extent that it becomes 
very insoluble in a mixture of ether and alcohol. The pro- 
cedure that the writer recommends is to allow it to become 
nearly dry in the water oven, and then to allow the last remains 
of water to evaporate spontaneously. A slight quantity of 
water is not hurtful to the emulsion, and if, after hard pres- 
sure with a spatula on a square of the pellicle, there is not 
sufficient moisture to damp blotting-paper, it may at once 
be transferred to a bottle to re- dissolve. The bottle em- 
ployed should be capable of holding twice the amount of 
solvents that will be used, as space is required for shaking. 

The solvents employed are equal parts of pure ether and 
absolute alcohol, 100 cc. being employed for every i| 
grammes of pyroxyline employed. 

With some pyroxyline the resulting images are deficient 
in vigour. To correct this, to the first wash water a strong 
solution of tannin, or salicine, &c., may be added. 

A modification of the above emulsion may be prepared 
by emulsifying with an excess (^say $ ^raxwKv^s^ of silver 



Drying the Plates. 



123 



nitrate, after 15 to 20 drops of strong nitric acid added to 
each 100 cc. of the collodion. Afterthe addition of the excess 
of silver nitrate a sufficient quantity of some metallic chloride, 
such as of cobalt, may be added, in order completely to 




neutralise that excess of silver, and to leave a slight excess 
of the soluble chloride. This method is due to Mr. Newton, 
and in his hands appears to work satisfactorily. The presence 
of a free chloride is not so destructive of sensitiveness as the 
free bromide, hence the preference that \s iweo. Wi ■&* 



124 Gelatino-bromide Process, 

former over the latter for neutralising any excess of silver 
nitrate. It is often useful to keep the pores of the col- 
lodion open by a little resinous matter, such as gum am- 
moniacum. This gum is very insoluble, and, if employed, 
a saturated solution of it in alcohol should be prepared, 
and the resulting varnish should replace the alcohol, em- 
ployed for re-dissolving the emulsion pellicle. With all the 
washed emulsion processes the plate is coated as with 
ordinary collodion and allowed to dry, no preservative being 
necessary. The dark heat which is radiated from a slab of 
iron, placed over a spirit lamp or Bunsen burner is recom- 
mended by Mr. Woodbury to cause the rapid evaporation of 
the solvents from the coated plate. The plate must not be 
exposed to the naked flame from these sources, as the blue 
colour is sufficiently intense to cause a veil to spread over 
its surface on development. Washed emulsion plates will 
keep indefinitely both before and after exposure, as will the 
emulsion if all excess of silver nitrate be washed away. The 
exposure necessary is largely dependent on the presence of 
soluble bromide or chloride and on their quantity. As a rule 
the plates require half as much exposure again as a wet plate. 
The development is conducted as laid down for the pre- 
vious process, and calls for no especial remark. 



CHAPTER XVIII. 

THE GELATINO-BROMIDE PROCESS. 

In this process we come upon another type of the emul- 
sion processes, and one which in some cases gives excessively 
rapid plates. The medium used for holding the sensitive 
salts in suspension is gelatine, which gives very valuable 
qualities to the image. It seems, however, to be doubtful 



Kennetfs Process, 125 

whether the emulsion can be prepared with an excess of 
silvernitrate, without incurring certain subsequent drawbacks. 
It is better to use a formula which has an excess of 
bromide, and an emulsion thus prepared will furnish plates 
which at all events are as rapid as a wet plate. The 
following is the method of proceeding in preparing the 
plates, as given by Mr. Kennett : — \ 2l kilogramme of 
Nelson's gelatine is soaked in 3 litres of distilled water, 
and after it is completely hydrated — known by all hard- 
ness having disappeared — the jar containing it is heated till 
solution is effected. It may be noted here that it has 
been recommended by the Rev. J. Palmer to substitute 
mild ale for half the quantity of water, but owing to the 
saccharine matter in it, a decomposition is apt to set in 
which destroys the plates when prepared. While the 
solution is still hot, 280 grammes of potassium bromide are 
added and thoroughly incorporated ; 380 grammes of silver 
nitrate are next dissolved in the smallest possible quantity 
of water, and added, with constant stirring, little by Httle 
to the bromised gelatine. This may be done in a subdued 
light, absolute exclusion from actinic rays being preferable, 
though not absolutely necessary. All subsequent opera- 
tions must^ however, be conducted in the dark room. The 
emulsion is next poured out into a dish and allowed to set 
firmly, after which it is cut into strips and washed in a jar 
with constant changes of water for six hours or more. 
When the water shows no milkiness on the addition of 
silver nitrate, it is to be presumed that all the soluble salts, 
including the excess of potassium bromide, have been washed 
out. If the gelatine be in very thin layers it may be 
allowed to dry spontaneously ; the aid of heat is necessary, 
however, if it be in thicker layers. In this pellicular state 
it will keep indefinitely. Mr. Kennett has obtained a patent 
for drying the gelatine emulsion by the aid of heat, and it need 
scarcely be said that the supply issued in this form is very 
convenient. In every 100 cc. of "walei, 9 ^^trasie.'Sk <^\ '^'^ 



126 



GelatitKhhromide Process. 



pellicle may be dissolved, by allowing it to soak and then 
by applying heat. A plate which has been properly cleaned 
and then slightly wanned is flowed over with the solution 
as if with collodion, and, after the gelatine has set, is placed 
on a level shelf or table to dry. In the last operation failure is 
most to be apprehended. Therie is always such a quantity 
of dust in the air, of greater or less size, that the danger of 
specks is really great. Perhaps the best method of avoiding 
dust is to cover the plates with a framework over which is 
stretched a piece of close muslin ; this filters out the larger 
J)articles of dust. These covers also have been found effec- 
tive in preparing gelatine plates for the mechanical printing 
processes. When the excess of moisture has been evaporated 
by the air, the plate is finally dried in a drying closet, the 
operation being accomplished in two or three hours. The 
exposure required, as already stated, is about equal to that 
of a wet plate. 

The following developing solutions are recommended by 
Mr. Kennett, of Maddox Street, who prepares these plates 
commercially : — ■ 



I. Pyrogallic acid . 
Distilled water . 




« 


I gramme 
. 100 cc. 


2. Potassium bromide 
Distilled water . 






5 grammes 
. 100 cc. 


3. Potassium bromide 
Distilled water . 


» < 


» 1 


.4 gramme 
I litre. 


4. Ammonia (-880). 
Water 


• 


• 
1 


I part 
16 parts. 


$. Gelatine . 
Distilled water . 




• 
• 


4*5 grammes 
I litre. 



The plate is first allowed to soak in No. 3 for five minutes, 
the temperature being kept about 1 5® C. To this solution in 
the dish is next added about 10 per cent, of No. 5, and after 
thorough soaking the plate is drained. To every 50 cc. of No. 
1, J cc. of No. 2 and 3 cc. of No. 4 are added, and flowed 
over the surface. The detail shoviVd ^adwalVj make its ap- 



Development, 1 27 

pearance, and, when well out, more, in equal parts, of Nos. 2 
and 4 should be added till sufficient density is obtained. In 
case this still leaves the ' whites ' of the negative deficient in 
opacity, the ordinary pyrogallic acid intensifying solution 
(p. 71) may be resorted to. The above development is 
efficacious when the plates are of the extra rapid kind supplied 
by Mr. Kennett ; if they are prepared exactly as given above, 
the preliminary soaking in No. 3 and No. 5 should not be 
carried out, but distilled water should be substituted for 
them. The fixing solution recommended by Mr. Kennett 
is the hyposulphite solution (p. 75). The cyanide can be 
equally well applied. The great secret of success seems 
to be in keeping ther- temperature of the washing water 
at about 15° C, and in giving the plates sufficient soaking 
previous to appl)dng the developer. The films do not seem 
to be affected after fixing by ordinary moisture, a change 
in the structure of gelatine apparently taking place. The 
image given by this process is generally of an olive green, 
which is highly adiactinic. 

The ordinary yellow glass of the developing room does 
not cut off sufficient of the actinic rays to afford complete 
protection against the veiling of these plates ; but the light 
passing through flashed ruby glass is found to be of a 
sufficiently adiactinic character to be a suitable light in 
which to conduct the developing operations. 

This process is well worthy of trial, particularly where 
rapidity of exposure is a desideratum. 



128 Calotype Process, 



CHAPTER XIX. 

CALOTYPE PROCESS. 

Had the historical order of photographic processes been 
followed, the calotype process would have been described 
immediately after the Daguerreotype process, but it seemed 
more likely that the details of the former would be better 
understood after a study of the collodion wet and dry 
processes. The original process which Fox Talbot intro- 
duced has been but little improved, and it is therefore given 
nearly as he described it, modifications being suggested where 
necessary. 

The paper employed should be as tough and grainless 
as possible, capable, however, of holding sufficient of the 
sensitive compoimd to give a body to the image. Good 
English paper of the consistency of medium Saxe answers 
every purpose. The great drawback to all papers of the 
present day seems to be the chance of transparent spots 
appearing during development, and a consequent damage 
to the image. What is the chemical nature of these spots 
is not known, but they can generally be got rid of by brush- 
ing a dilute solution of hydrochloric acid over the surface 
of the paper, and then thoroughly washing off all excess of 
acid. When dry the paper is ready for impregnating with 
silver iodide. This last is formed by taking — 

No. I. Silver nitrate .... 3 grammes. 
Distilled water . . . . 20 cc. 

„ 2. Potassium iodide . . .3 grammes 
Distilled water . . . . 20 cc. 

No. 2 is poured into the solution of No. i with constant 

stirring, and a precipitate of silver iodide is formed. The 

potassium iodide being in slight excess, a certain quantity 

of the silver iodide is held in so\m\.\oxi. Tl^a precipitate is 



Sensitising operations, 129 

allowed to settle at the bottom of the glass measure (in 
which we will suppose the two solutions to have been mixed) 
and the supernatant liquid is poiured off, water is again added, 
and after stirring it is again poiured off. This operation of 
washing is continued some three or four times, or until the 
soluble potassium nitrate is nearly eliminated. 

The silver iodide is next dissolved in a solution of potas- 
sium iodide. 

Potassium iodide . . • 30 grammes 
Water 60 cc. 

This is poured on the silver iodide and well stirred. 
As this quantity will not effect complete solution, crystals 
of the potassium salt must be added till after much stirring 
the solution is semi-transparent or milky. 

The paper is next cut to a convenient size, and is 
pinned on a flat board. The solution is applied by a brush 
of cotton-wool, a good form adapted for the put- fig. 27. 
pose being given in the figure, a is a glass tube / 

of about 20 centimetres long, and above i centi- ^ 
metre diameter. A loop of string, b, passes through 
the tube, across which is placed a thin tuft of cotton 
wool, c. The loop is then pulled up into the tube, 
a sufficiency ofcotton wool being allowed to remain 
externally to form the brush. It is advisable first to 
wash the wool in a weak solution of alkali in water, 
taking care, however, that none of the alkali re- 
mains in the fibre, and that it is thoroughly dried before 
being used as a brush. 

The solution is brushed up and down and across the 
paper, till the whole surface has received a uniform coating 
of the dissolved iodide. When partially dry the paper is 
immersed in a dish of distilled water, all air-bubbles being 
carefully removed from the surface. After soaking for a 
couple of minutes it is removed to a second dish, and sub- 
sequently to a third dish. The water Temo\^?» \!ci.^ ^Ck\a&- 

K 




1 30 Calotype Process. 

sium iodide, and leaves a primrose-coloured silver iodide on 
the surface of the paper. After the washing has been con- 
tinued two or three hours, the paper is hung up and (dried. 
In this state it is nearly insensitive to light (though not quite 
so, as the iodide is in the presence of organic matter), and 
can be stored away between the clean unprinied leaves of a 
book. When required for use, the paper is pinned on to 
the board as before, and a mixture of the following solutions 
is brushed over it : — 

No. I. Silver nitrate .... 5 grammes 
Glacial acetic acid . . . 8 cc. 
Water 50 cc. 

No. 2. Saturated solution of gallic acid in distilled water. 

To every cc. of No. i add 60 cc. of distilled water, 
next I cc. of No. 2, and finally 30 cc. of distilled water. 
If the temperature be high, the water must be increased 
to such an extent that immediate reduction of the silver 
nitrate may not take place. After well mixing, the solu- 
tion is applied lightly, but plentifully, to the iodised 
paper with the cotton -wool brush already described, and all 
excess blotted off on filtering paper of the purest descrip- 
tion. Two sheets are then placed back to back with blot- 
ting-paper between them. 

The paper is most sensitive in its moist state, but it is 
also capable of giving pictures when dry, or until the 
surface of the paper becomes discoloured by a reduction 
of the gallate of silver. For exposure in the camera a 
sheet may be placed between two pieces of glass, or the 
corners may be gummed on to a sheet of glass, the paper 
taking the position of the collodion film of the ordinary pro- 
cesses. The exposure varies considerably according to th6 
preparation of the paper ; and it should always be sufficiently 
prolonged to give a trace of the sky-line on the undeveloped 
paper. To develop the picture, the paper must be pinned 
on the board as before, and equal parts of No. i and Na 2 
applied, vnth similar quantities o£ vjaAfti as already indicated. 



Development of the Image, 1 3 1 

This is applied with the brush, and is continued till the 
developing action begins to flag. When this is the case, the 
gallic acid solution, No. 2, is applied very lightly, until the 
deep shadows begin to dim by transmitted light. The de- 
velopment must now immediately be arrested, otherwise 
the picture will be veiled. 

For an under-exposed picture more of No. i should be 
used than that given, and if over-exposure be feared (in- 
dicated by the picture being fairly visible). No. 2 should be 
in excess. A little consideration of these points will show 
how development may be equalised in dark parts. An 
artist in the production of these pictures will be able to 
produce a picture by a little attention to the above details, 
whilst a mere manipulator would probably produce nothing 
but an image wanting in delicacy and gradation. 

The negative is fixed by immersion in a sodium hypo- 
sulphite solution. 

Sodium hyposulphite . . .60 grammes 
Water i litre. 

The fixing being complete, which may be known by a 
total disappearance of the yellow of the iodide, when the 
paper is viewed by transmitted light, the picture is washed 
in abundant changes of water, until all the hyposulphite is 
thoroughly eliminated. This may be known by applying 
the test given at p. 151. The washing should take at least 
three or four hours even in running water. 

It may be advisable to call attention to the necessity of 
the addition of acetic acid to the sensitising solution. This 
is always advisable in warm climates, as it greatly restrains 
the reduction of the silver nitrate; the less added, however, 
the more sensitive the paper will be. 

When the paper negative is dry it is ready for waxing. 
A flat-iron (preferably a box- iron) is heated to such a tem- 
perature that it will readily melt white wax. A cake of this 
substance is brought in contact with tVve \ioxv, ^\v!^sX. ^^ 

K 2 



I 

132 Calotype Process. 

latter traverses the paper. The whole of the picture, 
except the sky, should be rendered translucent by it The 
superfluous wax is absorbed by blotting paper placed upon the 
negative, over which the hot iron is passed. It is a mistake 
in this last operation to heat the iron too much, over-heating 
causes the blotting-paper to take up too much of the wax, and 
leaves the grain of the paper visible. It sometimes happens 
that yellow spots occur in the whites^of the picture. These 
are generally removable by the application of a dilute 
solution of hydrochloric acid. It need scarcely be re- 
marked that this entails a thorough washing. The acid 
must never be applied till all the sodium hyposulphite is 
thoroughly eliminated, for if any remain in the paper it is 
decomposed by the acid, and the inevitable result will be a 
fading of the picture. Further remarks on this subject will 
be found in the chapter on silver printing. 

There are various processes for the production of paper 
negatives extant, amongst which may be mentioned those of 
Le Gray, Blanquart-Evrard, and Prichard. That due to Le 
Gray was at one time a great favoiuite, its distinguishing 
feature being that the paper is waxed before being sen- 
sitised. The waxed paper is immersed in a solution of 
potassium iodide and bromide, together with sugar of milk, 
and after drying is treated witli a solution of silver nitrate, 
acidified with glacial acetic acid. The development is 
carried on much in the same way as that indicated in the 
above process, the paper being submerged in the fluid. 
This last process, perhaps, is better adapted to careless 
manipulation, than that described above, as all danger of 
staining the back of the picture is avoided. 

The student is recommended at first to use these pro- 
cesses for the production of prints from glass negatives; 
any representing engravings or maps are, perhaps, the best 
adapted for a preliminary trial. An easy picture in half 
tone may next be experimented with, and then a picture in 
the camera. The advantage oi t\v\% ^xo^esme method of 



Silver Printing', 133 

attaining a knowledge of the process is this : with an ordi- 
nary gas burner, any exposure may be given, the time being 
diminished or increased at will. With the camera, on the 
other hand, the varying quality of light will often discourage 
the beginner in his first essays. 

The absolute necessity of the cleanliness of all developing 
cups and dishes, of neat manipulation, and of careful filtra- 
tion of the solutions, cannot be too strongly insisted upon. 
A neglect of any one of these will inevitably lead to failure and 
consequent annoyance. In no negative process is patience 
and good temper more a sine qud non than in these paper 
processes. 

Calotype is an admirable process for travellers, and is 
often practised in India at the present day, the amount of 
chemicals and apparatus necessary for the production of 
pictures being reduced to a minimum. A few dozen sheets 
of iodised paper, and a chest containing silver nitrate, gallic 
acid, and a bottle of acetic acid, are all the necessary 
chemicals, with the exception of the sodium hyposulphite. 
A box of scales and weights, the camera and its legs, and a 
couple of pieces of clean glass of the size of the slide, a 
few drawing pins, a folding dish, a cotton wool brush- 
holder, and candle-shade, complete the apparatus. 



CHAPTER XX. 

SILVER PRINTING. 



In the fourth chapter the results of the action of light on 
silver chloride and organic compounds of silver were shown. 
In this part it is proposed to treat the subject rather more 
fully, as silver printing entirely depends upon it. 

The student would do well to make the following experi- 
ments for himseJi^ ^^ by so dping the laUoivaX^ ol ^^ N'^i>ar 



134 Silver Printing. 

tions in the processes will become familiar to him, and many 
failures will be avoided by a study of the theory. 

Take any ordinary paper which contains size of some 
description, and immerse it in a solution of sodium chloride. 

Sodium chloride .... I gramme 
Water 50 cc 

Hang it up and allow it to dry, and in non-actinic light 
(adopting the manipulations which will be presently de- 
scribed) float several pieces of convenient dimensions on a 
solution of silver nitrate for three minutes. 

Silver nitrate .... 5 grammes 
Water 50 cc. 

When dry to the touch, place one of these pieces under 
a negative in a printing frame, and expose it to the action 
of the sunlight ; after a few seconds open the frame in 
a subdued light, and note the result. The parts acted 
upon by light will have a violet tint, and if ammonia 
be applied to a portion of the darkened paper it will be 
found that the image almost entirely disappears. For 
reasons already given this will indicate that the silver 
chloride is dissolved. Allow further play of sunlight, 
say for a couple of minutes, and again note the result 
It will be found that the image is much redder in 
colour, and that ammonia fails to remove all the coloura- 
tion. From this we infer that the organic compound 
formed by the size of the paper and the silver nitrate is 
acted upon. Next take another sheet of the same paper 
and wash out all excess of silver nitrate, and expose under 
a negative, and examine the print at the same intervals as 
before. It will be found that the short exposure produces 
hardly any perceptible darkening, whilst with the longer it 
is much less than in the previous experiment. From the 
results of experiments already detailed in the fourth chapter, 
it will he seen that the absence of silver nitrate prevents the 
darkening of the silver chloride, aivd xJwaX ^^ ot^xAc com- 



Experiments with Sensitive Papers, 135 

pound is the more impressionable. A minute examination 
of the image will also show that there is a spotted irregular 
appearance in the darkest parts. There is an easy 
theoretical explanation of this. The chlorine liberated 
from the darkening silver chloride is taken up by the 
organic compound, bleaching it to a certain extent, forming 
white chloride of silver, which in its turn is capable of being 
acted upon by light. Experiments with similarly washed 
paper will show that, though the first darkening is much slower 
than in the unwashed paper, yet the action of the former 
approaches more nearly the rapidity of the latter, as the 
organic silver oxide, which is greedy of chlorine, is formed. 
Next, fix these paper images by immersion in the bath of 
sodium hyposulphite, as given at a subsequent portion of 
this chapter. Both prints will assume a foxy-red colour, 
that containing no free silver nitrate losing least in depth. 
The reason will be apparent. The silver subchloride, Ag2Cl, 
formed is soluble as far as one atom of silver and one 
atom of chlorine is concerned (AgCl), leaving behind one 
atom of metallic silver. Since it is only the surface of a 
particle of silver chloride that is blackened, the darkening 
of the compound is in itself a protection from the penetra- 
tion of light into it. It can be shown that the depth to 
which such penetration can take place with ordinary ex- 
posure is very superficial, hence the metallic silver left 
behind must be exceedingly minute ; so small is it indeed 
that the most delicate balances are too coarse to weigh 
it It may be of interest to note an experiment which 
was carried out to test this. One thousand square centi- 
metres of a glass plate were coated with a layer of silver 
chloride held in situ by inert collodion, and exposed to 
sunlight in the presence of an excess of silver for five 
minutes. The original amount of chloride was 102 centi- 
grammes, and after fixing in the bath the metallic deposit 
was dissolved off in nitric acid, and estimated volumetrically^ 
and iound to give only 55 milligrammes oi me\i)K\c ^s^^x. 



1 36 Silver Printing, 

The organic substances employed in printing may now 
briefly be considered. Albumen undoubtedly comes first, 
owing to the properties it possesses of giving a good tone 
when converted into albuminate of silver if in contact with 
silver chloride and excess of silver nitrate, and also on 
account of its insolubility after coagulation. 

The formula for albumen is taken as C72H|08Ni8SO22> 
though it can scarcely be said to be established with any 
certainty. Albumen coagulates in the presence of nitric 
acid, and also at 65° C. It is precipitated, but not coagu- 
lated by alcohol. It combines with the metals, prominent 
amongst which is the compound it forms with silver. Silver 
albuminate is white, turning a dark red-brick colour in the 
presence of white or other actinic lights. The change is 
speedily effected, and, like other organic compounds of 
silver, is not dissolved by ammonia after darkening, though 
the addition of an alkali speedily dissolves the white albumi- 
nate. This alone prevents the adoption of an alkaline solution 
of silver, such as the ammonio-nitrate of silver for sensitising 
paper coated with this and some soluble chloride, as the 
effect would be simply to dissolve it. Gelatine, which is the 
size used in some papers, combines with silver, and forms a 
red tint on exposure to light; owing to its colour, and the 
greater difficulty of toning, it is not usually employed in print- 
ing operations. Starch (CgHjoOs) forms a compound with 
silver, which on exposure to light darkens to a more violet 
colour than either of the preceding. It is largely used in 
sizing paper, and it is consequently necessary to note this 
colour. 

From the foregoing remarks it will be seen that all the 
bodies which are employed in sizing ordinary paper will 
combine with silver, but there are other reasons why albumen 
is that which is usually employed. Its adoption is due to 
the fact that it remains on the surface of paper, forming a 
smooth and thin layer, which is capable of holding in situ 
the different chlorides, and that on tYve a^^\\ca.\.\avi of silver 



Collodio'CJtloride, 137 

nitrate this delicate film is converted into an organic salt of 
silver together with the silver chloride. In all printing 
operations one point is a desideratum, viz., that the image 
should be on the surface of the paper and not sunk into it. 
The importance of this may be tested by sensitising albu- 
menised paper on the reverse side, and endeavouring to 
obtain a print on the albumen surface in the ordinary 
manner. It will be found that the image will appear feeble 
by reflected light, though by transmitted light it will appear 
well-defined and dense. When albumen is used fresh, and in 
a slightly alkaline condition, the resulting print possesses 
greater stability than any of the foregoing substances; as re- 
gards delicacy of image it cannot be surpassed. Unfortu- 
nately albumen is most easily applied to the surface of paper 
when slightly acid, the acidity being due to decomposition, 
and the resulting compounds formed are more liable to 
change. 

It may be asked, why not print in pure silver chloride 
alone, held in situ by some vehicle, such as collodion ? This 
is not impossible though impracticable, as the reduction of 
silver chloride by the fixing solutions is so great, that the 
print would be wanting in vigour. With the addition of 
some organic compound, however, it becomes quite feasible ; 
but then, be it remembered, the depth obtained is due to that 
organic substance. Thanks to the discovery of Mr. Wharton 
Simpson, that silver chloride and some organic compounds of 
silver (amongst which we may name citrate of silver) will 
emulsify in collodion, prints are readily obtained by the 
collodio-chloride, and possess a beauty which cannot be sur- 
passed In the next chapter this process will be described ; 
merely mentioning en passant, that in this process, as in 
any other in which printing on silver chloride takes place, 
an excess of silver nitrate is essential. 

The colour of prints obtained is always objectionable if 
fixed directly after taking out of the printiu^ ftarcv^^ \ ^xsjI 
TtsoTt is had to an operation called tonmglo i^xii^'^tSx \s\ss«. 



138 Silver Printing, 

pleasing. This toning may consist of gilding the silver 
image, platinising it, or substituting some other metal for it 
The colour of the silver print when appearing through this 
other metal may give a pleasing tint, or it may fail to do so, 
according to the extent to which the operation is carried. It 
will be seen in the practical instructions in printing, that the 
picture is more or less washed in water before toning. By 
immersion in water, the violet-colovu-ed image becomes of a 
red colour : to what this change is due is rather imcertain. 
It has been held that it is due to the water dissolving a cer- 
tain part of the silver oxide. It may be due to a different 
compound being formed by the combination of water with 
the altered compound, but this is doubtful. In order to tone 
the picture, certain solutions of gold, platinum, or other 
metals are made, and the print immersed in them ; the first 
of these metals, in the shape of gold trichloride, is that 
usually employed. It is, therefore, proposed chiefly to con- 
fine the remarks on toning to that process in which that metal 
is principally employed. The gold trichloride has the 

' formula AUCI3, which is a fairly stable compound. If its 
temperature be raised to 170° C. it becomes decomposed, 
a pale yellow and insoluble powder, gold chloride, AuCl, 
resulting, chlorine being evolved. When the former salt of 
gold is mixed with a solution of silver nitrate, the chlorine 
leaves the gold to form silver chloride, and the latter salt of 
gold is formed. Acetates, as also the carbonates of the 
alkalis, are capable of precipitating gold from a neutral solu- 
tion in the presence of any disturbing cause— such as 
organic matter. 

It will be seen from the formulae given for toning solu- 
tions p. 148, that one contains chloride of lime, and as an 
example of one kind of toning this one will be considered. 
If a print, so thoroughly washed that all excess of silver 
nitrate is eliminated, be immersed in this solution, it will be 
found that the gold deposits very slowly, and that the image 

becomes feeble and spotted in appeaiaTice,^\v^x^"a&^tha 



Theory of To7iing, 139 

print in which the excess of silver nitrate has been but 
partially removed, the toning or gilding action takes place 
much more readily. 

Chloride of lime is a mixture of calcium chloride (CaCl2) 
with calcium hypochlorite (CaCl202) being made by passing 
chlorine over calcium hydroxide, or common slaked lime. 

Calcium hydrate + Chlorine = Water + Calcium chloride + pochlorite 
2CaH20 + 2CI2 «2H20+ CaClg + CaCLjOj 

Now gold trichloride, when uncombined with an alkali, 
is generally in an acid condition, due to the presence of 
hydrochloric acid, and in order to neutralise this, calcium 
carbonate forms part of the toning bath. On immersing the 
silver print the equilibrium is disturbed, and the gold begins 
to deposit; and, consequently, chlorine is liberated. In 
the directions for use of the toning bath it is stated that the 
solution should be made with hot water if required for im- 
mediate use, whilst if made with cold it must st^^nd twenty- 
four hours. This causes a certain quantity of the hypo- 
chlorous acid to be evolved, and leaves a small portion of 
calcium hydrate in solution. 

In the case of the thoroughly washed print, the chlorine 
attacks the silver subchloride or the organic silver oxide, 
and forms the white silver chloride ; for, be it remarked, the 
gold which is situated nearest the printed surface is first 
reduced, and the image is therefore in close proximity 
to the chlorine, and readily attackable by it. At the 
same time the silver oxide and sub-chloride are in proxi- 
mity to the chloride of lime, which reduces them also to 
the state of chloride, particularly if it be rich in hypochlo- 
rous acid. In the case of the slightly washed print the same 
action takes place, but we have a new element to deal with. 
In this case the solution or excess of the silver nitrate held 
in the pores of the paper gradually becomes diffused to 
the surface, combines at once with the liberated chlotvas.^ 
and ioTTtts chloride of silver, but not at the expense oj iKe 



140 Silver Printing'. 

image. It will be noticed in toning operations that this 
chloride is absolutely formed on the surface of the print, and 
can be removed by a slight rub with the finger. On fixing 
the print, the silver chloride is in both cases removed ; in 
the one firom the image itself, in the other from the paper. 
This accounts for the spotted appearance in the one case, 
and its absence in the other. 

To test this theory, the following experiments were under- 
taken by the writer. A print was thoroughly washed, then 
immersed in a solution of lead nitrate, and again slightly 
washed. On applying the chloride of lime toning bath, the 
print quickly changed to a rich brown colour, and, after 
nxing, had all the qualities of a properly toned print In 
this case the lead combined with the chlorine, and acted 
like the silver nitrate. Another toning bath, consisting of 
lime water and gold trichloride, was prepared. Two well- 
washed prints were immersed, and left respecrively for 
three minutes and fifteen minutes ; on the latter a slight 
deposit of gold was visible, and also a diminution in the 
depth of the print after fixing. With the former the print 
was less affected. Prints in which silver nitrate and lead 
nitrate were present both toned admirably, but rather too 
rapidly for safety. On examination a trace of hypochlorous 
acid was found in the toning solution. A shght addition 
of chloride of lime was next made, and prints in which silver 
and lead nitrate were present were immersed in the solution ; 
they toned gradually and regularly. This last experiment, 
which was confirmed by others, showed that the calcium 
hypochlorite contained in the chloride of lime, acted as a 
retarder to the toning operation, as the chlorine contained 
in hypochlorous acid combined with the silver nitrate 
equally with that evolved firom the precipitating gold. This 
manifestly would check the deposition of the gold. 

Another toning solution used is one made with sodium 

acetate and gold. In practice it is found that toning takes 

place most regularly when the piml V\as b^^w previously well 



Theory of Toning, 141 

washed. On adding a solution of sodium acetate to silver 
nitrate, a sparingly soluble silver acetate and sodium 
nitrate are formed by double decomposition. If then 
the silver nitrate be present in the print, the greatest 
portion of the adjacent sodium acetate is decomposed, 
and sodium nitrate left in its place. The sub-chloride 
and oxide of silver both seem to be as readily at- 
tacked by chlorine as the silver acetate. Hence the chlorine, 
having nothing at hand to absorb it (sodium nitrate not being 
able to do so), attacks the silver of the print and produces 
the bleaching action already referred to. When all the free 
silver nitrate, however, is washed away, the conditions are 
changed ; the sodium acetate will absorb chlorine, and form 
a chloracetate and hydrochloric acid, as indicated in the 
following equation :- 

Gold Sodium ^ r \A a- So^i^^^ trichlor- Hydrochloric 

trichloride acetate = o + acetate acid 

2AuCl, +NaC2H30,= 2Au + NaCjClgOj + 3HCI. 

Eventually an evidence of this reaction may be traced 
in the fact that the solution becomes acid, and refuses to 
tone. 

The foregoing experiments exemplify the following 
laws : — 

(i.) That a neutral solution of the gold toning bath is 
necessary. 

(2.) That some active soluble chlorine absorbent must be 
present, either in the print or in the solution. 

(3.) That when the affinity of the absorbent for chlorine 
is violent its action must be retarded. 

In considering any toning solution, these three qualifica- 
tions must be taken into account, and if one of them be 
violated, a perfect print must not be expected. 

The theory of fixing prints is the same as already in- 
dicated at page 74, and need scarcely be touched upon. 
The reason why potassium cyanide cannot be M's^e^xsMq «s^- 
jAoyed as a fixing agent has been already sVowci\.o\ie ^^\^ 



142 Silver Printmg, 

the fact that the organic oxide of silver is soluble in its solu- 
tion. It must be strongly impressed upon the student that 
two forms of double hyposulphites of silver and sodium are 
formed, one of which is soluble and the other insoluble, see 
p. 74. The soluble form undergoes a change in light, which 
renders it insoluble; hence fixing the print in daylight 
should be avoided. 

When prints are immersed in a solution of sodium hypo- 
sulphite, a certain portion of this salt is combined with the 
double salt of silver formed. Every print immersed there- 
fore leaves a smaller quantity of the imcombined sodium 
hyposulphite in solution ; and since the double salt of silver 
and sodium is soluble in the uncombined hyposulphite, it 
follows that care must be taken not to fix too large 
an area of print in the same solution, otherwise the in- 
soluble salt will be formed in the pictures. The eflfect of 
this is seen in the fading of prints. No amount of washing 
will eliminate this insoluble form. The acid vapours to be 
found in the air will decompose it, and cause a liberation 
of sulphur compounds, which gradually bleach the black 
portions of the image, and give the whites a jaundiced 
appearance. 

Even where the soluble double hyposulphite has been 
formed, washing the prints in a thorough manner is 
essential for permanency, for any trace of it will decom- 
pose in a similar manner, as will also the sodium hypo- 
sulphite itself. In the writer's opinion the prints should be 
immersed in two separate solutions of the sodium hypo- 
sulphite ; the first will form the necessary soluble salt, and 
the latter will cause it almost entirely to disappear, all traces 
being subsequently eliminated by the washing water. Some 
American writers have proposed to shorten the washing of 
the print by a final immersion in a solution of iodine, tetra- 
thionate being formed ; the following reaction would be 
as ioWows : — 
Sodium hyposulphite + Iodine *= Sodium lelTaAi\ox«l^-v^Q^vMS!L\Qd^^ 



Developed Prints, 143 

Both these salts are soluble in water, but less so than the 
sodium hyposulphite, and the tetrathionate appears to be 
more readily decomposed. If a silver compound be pre- 
sent with the hyposulphite, the same reaction apparently takes 
place, though an exact analysis of it has not as yet been 
undertaken. The quality of the washing water is important 
— it should if possible be rain water, odierwise pure spring 
water, to which a little alkali should be added for the first 
washing. This renders the elimination of the soluble salts 
more complete. 

The causes of instability in a print are as yet imperfectly 
recognised ; and, owing to the want of chemical knowledge 
on the part of some photographers, the fading has assumed a 
more mysterious aspect than is warranted. Take the case of 
the acetate toning solution. It has been shown that it be- 
comes acid, and the prints are often taken direct from this 
bath to the hyposulphite solution. An acid immediately 
commences to decompose the latter, and fading necessarily 
results. Washing between each operation should be in- 
sisted upon, and then the chances of fading are reduced to 
a minimtun. 

Sometimes it is convenient only partially to print a 
picture on paper and then to develop in a similar 
manner to that employed in the calotype picture. The 
theory is the same, and the manipulations are the same. 
The advantages obtained by this method of production are 
more than counterbalanced by the defective tone they 
usually take, and the lack of purity in the whites. The 
reason of the difference of tone is accounted for by the 
fact that the metallic silver preponderates so largely over 
the organic oxide. The blending of the colour of the latter 
with that of the precipitated gold is the charm which exists 
in the ordinary print on albuminised paper. For some pur- 
poses the cold tone usually produced does not signify, and 
then the developing process may be found useful. 



144 Silver Printing. 



CHAPTER XXL 

MAXIPULATIOVS IN SILVER PRINTING. 

The papers employed in silver printing are known as 
Saxe and Rive, the former being suitable for large pictures, 
whilst the latter are preferable for smaller sizes. The fol- 
lowing formula may be used for the albumen solution, with 
which to coat the paper : — 

AminoniniTi chloride . . .10 grammes 
Spirits of wine . . . . 15 cc. 

Water 135 cc 

Albumen .... 450 cc. 

The first three are thoroughly mixed, and the albumen, 
derived from the whites of eggs, is gradually added to the 
solution. Perhaps the simplest way of effecting solution 
and perfect mixture is to half fill a bottie with the albumen, 
and then to add a fair supply of roughly powdered glass. 
Shaking the bottle will cause the flocculent matter to be 

Fig. 28. 




broken up, and leave it in a state ready for filtering through 
sponge or well-washed tow. The paper, having been cut 
into sheets of convenient size, is floated on the fluid, con- 
tained in a dish, the hands grasping its two oppo»te 
corners. The convex surface of the paper thus formed is 
^rst brought in contact with the solution. As the hands are 



Dull Surface Prints, 145 

drawn apart, the paper pushes out all air-bubbles before it, 
and at length lies in perfect contact with the solution. It is a 
wise precaution to take, however, to raise the paper from one 
comer, to make certain of the absence of all air-bubbles, and 
then to allow it to remain at rest on the solution for a minute. 
It is next removed, and hung on a line to dry, being held 
by a couple of American clips, or thrown over a stretched 
cord. This last plan is apt to cause markings, though it is pro- 
bably necessary when large sheets of paper are manipulated, 
owing to their tendency to tear if only suspended by two 
comers. When dry the paper will not be flat, and should 
therefore be rolled and put away between flat boards. 
When a print having a dull surface is required, the following 
formula is sometimes used : — 



Ammonium chloride . 
Gelatine . 
Water 



6 grammes 
*6 gramme 
300 cc. 



The gelatine is first dissolved in hot water, and then the 
remaining salt added. The paper is floated for three minutes 
on this solution. 

Another mode of producing a dull surface, and which 
is very effective, is to use resinised paper. The annexed 
formula is workable, and is due to Mr. H. Cooper, jun. : — 

Frankincense . . . . i gramme 

Mastic *8 gramme 

Calcium chloride . from '5 to i gramme 
Alcohol 45 cc. 

Good Rive paper is immersed in this solution for half a 
minute, after which it is ready for floating on a moderately 
strong sensitising bath. 

The Sensitising Bath. 

When a paper is weakly salted, say, having half the 
amount of chloride given in the formula for albumenising 
paper, a weak sensitismg bath is usually eTii^\o^t'&.,^V<et^a& 



146 Silver Printing-. 

with paper strongly salted, or for the resinised paper, one 
somewhat stronger is necessary. The following formulae 
will show what the extreme strengths of solution should 
be : — 



and 



Silver nitrate .... 6 grammes 
Water ^ . . . , 100 cc. 

Silver nitrate . . . .15 grammes 
Water 100 cc. 



The paper is floated on either of these solutions in the 
manner given for albumenising paper, the time of contact 
varying from three minutes in hot to five minutes in cold 
weather. It should be removed slowly from the sensitising 
bath to prevent waste of solution, and when hung up to dry 
by an American clip in the dark room, the drainings should be 
collected by attaching a slip of blotting-paper to the bottom 
comer. It is always advisable to have one comer lower 
than the others, as the sensitising solution thus drains more 
equally away. 

In order to preserve sensitised paper from colouration 
due to the decomposition of the organic salt of silver, it may 
be placed between sheets of blotting-paper impregnated 
with sodium carbonate. Other devices have been adopted 
for the same purpose ; descriptions of them will be found 
in the various manuals. 

Printing the Picture. 

Printmg operations are rarely carried on in the same tem- 
perature and state of atmospheric moisture as those in which 
the paper is dried ; hence it is advisable to allow the paper 
to assume the conditions of the former before rigidly con- 
fining in the frame. The fact that paper expands in moist 
air at once shows that the dimensions of a photograph can 
never be relied upon as being accurate. Measurements have 
shown that a. drawing, for instance, ^\ \^i^ ^s much as i per 



The Printing Frame. 



147 



cent, in ceitain coDditions of the atmosphere. The same 
remark applies to plans printed in the ordinary lithographic 
press J the scale is never correct except under fixed con. 
ditions. 

The n^ative should be placed in the printing fi'ame 
with the vaniished side next the paper. A convenient form 
of irame, and one which is usually employed by photo- 
graphers, is shown in the diagram, b is a sheet of thick 
plate-glass, which rests in a frame, a. The negative is 
placed on the glass, the sheet of paper over it, then a smooth 




felt pad, and over this a back, c, hinged in the centre. Two 
cross-bars, d d, to which are affixed springs, cause ihe back 
to press on the pad, and are held in position as shown by 
E E. The use of the hinged back is to allow the print to 
be examined when required. During such an exami- 
nation one of the catches e is loosened, and the portion 
of the picture beneath one half of the back can be in- 
spected without any danger of the relative position of 
the paper and the negative being changed. The depth 
of the print is an important point to attend to. It xwist. 
be remembered iba,t much of the apparent v\^o%'[ SaV^Vx^ 



148 Silver Printing. 

the subsequent operations of toning and fixing, and due 
allowance must be made for this. It requires considerable 
practice to judge correctly of the proper depth, and no 
fixed rule can be given, so much depending on the relative 
proportions of the chloride to the organic compound of 
silver, and on the nature of the toning bath. Much might 
be said about the artisticnianipukt'ionofprints, but it hardly 
enters into the scope of this work, though some hints will be 
given in the chapter on the picture. 



The following toning baths may be considered as 
ilandanls : — 



GoW trichloride . . 


. -asgisnuiie 


Chl>.ride oflime 


- ■2Se™nnw 


Chalk (precipiuted) . . 


. I teaspoonfiil 


Water . . . . 


. lUtre. 



The water should be boiling if the solution be required 
lo be uiied at once, otherwise it shouldstand in sn uncorked 
boHle for twenty-four hours. 



CioKl IricUoride 
Stxtium acetaie 
Wniet . 



Thin nhould be raised a day before being used. Before 
\wmii. the tiolutions should be filtered in a clean dish 
ulitlhUy wanttetl. if the weath« be cold. The prints are 
HU«*a in W«tCT of about 15= C, and the washing continued 
«MJjUwrt«l in the last chapter, ac«»ding to the toning 
loywl. llwy are then immeised in ttie tcming so- 
~ or fout at a tinw, and the dish is kept in con- 
>> *u ax to allow an equal toning action through- 
Vcwioe mwuial tbat no two prints should stick 
Uw HMil maoBk AoEwSm^ i» the colour 




Illiki 



Toning^ and Fixing the Print 149 

of the print desired, so must the continuation of the toning 
action be regulated. If a rich chestnut brown be required, 
but very little apparent change in the colour of the print is 
necessary, whereas if an engraving black tone is sought, the 
action must be continued till the image is decidedly blue. 
It is not to be inferred that these rules are absolute in 
every case ; so much depends on the sizing of the paper 
and on the amoimt of chloride present that they are not 
applicable in all cases, but with Saxe paper, prepared as 
given in the foregoing formulae, they will hold good. 

Fixing the Print. 
The fixing solution is made up as follows : — 

Sodium hyposulphite . . . 200 grammes 
Water i litre. 

Between toning and fixing it is essential that the prints 
should be well washed. The necessity of this may be under- 
stood by referring to p. 141. It has been sometimes recom- 
mended to acidify the washing water, but the proposer of this 
plan can have had no thought of the danger to the permanency 
of the prints which he thereby introduced ; an acid at once 
begins the decomposition of the hyposulphite. The writer 
strongly urges the necessity of a strongly alkaline condition 
of this bath, and in practice he adds 50 cc. of strong ammo- 
niiun hydrate to it when fixing prints. Mr. J. Spiller was the 
first to point out the use of ammonium carbonate in the 
solution; he showed that it dissolved out a certain compound 
left in the whites of the picture, which otherwise was in- 
soluble, and which readily decomposed under atmospheric 
action. The pictures should be immersed in the solution for 
ten or fifteen minutes, the time varying according to the thick- 
ness of the paper ; they should then be washed (unless they 
1^ placed in a second solution of hyposulphite as already 
suggested), rapidly at first and afterwards more slowly. 
Perhaps the best way of eliminating lYve ^Tta\.ex ^^xX. ^\ "*^^ 



ISO 



Silver Printing, 



hyposulphite is to place the prints in a large tub of water, 
which is kept in motion, and after five minutes' washing 
to place them in a smaller quantity of water- After this 
they may be removed to a washing trough, where the water 
will be changed several times an hour. The accompanying 
idea of a washing trough may prove useful. It is one which 
was designed and is employed by Mr. England, and has 

Fig. 3a 




answered well the purpose for which it is intended, a is a 
trough, at the side of which is a syphon, s, the inside leg 
reaching to within 2 or 3 millimetres of the bottom, the 
bend of which is a little below the top of the trough, b is 
a cradle, pivoted on a rod, e, which passes through the 
sides of A as shown, c is a water-wheel attached to the 
wall on to which a gentle stream of water from the tap, f, 
plays. G is a small arm attached to the axle of the wheel, 
having a rod suspended fi*om it, which is attached to the 
cradle, b. As the wheel slowly turns the rod is raised, and 
Uie prints are caused to move about in the water. The 
water runs into the trough through the pipe, h, and when 
it reaches the top of syphon pipe, the trough gradually 
empties itself, leaving the prints on the gutta-percha strips 
which form the bottom of the cradle. It will be seen 
that the supply of water must a\wa'^^^3^xa.^^x\^'Si^^^X!L\i\si.t 



Tests for Sodium Hyposulphite, 151 

which the S)rphori is capable of carrying away* A useful 
addition to the trough is a horizontal pipe attached to the 
well of the wheel, moving from side to side by the motion of 
the wheel, and thus distributing the entering water over the 
sur&ce of the prints by means of a fine rose. This prevents 
the chance of any of the prints getting surface-dry, which 
sometimes happens. In a trough of this description twelve 
hoiurs suffice to ensure the total removal of the hyposulphite. 
Should this mode of washing be inapplicable, the prints may 
be placed in dishes, changing the water every quarter of an 
hour for the first hour, and every half-hour subsequently for 
six hours. If during this time they are well sponged twice 
or three times with a soft sponge, it will be found on apply- 
ing one of the following tests that the hyposulphite is 
eliminated. 

The first test is based on the reaction of iodine with 
sodium hyposulphite, shown at p. 142. Take a small piece 
of starch the size of a pea, powder it and boil it in 
10 cc. of water, till a clear solution is obtained ; add 
5 cc. of a saturated solution of iodine in alcohol to the 
clear liquid. A dark blue colour due to starch iodide 
will now be apparent. Drop 2 drops of the solution 
into two clean test-tubes, and fill up one with dis- 
tilled water, and the other with the water to be tested. A 
faint blue colour should be perceptible in the first test- 
tube, whilst the presence of hyposulphite in the other will 
be shown by the total absence of colour. The contents 
of the two solutions in the test-tubes can be best com- 
pared by placing a piece of white paper behind them and 
examining them by reflected light. The sodium hyposul- 
phite may not be found in the washing water, yet a trace 
may remain in the prints. If a very weak solution of iodine 
be brushed across the back of a print, the absence of all 
colour will indicate the presence of the hyposulphite. One 
selected out of a batch of prints may thus be testtid^ ^\Nav\^ 
it is rarely necessary, if the water indical^^ \)waX. \Jcv^ ^^^wsv^i. 
Aas been thoroughly effected. 



152 Silver Printing. 

The dishes that are used for holding the fixing solutions 
must in no case be employed for any other purpose. The 
material of which they are made should be, if possible, glass 
or porcelain, and never tin or zinc. 

The defects in prints due to defective manipulation and 
not to want of artistic skill are but few in number. Red 
marks that repel the toning solution can usually be traced 
to contact with hot and moist fingers. A red tone after 
fixing is due to an insufficient deposit of gold, and a blue 
tone to an excessive deposit. The whites may appear 
yellow through the fixing solution being of insufficient 
strength, or through paper being used when the sensitive 
surface shows signs of discolouring through too long keep- 
ing. The general cause of the fading of prints has already 
been detailed. 



CHAPTER XXII. 

COLLODIO-CHLORIDE PROCESS. 



This process is intended to be employed for printing on 
glass or paper, and for permanent silver prints nothing 
better can be desired. The following is a formula which is 
taken from the published process of Mr. Wharton Simpson: — 

No. I. Silver nitrate .... 4 grammes 
Water 4 cc. 

No. 2. Strontium chloride . . 4 grammes 

Alcohol . . . . . 60 cc. 

No. 3. Sodium citrate . . . • 5 *5 grammes 
Alcohol . . . . . 60 cc. 

To every 50 cc. of plain collodion, i cc. of No. i is 

added, being previously mixed with 2 cc. of alcohol, in 

order to prevent precipitation of the pyroxyline. Next 2 cc. 

of No. 2 are added with constaivt %\v3i^dTv^, ;m.^^'wa\Vj \ cc. 



Collodio-Chloride Process, 153 

of No. 3. In a quarter of an hour it is fit for use. It will 
be uoted that there is a large excess of silver nitrate present. 
The amount necessary to combine with the strontium chloride 
is only '29 cc. and with the sodium citrate 'iS cc. of silver 
nitrate ; there is, therefore, present more than double the 
amount of silver nitrate necessary to combine with them. As 
already shown, this excess is necessary. In practice, par- 
ticularly when printing on glass, it has been found very 
difficult to prevent the salts crystallising in the film whilst 
drying; and in order to overcome this source of annoyance, 
a method analogous to that of the washed bromide emulsion 
process may be employed. The above proportions of stron- 
tium chloride and sodium citrate may be kept, but the silver 
nitrate should be reduced to one-half. The plain collodion 
is made up with half the solvents usually employed to dissolve 
the pyroxyline, and consequently only half the above quan- 
tity is used in mixing the coUodio-chloride. After the 
emulsion is formed it is poured into a dish, allowed to set, 
well washed, dried, and then dissolved up in the proper 
proportions of solvents, in the alcohol of which \ cc. of the 
silver nitrate solution has previously been added. In this 
state the coUodio-chloride contains the same necessary 
excess of silver nitrate, but the strontium and sodium ni- 
trates are absent. This diminishes the risk of crystalli- 
sation taking place in the film, and with a certain class of 
pyroxyline this is entirely avoided. The silver citrate supplies 
the necessary organic matter by which a vigorous image is 
obtained. 

If a glass plate has to be coated with the emulsion, 
the same directions as those given for coating emulsion 
plates should be followed, with the addition that it is well to 
dry the film before a fire, and to print whilst it is still warm. 
When a paper has to be coated, more difficulty is found. 
The paper must be strongly sized; ordinary paper allows 
the collodion to penetrate through its pores^ atvd a. xxss.-a.Vsi 
appearance is sometimes the result. Aitovftoo\.^^^^^-,'»cs:^- 



1 54 Silver Printing. 

plied by most dealers in photographic materials, is perhaps 
the best kind. Obernetter, of Munich, uses an enamel 
paper as a support A similar paper is prepared by 
coating ordinary paper with a strong solution of gelatine, 
in which barium sulphate, known as * Mountain snow,' is 
mixed. When dry, this gives an impervious skin to the 
surface of the paper. The paper is pinned on to a boards 
the edges being turned up 2 or 3 millimetres, and at one 
corner a spout is formed, from which the collodion is poiured 
off. The emulsion is now applied as if to a glass plate. 
Some operators find that by fuming the film with the vapour 
of ammonia, after thorough drying, increased vigour is 
imparted to the print. In any case this end may be 
attained by applying a solution of gallic acid and acetate of 
lead, together with a few drops of a solution of silver nitrate. 
The print may be toned in any of the ordinary toning baths. 
Ammonium sulpho-cyanide and gold have been recom- 
mended, but the tones thus obtained vary greatly in richness. 
For printing on glass, a special printing frame has been 
designed, but this is not required if the precaution be taken 
to gum a strip of paper along the corresponding edges of the 
sensitive plate and of the negative. They may then be 
separated one from the other with the certainty that they 
will fall into their original position. The prints are fixed in 
sodium hyposulphite, made as under : — 

Sodium hyposulphite . . "33 grammes 
Water , I litre. 

An immersion of eight minutes in this solution is sufficient 



Printing with Ferric Salts,, 155 



CHAPTER XXIII. 

PRINTING WITH IRON AND URANIUM COMPOUNDS. 

As already stated, these processes Nare not very generally 
employed for the production of prmts, but still they are 
useM for certain purposes, such as copying engravings, maps, 
&c, by contact 

Printing Processes with Salts of Iron, 

Sir John Herschell investigated the relative sensitive- 
ness of the different salts of iron, and came to the con- 
clusion that the double citrate of iron and ammonia was 
more readily acted upon by light than any other, whilst 
after it came the double oxalate of iron and potassium. 
To produce the former salts, take a weighed quantity of 
ferrous sulphate, dissolve in water, and boil with nitric 
acid till it is thoroughly oxidised and in the ferric state; 
next precipitate with ammonium hydrate, and wash the 
ferric oxide in warm water to get rid of all the soluble salts. 
Transfer the washed oxide into a glass beaker and gradually 
add a solution of citric acid, and warm. When a small 
trace of ferric oxide remains undissolved, the addition of the 
citric acid should be stopped. Take the same amount of 
citric acid already added to the ferric oxide, and care- 
fully neutralise it with ammonium hydrate, testing the 
operation with litmus-paper. Then mix the two solutions 
together and evaporate to dryness over the water bath, and 
when sufficiently concentrated, allow the crystals of the 
double citrate of iron and ammonium to separate out. After 
carefully drying between blotting-paper they are ready for 
use. The double oxalate of iron and potassium may be 
prepared in a similar manner. When required to x^xsjisx 
paper sensitive the following proportions s\vo\A'^\:>^ \a^'e^\ — 



156 Printing with Ferric Salts. ' 

Double citrate of iron and ammonium . 10 g^rammes 
Water (distilled) .... 100 cc. 

This is applied to the paper with a brush, or else the paper 
may be floated on it. When dry it is exposed beneath a 
negative from a minute in bright sunshine to a quarter of 
an hour in diffused light, when it is ready for development; 
though the image will be barely visible. If a blue picture be 
required, all that is necessary is that the print should be 
immersed in a solution of potassium ferri-cyanide. After a 
few seconds the image will be found perfectly developed. 
A copious washing in water (in which a little citric acid has 
first been dissolved for the first washing) will dissolve out 
all the soluble salts, and leave the blue image unchanged. 
The theory of this reaction has already been explained in 
chap. IV., and need not again be discussed. When pictures 
were developed by this method the process was called 
cyanotype by Sir J. Herschell. 

Instead of developing with the potassium ferri-cyanide, 
the exposed paper may be immersed in a dilute and neutral 
solution of gold trichloride. The gold gradually deposits 
on the exposed portions and gives a purple image. This 
method of producing pictures on an iron salt has been 
called the chrysotype. The reduction of the gold follows 
from the fact that the ferrous salts are capable of reducing 
salts of gold to the metallic state when coming in contact 
with them in solution. In the case of pictures taken by 
means of the double oxalate of iron and ammonium, it is 
well to add to the gold solution a little neutral ammonium 
oxalate. The development in this case takes place very 
rapidly. To fix the pictures they should be immersed 
in water slightly acidified with hydrochloric acid, and then 
be thoroughly washed. 

An exposed paper prepared with any double salt of 

iron and ammonium may be developed by floating it on 

a solution of silver nitrate, to which a trace of gallic acid 

and acetic acid have been addtd*, \5cv^ ^^Tta\3& ^\.\^^c&^ the 



Willis's Platinum Process. 1 57 

silver nitrate, and causes the metallic silver to deposit where 
the ferrous salt existed. The gallic acid subsequently causes 
a further reduction of the silver nitrate, and the first de- 
posit of silver attracts the following. An image is thus 
built up. 

Founded on the same reaction pictures may be obtained 
by means of platiniun tetrachloride, mercuric chloride, and 
potassium dichromate, &c., though greater exposure with 
these is necessary. 

One of the most recent advances in printing with iron 
salts is due to Mr. W. Willis, junr., and he has made it 
the subject of a patent. 

He floats a paper for a short time on a solution of silver 
nitrate and dries it He next brushes over the paper a solu- 
tion of the double oxalate of potassium and iron, together 
with a solution of * chloro-platinite ' of potassium. The paper 
is exposed under a negative, and a feeble image, due to the 
silver nitrate, is produced. It is then floated on a warm 
solution of potassium oxalate, and this latter at once reduces 
the platinimi to the metallic state where it is in contact with 
the ferrous salt (produced by the action of light on the ferric 
salt), and an image in platinum black is obtained. It was 
found in practice that, unless a preliminary wash of silver 
nitrate was given to the paper, the platinimi, to form the 
image, was loosely deposited, and fell back into the solution. 
By causing a small amoimt of silver nitrate to be reduced at 
first to the state of oxide, a nucleus was given, to which the 
subsequently reduced metal could adhere. To fix these 
prints, the paper is immersed in sodium hyposulphite, and 
then in potassium oxalate (the former to remove all traces of 
the silver nitrate), and afterwards is thoroughly washed in 
cold water. The prints produced by this process are ex- 
ceedingly beautiful, and, as platinum black forms the image, 
they may be considered as being far more permanent than a 
silver print 

About t8s7 Salmon and Garniex btow^X.ov^x.^V^^^^^'s.^ 



148 Silver Printing, 

the subsequent operations of toning and fixing, and due 
allowance must be made for this. It requires considerable 
practice to judge correctly of the proper depth, and no 
fixed rule can be given, so much depending on the relative 
proportions of the chloride to the organic compound of 
silver, and on the nature of the toning bath. Much might 
be said about the artistic manipulation ofprints, but it hardly 
enters into the scope of this work, though some hints will be 
given in the chapter on the picture. 

Toning. 

The following toning baths may be considered as 

standards : — 

I. 

Gold trichloride . . . '25 gramme 

Chloride of lime . . • '25 gramme 

Chalk (precipitated) . . .1 teaspoonful 

Water . . . . .1 litre. 

The water should be boiling if the solution be required 
to be used at once, otherwise it should stand in an uncorked 
bottle for twenty-^four hours. 

II. 

Gold trichloride . . . *25 gramme 
Sodium acetate ... 7 grammes 
Water i litre. 

This should be mixed a day before being used. Before 
toning the solutions should be filtered in a clean dish 
slightly warmed, if the weather be cold. The prints are 
placed in water of about 15° C, and the washing continued 
as indicated in the last chapter, according to the toning 
bath employed. They are then immersed in the toning so- 
lution three or four at a time, and the dish is kept in con- 
stant motion, so as to allow an equal toning action through- 
out It is likewise essential that no two prints should stick 
together, for the same leason. jk.ccox^\xi% xa NJwt colour 



Toning^ and Fixing the Print 149 

of the print desired, so must the continuation of the toning 
action be regulated. If a rich chestnut brown be required, 
but very little apparent change in the colour of the print is 
necessary, whereas if an engraving black tone is sought, the 
action must be continued till the image is decidedly blue. 
It is not to be inferred that these rules are absolute in 
every case ; so much depends on the sizing of the paper 
and on the amoimt of chloride present that they are not 
applicable in all cases, but with Saxe paper, prepared as 
given in the foregoing formulae, they will hold good. 

Fixing the Print. 
The fixing .solution is made up as follows : — 

Sodium h)rposulphite . . . 2CX) grammes 
Water i litre. 

Between toning and fixing it is essential that the prints 
should be well washed. The necessity of this may be under- 
stood by referring to p. 141. It has been sometimes recom- 
mended to acidify the washing water, but the proposer of this 
plan can have had no thought of the danger to the permanency 
of the prints which he thereby introduced ; an acid at once 
begins the decomposition of the hyposulphite. The writer 
strongly urges the necessity of a strongly alkaline condition 
of this bath, and in practice he adds 50 cc. of strong ammo- 
nium hydrate to it when fixing prints. Mr. J. Spiller was the 
first to point out the use of ammonium carbonate in the 
solution; he showed that it dissolved out a certain compound 
left in the whites of the picture, which otherwise was in- 
soluble, and which readily decomposed under atmospheric 
action. The pictures should be immersed in the solution for 
ten or fifteen minutes, the time varying according to the thick- 
ness of the paper ; they should then be washed (unless they 
be placed in a second solution of hyposulphite as already 
suggested), rapidly at first and aftetyjaid^ xaot^ ^cs^. 
Verhaps the best way of eliminating lYve g;tea.\.tx ^^^ ^^ "^^ 



148 Silver Printing. 

the subsequent operations of toning and fixing, and due 
allowance must be made for this. It requires considerable 
practice to judge correctly of the proper depth, and no 
fixed rule can be given, so much depending on the relative 
proportions of the chloride to the organic compound of 
silver, and on the nature of the toning bath. Much might 
be said about the artistic manipulation of prints, but it hardly 
enters into the scope of this work, though some hints will be 
given in the chapter on the picture. 

Toning, 

The following toning baths may be considered as 

standards : — 

I. 

Gold trichloride . . . '25 gramme 



Chloride of lime 
Chalk (precipitated) 
Water 



'25 gramme 
I teaspoonful 
I litre. 



The water should be boiling if the solution be required 
to be used at once, otherwise it should stand in an uncorked 
bottle for twenty-four hours. 

11. 

Gold trichloride . . • '25 gramme 
Sodium acetate ... 7 grammes 
Water i litre. 

This should be mixed a day before being used. Before 
toning the solutions should be filtered in a clean dish 
slightly warmed, if the weather be cold. The prints are 
placed in water of about 15® C, and the washing continued 
as indicated in the last chapter, according to the toning 
bath employed. They are then immersed in the toning so- 
lution three or four at a time, and the dish is kept in con- 
stant motion, so as to allow an equal toning action through- 
out It is likewise essential that no two prints should stick 
together, for the same leason. jkccoxivci^ \ft \3afc colour 



Toning and Fixing the Print 149 

of the print desired, so must the continuation of the toning 
action be regulated. If a rich chestnut brown be required, 
but very little apparent change in the colour of the print is 
necessary, whereas if an engraving black tone is sought, the 
action must be continued till the image is decidedly blue. 
It is not to be inferred that these rules are absolute in 
every case ; so much depends on the sizing of the paper 
and on the amount of chloride present that they are not 
applicable in all cases, but with Saxe paper, prepared as 
given in the foregoing formulae, they will hold good. 

Fixing tJie Print. 
The fixing solution is made up as follows : — 

Sodium hyposulphite . . . 200 grammes 
Water i litre. 

Between toning and fixing it is essential that the prints 
should be well washed. The necessity of this may be under- 
stood by referring to p. 141. It has been sometimes recom- 
mended to acidify the washing water, but the proposer of this 
plan can have had no thought of the danger to the permanency 
of the prints which he thereby introduced ; an acid at once 
begins the decomposition of the hyposulphite. The writer 
strongly urges the necessity of a strongly alkaline condition 
of this bath, and in practice he adds 50 cc. of strong ammo- 
nium hydrate to it when fixing prints. Mr. J. Spiller was the 
first to point out the use of ammonium carbonate in the 
solution; he showed that it dissolved out a certain compound 
left in the whites of the picture, which otherwise was in- 
soluble, and which readily decomposed under atmospheric 
action. The pictures should be immersed in the solution for 
ten or fifteen minutes, the time varying according to the thick- 
ness of the paper ; they should then be washed (unless they 
be placed in a second solution of hyposulphite as already 
suggested), rapidly at first and afteivjaida xaot^ ^o^. 
V&rhaps the best way of eliminating lYve g;te^Xei ^^^ ^"^ "^^ 



1 62 Printing with Chromium Salts, 

negative will have become insoluble to a depth correspond- 
ing to the intensity of light entering, and as there will be 
but little of the negative which will not allow some light to 
pass through, it may be considered that the whole of the 
exterior surface of the gelatine has become insoluble, whilst 
the soluble portions remain enclosed between the insoluble 
layer and the surface of the paper. If such a print were 
immersed in hot water to dissolve away the unaltered gela- 
tine, the viscid solution would remain imprisoned, and no 
development of the image would be possible. This difficulty 
Swan overcame by cementing the insoluble smiace to 
paper by a solution of india-rubber. On immersion in hot 
water the original paper easily strips off, leaving the water 
free access to the soluble gelatine. When this is com- 
pletely dissolved away, an image in pigmented gelatine 
remains on the india-rubbered paper, though reversed as 
regards left and right This defect again was over- 
come in one of two ways — either by using a negative 
reversed as regards left and right, or by the following pro- 
cedure. Another piece of paper coated with starch or 
gelatine, was applied to the image, and allowed to dry in 
contact. The india-rubbered paper was then moistened with 
benzine or some other india-rubber solvent, and detached 

It will be well to draw the student* s attention to the 
reason why the portions of the film of gelatine become 
insoluble to depths corresponding to the intensity of light, 
instead of becoming only partially insoluble through 
their whole depth. The light that is chiefly effective in 
causing the reduction of the dichromate is the blue. Now, 
since the dichromate is of an orange colour, it is evident 
that an absorption of the blue will take place, and experi- 
ment has shown that a small thickness of gelatine coloured 
by it will prevent any effective ray being transmitted. In 
order to cause the reduction of the chromium compoimd, 
the amplitude, multiplied by the number of the waves, must 
be of a certain constant numerica\ \a\u^\\i \5cifc^\Ck^N3LcX(aIls 



The Temporary Support 163 

short of this constant no change will be effected. On this 
assumption it will be readily seen that insolubility will take 
place only to certain depths, depending on the length of ex- 
posure and intensity of the light. At the same time it will 
be seen that it does not necessarily follow that the whole of 
the gelatine or other organic body becomes insoluble to that 
depth, but that the ratio of soluble to insoluble matter in- 
creases as the depth becomes greater. This last point is 
important, for it seems that the photo-mechanical printing 
processes are really dependent on it 

In order to obtain perfection in prints formed in gelatine, 
the image should be dark-coloured and transparent or 
translucent, in order that the minutest difference in shades 
may be observable; in other words, the white ground of the 
picture must play its part in this as in silver printing. 

J. R. Johnson was the first to improve upon Swan's 
process; he found that the insoluble gelatine could be 
caused by atmospheric pressure to adhere to any impervious 
surface. This he effected in the following ingenious manner. 
The undeveloped picture printed in the usual manner on 
gelatinised paper was immersed in cold water, and allowed to 
absorb a certain quantity of the fluid, causing the unaltered 
gelatine to swell slightly. Immediately that the curling of 
the paper in the water showed that sufficient fluid had been 
imbibed, he brought the surface of the gelatine and a metal 
plate nearly in contact, a thin layer of water being allowed 
to separate them. This water he squeezed out, and the 
gelatine continuing to swell owing to the fluid remaining in 
the pores of the paper, a partial vacuum was created between 
the two surfaces, and the insoluble gelatine was found to 
adhere firmly to the metal plate. When the picture, so held, 
was immersed in hot water the paper backing could be 
stripped off, and development took place on the temporary 
metallic support. Gelatinised paper applied to the image 
could then be employed as a final swppoxX.. \ti y^^^^^ ^ 
was found that this method of deve\opxsi"eiv\. ^w^'s* \«^^ "^^ 

M 2 



ISO 



Silver Prwtting, 



hyposulphite is to place the prints in a large tub of water, 
which is kept in motion, and after five minutes* washing 
to place them in a smaller quantity of water- After this 
they may be removed to a washing trough, where the water 
will be changed several times an hour. The accompan)dng 
idea of a washing trough may prove useful. It is one which 
was designed and is employed by Mr. England, and has 

Fig. 30. 




answered well the purpose for which it is intended, a is a 
trough, at the side of which is a syphon, s, the inside leg 
reaching to within 2 or 3 millimetres of the bottom, the 
bend of which is a little below the top of the trough, b is 
a cradle, pivoted on a rod, e, which passes through the 
sides of A as shown, c is a water-wheel attached to the 
wall on to which a gentle stream of water from the tap, f, 
plays. G is a small arm attached to the axle of the wheel, 
having a rod suspended firom it, which is attached to the 
cradle, b. As the wheel slowly turns the rod is raised, and 
the prints are caused to move about in the water. The 
water runs into the trough through the pipe, h, and when 
it reaches the top of syphon pipe, the trough gradually 
empties itself, leaving the prints on the gutta-percha strips 
which form the bottom of the cradle. It will be seen 
that the supply of water must always be Ta\)J\ex\e^^^^si\isaX 



Tests for Sodium Hyposulphite. 151 

which the syphon is capable of carrying away* A useful 
addition to the trough is a horizontal pipe attached to the 
well of the wheel, moving from side to side by the motion of 
the wheel, and thus distributing the entering water over the 
surface of the prints by means of a fine rose. This prevents 
the chance of any of the prints getting surface-dry, which 
sometimes happens. In a trough of this description twelve 
hours suffice to ensure the total removal of the hyposulphite. 
Should this mode of washing be inapplicable, the prints may 
be placed in dishes, changing the water every quarter of an 
hour for the first hour, and every half-hour subsequently for 
six hours. If during this time they are well sponged twice 
or three times with a soft sponge, it will be found on apply- 
ing one of the following tests that the hyposulphite is 
eliminated. 

The first test is based on the reaction of iodine with 
sodium hyposulphite, shown at p. 142. Take a small piece 
of starch the size of a pea, powder it and boil it in 
10 cc. of water, till a clear solution is obtained j add 
5 cc. of a saturated solution of iodine in alcohol to the 
clear liquid. A dark blue colour due to starch iodide 
will now be apparent. Drop 2 drops of the solution 
into two clean test-tubes, and fill up one with dis- 
tilled water, and the other with the water to be tested. A 
faint blue colour should be perceptible in the first test- 
tube, whilst the presence of hyposulphite in the other will 
be shown by the total absence of colour. The contents 
of the two solutions in the test-tubes can be best com- 
pared by placing a piece of white paper behind them and 
examining them by reflected light. The sodium hyposul- 
phite may not be found in the washing water, yet a trace 
may remain in the prints. If a very weak solution of iodine 
be brushed across the back of a print, the absence of all 
colour will indicate the presence of the hyposulphite. One 
selected out of a batch of prints may thus be test^id^ ^\vQViJ^ 
it is Taiely necessary, if the water ind\caX.t?» \)cva.\. \N\^ ^^.^wss^^ 
Jjas been thoroughly effected. 



ISO 



Silver Prwtting, 



hyposulphite is to place the prints in a large tub of water, 
which is kept in motion, and after five minutes* washing 
to place them in a smaller quantity of water- After this 
they may be removed to a washing trough, where the water 
will be changed several times an hour. The accompanying 
idea of a washing trough may prove useful. It is one which 
was designed and is employed by Mr. England, and has 

Fig. 3a 




answered well the purpose for which it is intended, a is a 
trough, at the side of which is a syphon, s, the inside leg 
reaching to within 2 or 3 millimetres of the bottom, the 
bend of which is a little below the top of the trough, b is 
a cradle, pivoted on a rod, e, which passes through the 
sides of A as shown, c is a water-wheel attached to the 
wall on to which a gentle stream of water from the tap, f, 
plays. G is a small arm attached to the axle of the wheel, 
having a rod suspended from it, which is attached to the 
cradle, b. As the wheel slowly turns the rod is raised, and 
the prints are caused to move about in the water. The 
water runs into the trough through the pipe, h, and when 
it reaches the top of syphon pipe, the trough gradually 
empties itself, leaving the prints on the gutta-percha strips 
which form the bottom of the cradle. It will be seen 
that the supply of water must always be Ta\5cvex\^^^^^si\isa.t 



Tests for Sodium Hyposulphite. 151 

which the syphon is capable of carrying away^ A useful 
addition to the trough is a horizontal pipe attached to the 
well of the wheel, moving from side to side by the motion of 
the wheel, and thus distributing the entering water over the 
surfece of the prints by means of a fine rose. This prevents 
the chance of any of the prints getting surface-dry, which 
sometimes happens. In a trough of this description twelve 
hoiurs suffice to ensure the total removal of the hyposulphite. 
Should this mode of washing be inapplicable, the prints may 
be placed in dishes, changing the water every quarter of an 
hour for the first hour, and every half-hour subsequently for 
six hours. If during this time they are well sponged twice 
or three times with a soft sponge, it will be found on apply- 
ing one of the following tests that the hyposulphite is 
ehminated. 

The first test is based on the reaction of iodine with 
sodium hyposulphite, shown at p. 142. Take a small piece 
of starch the size of a pea, powder it and boil it in 
10 cc. of water, till a clear solution is obtained ; add 
5 cc. of a saturated solution of iodine in alcohol to the 
clear liquid. A dark blue colour due to starch iodide 
will now be apparent. Drop 2 drops of the solution 
into two clean test-tubes, and fill up one with dis- 
tilled water, and the other with the water to be tested. A 
faint blue colour should be perceptible in the first test- 
tube, whilst the presence of hyposulphite in the other will 
be shown by the total absence of colour. The contents 
of the two solutions in the test-tubes can be best com- 
pared by placing a piece of white paper behind them and 
examining them by reflected light. The sodium hyposul- 
phite may not be found in the washing water, yet a trace 
may remain in the prints. If a very weak solution of iodine 
be brushed across the back of a print, the absence of all 
colour will indicate the presence of the hyposulphite. One 
selected out of a batch of prints may thus be test^d^ tKowj^ 
it is rarely necessary, if the water indicale^ XJcvaX. \Js\fe ^^.^^wccs-^x, 
Jjas been thoroughly efifected. 



ISO 



Silver Printing. 



hyposulphite is to place the prints in a large tub of water, 
which is kept in motion, and after five minutes* washing 
to place them in a smaller quantity of water- After this 
they may be removed to a washing trough, where the water 
will be changed several times an hour. The accompanying 
idea of a washing trough may prove useful. It is one which 
was designed and is employed by Mr. England, and has 

Fig. 3a 




answered well the purpose for which it is intended, a is a 
trough, at the side of which is a syphon, s, the inside leg 
reaching to within 2 or 3 millimetres of the bottom, the 
bend of which is a little below the top of the trough, b is 
a cradle, pivoted on a rod, e, which passes through the 
sides of A as shown, c is a water-wheel attached to the 
wall on to which a gentle stream of water from the tap, f, 
plays. G is a small arm attached to the axle of the wheel, 
having a rod suspended from it, which is attached to the 
cradle, b. As the wheel slowly turns the rod is raised, and 
the prints are caused to move about in the water. The 
water runs into the trough through the pipe, h, and when 
it reaches the top of syphon pipe, the trough gradually 
empties itself, leaving the prints on the gutta-percha strips 
which form the bottom of the cradle. It will be seen 
that the supply of water must always be ia\5[ve\\t.'sSk^^si\ivaX 



Tests for Sodium Hyposulphite. 151 

which the syphon is capable of carrying away* A useful 
addition to the trough is a horizontal pipe attached to the 
well of the wheel, moving from side to side by the motion of 
the wheel, and thus distributing the entering water over the 
surfece of the prints by means of a fine rose. This prevents 
the chance of any of the prints getting surface-dry, which 
sometimes happens. In a trough of this description twelve 
hours suffice to ensure the total removal of the hyposulphite. 
Should this mode of washing be inapplicable, the prints may 
be placed in dishes, changing the water every quarter of an 
hour for the first hour, and every half-hour subsequently for 
six hours. If during this time they are well sponged twice 
or three times with a soft sponge, it will be found on apply- 
ing one of the following tests that the hyposulphite is 
eliminated. 

The first test is based on the reaction of iodine with 
sodium hyposulphite, shown at p. 142. Take a small piece 
of starch the size of a pea, powder it and boil it in 
10 cc. of water, till a clear solution is obtained j add 
5 cc. of a saturated solution of iodine in alcohol to the 
clear liquid. A dark blue colour due to starch iodide 
will now be apparent. Drop 2 drops of the solution 
into two clean test-tubes, and fill up one with dis- 
tilled water, and the other with the water to be tested. A 
faint blue colour should be perceptible in the first test- 
tube, whilst the presence of hyposulphite in the other will 
be shown by the total absence of colour. The contents 
of the two solutions in the test-tubes can be best com- 
pared by placing a piece of white paper behind them and 
examining them by reflected light. The sodium hyposul- 
phite may not be found in the washing water, yet a trace 
may remain in the prints. If a very weak solution of iodine 
be brushed across the back of a print, the absence of all 
coloiu: will indicate the presence of the hyposulphite. One 
selected out of a batch of prints may thus be test^d^ tkow^ 
it is rarely necessary, if the water indicale^ XJcvaX. ^^ ^^i^kcsvx, 
Jjas been thoroughly efifected. 



ISO 



Silver Printing. 



hyposulphite is to place the prints in a large tub of water, 
which is kept in motion, and after five minutes* washing 
to place them in a smaller quantity of water- Aftei* this 
they may be removed to a washing trough, where the water 
will be changed several times an hour. The accompanying 
idea of a washing trough may prove useful. It is one which 
was designed and is employed by Mr. England, and has 

Fig. 3a 




answered well the purpose for which it is intended, a is a 
trough, at the side of which is a syphon, s, the inside leg 
reaching to within 2 or 3 millimetres of the bottom, the 
bend of which is a little below the top of the trough, b is 
a cradle, pivoted on a rod, e, which passes through the 
sides of A as shown, c is a water-wheel attached to the 
wall on to which a gentle stream of water from the tap, f, 
plays. G is a small arm attached to the axle of the wheel, 
having a rod suspended from it, which is attached to the 
cradle, b. As the wheel slowly turns the rod is raised, and 
the prints are caused to move about in the water. The 
water runs into the trough through the pipe, h, and when 
it reaches the top of syphon pipe, the trough gradually 
empties itself, leaving the prints on the gutta-percha strips 
which form the bottom of the cradle. It will be seen 
that the supply of water must aLwa-ysbe T^x5cv^\\ts&^^si\ivax 



Tests for Sodium Hyposulphite, 151 

which the syphon is capable of carrying away* A useful 
addition to the trough is a horizontal pipe attached to the 
well of the wheel, moving from side to side by the motion of 
the wheel, and thus distributing the entering water over the 
sur^e of the prints by means of a fine rose. This prevents 
the chance of any of the prints getting surface-dry, which 
sometimes happens. In a trough of this description twelve 
hours suffice to ensure the total removal of the hyposulphite. 
Should this mode of washing be inapplicable, the prints may 
be placed in dishes, changing the water every quarter of an 
hour for the first hour, and every half-hour subsequently for 
six hours. If during this time they are well sponged twice 
or three times with a soft sponge, it will be found on apply- 
ing one of the following tests that the hyposulphite is 
eUminated. 

The first test is based on the reaction of iodine with 
sodium hyposulphite, shown at p. 142. Take a small piece 
of starch the size of a pea, powder it and boil it in 
10 cc. of water, till a clear solution is obtained j add 
5 cc. of a saturated solution of iodine in alcohol to the 
clear liquid. A dark blue colour due to starch iodide 
will now be apparent. Drop 2 drops of the solution 
into two clean test-tubes, and fill up one with dis- 
tilled water, and the other with the water to be tested. A 
faint blue colour should be perceptible in the first test- 
tube, whilst the presence of hyposulphite in the other will 
be shown by the total absence of colour. The contents 
of the two solutions in the test-tubes can be best com- 
pared by placing a piece of white paper behind them and 
examining them by reflected light. The sodium hyposul- 
phite may not be found in the washing water, yet a trace 
may remain in the prints. If a very weak solution of iodine 
be brushed across the back of a print, the absence of all 
coloiu: will indicate the presence of the hyposulphite. One 
selected out of a batch of prints may tlius be test^d^ tivcwj^ 
it is Taieiy necessary, if the water indicale^ XJcvaX. \Js\fe ^^!^kcs\^ 
lias been thoroughly effected. 



I ^2 Willies A niline Process. 

softened transfer paper is applied to it in the same manner 
as was adopted for causing the undeveloped gelatine image 
to adhere to the zinc plate. 

After drying, the picture- will peel off from the plate and 
adhere to the transfer paper. The carbon print is then 
complete. 

The same manipulations, with a few evident modifica- 
tions, are necessary when the temporary support is pliable. 

When a reversed negative is employed, the image may 
be developed on the final support 



CHAPTER XXV. 

Willis's aniline process. 



Willis's aniline process may next be briefly described. It is 
dependent on the action of dichromates on organic matter, 
though the printed image is given colour by means of aniline. 
Sized paper is floated in potassium dichromate, to which 
a little phosphoric acid has been added. It is then ex- 
posed beneath a transparent or translucent positive, such as a 
plan or map, and when the image is perfectly visible, it is 
exposed to the action of aniline Vapour. Aniline salts have 
the property of striking a green, black, or reddish colour 
when brought in contact with acidified dichromates ; hence 
those parts which have not been exposed to light, or have 
been shielded from it (as is the case with the lines of the 
positive print), are deeply coloured, the rest of the paper 
remaining of the faint colour due to the reduced chromium 
oxide. In developing these prints, aniline is dissolved in 
spirits of wine, and the mixed vapours are allowed to come 
in contact with the print. It will at once be evident what 
an extremely valuable process this is for copying engravings, 
plans, and tracings. All that is required is a sensitising 
solution, a sheet of glass to pVace o\« tVv^ -^laxv^ &c, (which, 



The Powder Process. 173 

when exposed, should have its back in contact with the 
sensitive paper), to keep them in contact, and the sensitised 
paper. A rough box with a lid, on which can be stretched 
the printed paper, a basin to contain the aniline solution, 
and a spirit lamp to warm it, complete the outfit. 

The prints can be washed, and are then tolerably per- 
manent 

This process is the subject of a patent secured by the 
inventor, Mr. W. Willis, and is worked commercially by one 
establishment On the expiration of the patent it will no 
doubt be largely employed by engineers and others for the 
purposes indicated. There are various modifications of this 
method of printing by using coloured aniline dyes, such as 
rosaniline. For some purposes they are useful, but as a 
rule, they are better for the reproduction of subjects executed 
in line than for half-tone negatives. 

THE POWDER PROCESS. 

Reference has already been made to Poitevin*s process, 
in which originally salts of iron were employed to sensitise 
gelatine, the development being effected by the application 
of plumbago, or other impalpable powder. The dichromates 
subsequently were found to answer better than the ferric 
salts, tiie development of the prints being somewhat more 
easy with them. A mixture of gum-arabic, sugar, and 
a little glycerine, together with a sensitising solution of 
potassium dichromate, is prepared and poured over a glass 
plate, or other impervious surface, and allowed to dry in a 
warm temperature. The plate thus prepared is at once 
exposed for a few minutes beneath a transparent positive 
and withdrawn. Those parts acted upon by light will be found 
to be hygroscopic in the ratio of the time of exposure and in- 
tensity of the light. Any impalpable powder brushed over 
the plate will now be found to adhere to these hY^o^i-cy^vo. 
psLTts in proportion to the moVslMte -wV^cJci ^^^ VOsSs., 



174 ^ Voodburytype. 

Hence a positive, reversed as regards left and right, will 
result When the image is developed it is coated with 
collodion, and can then be transferred to paper, &c., in an 
unreversed position. The soluble dichromate will be washed 
out during the process of transferring. This process is some- 
times employed for obtaining images which can be burnt in 
on glass or enamels. For those who wish to try the process 
the following formula for the sensitive compound will be 
found efficient. It is due to Obemetter, of Munich : — 

Dextrine 4 parts 

White sugar 5 parts 

Ammonium dichromate ... 2 parts 

Glycerine ...... 2 to 8 drops for every 

100 cc. of water 

Water 96 parts 

It is sometimes recommended to give the glass plate a pre- 
liminary coating of plain collodion. The powder must be 
very gently applied with a cotton-wool brush or fine cameFs- 
hair brush. 

WOODBURYTYPE. 

The Woodburytype process is an exceedingly ingenious 
method of obtaining a mould of a gelatine print, from which 
other prints may be obtained. A rather thick film of 
sensitive gelatine is prepared, resting on a tough film of 
collodion. This is placed beneath a negative, the collodion 
side being next the image. It is then exposed to light pro- 
ceeding from a point, or to sun-light, arranged in such a 
manner that it always receives the rays in one direction. 
Uncontrolled diffused light will not do, as, owing to the thick- 
ness of the gelatine, the image on development would be 
ill-defined. When sufficiently printed, the gelatine picture 
is developed as if it were an autotype print, presenting the 
image in considerable relief. When dried, the gelatine pic- 
ture is placed on a perfectly flat metal plate, and a sheet of 
soft metal (lead, for instance) \s ptes^t^ oti*\\.\yj tc^ftwa&Ql an 




Production of Prints by Woodburytype. 175 

hydrauUc press. This latter presenting an exact mould 
of the former, is then placed in a press made as in ac- 
companying figure. Gela- 
tine is next dissolved in 
hot waler and fine pig- 
ment or permanent dye 
added to it, and the vis- 
cous solution thus pre- 
pared is poured on to the 
mould. Paper of a very 
even texture, and which 
has been strongly sized, 
is placed on the top of the 
pool of liquid gelatine and 
the top plate of the press, 
hinged as shown in fig. - 
35, is brought down on _ 

to the mould and firmly ~ 

locked by the catch, also shown in the same figure, squeezing 
out the superfluous gelatine. When it is judged that the 
gelatine has set (which it soon does, owing to its contiguity 
to a mass of cold metal), the top is raised and the paper, 
which now bears the picture, is detached. The print is im- 
mersed in a solution of alum to render the picture insoluble. 
The top plate, which is of glass, must be a perfect plane, 
otherwise the liquid gelatine will not be squeezed out from 
the portions that are to remain white, and the print will be 
uneven and mottled. The great relief of the original image 
is necessary in order to give sufficient intensity to the re- 
production, for it must be recollected that the gelatine solu- 
tion, filling even the greatest depths of the mould, will 
present but a very thin layer on drying. If the mould were 
obtained from an ordinary gelatine print there would not 
be sufficient depth of colour properly to represent the 
various gradations of shade. The pictures produced by this 
process are presumably permanent, aiii ca-ti. \«. ■^x'^-«:r.&. 



1/6 Photo-Lithographic Transfers, 

at a cost but little in excess of the gelatine solution and 
the paper employed. As can be understood, there are diffi- 
culties in the way of producing any large surface which 
should be represented by pure white, since, however homo- 
geneous a paper may be, it is invariably slightly thicker in 
some parts than others, and this prevents the glass plate 
attached to the lid of the press from fulfilling its functions 
in an absolutely perfect manner. 



CHAPTER XXVI. 

PHOTO-LITHOGRAPHIC TRANSFERS, 

Another process, to which reference must be made, is that 
perfected by Colonel De C. Scott, R.E., and Sir Henry James, 
late Director of the Ordnance Survey of Great Britain. It 
also is dependent on the insolubility of gelatine when treated 
with a dichromate and exposed to light. It will be described 
in detail, as it is capable of producing prints in printer's 
ink, as well as in ink suitable to give a transfer on to zinc or 
stone. From such transferred prints the original drawing 
can be reproduced by ordinary surface printing. It may 
be well to notice the requisites for these transferrable 
prints. First, the image should be made in an ink which 
is readily held by a lithographic stone or mulled zinc 
plate. Secondly, it must be capable of a fair amount 
of resistance to pressure; that is, it must be tolerably 
hard and cohesive, otherwise the act of passing a paper 
holding the image through a lithographic press would cause 
a spreading of the ink, and a consequent want of sharp- 
ness in all the impressions taken firom the stone. Thirdly, 
the ink must be of such a quality that a very thin coating is 
sufficient to leave a sharp and firm impression on the stone 
or zinc plate. Fourthly, the paper on which the image is de- 
re/oped must be tough, and not easily torn or stretched. 
These requisites are fulfilled li \he icJ^oVm^ ^w^kXxqqs are 
attended to. The best paper to ^e\ecx\^ >i:cv^x>sxiwrii^\ws^ 



Preparation of Gelatinised Paper, i yj 

post paper, which is not highly sized. If it be, the sizing 
should be removed by immersion in boiling water, previous 
to coating it with the gelatine solution. The solution is pre- 
pared according to this formula: — 

Potassium dichromate . . .44 grammes 

Gelatine 44 to 6d grammes 

Glycerine . . . . . . 2 cc. 

Water I litre. 

The varying quantity of gelatine is due to the fact that 
some gelatines give much more body to the solution than 
others. Thus, if fine-cut gelatine be employed it has been 
found in the writer's experience that the larger quantity will 
be necessary, whilst if flake gelatine be employed the smaller 
quantity will usually suffice. The gelatine is of course 
thoroughly softened in half the above quantity of water, and 
then the remaining half, in which the dichromate has been 
dissolved, is added in a boiling state. The solution is poured 
into a dish, and placed over the hot-water tin, as described 
at p. 167. A sheet of paper of the proper size is floated on 
it for three minutes, and then hung up by two corners to dry. 
This causes the coating to be thicker at the bottom comers 
than the top, to avoid which resort may be had to the 
artifice shown in the figure, p. 167. In any case a second 
coating is required, and this is given in a similar manner. If 
the paper have been hung up to dry previous to the setting 
of the gelatine, the opposite corners to those by which the 
sheet was first suspended should be hung lowest. This 
secures a fairly even coating. The paper in this condition, 
even when damp, is slightly sensitive, and therefore it 
should be dried in a room which only admits non-actinic 
light. It is exposed in the ordinary manner beneath a 
negative, which should be of a line engraving, and not in 
half tint.* When the lines appear of a well-defined fawn 
colour on a yellow ground, the paper should be removed to 
the dark room for subsequent treatment. 

* Partial success has been obtained by S\t Wervt^ ^wxvt's.Vsv \<t\A«tx- 
mg even this latter class of work. 

N 






.. : -: z'tr.':t zizsz be made, j 
. ?• . T. J- Z.. iz.'i Sir Heniy J 
.rr-ii .r "f-jT r'- :: Great Britai 
••- . •..- :czeli£newhen L 
.: r*r : - . j'xi- I: will be dfc 
. -: .. ^rid'nrz prints in y 
"-^-r "": ~r X irsnsfer on to^ 
".riiT—ri zmzs ±« <x^nal 
.:-::n^— >iriice printing. * 
: -r:;::^-is 5oc these tear* 
^: ?c:.i..i re zzode inani ^ 
:."i:.:sr::LTcic sccxie or mr* 
1.U-C r»; rarable of a fai'f^ 
^:ri ±j.: is. iz must 1: 



178 PhotO'Lithographic Transfers, 

If the object be to make a print to transfer to stone or 
zinc, the following ink should be prepared (though any 
lithographic ink will answer fairly well): — 

Lithographic printing ink . . .16 parts by weight 
Middle linseed varnish . . 8 parts 

Burgundy pitch 6 parts 

Palm oil . . . . . .1 part 

White wax I part 

Bitumen 2 parts 

The ink and varnish are first mulled together with a 
muller, the Burgundy pitch and bitumen are next melted 
over a clear fire till all water is driven off, the wax next 
melted, and finally the palm oil. When properly melted 
they should readily catch fire, which shows that certain gases 
are being liberated. The ink and varnish are now well 
stirred into it, and the mixture run into the pots for storage. 
Should it be desired only to make a single print, the best 
ordinary chalk lithographic ink may be employed. 

Where a lithographic press is available, a fine and even 
coating of one of these inks is usually given to a stone by 
means of a lithographic roller, the paper bearing the picture 
is then placed face downwards on it, and pulled through 
the press, by which plan a thin coating of ink is given to 
the entire sheet of paper. In the absence of a press the ink 
may be rendered liquid with turpentine, and an even film of 
ink may be given with a fine sponge. 

To develop the picture the print is floated back down- 
wards on a dish of water, having a temperature of about 
50® C, and is allowed to remain on it till the lines are seen 
as depressions. It is then removed on to a sloping board, 
and a stream of warm water, of about 70** C, is poured over 
the surface; the soluble gelatine being in a hydrated condi- 
tion, is carried away together with the ink that covered 
it, and the image is left, formed of ink resting on slighdy- 
raised ridges of insoluble gelatine. A very soft sponge 
dipped in the hot water and a^^lkd to the surface 
aids the rfeveJopment, in fact \t caxv t^t^Vj \i^ ^^t^oov- 



Lithographic Press. 




i8o Photo-Lithographic Transfers. 

plished without it ; but the most deKcate touch is required 
for this part of the operation, as the ink on the fine lines is 
very liable to be carried away. The developed print is next 
washed in cold water, and then hung up to dry. In this state 
it is ready for transfer to stone or zinc, if transfer ink have 
been employed. It is beyond the scope of this book to 
describe the transferring operations : these are described 
in other works.* A very convenient lithographic press, suit- 
able for amateurs, has lately come under the notice of the 
writer ; the figure on the preceding page will give some 
idea of its form. It is cheap and well adapted for this 
process, as well as for certain photo-mechanical printing 
processes. 

A A is a cast-iron carriage of the form shown, b is the 
bed of the press, which is caused to move in the carriage 
by means of a roller, to which is attached the arm h. 
D D are drawers containing the necessary plant, c is a litho- 
graphic stone, shown in position, and held firm by means 
of the cross pieces of angle-iron fitting into the slots as 
shown. E is a substitute for the usual scraper ; it con- 
sists of a roller, round which, as well as round a smaller 
roller, is passed a band of flannel. A downward pressure 
can be given to the roller by an ingeniously devised screw- 
motion, F, which, whilst giving the necessary pressure, yet 
causes it to take the natural bearing of the stone or plate. 
K K are the clamps by which it can be attached to a table, 
and R is the roller supplied for inking it. With this machine 
the impressions pulled are excellent, and it is very portable. 

Plates made of composition similar to solder are sup- 
plied by the manufacturer. They are excessively sensitive 
to greasy ink, and a number of impressions can be pulled off 
without clogging the work. To clean these plates all that 
is required is to wash out the old work with a solution 
of caustic potash, and then to scour the surface with fine 

' Instruction in Photography y published by Messrs. Piper & Carter ; 
or in Sir Henry James' work, p\ib\\%\ved >q^ 'Nl^?sR,.\Axv^caaxvs. 



P/totO'Engraving Processes. i8i 

emery powder. A dilute solution of acid is poured over the 
plate, and after washing under the tap, and gently warming, 
it is/eady to receive a transfer. 

Should only one copy of the picture be required, the 
print, which should in that case have been printed in litho- 
graphic ink, is placed in a copying, lithographic, or typographic 
press, face up, and a slightly damped piece of white or 
other paper placed over it. When the pressure is brought to 
bear, the ink is retained by the latter, and a good impression 
is thus obtained. This method has been named by Sir H. 
James as the papyrograph. It must not be mistaken for 
another process, used for copying letters or circular?, and 
known by the same name. 

Various modifications of this process have from time 
to time been proposed, such as coating the gelatine with 
albumen, but in the writer's experience, when a picture is to 
be obtained by dissolving away the gelatine, no better pro- 
cess than the above can be used. 



CHAPTER XXVII. 

PHOTO-ENGRAVING AND RELIEF PROCESSES. 

Ni^pce's process, it will be recollected, was founded on 
the fact that bitumen of Judaea, when exposed to light, 
became insoluble in its ordinary solvents if partially 
saturated. Silver plates were coated with bitumen, and 
after exposure the unaltered portions were dissolved away 
and iodine applied. The . remaining bitumen was then re- 
moved, and the image was consequently formed of metallic 
silver on a ground of silver iodide. Had Nidpce removed 
the iodide by any proper solvent, he would have obtained 
a plate slightly engraved. Most of the present processes 
for photographically obtaining relief blocks, ^xv^ "^^^^ ^^- 
graved plates, are based on the same pxvcvcv^^ ^& ^x^^^^^^^s 



1 82 PhotO'Engraving Processes. 

in fact, there is very little departure from his mode of work- 
ing until the biting-in commences. The student must dis- 
tinguish between a relief and an engraved plate. The former 
is intended to be printed in the ordinary printing press, the 
portions representing the lines of the sketch being raised as 
in a wood- cut, whilst with the latter they are in depression. 
From the nature of the processes to be described it will be 
seen that the objects to be copied must be drawings in lines 
and not in half-tint, and up to the present time there is no 
process of which the details are published, which is capable 
of giving a good print either from an engraved plate or 
from a relief block, preserving the proper gradations in 
light and shade. Secret processes, however, there are, which 
furnish excellent results : some of these will be mentioned at 
the end of the chapter. An outline of a successfril pro- 
cess for the production of either a relief block or an en- 
graved plate will now be given. • 

A plate is coated with a thin film of asphaltum or 
bitumen of Judaea, dissolved in chloroform or other con- 
venient solvent, and after drying it is ready for exposure 
beneath a subject. If an engraved plate be required, the 
parts that have to be bitten in are the lines ; hence those 
portions must be protected from the action of light, since in 
order to lay the surface of the metal bare they should be 
covered with the soluble asphaltum. In taking a print from 
an engraved plate, the latter is reversed as regards left and 
right, therefore it is evident that a reztersed positive should 
be employed, from which to print on the metal plate* 
For the production of a relief block, by similar reasoning 
it will be found that an ordinary unreversed negative pic- 
ture is required, as it is from its nature reversed as regards 
right and left. Whether a positive or a negative be em- 
ployed, the opacity should be extreme on those portions 
which are to protect the sensitive layer from light Such a 
positive or negative is placed in contact with the plate, and 
exposure given till it is judged t\val svaS^d^x^X \sv^ON&k>L\t«f is 



Ekrard's Process, 183 

given to the exposed portions. The soluble portions are 
then dissolved away by a solvent which is nearly saturated 
with the asphaltum. If the manipulations have succeeded, 
the metal should be perfectly bare in parts. Steel, copper, 
or zinc plates may be employed for this work; the two 
former are more especially suitable for engraving. The 
mordant usually employed for these may be a mixture of 
hydrochloric acid with potassium chlorate, which causes an 
evolution of chlorine. For zinc, hydrochloric acid alone 
may be employed, though it is well previously to dip the 
plate in a solution of copper sulphate. For an engraving 
the biting-in need be but very slight, though much of course 
must depend on the nature of the work as shown by the 
thickness of the lines. The thicker the lines the deeper 
must be the biting-in. For a relief block the biting-in has 
to be carried to a far greater extent ; in fact, as deeply as 
yseen in !an ordinary wood-cut. This involves very tedious 
manipulation after the first biting. The plate has to be 
warmed, dusted with resin ; again heated to slightly melt the 
bitumen, so as to allow it to flow down the sides of the 
bitten-in lines. This process has to be repeated till a suffi- 
cient depth is attained. When there are larger spaces of 
white in the print, the metal is usually removed by a fine 
saw, or a graver. Relief-block making is essentially difficult 
in almost every stage, and rarely repays an amateur the 
labour he may bestow upon it. 

Ehrard, of Paris, has another method of producing en- 
gravings, which is also dependent on biting in. He prepares 
a transfer, as for zincography, and, after going through the 
usual manipulations to transfer it to a copper-plate, he plunges 
it into an electro-plating bath for a few minutes, thus covering 
the copper with a thin film of silver, the lines of the engraving 
being protected by the greasy ink. After a rinse in dilute 
acid the plate is transferred to a bath of mercuric chloride, 
where the silver is converted into the dov\bl^ eVvVsnAR.* ^5x^x 
wasbjij^^ the ink is removed, and the V\tvi\% ^xoc^ss "^^^^^^^ 



1 84 Photo-Engraving Processes, 

to proceed. The details of this process are a secret, but 
what is stated above gives a general idea of the process. 
The analogy that exists between this and Fox Talbot's 
process of engraving a daguerreotype plate is obvious. 

Another process for obtaining the same results, various 
modifications of which have from time to time been an- 
nounced, is due to Talbot. It consists of printing the nega- 
tive on a gelatine film, washing away the unaltered gelatine, 
and making an electrotype from it. In the trade there are 
several firms who practise either photo- engraving or relief- 
block making, but it is not known which methods they 
adopt, as the several processes are kept secret. Amongst 
these may be named Goupil, Gillot, and Dujardin, of Paris ; 
Dallas, and Leitch & Co., of London. Scamoni, of St. 
Petersburg, also makes very beautiful reproductions of 
engravings, &c. His method seems to be based on the 
building up of a relief on the negative itself, and then taking 
an electrotype. Fig. 37 is a print from a photo-relief plate 
by Warnerke, produced by a process of which the details are 
not as yet pubHshed. 

As already stated, all these processes seem adapted to 
the reproduction of line work in contradistinction to half- 
tone drawings or photographs from nature. Woodbury, 
however, introduced a method of making plates to give 
mezzo-tint prints from ordinary negatives, which has been 
adopted by Roussillon, the manager of Goupil & Cc's works 
at Asnibres. It is founded on the Woodburytype process, 
a grain being given by the action of light, the dimensions of 
the granules depending on the extent to which the light has 
acted. The student must picture to himself the formation of 
a Woodburytype mould made as already described having 
the grain, and he will then readily see that the desired de- 
pressions are present for copper-plate printing. It appears 
that the graver has to be employed to touch up these 
plates, and it is difficult to know how much is due to the 
merely mechanical reproductiotv, aivd Vow \«wOft.\.Q ^e artist 
employed upon them. 



Pkoto-Relief Plate. 185 




1 86 Photo- Collotype Processes, 

Mr. Dallas, of Gray's Inn Road, has produced photo- 
relief blocks for the reproduction of half-tone prints, but 
the details of his process are kept secret. Some of the 
specimen prints produced by this gentleman leave but 
little to be desired, especially if they have undergone no 
retouching with the graver. 

We cannot quit this subject without remarking that some 
very beautiful half-tone typographical blocks were produced 
by Pretsch as early as 1858, his process being based on that 
of Talbot, already mentioned. 



CHAPTER XXVIII. 

PHOTO-COLLOTYPE PROCESSES. 



By a photo-collotype process is meant a 'surface print- 
ing' process, by which prints are obtained from the 
surface of a film of gelatine, or other kindred sub- 
stance. The general methods by which such surfaces 
are formed are based upon the one fact already pointed 
out at p. 161, that gelatine, like other similar bodies, when 
impregnated with potassium dicKromate, becomes in- 
capable of absorbing moisture after full exposure to light ; 
and that where light has partially acted, there it becomes 
only partiaJly Absorbent, when compared with the amount it 
will absorb when entirely guarded from light. Suppose we 
prepare a film of gelatine with which has been mixed some 
potassium dichromate, by floating a warm solution of the 
mixture over a smooth surface, such as a thick glass plate, 
and when dry expose it bf»eath a negative in which we 
have different degrees of light and shadow, as in a land- 
scape or a portrait negative; on immersing the film in cold 
water, we shall have a picture impressed in which the different 
de^ees of shadow are represented by different degrees of 
relief. If the back of a siraAaiVy U^^X^^ ^^Vaxxaa film be 



Character of the Printing Surface, 1 87 

exposed to light previously to its immersion, the relief after- 
wards will be found to be much slighter. This is evidently a 
necessary consequence. If over either of these surfaces, when 
all superfluous moisture has been removed, a smooth soft 
roller carrying a fine layer of greasy ink be passed, it will be 
found that the greasy ink will adhere to the parts exposed 
to light in nearly exact proportion to the intensity of light 
which has acted on them. 

With the film in which the relief is high the ink will take 
less readily, because the roller, even when tolerably soft, will 
fail to come in contact with the exposed parts. With the 
film having but small relief the difficulty will not be found. 
If such a film as the latter be now placed in a printing 
press, an impression from it may be obtained, but it will 
be foimd that as regards right and left the pictures will be 
reversed. A reversed negative is therefore necessary. Theo- 
retically the number of impressions which can be pulled from 
the surface is not limited, if the surface.be kept damp, and if a 
fresh application of ink be given by the roller. It will be 
found, however, that after each pull there is a tendency of the 
unexposed gelatine to adhere to the paper, and thus to 
spoil the printing surface. In order to prevent this it has 
become customary to introduce into the gelatine some sub- 
stance which will harden it. Certain gum resins, alum, 
chrome alum, and kindred substances effect this hardening, 
and one or other of them is to be found in the formulae given 
for most of these processes. Albert, of Munich, may be 
said to have first discovered a thoroughly workable process, 
based on the above principles, and we shall briefly give an 
outline of the method he adopted as being a typical one, 
and unencumbered with any of the large number of modi- 
fications introduced at various times by other experi- 
menters. 

A piece of plate glass some 2 centimetres in thickness 
is coated with a gelatine mixture made as follows ; — 



i88 Photo-Collotype Processes, 

I. 

Good glue lo grammes. 

Water 80 cc. 

II. 

Potassium dichromate ... 3 grammes. 

Water 40 cc. 

These are dissolved separately and mixed warm. The 
plate is then coated and dried by heat, 5 or 6 hours expo- 
sure to a temperature of about 60® C. being sufficient to 
effect desiccation. The plates are now exposed back upper- 
most to light for about a quarter of an hour, the gelatine 
films resting on a smooth black surface, after which they re- 
ceive over the first a second coating made as follows : — 

I. 

Gelatine 8 grammes. 

Water loo cc. 

II. 

Potassium dichromate ... 3 grammes. 

Water 40 cc. 

To No. I is added 60 cc. of white of egg, and after 
heating to 60° C, No. 2 is mixed with it, and the solu- 
tion is filtered through cotton-wool. This coating is 
dried, and the plate is ready for printing. The expo- 
sure depends upon the quality of the light; it must be 
continued till the whole of the details are visible on 
the gelatine, and much of the success depends upon 
the depth to which it is carried. When judged suffi- 
ciently printed, the back of the plate is again exposed to 
light to such a degree that the resulting relief when the 
film is wetted will be small. The film is now washed to re- 
move all excess of the dichromate, and is again allowed to 
dry. The dried plate is next placed for 5 minutes face up- 
permost in a dish containing a 25 per cent solution of 
glycerine in water. The bacV '\s xWw ew^a^^^t^ ^w t\ve bed 



Helioiype. 189 

of a lithographic press by means of plaster of Paris, and is 
lightly rubbed over with linseed oil, and again slightly 
damped with water. A soft roller, charged with greasy ink, 
is then passed over the surface,, when it is found that a 
perfect print appears on the surface. The plate, the surface 
of which is in contact with a piece of paper, is now passed 
beneath the press, and an impression pulled. Such a press 
as that in fig. 36 may be employed. 

Mr. Ernest Edwards introduced an important modifica- 
tion of the above by mixing chrome alum with the gelatine 
to harden the gelatine film. He only uses one coating to 
the glass plate, and when dried strips it from the glass 
surface, and prints it in this condition. He retransfers 
the film to a pewter or other metal plate, and pulls im- 
pressions from it when thus supported. By this device the 
danger of destroying the printing surface, owing to the 
possible breakage of the glass plate, is overcome, and in 
consequence the cost of production is diminished. For a 
full description of the process, which is named * Heliotype ' 
•by the inventor, the student is referred to another work.^ 

The most recent modification of these collotype pro- 
cesses is by Capt Waterhouse, Assistant Surveyor General 
of India, to whom many improvements in facilitating the 
production of mechanical prints are due. We give the pro- 
cess as he describes it in the * Year Book of Photography, 
1877:— 

* As a support for the sensitive film I use flat plates of 
copper, the same as used for engraving, finely grained on 
one side. 

' Having been levelled on the drying apparatus, the 
plates are washed with warm water, and coated on the 
grained side, while wet, with a mixture composed of — 

Gelatine (Nelson's opaque) . . . 15 grammes. 

Water 100 cc. 

Potassium dichromate, in powder . . 4 grammes. 

Formic acid (when the former are d\sso\\ed'\ /^ t^. 



' fns/ructicn in Photography^ P\pet axvA C-axVex, 



1 90 Photo- Collotype Pi^ocesses, 

* This is applied like collodion, the excess being poured 
away, so as to leave just sufficient on the plates to give a 
thin, even coating. About 6 grammes of gelatine in solu- 
tion is sufficient to coat 1,000 square centimetres. 

*When coated, the plates are replaced in the drying 
apparatus, covered over, and left to dry. If the coating has 
been well applied, and the plates are fairly flat, the films 
dry up in the course of an] hour or two ' with a fine, even, 
glossy surface, perfectly free fi^om the streaks and waviness 
so common when working with thick films. 

* As the formic acid does not exercise a strong reducing 
action on the bichromate, it is well not to use the plates 
quite firesh, but to let them harden for a day or two ; other- 
wise the film will be tender, and adhere to the paper in 
printing. This might, however, be remedied, if desired, by 
the cautious addition of some hardening agent, such as 
chrome alum, glycerine, glucose, honey, &c. 

* The films are very sensitive, and do not require long 
exposure. From 10 to 20 minutes in the shade is sufficient 
for negatives of ordinary density. 

* After exposure the plates are plunged into a trough of 
water, and washed to remove the dichromate, and are then 
ready for the press. 

* Glue rollers are the best for inking, and though I have 
not had an opportunity of thoroughly testing the endurance 
of the plates, the wear appears to be very little ; while there 
is no tendency at all for the film to chip or break away from 
the plate. Owing to the relief being very slight, as well as 
to the composition of the films, the plates take the ink very 
readily. 

*Any kind of suitable printing press may be used ; but 
vertical pressure is, perhaps, the best, as being least wearing 
to the printing surface, and preserving the flatness of the 
copper. 

* This refers to an Indian climate ; the plates in our colder climate 
should be kept in a room at a tempeiaXxrct o^ ^iJoovil ^o c. 



Lenses. 



191 



*The disadvantage of the process is the difficulty of 
retaining the flatness of the copper plates, and, conse- 
quently, of securing their perfect contact with the negative 
in the pressure frame. With thin plates this may be over- 
come by strong pressure ; but it is better, when practicable, 
to transfer the negative film to the gelatine surface in a bath 
of alcohol/ 

It is scarcely possible to enumerate all the different col- 
lot)rpe processes : we may mention the autotype mechani- 
cal process, Pumphrey's and ThieFs, as amongst the most 
successful 



CHAPTER XXIX. 



THE LENS. 



Fig. 38. 



Without entering into any discussion as to the- theory of 
light, it will suffice to glance at the more general laws of 
geometrical optics, such being sufficient to show the prin- 
ciples on which photographic lenses have been designed. 

A ray of light, whilst passing through a medium of uniform 
density, travels in straight lines, and when a ray of light 
passes from any medium to one 
more dense, at any angle less than a 
right angle to the tangent of the 
common surface, the direction of the 
ray of light is bent in towards the 
normal of the common surface ; and 
if the ray pass from a medium to 
one less dense, it is bent away from 
the normal. Fig. 38 explains what 
is meant by the above. Let g g 

represent the section of a thick sheet of glass with parallel 
surfaces. Let a ray of light, a b, strike the top surface of 
the glass at b. Glass being a denser medium than air, the 
ray will be bent in towards the normaX, ^ ^^ cA >^'^ ^vis^^';:)^^ 




192 Lenses. 

and strike the lower surface of the glass at c ; on the ray of 
light emerging from c to the air it will be again bent away 
from the normal n' n', and move in the direction c»d, which 
is parallel to a b, since the surfaces of the glass plate are 
supposed to be parallel 

It is found experimentally that the sines of the angles 
which the ray makes with the normal at the surface of the 
two media have a fixed ratio to one another, and that this 
coefficient is dependent on the media through which the ray 
passes. Thus from air to ordinary flint-glass the coefficient 
is about I '5, and from the flint-glass to air the reciprocal 
about yV or '66. Applying plane trigonometry to this expe- 
rimental fact, it will be found that there is a limit to the angle 
at which a ray of light can pass from any medium to one 
less dense, since the limit of the sine of an angle is unity. 
When the ray strikes the surface at this particular angle or at 
a greater the rays are reflected back, and the limiting angle 
itself is called the critical angle, or angle of total reflection, for 
these two media. A reference to this has been made at p. 88. 
Instead of the surfaces' of the glass being parallel we may 
have them inclined at an angle to one another, and in this 
case the refraction at each surface will follow the same law. 
An object which is really at k, fig. 39, will apparently occupy 

the' position k', which is 
F>G 39. ^ equivalent to saying that 

a ray of (monochromatic) 
light projected in the di- 
rection KA would have a 
direction c e after passing 
through the prism. If the 
projected beam of light in 
the direction k a be white, it will be found, as already noted 
in the second chapter, that on emerging from c it is split up 
into rays of the different rainbow tints. If we take any three 
distinctive rays in the red, yellow, and blue, we shall find that 
the red is least refracted and faWs mad\cecUonR,fig. 40, the 




Dispersion, 



193 



blue most and falls at 6, and that the yellow occupies the 
intermediate position. This difference in the index or co- 
efficient of the refractive power of the media for diflferent 
coloured rays gives the phenomenon known as dispersion. 



Fig. 40. 




It is found by experiment that the angles formed 
by the directions of the different rays of light vary ac- 
cording to the composition of the glass employed for the 
prism; that with one specimen, for instance, the angle 
R and Y does not bear the same ratio to the angle formed 
by Y and b that it does when another specimen is em- 
ployed. It is owing to this diflference in dispersive 
power of various glasses, that it has been found pos- 
sible to cause the component rays of white light to be 

Fig. 41 




nearly equally refracted, and yet to show no appreciable 
colour, due to dispersion. It will be seen in fig. 41 that, 
by employing opposing prisms of different composition, the 
dispersion may be almost entirely overcome. Thus it 
may happen that by placing a prism b of the dimensions, 
and in the position shown, the rays originally forming 
white light, and which were decoiivpo^^d \>^ ^^ T^\v^\a. 

o 



194 Lenses, 

A, might be so bent, owing to the difference in the dis- 
persive power of two media, that they emerge from b 
parallel to each other, instead of each ray forming a defi- 
nite angle with its neighbour, and that still the original ray 
may be refracted. Supposing b and a to be of the same 
homogeneous medium, it is evident that the same result 
would not be obtained. If the distance between b and a 
were diminished till the adjacent surfaces touched, the paral- 
lelism of the rays emerging from b would still be obtained, 
and, owing to the small dispersion of the rays in a, an inci- 
dent ray of white light would emerge as white light The 
two media we have been supposing to be employed are 
only hypothetical. Unfortunately up to the present date no 

Fig. 42 




two media have been found whose dispersive power can be 
utilized so as absolutely to correct one another. 

Supposing we have a series of prisms and their frusta 
joined together as shown, it is evident that the surfaces may 
be worked at such angles that the rays of light proceeding 
from an object at any distance from them may cut in one 
point and form an image of that object. In the figure 42 we 
have supposed the luminous object to be infinitely distant and 
to form one single image. By roundmg off the angles the 
same result may still be obtained and will form a lens. The 
curve that a glass would take, to give such theoretically 
perfect results, would be practically unsuitable, owing to the 
difficulty of grinding it; and also because it would only 
he correct for a particular d\s\aiie^ ^.iid dkection of object 



The Use of the Diaphragm, 



19s 




In practice lenses are worked to spherical surfaces, as 
being most convenient, and being capable of approximate 
accuracy. 

We will first glance at the inaccuracy that the spherical 
surface may cause fig. 43. 

when uncorrected by 
other means. If the 
rays of light, striking 
the lens obliquely, or 
along its axis, be re- 
flected from any dis- 
tant object, they will 

be practically parallel rays, and if different annuli of the 
lens be covered up, it will be found that the point of 
intersection of the rays will vary, the intersection of the 
marginal rays will be nearer to the lens than that of the 
central rays, fig. 43 ; thus when the whole of the lens is 
utilised the object will appear wanting in definition owing 
to what is called Fig 44. 

* spherical aberration.' 
This defect is over- 
come in a great mea- 
sure by placing a 
diaphragm in front of 
the lens, fig. 44, the 
oblique rays and the 
central rays, passing through which, can be brought approxi- 
mately infocus on a plane at right angles to the axis of the lens. 
Again, a diaphragm has a further advantage in that it allows 
the focus of a distant and a near object to lie on one plane. 
The nearer an object from which the rays proceed is to the 
lens, the longer will be the focus after they pass through the 
lens. Let the rays issue from a distant and a near object. 
From fig. 45 it is apparent that if the whole lens be 
used (supposing spherical aberration eVvKvvcva.x.^^ "C5es&\<^ 
would he no plane, x x, on whicVv l\ve Vv^o oV^^^v^^^'^^ 

o 2 




196 



Lettses. 



appear at all defined. The effect of a diaphragniy or *• stop/ 
as it is technically called, is to narrow both pencils of light so 
that neither of them is much out of focus at any point inter- 
mediate between the foci of the extreme rays. See fig. 46. 
This will be entered into further on in this chapter. 

Supposing that the rays from the near object formed an 

Fig. 45. 




angle with the axis of the lens, and those from the distant 
object coincided with it, a larger diaphragm might be em- 
ployed if the plane on which the images of the objects 
have to be received makes an angle with the axis of the lens, 

Fig 46 




fig. 47. It will be seen that the swing-back of a camera 
serves this purpose. 

It is to be observed that nearly the same results can be 
obtained by placing the diaphragm behind the lens instead 
of in front, fig. 48 ; and also that the size of the diaphragm 
determines the brightness of the image, for only a portion of 
the lens is utilised. 

With a lens such as shown there is a difference in the 
resulting images when the diaphragm is placed in front or 
behind the lens. In both cases we have distortion, but the 
distortion in the one case is the reverse of that in the other. 
V^hcn the diaphragm is in front of the lens the image of a 
square would be barreV-shaptd. '^V^^x \\. \s behind the 



The Use of the Diaphragm, 



197 



curvature would be reversed, fig. 49. It would be useless in 
either case to take an architectural subject with such a lens 



Fig. 47. 




unless the building occupied but a small proportion of the 
picture. The reason of this distortion will be apparent 
when it is remembered that the margin of the lens, its sur- 
faces being portions of spheres, will cause greater refraction 



Fig. 48. 




than the central portion. When the diaphragm is in front 
of the lens it is the margin of the lens which gives the 
image of the comer of the square. The image of the centre 
of each side is formed by a portion of the lens which is 
more central, and therefore is less proportionally bent. 
WTien the diaphragm is behind the lens different portions of 
the lens are used to form the image, and consequently the 
distortion is reversed. By placing a lens on each side of the 
diaphragm it is evident that distortion due to this cause 
may be entirely overcome, and thus we get what is called a 
doublet lens. It will be found that wiftv c^TVi\xv\t.Tvsfcs^'^^^ 



198 



Lenses. 



attempt to obtain a sharp focus of horizontal and verti- 
cal lines near the margin of the focussing screen, we shall 
fail ; either the one or the other will be indistinct. This is 

Fig. 49. 





due to astigmatism^ a defect also caused by the spherical 
form given to the surfaces of lenses. 

Lenses have various shapes given to them ; the following 
are the different forms: — 

Fig. 50. 






M is the double convex ; n, a plano-convex ; o, a con- 
cavo-convex ; p, a double concave ; Q, a plano-concave ; r, 
a meniscus. Lenses in which the concavity is greater than 
the convexity can have no actual but only a virtual focus, 
as may be seen by making a diagram. All such, when 
combined with other lenses, in which the convexity prepon- 
derates,^ will either increase the focal length or give a 
virtual focus to the combination. In photographic lenses 
the chief use of concave lenses is, by making them of suitable 
glass, to secur achromatism. 

The principal focus of a lens is the point where rays 
which enter parallel meet on emergence. As an example 
we may refer to fig. 42. 

' The material in which the lenses are worked must be taken into 
consideration in determining lh\s. 



The Optical Centre, 



199 



The optical centre of a lens is that point in the axis of 
the lens through which lines joining any points in an object 
and their images would intersect. 

Any point in any object, and the image of that point 
are said to be the conjugate foci of the lens; and the 
conjugate focal distances are said to be the distances of the 
optical centre of the lens from these two points. 

The equivalent focus of a lens is a term applied to a 
compound lens. It is the focus of parallel rays entering the 
lens. It is termed equivalent from being compared with a 
single lens that would produce the same sized image at the 
same distance from the object. 

To find the optical centre of a combination of lenses 
measure a distance of say 50 metres away from some fixed 
point, and place a rod at the extremity. From this rod 
measure a line of say 10 metres in length, exactly at right 
angles to tlie first line, and place a rod over this point. Now 

Fig. 51. 




place the front of the camera exactly over the starting-point 
of the first line and level it, the lens being in the direction 
of the first line. Having marked a central vertical line in 
the ground-glass with a pencil, focus the first rod accurately, 
so that it falls on the pencil line in the ground-glass. Take 
a picture of the two rods in the ordinary manner, and 
measure back as accurately as practicable the distance of the 
centre of the ground -glass from the starting-point, and also 
on the negative the distance apart, at their base, of the 
images of the two rods. 



200 Lenses, 

Suppose the first measured line — 
A B to be 50 metres; bd, the 2nd line (the distance apart of 
the rods), to be 10 metres; a c to be 30 centimetres; and 
E c, the distance apart of the images of the bases of the two 
rods, to be 6 centimetres. 

Then bd + ce:cb:: ce:cf, which is the equivalent 

focal distance. 

(Co + '^^ *o6 
CF= ^^ ^' =30 centimetres. 

10 -06 



• • 



It is, therefore, this distance along its axis from the ground- 
glass of the camera to the optical centre of the lens. 

The student will readily devise tlie means of setting off 
the distance thus found on the brasswork. 

The relation of the conjugate foci to one another is ex- 
pressed by the following formula : — 

11 I 

V ~ f u ' 

Where v is the distance of the optical centre of the lens from 
the ground-glass, u is the distance of the optical centre of 
the lens from the object to be photographed, /is the equiva- 
lent focal distance. From this it will be seen that if u is 

very gr^at, then - is so small that it may be neglected, and 

there remains v =/, That is, the image of an object at a 
great distance will be at the equivalent focal distance. 

Applying the above formula, suppose we have a lens 
where / = 30 centimetres and « = 40 centimetres : — 

I I I _ I 

V 30 ~ 40 "" 120' 

That is z' = 120 centimetres, or the distance of the ground- 
glass from the centre of the lens must be 120 centimetres 
to bring it into focus. 

Let it be required that u s\vo>3\^ \i^ n xijcci^ ^eater than 



On the CJioke of a Lens, 201 



v^ which is the same as saying that* the image must be ^ 
the size of the object. 
Then— 



n 



f 



or — 



= 


I 

V 


I 
+ - 

nv 


• 

nv * 


V 


s 


n 





Suppose, as before, / = 30 centimetres, and it is required 

to diminish the image of an object to J of the size of the 

original : — 

30(4+1) 
V ■■ '^ — — ■ « 37.5 centimetres, 

4 
M a ^v = 4x37.5 = 150 centimetres, 

or the ground-glass must be 37*5 centimetres and the object 
150 centimetres from the lens. 

By similar reasoning, if the object is to be enlarged 4 
limes, it will be found that the above distances must be 
reversed. 

In choosing a photographic lens the purpose for which 
it is required must be kept in view, for it will be evident 
that the requirements necessary may be different. In a 
lens for taking portraits we have, for instance, certain 
properties which are not essential, and even might be detri- 
mental in a lens for taking landscapes. With the former 
the objects to be photographed are generally within a few 
feet of it, and there are a variety of points situated in dif- 
ferent planes which ought to be impressed with sharpness 
on the photographic plate, and that without any distor- 
tion. The last desideratum puts the employment of a 
single lens out of the question, and it is evident that a 
double lens must be used. Starting with this it is quite 
evident that the curves of the surfaces of portrait lenses 
must vary from those for landscape work, and must be so 
designed as to be capable of delinealmg ^om\s» vcv ^i^^^^s^ 



202 



Lenses. 



Fig. 52. 



planes Aot far from the lens itsel£ It will be found that this 
can be secured by combining lenses of the same or diflferent 
focal lengths, separating the pairs by a long interval. This 
limits the extent of field and necessitates the employment of 
object glasses of wide diameter in order to cover a sufficient 
area. In practice the lenses are so far separated that the 
amount of surface of the photographic plate which can 

be utilised for some purposes, 
scarcely exceeds the diameter 
of the lens itself. Again, rapid- 
ity is an essential quality of 
a good portrait lens, and the 
curves of the surfaces of the 
lenses, and their separation, 
must be so adapted that, with- 
out the use of any diaphragm, 
they shall give a fairly sharp 
image of a figure or part of 
a figure when placed at a 
reasonable distance. Spherical aberration is a positive ad- 
vantage for some of these requisites. Fig. 52 gives an idea of 

the curves and also the amount 
of separation which is given to 
the lenses of a Petzval portrait 
combination, on the pattern of 
which many of the modem ones 
are still constructed. The dark 
shaded portions show the crown 
glass, and the light shaded por- 
tions the flint glass lenses. 

In one of the beautiful por- 
trait lenses introduced by Dall- 
meyer we have a decided varia- 
tion from this model. The advantage of this lens, fig. 53, is 
that two components of the back combination are capable 




Fig. S3. 




Landscape Lenses. 203 . 

of being slightly separated, giving a greater depth (though a 
more difiused) focus than ordinarily obtainable. 

For landscape lenses it is not so necessary that points 
lying on difTerent planes near the lens should be brought in 
focus on to the photographic plate, but that objects at a dis- 
tance from the camera, though lying in far different planes, 
should be sharply defined, and also that objects lying at 
a considerable angle from the axis of the lens should be 
in good focus. This latter requisite does not exist to 
nearly so large an extent in a portrait combination; hence, 
evidently, the curvatures of the lenses must be different, as 
also the amount of separation between the two lenses, when 
a double combination is employed. For ordinary landscape 
work there is nothing to prevent the adoption of a single lens, 
since the distortion produced by it would pass unnoticed, 
though, as already pointed out, architectural subjects de- 
mand freedom from all distortion, and, therefore, a com- 
bitiation of lenses has to be resorted to. All single lenses, 
for certain optical reasons, have the meniscus form given to 
them, and fig. 54 gives an idea of the forms adopted by 
some of the best makers. 

As already pointed out, the lenses are rendered achro- 
matic, the achromatism being adapted pj^ 
for the actinic rays more than for ^ g 
the visual rays. Fig. No. 1 shows a 
meniscus flint lens cemented to two 
crown concavo-convex lenses No 
2 has a crown double con\e\ ce 
mented to a double concave flmt 
lens, whilst No. 3 shows a crown 
concave convex lens cemented to a meniscus flint lens. 

Of a combination of lenses for architectural work we 
show three examples. The first is of the 'rapid recti- 
linear' type, as made by Dallmeyer, fig. 55. It is formed 
by a symmetrical pair of lenses of flint and crown ; the 
concave surfaces of the lenses face ea<i\ o'Cn^t. "Vl -w^ '::iJs- 




204 



Lenses, 



Fig. 55. 



the focal lengths of the combination io'5 in., the focal lengths 
of each lens will be found to be about 20, and the separa- 
tion between the two lenses to be 
about 2 inches. 

It may be useful to give a 
rule for ascertaining the focal 
length of any pair of lenses when 
combined. 

Multiply the focal length of 
one lens by that of the other, 
and divide by the sum of their 
focal lengths less the distance of 
separation. In the above case we 
have — 




/ = 



20 X 20 



400 



= 10-526. 



Fig. 56. 



40-2 - 38 

The diaphragms for this combination occupy a position 
half-way between the symmetrical lenses, and, therefore, 
give no distortion. This lens covers an angle of about 60°. 
The next lens, fig. 56, is what is known as a * wide angle ' 
doublet, in which the separation between the lenses is very 
small, and their foci considerably shorter, in proportion to 

the area of the circle that it is to cover. 
Some of these combinations are made so 
as to cover a circle whose diameter sub- 
tends an angle of 90° from the optical 
centre. The objection to these lenses is 
the unequal illumination and the small 
stop that is obliged to be employed with 
them, and their consequent slowness. 

The following diagram (fig. 57) shows 
a section of the 'triplet lens,' in which 
the place ordinarily occupied by the dia- 
phragm is. replaced by a 3rd compound 
were certain advantages connected 
tame 'wVveiv \\. '^^^ vctao^xsjasA^ Wt,' 




Flare Spots, 



205 



Fig. 57. 




since the manufacture of non-distorting doublets giving 
a fairly flat field has been perfected, they are compara- 
tively obsolete. It is, however, a good illustration of the 
ingenuity with which opticians 
aimed to meet the requirements 
of photographers. 

In the doublet lens the posi- 
tion of the diaphragm is important, 
otherwise — as can well be under- 
stood — the second lens will not 
correct the distortion of the first. 
In the case of a doublet in which 
both lenses are symmetrical, the 
diaphragm should naturally occupy * 
a position half-way between them. 

If the focal length of the front lens be different from that of 
the back, the diaphragm must occupy a position proportional 
to the focal length of the lenses. 

With certain classes of doublet lenses as formerly con- 
structed there was formed a fogged central patch on the 
exposed plate. This was due to what is called a 'flare 
spot,' which is a circular patch of light seen on the 
ground' glass immediately in a line with the axis of the 
lens. It is, in reality, an image of the opening in the dia- 
phragm. If glass were perfectly transparent, such a defect 
could not exist ; but, owing to its reflecting light from its 
surfaces, it has a reality which is often very troublesome. 
The surface of the lens reflects the aperture in the diaphragm 
and forms a distinct image of it, and if this image happen 
to coincide with the focal distance of the lens, the flare spot 
is sure to make its appearance. By slightly altering the 
position of the stop this defect is overcome. But as will have 
been noticed before, the position of the diaphragm in a 
doublet lens is of importance for eliminating distortion; 
hence by curing this defect distortion might be introduced. 
By previously altering the distance o^ >i!^e ^^-N^^-aiCNss^ 'cJ^ 



204 



Lenses, 



Fig. sj. 



the focal lengths of the combination 10*5 in., the focal lengths 
of each lens will be found to be about 20, and the separa- 
tion between the two lenses to be 
about 2 inches. 

It may be useful to give a 
rule for ascertaining the focal 
length of any pair of lenses when 
combined. 

Multiply the focal length of 
one lens by that of the other, 
and divide by the sum of their 
focal lengths less the distance of 
separation. In the above case we 
have — 




/ = 



20 X 20 



400 



= io'526. 



Fig. 56. 



40-2 38 

The diaphragms for this combination occupy a position 
half-way between the symmetrical lenses, and, therefore, 
give no distortion. This lens covers an angle of about 60°. 
The next lens, fig. 56, is what is known as a * wide angle ' 
doublet, in which the separation between the lenses is very 
small, and their foci considerably shorter, in proportion to 

the area of the circle that it is to cover. 
Some of these combinations are made so 
as to cover a circle whose diameter sub- 
tends an angle of 90° from the optical 
centre. The objection to these lenses is 
the unequal illumination and the small 
stop that is obliged to be employed with 
them, and their consequent slowness. 

The following diagram (fig. 57) shows 
a section of the * triplet lens,' in which 
the place ordinarily occupied by the dia- 
phragm is. replaced by a 3rd compound 
meniscus lens. There were certain advantages connected 
with this lens at the time wYveiv *\V ^^^ Ycteo^>\c&^^ Wt," 




Flare Spots. 



205 



Fig. 57. 




since the manufacture of non-distorting doublets giving 
a fairly flat field has been perfected, they are compara- 
tively obsolete. It is, however, a good illustration of the 
ingenuity with which opticians 
aimed to meet the requirements 
of photographers. 

In the doublet lens the posi- 
tion of the diaphragm is important, 
otherwise — as can well be under- 
stood — the second lens will not 
correct the distortion of the first. 
In the case of a doublet in which 
both lenses are symmetrical, the 
diaphragm should naturally occupy * 
a position half-way between them. 

If the focal length of the front lens be different firom that of 
the back, the diaphragm must occupy a position proportional 
to the focal length of the lenses. 

With certain classes of doublet lenses as formerly con- 
structed there was formed a fogged central patch on the 
exposed plate. This was due to what is called a 'flare 
spot,' which is a circular patch of light seen on the 
ground- glass immediately in a line with the axis of the 
lens. It is, in reality, an image of the opening in the dia- 
phragm. If glass were perfectly transparent, such a defect 
could not exist ; but, owing to its reflecting light from its 
surfaces, it has 3, reality which is often very troublesome. 
The surface of the lens reflects the aperture in the diaphragm 
and forms a distinct image of it, and if this image happen 
to coincide with the focal distance of the lens, the flare spot 
is sure to make its appearance. By slightly altering the 
position of the stop this defect is overcome. But as will have 
been noticed before, the position of the diaphragm in a 
doublet lens is of importance for eliminating distortion; 
hence by curing this defect distortion might be introduced. 
By previously aXttrmg the distance o^ >l\v^ •s^^^^ax^aSaofcs. <^V 



204 



Lenses, 



Fig. 55. 



the focal lengths of the combination 10*5 in., the focal lengths 
of each lens will be found to be about 20, and the separa- 
tion between the two lenses to be 
about 2 inches. 

It may be useful to give a 
rule for ascertaining the focal 
length of any pair of lenses when 
combined. 

Multiply the focal length of 
one lens by that of the other, 
and divide by the sum of their 
focal lengths less the distance of 
separation. In the above case we 
have — 




/ = 



20 X 20 



400 



= 10*526. 



Fig. 56. 



I 



40-2 38 

The diaphragms for this combination occupy a position 
half-way between the symmetrical lenses, and, therefore, 
give no distortion. This lens covers an angle of about 60®. 
The next lens, fig. 56, is what is known as a * wide angle ' 
doublet, in which the separation between the lenses is very 
small, and their foci considerably shorter, in proportion to 

the area of the circle that it is to cover. 
Some of these combinations are made so 
as to cover a circle whose diameter sub- 
tends an angle of 90° from the optical 
centre. The objection to these lenses is 
the unequal illumination and the small 
stop that is obliged to be employed with 
them, and their consequent slowness. 

The following diagram (fig. 57) shows 
a section of the * triplet lens,' in which 
the place ordinarily occupied by the dia- 
phragm is. replaced by a 3rd compound 
meniscus lens. There were certain advantages connected 
with this lens at the time wheiv \\. ^^^ yoXxo^x^k.^^, Wt,' 




Flare Spots. 



205 



Fig. 57. 




since the manufacture of non -distorting doublets giving 
a fairly flat field has been perfected, they are compara- 
tively obsolete. It is, however, a good illustration of the 
ingenuity with which opticians 
aimed to meet the requirements 
of photc^raphers. 

In the doublet lens the posi- 
tion of the diaphragm is important, 
otherwise — as can well be under- 
stood — the second lens will not 
coirect the distortion of the first. 
In the case of a doublet in which 
both lenses are symmetrical, the 
di^^hragm should naturally occupy * 
a position half-way between them. 

If the focal length of the front lens be different firom that of 
the back, the diaphragm must occupy a position proportional 
to the focal length of the lenses. 

With certain classes of doublet lenses as formerly con- 
structed there was formed a fogged central patch on the 
exposed plate. This was due to what is called a 'flare 
spot,' which is a circular patch of light seen on the 
ground glass immediately in a line with the axis of the 
lens. It is, in reality, an image of the opening in the dia- 
phragm. If glass were perfecdy transparent, such a defect 
could not exist ; but, owing to its reflecting light from its 
surfaces, it has a reality which is often very troublesome. 
The surface of the lens reflects the aperture in the diaphragm 
and forms a distinct image of it, and if this image happen 
to coincide with the focal distance of the lens, the flare spot 
is sure to make its appearance. By slightly altering the 
position of the stop this defect is overcome. But as will have 
been noticed before, the position of the diaphragm in a 
doublet lens is of importance for eliminating distortion; 
hence by cxuing this defect distortion might be introduced. 
By previously altering the distance o^ >l\v^ "s^^^^ax^Qfcs. <^S. 



204 



Lenses, 



Fig. 55. 




the focal lengths of the combination 10*5 in., the focal lengths 
of each lens will be found to be about 20, and the separa- 
tion between the two lenses to be 
about 2 inches. 

It may be useful to give a 

3 rule for ascertaining the focal 
length of any pair of lenses when 
combined. 
Multiply the focal length of 
one lens by that of the other, 
and divide by the sum of their 
focal lengths less the distance of 
separation. In the above case we 
have — 



Ti 



/ 



400 



-I 10*526. 



Fig. 56. 



20 X 20 
40-2' " 38 

The diaphragms for this combination occupy a position 
half-way between the symmetrical lenses, and, therefore, 
give no distortion. This lens covers an angle of about 60®. 
The next lens, fig. 56, is what is known as a * wide angle' 
doublet, in which the separation between the lenses is very 
small, and their foci considerably shorter, in proportion to 

the area of the circle that it is to cover. 
Some of these combinations are made so 
as to cover a circle whose diameter sub- 
tends an angle of 90° from the optical 
centre. The objection to these lenses is 
the unequal illumination and the small 
stop that is obliged to be employed with 
them, and their consequent slowness. 

The following diagram (fig. 57) shows 
a section of the 'triplet lens,' in which 
the place ordinarily occupied by the dia- 
phragm is. replaced by a 3rd compound 
meniscus Jens. There were certain advantages connected 
with this lens at the time wYvetv \V ^^l'^ vcyXxo^xs.^^^.^ \iw.t," 





Ftan Spots. 205 

since ihe nuunoGiclure of non -distorting doublets giving 

a £iirly flat field has been perfected, they are com|>;ira» 

lively obsolete. It is» however, a good illustration of the 

ingenuity with which opticians 

aimed to meet the ret|uirements fk** sr 

of photographers. 

In the doublet lens the posi- 
tion of the diaphragm is important, 
otherwise — as can well be under- 
$tood-*the second lens will not 
correct the distortion of the first. 
In the case of a doublet in which 
both lenses are symmetricaL the 
diaphragm should naturally occupy * 
a position half-way between thom. 

If the focal length of the front lens l>c different from that of 
the back, the diaphragm must ocxupy a iK)$ition proiK>rtional 
to the focal length of the lenses. 

With certain classes of doublet lenses as formerly con- 
structed there was formed a fogged central patch on the 
exposed plate. This \i^as due to what is called a * flare 
spot/ which is a circular {Kitch of light seen on the 
ground glass immediately in a line \%Hth the axis of the 
lens. It is in reality, an image of the opening in the dia- 
t^hragni. If glass were |K*rtectly transfwrent. such a defect 
could not exist : but. owing to its reflei ting light from its 
suriaces, it has a reality which is often \cry troublesome. 
The surface of the lens reflects the ajK^rture in the diaphragm 
and forms a distinct image of it. and if this image happen 
to coincide with the focal distance of the lens, the flare s(>ot 
is sure to make its appearance. By slightly altering the 
{tosition of the stop this defect is overcome. Hut as will ha\ e 
been noticed t>efore, the t^sition of the diaphragm in a 
doublet lens is of im)K)rtance for eliminating distortion; 
hence by curing this defect distortion might l>e introduceil. 
Hy previously altering the dUiatxce o< \W 'WiV^wSSsscv >^^ 



204 



Lenses, 



Fig. 55. 



the focal lengths of the combination 10*5 in., the focal lengths 
of each lens will be found to be about 20, and the separa- 
tion between the two lenses to be 
about 2 inches. 

It may be useful to give a 
rule for ascertaining the focal 
length of any pair of lenses when 
combined. 

Multiply the focal length of 
one lens by that of the other, 
and divide by the sum of their 
focal lengths less the distance of 
separation. In the above case we 
have — 




/ 



20 X 20 



400 



= 10*526. 



Fig. 56. 



40-2 38 

The diaphragms for this combination occupy a position 
half-way between the symmetrical lenses, and, therefore, 
give no distortion. This lens covers an angle of about 60°. 
The next lens, fig. 56, is what is known as a ' wide angle' 
doublet, in which the separation between the lenses is very 
small, and their foci considerably shorter, in proportion to 

the area of the circle that it is to cover. 
Some of these combinations are made so 
as to cover a circle whose diameter sub- 
tends an angle of 90° from the optical 
centre. The objection to these lenses is 
the unequal illumination and the small 
stop that is obliged to be employed with 
them, and their consequent slowness. 

The following diagram (fig. 57) shows 
a section of the 'triplet lens,' in which 
the place ordinarily occupied by the dia- 
phragm is. replaced by a 3rd compound 
meniscus Jens. There were certain advantages connected 
with this lens at the time wYveiv \V ^^^ Sxv\xo^>\^^^, Wt,' 




Flare Spots, 



205 



Fig. 57. 




since the manufacture of non-distorting doublets giving 
a fairly flat field has been perfected, they are compara- 
tively obsolete. It is, however, a good illustration of the 
ingenuity with which opticians 
aimed to meet the requirements 
of photographers. 

In the doublet lens the posi- 
tion of the diaphragm is important, 
otherwise — as can well be under- 
stood — the second lens will not 
correct the distortion of the first. 
In the case of a doublet in which 
both lenses are symmetrical, the 
diaphragm should naturally occupy * 
a position half-way between them. 

If the focal length of the front lens be different firom that of 
the back, the diaphragm must occupy a position proportional 
to the focal length of the lenses. 

With certain classes of doublet lenses as formerly con- 
structed there was formed a fogged central patch on the 
exposed plate. This was due to what is called a 'flare 
spot,' which is a circular patch of light seen on the 
ground- glass immediately in a line with the axis of the 
lens. It is, in reality, an image of the opening in the dia- 
phragm. If glass were perfectly transparent, such a defect 
could not exist ; but, owing to its reflecting light from its 
surfaces, it has a reality which is often very troublesome. 
The surface of the lens reflects the aperture in the diaphragm 
and forms a distinct image of it, and if this image happen 
to coincide with the focal distance of the lens, the flare spot 
is sure to make its appearance. By slightly altering the 
position of the stop this defect is overcome. But as will have 
been noticed before, the position of the diaphragm in a 
doublet lens is of importance for eliminating distortion; 
hence by curing this defect distortion might be introduced. 
By previously altering the distance o^ V\v^ «5»^^^as^^\Nss^ ^'^ 



204 



Lenses, 



Fig. 55. 



the focal lengths of the combination 10*5 in., the focal lengths 
of each lens will be found to be about 20, and the separa- 
tion between the two lenses to be 
about 2 inches. 

It may be useful to give a 
rule for ascertaining the focal 
length of any pair of lenses when 
combined. 

Multiply the focal length of 
one lens by that of the other, 
and divide by the sum of their 
focal lengths less the distance of 
separation. In the above case we 
have — 




/ = 



20 X 20 



400 



=s io*526. 



Fig. 56. 



40-2 38 

The diaphragms for this combination occupy a position 
half-way between the symmetrical lenses, and, therefore, 
give no distortion. This lens covers an angle of about 60°. 
The next lens, fig. 56, is what is known as a * wide angle ' 
doublet, in which the separation between the lenses is very 
small, and their foci considerably shorter, in proportion to 

the area of the circle that it is to cover. 
Some of these combinations are made so 
as to cover a circle whose diameter sub- 
tends an angle of 90° from the optical 
centre. The objection to these lenses is 
the unequal illumination and the small 
stop that is obliged to be employed with 
them, and their consequent slowness. 

The following diagram (fig. 57) shows 
a section of the * triplet lens,' in which 
the place ordinarily occupied by the dia- 
phragm is. replaced by a 3rd compound 
meniscus lens. There were certain advantages connected 
with this lens at the time wYvetv \\. ^^s» \tAxcA>\«A, but," 




Flare Spots, 



205 



Fig. 57. 




since the manufacture of non-distorting doublets giving 
a fairly flat field has been perfected, they are compara- 
tively obsolete. It is, however, a good illustration of the 
ingenuity with which opticians 
aimed to meet the requirements 
of photographers. 

In the doublet lens the posi- 
tion of the diaphragm is important, 
otherwise — as can well be under- 
stood — the second lens will not 
correct the distortion of the first. 
In the case of a doublet in which 
both lenses are symmetrical, the 
diaphragm should naturally occupy * 
a position half-way between them. 

If the focal length of the front lens be different from that of 
the back, the diaphragm must occupy a position proportional 
to the focal length of the lenses. 

With certain classes of doublet lenses as formerly con- 
structed there was formed a fogged central patch on the 
exposed plate. This was due to what is called a 'flare 
spot,' which is a circular patch of light seen on the 
ground' glass immediately in a line with the axis of the 
lens. It is, in reality, an image of the opening in the dia- 
phragm. If glass were perfectly transparent, such a defect 
could not exist ; but, owing to its reflecting light from its 
surfaces, it has ^ reality which is often very troublesome. 
The surface of the lens reflects the aperture in the diaphragm 
and forms a distinct image of it, and if this image happen 
to coincide with the focal distance of the lens, the flare spot 
is sure to make its appearance. By slightly altering the 
position of the stop this defect is overcome. But as will have 
been noticed before, the position of the diaphragm in a 
doublet lens is of importance for eliminating distortion; 
hence by curing this defect distortion might be introduced. 
By previously altering the distance o^ \)cve ^^-^^^-ajoiss^ 'cJ^ 



204 



Lenses, 



Fig. 55. 



the focal lengths of the combination 10*5 in., the focal lengths 
of each lens will be found to be about 20, and the separa- 
tion between the two lenses to be 
about 2 inches. 

It may be useful to give a 
rule for ascertaining the focal 
length of any pair of lenses when 
combined. 

Multiply the focal length of 
one lens by that of the other, 
and divide by the sum of their 
focal lengths less the distance of 
separation. In the above case we 
have — 




/« 



20 X 20 



400 



= 10*526. 



Fig. 56. 



40-2 38 

The diaphragms for this combination occupy a position 
half-way between the symmetrical lenses, and, therefore, 
give no distortion. This lens covers an angle of about 60°. 
The next lens, fig. 56, is what is known as a * wide angle ' 
doublet, in which the separation between the lenses is very 
small, and their foci considerably shorter, in proportion to 

the area of the circle that it is to cover. 
Some of these combinations are made so 
as to cover a circle whose diameter sub- 
tends an angle of 90° from the optical 
centre. The objection to these lenses is 
the unequal illumination and the small 
stop that is obliged to be employed with 
them, and their consequent slowness. 

The following diagram (fig. 57) shows 
a section of the 'triplet lens,' in which 
the place ordinarily occupied by the dia- 
phragm is. replaced by a 3rd compound 
meniscus lens. There were certain advantages connected 
with this lens at the time wYvetv \X. ^^s SxvXxo^Mc^es., Xsvs.v.,- 




Flare Spots, 



205 



Fig. 57. 




since the manufacture of non-distorting doublets giving 
a fairly flat field has been perfected, they are compara- 
tively obsolete. It is, however, a good illustration of the 
ingenuity with which opticians 
aimed to meet the requirements 
of photographers. 

In the doublet lens the posi- 
tion of the diaphragm is important, 
otherwise — as can well be under- 
stood—the second lens will not 
correct the distortion of the first. 
In the case of a doublet in which 
both lenses are symmetrical, the 
diaphragm should naturally occupy % 
a position half-way between them. 

If the focal length of the front lens be different firom that of 
the back, the diaphragm must occupy a position proportional 
to the focal length of the lenses. 

With certain classes of doublet lenses as formerly con- 
structed there was formed a fogged central patch on the 
exposed plate. This was due to what is called a 'flare 
spot,' which is a circular patch of light seen on the 
ground- glass immediately in a line with the axis of the 
lens. It is, in reality, an image of the opening in the dia- 
phragm. If glass were perfectly transparent, such a defect 
could not exist ; but, owing to its reflecting light from its 
surfaces, it has 4 reality which is often very troublesome. 
The surface of the lens reflects the aperture in the diaphragm 
and forms a distinct image of it, and if this image happen 
to coincide with the focal distance of the lens, the flare spot 
is sure to make its appearance. By slightly altering the 
position of the stop this defect is overcome. But as will have 
been noticed before, the position of the diaphragm in a 
doublet lens is of importance for eliminating distortion; 
hence by curing this defect distortion m\^\.\i^ \s&L^^!i»K-'^^. 
By previously altering the dV^Utvee^ ^^ ^^ 's.^^^'ascvss^ ^"^ 



2o6 Lenses. 

the two lenses, both evils may be avoided. At the best it 
seems, however, that the flare spot is really only distributed 
over the entire area which the lens covers. This reflection 
from the surface seems to account in a measure for the veil 
on negatives, which is often apparent when using certain 
slow lenses where bright objects have been photographed, 
and the exposure prolonged to enable the details in dark 
shadow to be capable of development. The veil is pro- 
bably the photograph of the illuminated lens. 

We must again revert to the diaphragm, or * stop,' in 
order to give some further idea of its use, and also of 
the necessity which may exist for using one of large or 
small aperture. In the case of a single lens we have 
already shown that the position of a stop aff"ects the shape 
of the distortion, depending whether it be placed in front 
or rear of the lens. It may now be stated — and the 
reason will be apparent on examining the previous figures 
— that on the distance of the diaphragm from the lens 
is dependent the amount of distortion, as is also the size 
of the picture which the lens is capable of defining; 
whilst at the same time the flatness of the field is also in 
a great measure due to a large distance being maintained 
between them. In constructing a lens, then, an optician 
has to hit a mean in order to give a satisfactory result. 
From these remarks it will be evident that a lens which 
embraces a wide angle should give least distortion, because 
the diaphragm must be necessarily closer to the lens than 
when the angle is curtailed. It is for this reason that the 
employment of a wide angle lens, with a plate of a size 
larger than that it was constructed to cover, is found to 
yield more satisfactory pictures than if a lens capable of 
embracing a less angle be employed. Thus a wide angle 
landscape lens intended to be used for a 40 x 30 centi- 
metre plate, gives more accurate pictures on a 20 x 16 
centimetre plate than does a lens embracing a more mode- 
rate angle when used for the same svzed ^\ax^. 



Illuminating Power. 207 

When a diaphragm is used, with the ordinary landscape 
lens or a double combination of lenses, there is a certain 
inequality of illumination of the field. The aperture of the 
diaphragm is for obvious reasons circular, and when the 
rays of light strike this in any direction but axially, it is evi- 
dent that the admitted light must be diminished, varying in 
fact as the cosine of the angle the rays make with the axis of 
the lens. Thus the margins of the picture will on this 
account have less illumination than the centre. Another 
cause of the falling off of illumination is this : — If we have 
two equal, and equally bright and equidistant, objects, so placed 
that the image of one faUs on the margin of the plate and of 
the other at the centre, the area occupied by the first image 
will be greater than that occupied by the second, and conse- 
quently the marginal illumination will be less. Mr. Dallmeyer 
states in the first of two articles ^ which he has written on this 
subject, that * the diminution of light from the centre towards 
the margins of the pictures from both these causes increases 
rapidly with any increase of angle of view beyond 40° At 
this obliquity the extreme margins only receive 80 per cent, 
of the light falling upon the centre, at 50® it is reduced to 
70 per cent., at 60'' to 55 per cent, at 70° to 45 per cent., or 
less than one half. Therefore the larger the angle included 
in the picture the more apparent becomes the defect.' In the 
same article Mr. Dallmeyer insists that the aperture of a 
diaphragm should always be expressed in terms of the focal 
length. Thus an aperture of 5 centimetres when used with a 
lens of 50 centimetre focus, should be called -^ aperture, 
which is a means of expressing the intensity of a lens. The 
aperture of the diaphragm also determines the amount of 
depth of focus, and this increases as the diameter of the 
aperture diminishes. Any point which is out of focus is re- 
presented by a disc of confusion, and when such a disc does 
not exceed a certain diameter, the eye is unable to distinguish 
it from a point. In practice i minute of arc is taketv ^s. 
' Year Book of Photography ^ l%^6 ».xv^ \^*n . 



2o8 Lenses. 

the limit. When the diameter of this disc, as viewed from 
an ordinary distance for examining a picture (40 to 50 
centimetres) subtends more than a minute of arc, the 
object will appear to be out of focus, whilst if less it will be 
in focus. Hence we may argue that the smaller the aperture 
of the diaphragm the greater the depth of focus there will be, 
since the focus of nearer objects and distant ones may all 
be made to fall within this limiting angle by diminishing it. 
A reference to fig. 43 will aid the student in comprehending 
this. Taking a disc of '25 millimetre diameter, which is about 
a minute of arc as seen from a distance of 50 centimetres, 
as the greatest admissible diameter of disc of confusion, a 
table is readily constructed of the nearest point which will 
be in focus when any aperture of diaphragm is employed. 
Suppose we know the equivalent focus of the lens in ques- 
tion to be 25 centimetre focus, and that we are to use an 
aperture of 2 '5 centimetres : 
Taking the formula — 

i - 1 _i 

V f «' 

when the distance is in focus, the nearest part of the fore- 
ground which can be considered sharp will have a focus 
which is longer than the equivalent focus by -25 centimetre, 
for— 

Cent. Cent. Millimetre. 

2-5 : 25 :: '025 : x 

jc = '25 centimetre ; 

. I ^ J _ ^ ^ '3 

* ' V 25 25 +-25 25x25-25 

I 



2525 
.*. V = 25-25 metres. 

That is to say, all parts of the picture lying beyond 25 
metres will appear to the eye to be in focus. The following 
table has been constructed on that basis : — 



Advantage of Short FocL 



209 



T *. •* 


Relative 
Exposures 


Focal length of lenses in centimetres 


Intensity, 

or 

Aperture 

Ratio 


10 


15 


20 


25 


30 


40 


50 


Distance of nearest distinct objects in metres 


10 

% 

40 


I 
2-25 

4 

9 
16 


41 
27 

2'I 

1*4 
i-l 


91 
6-1 
4-6 

31 
-•4 


i6-2 

IO-8 

8-2 

5-5 
4-2 


252 

16-9 

127 

8-6 

6-5 


36-3 

243 

183 
123 

9*3 


64-4 

431 

32-4 
217 

i6'4 


100-5 
67-2 

50-5 
33*8 
25-5 



The annexed formula will approximately give the nearest 
point/ which will appear in focus when the distance is accu- 
rately focussed, supposing the admissible disc of confusion to 
be '025 centimetres : — 

When/ as the focal length of the lens in centimetres, 
a = the ratio of the aperture to the focal length, 

the result is in metres. 

In the application of the foregoing formula the stu- 
dent should note the advantage of using a lens of short 
focus in lieu of one of long focus, viz., that more of 
the foreground can be placed in the picture without any 
detriment to it through * fuzziness.* It can also be shown 
that an enlargement from a small negative is better than 
a picture of the same size taken direct as regards sharp- 
ness of detail. Suppose, for instance, we wish to com- 
pare for sharpness a picture taken with a lens 50 centimetres 
focus with an enlargement of the same size, from an original 
negative taken with a lens of only 10 centimetres focus, 
both having the same aperture ratio, say y^^. The negative 
in the last case would be only ^ the size (linear) of the 
former. To compare the two the disc of confusion in this 
latter should only be '005 centimetres diameter, axvd iVwv's. 



2IO Lenses, 

should give the distance of the nearest distinct object, since, 
when enlarged 5 times it will give a disc of '025 centimetres 
diameter, which we have plready taken as the limit of dis- 
tinctness. Calculating as before for the lens of smaller focal 
length, 

•5 : 10 :: -005 : x 

X = *i centimetre 

I _ I I -I 

z/ "" 10 10+ 'I ^ 'lOIO 

/. V = lO'i metres, 

that is, after enlarging a pictiu"e to the size given by a 
lens of 50 centimetres focal length, an object io*i metres will 
still appear in focus. In looking at the table, it will be found 
that with the direct picture of the same size the nearest object 
in focus will be at 50*5 metres distance. Calculation shows 
that the gain in an enlargement, compared with a direct 
negative, is inversely proportional to the focal lengths of the 
lenses. This, of course, refers only to an aplanatic lens, and 
care must be taken to distinguish between the advantages 
to be gained in enlargement by the use of a smaller lens, 
with the disadvantages that ensue from the deterioration in 
the relative values of light and shade (see p. 257). 

The student should remark that in doublet lenses the 
apertures in the diaphragms do not show accurately the 
available aperture of the lens. In order to ascertain their 
correct value, a distant object should be focussed in the 
camera, in order that the focussing screen may be at the 
equivalent focus of the lens ; this screen is then removed and 
replaced by a glass over which is pasted any opaque paper. 
A candle is brought near the centre of the opaque screen in 
which a small hole has been punctured. The front com- 
bination of the lens is illuminated by the rays of light coming 
through the orifice, and the diameter of the disc of light 
seen on the front of the lens gives the available aperture of 
the lens when used with that diaphragm. 



Apparatus. 



CHAPTER XXX. 

APPARATUS. 

It will be unnecessary to describe much of the apparatus 
that is in daily use by the photographer, as some have already 
been described in various chapters, and some must be left 
to individual taste. In those kinds of apparatus which are to 
be described it must be borne in mind that the recom- 
mendations made are merely the results of the writer's indi- 
vidual experience, and it is not improbable that something 
better may be known to others. 

Cameras. — It should be considered an essential in every 
camera, excepting one used for copying and enlarging, that 
it should have a back that at least will swing at an angle 
away from the vertical plane, and it is a great comfort when 
a movement in a horizontal plane can also be given to it. 
Technically, the backs which can move thus are termed 
'swing-backs.' The accompanying figure will give an idea of 
a double swing-back, and also 
of the kind of camera which ^''■- ?* 



for landscape work seems 

everything to be desired, a is 

the front of the camera into 

which screws the lens l. The 

lens can be caused to occupy a 

position out <ii the centre of 

the camera by the double mo ve - 

ment shown in fig. 59. a is 

the board to which the lens is 

attached by means of its tlange 

sliding in the grooves b b, which are fixed on to the main 

movable front c c. This front also slides in grooves d rf, 

attached to the body of the cameia. TVese. ItoiA% ^\^ 'k**^ 




212 



Apparatus, 







Fig. 59. 




d 


^ /& 




6 




'^o - 


A 


d 


c 





a any point required by means of the screws e and / which 
run in the slots as shown. Reverting to fig. 58 it will be seen 

that the camera has what is known 
as the * bellows ' form, the bellows 
B being attached to a and also 
to the swinging framework d. e 
is connected with r by means of a 
rod, passing through the side of 
the framework, and terminated 
by a clamping screw k. r can 
be made to approach or recede 
from A by means of a slow-motion 
screw turned by the handle x. d is connected with m by 
pivots which work in the brass plates h, and since c is fixed 
as regards the vertical plane, it is evident that d can move 
through any small angle about h, without in any way interfer- 
ing with the other movements of the camera, and the angle can 
he maintained by clamping the screw, which works in a slot 
as shown. Thus, then, a swing away from the vertical plane 
is secured. The motion of d in a horizontal plane is secured 
by pivoting the frame m on to R. If the clamping screw k 
be loosened, m, and therefore t>, can be moved through any 
small angle in a horizontal plane, and can be fixed in that 
position by tightening k. The double swing motions are 
therefore secured, f is a bar with a long slot cut in it, so 
arranged that clamping screws in c and a can fix it and give 
additional rigidity to the camera. When r has been moved 
along the tail-board q, so that c touches a where the 
clamping screws m and k are loosened, the latter is free 
to turn up against the ground glass g. When a small pin 
at s is withdrawn from p, this board, being hinged as shown, 
folds round the tumed-up tail-board and q, is kept in 
position by means of a small snap spring fixed to the bottom 
of the camera. 

The camera itself can be attached to the stand by the 
tail-board Q, in which poakiotv \}cve. ^^^\.^^\. X^-wglcv of the 



The Woodbury Camera. 




213 

picture is h>jTizontal, or by e when the height of the picture 
has to be longer than its breadth. A camera 21 x 16 centi- 
metres of this form, when packed in a leather case, weighs 
about 6 kilogrammes, or 1 4 lbs. For work in the studio where 
the diminution of weight is no object, a rigid form of camera 
can be adopted. Such 
a form we give in fig, 60- f™ f^- 

This is a camera adapt- 
ed for taking cartes de 
visite, and it will be 
noticed that the alter- 
ation in focus is se- 
cured by a different 
arrangement to that in 
the last The front part, 
which carries the lens, slides outside the back part, the move- 
ment being effected by a pair of racks fastened on the base 
board, on which a long pinion works. Some photographers 

Fio. 61. 

prefer this motion to that given by the screw, since the hands 
do not interfere with the position of the body whilst viewing 
the image on the screen. 

It will also be noticed that there is a long carrier for the 
dark slides, and that the dark slide is more than double tKa 
length necessary to secure one pVctviw. TVed^i\e.o,c&.'<iiJ.\'& 



204 



Lenses, 



Fig. 55. 



the focal lengths of the combination 10*5 in., the focal lengths 
of each lens will be found to be about 20, and the separa- 
tion between the two lenses to be 
about 2 inches. 

It may be useful to give a 
rule for ascertaining the focal 
length of any pair of lenses when 
combined. 

Multiply the focal length of 
one lens by that of the other, 
and divide by the sum of their 
focal lengths less the distance of 
separation. In the above case we 
have — 




/ = 



20 X 20 



400 



=: 10*526. 



Fig. 56. 



40-2 38 

The diaphragms for this combination occupy a position 
half-way between the symmetrical lenses, and, therefore, 
give no distortion. This lens covers an angle of about 60°. 
The next lens, fig. 56, is what is known as a * wide angle ' 
doublet, in which the separation between the lenses is very 
small, and their foci considerably shorter, in proportion to 

the area of the circle that it is to cover. 
Some of these combinations are made so 
as to cover a circle whose diameter sub- 
tends an angle of 90° from the optical 
centre. The objection to these lenses is 
the unequal illumination and the small 
stop that is obliged to be employed with 
them, and their consequent slowness. 

The following diagram (fig. 57) shows 
a section of the * triplet lens,' in which 
the place ordinarily occupied by the dia- 
phragm is. replaced by a 3rd compound 
meniscus Jens. There were certain advantages connected 
with this lens at the time w\vetv \\. ^^?» vcvXxci^wK.^^, Wt,* 




Plare Spots, 



205 



Fig. 57. 




since the manufacture of non -distorting doublets giving 
a fairly flat field has been perfected, they are compara- 
tively obsolete. It is, however, a good illustration of the 
ingenuity with which opticians 
aimed to meet the requirements 
of photographers. 

In the doublet lens the posi- 
tion of the diaphragm is important, 
otherwise — as can well be under- 
stood — the second lens will not 
correct the distortion of the first. 
In the case of a doublet in which 
both lenses are symmetrical, the 
diaphragm should naturally occupy » 
a position half-way between them. 

If the focal length of the front lens be different firom that of 
the back, the diaphragm must occupy a position proportional 
to the focal length of the lenses. 

With certain classes of doublet lenses as formerly con- 
structed there was formed a fogged central patch on the 
exposed plate. This was due to what is called a 'flare 
spot,' which is a circular patch of light seen on the 
ground glass immediately in a line with the axis of the 
lens. It is, in reality, an image of the opening in the dia- 
phragm. If glass were perfectly transparent, such a defect 
could not exist ; but, owing to its reflecting light from its 
surfaces, it has ^ reality which is often very troublesome. 
The surface of the lens reflects the aperture in the diaphragm 
and forms a distinct image of it, and if this image happen 
to coincide with the focal distance of the lens, the flare spot 
is sure to make its appearance. By slightly altering the 
position of the stop this defect is overcome. But as will have 
been noticed before, the position of the diaphragm in a 
doublet lens is of importance for eliminating distortion; 
hence by curing this defect distortion might be introduced. 
By previous]/ altering the distatvee oi \\\e. ^^^^ct'^XM^^ ^'^ 



2o6 Lenses, 

the two lenses, both evils may be avoided. At the best it 
seems, however, that the flare spot is really only distributed 
over the entire area which the lens covers. This reflection 
from the surface seems to account in a measure for the veil 
on negatives, which is often apparent when using certain 
slow lenses where bright objects have been photographed, 
and the exposure prolonged to enable the details in dark 
shadow to be capable of development. The veil is pro- 
bably the photograph of the illuminated lens. 

We must again revert to the diaphragm, or *stop,' in 
order to give some further idea of its use, and also of 
the necessity which may exist for using one of large or 
small aperture. In the case of a single lens we have 
already shown that the position of a stop affects the shape 
of the distortion, depending whether it be placed in front 
or rear of the lens. It may now be stated— and the 
reason will be apparent on examining the previous figures 
— that on the distance of the diaphragm from the lens 
is dependent the amount of distortion^ as is also the size 
of the picture which the lens is capable of defining ; 
whilst at the same time the flatness of the field is also in 
a great measure due to a large distance being maintained 
between them. In constructing a lens, then, an optician 
has to hit a mean in order to give a satisfactory result. 
From these remarks it will be evident that a lens which 
embraces a wide angle should give least distortion, because 
the diaphragm must be necessarily closer to the lens than 
when the angle is curtailed. It is for this reason that the 
employment of a wide angle lens, with a plate of a size 
larger than that it was constructed to cover, is found to 
yield more satisfactory pictures than if a lens capable of 
embracing a less angle be employed. Thus a wide angle 
landscape lens intended to be used for a 40 x 30 centi- 
metre plate, gives more accurate pictures on a 20 x 16 
centimetre plate than does a lens embracing a more mode- 
rate anglQ when used for the same me^ ^\aXe. 



Illuminating Power. 207 

When a diaphragm is used, with the ordinary landscape 
lens or a double combination of lenses, there is a certain 
inequality of illumination of the field. The aperture of the 
diaphragm is for obvious reasons circular, and when the 
rays of light strike this in any direction but axially, it is evi- 
dent that the admitted light must be diminished, varying in 
fact as the cosine of the angle the rays make with the axis of 
the lens. Thus the margins of the picture will on this 
account have less illumination than the centre. Another 
cause of the falling off of illumination is this : — If we have 
two equal, and equally bright and equidistant, objects, so placed 
that the image of one faUs on the margin of the plate and of 
the other at the centre, the area occupied by the first image 
will be greater than that occupied by the second, and conse- 
quently the marginal illumination will be less. Mr. Dallmeyer 
states in the first of two articles ^ which he has written on this 
subject, that * the diminution of light from the centre towards 
the margins of the pictures from both these causes increases 
rapidly with any increase of angle of view beyond 40°. At 
this obliquity the extreme margins only receive 80 per cent, 
of the light falling upon the centre, at 50° it is reduced to 
70 per cent., at 60° to 55 per cent, at 70° to 45 per cent., or 
less than one half. Therefore the larger the angle included 
in the picture the more apparent becomes the defect.' In the 
same article Mr. Dallmeyer insists that the aperture of a 
diaphragm should always be expressed in terms of the focal 
length. Thus an aperture of 5 centimetres when used with a 
lens of 50 centimetre focus, should be called ^ aperture, 
which is a means of expressing the intensity of a lens. The 
aperture of the diaphragm also determines the amount of 
depth of focus, and this increases as the diameter of the 
aperture diminishes. Any point which is out of focus is re- 
presented by a disc of confusion, and when such a disc does 
not exceed a certain diameter, the eye is unable to distinguish 
it from a point. In practice i minute of arc is taken a& 
' Year Book of Photography^ l%^6 aiv^ \%ni . 



2o8 Lenses. 

the limit. When the diameter of this disc, as viewed from 
an ordinary distance for examining a picture (40 to 50 
centimetres) subtends more than a minute of arc, the 
object will appear to be out of focus, whilst if less it will be 
in focus. Hence we may argue that the smaller the aperture 
of the diaphragm the greater the depth of focus there will be, 
since the focus of nearer objects and distant ones may all 
be made to fall within this limiting angle by diminishing it. 
A reference to fig. 43 will aid the student in comprehending 
this. Taking a disc of '25 millimetre diameter, which is about 
a minute of arc as seen from a distance of 50 centimetres, 
as the greatest admissible diameter of disc of confusion, a 
table is readily constructed of the nearest point which will 
be in focus when any aperture of diaphragm is employed. 
Suppose we know the equivalent focus of the lens in ques- 
tion to be 25 centimetre focus, and that we are to use an 
aperture of 2*5 centimetres : 
Taking the formula — 

V f u* 

when the distance is in focus, the nearest part of the fore- 
ground which can be considered sharp will have a focus 
which is longer than the equivalent focus by -25 centimetre, 
for — 



Ce 
2" 


lit. 
5 


Cent. 

: 25 


Millimetre. 

:: -025 : 


X 




X 


= 


•25 centimetre ; 






I 

• 


« 


I 
25"" 


I 


25 


•25 


V 


25 + -25 


X25-25 




- 


I 

2525 








.\ V 


T 


25-25 


metres. 








;^ all parts of the picture lying beyond 25 
to the eye to be in focus. The following 
nstructed on that basis : — 



Advantage of Short Foci, 



209 



Intensity, 

or 

Aperture 

Ratio 


Relative 
Exposures 


Focal length of lenses in centimetres 


10 


15 


20 


25 


30 


40 


50 


Distance of nearest distinct objects in metres 


10 


I 
2-25 

4 

9 
16 


41 
27 

2' I 

1*4 
i-i 


91 
61 
4-6 
31 
-•4 


162 

IO-8 

8-2 

5-5 
4-2 


252 

i6'9 

127 

?^'(> 

6-5 


363 
243 
i8-3 

12-3 

93 


64-4 

431 

32-4 
217 

16-4 


1005 
672 

50-5 
33*8 
25-5 



The annexed formula will approximately give the nearest 
point/ which will appear in focus when the distance is accu- 
rately focussed, supposing the admissible disc of confusion to 
be '025 centimetres : — 

/ = -4l x/'xtf 
When/ = the focal length of the lens in centimetres, 
a = the ratio of the aperture to the focal length, 

the result is in metres. 

In the application of the foregoing formula the stu- 
dent should note the advantage of using a lens of short 
focus in lieu of one of long focus, viz., that more of 
the foreground can be placed in the picture without any 
detriment to it through * fuzziness.* It can also be shown 
that an enlargement from a small negative is better than 
a picture of the same size taken direct as regards sharp- 
ness of detail. Suppose, for instance, we wish to com- 
pare for sharpness a picture taken with a lens 50 centimetres 
focus with an enlargement of the same size, from an original 
negative taken with a lens of only 10 centimetres focus, 
both having the same aperture ratio, say y^^. The negative 
in the last case would be only \ the size (linear) of the 
former. To compare the two the disc of confusion in this 
latter should only be '005 centimetres diameter, and this 

p 



2o8 Lenses. 

the limit. When the diameter of this disc, as viewed from 
an ordinary distance for examining a picture (40 to 50 
centimetres) subtends more than a minute of arc, the 
object will appear to be out of focus, whilst if less it will be 
in focus. Hence we may argue that the smaller the aperture 
of the diaphragm the greater the depth of focus there will be, 
since the focus of nearer objects and distant ones may all 
be made to fall within this limiting angle by diminishing it. 
A reference to fig. 43 will aid the student in comprehending 
this. Taking a disc of '25 millimetre diameter, which is about 
a minute of arc as seen from a distance of 50 centimetres, 
as the greatest admissible diameter of disc of confusion, a 
table is readily constructed of the nearest point which will 
be in focus when any aperture of diaphragm is employed. 
Suppose we know the equivalent focus of the lens in ques- 
tion to be 25 centimetre focus, and that we are to use an 
aperture of 2*5 centimetres : 
Taking the formula — 

1-1 _1 

V f u^ 

when the distance is in focus, the nearest part of the fore- 
ground which can be considered sharp will have a focus 
which is longer than the equivalent focus by '25 centimetre, 
for — 

Cent. Cent. Millimetre. 

2'5 : 25 :: -025 : x 

X = *2^ centimetre ; 
III '25 

" * V 25 25 +-25 25x25-25 
I 



2525 
/. V =s 25-25 metres. 

That is to say, all parts of the picture lying beyond 25 
metres will appear to the eye to be in focus. The following 
tab'\e has been constructed on that basis : — 



Advantage of Short FocL 



209 



1 

1 


Relative 
Exposures 


Focal length of lenses in centimetres 


Intensity, 

or 

Aperture 

Ratio 


10 


15 


20 


25 


30 


40 


50 


Distance of nearest distinct objects in metres 


10 

1 

¥ 

3o 

• 


I 
225 

4 
?6 


41 

27 
2T 

1-4 


91 
61 

4-6 

31 
24 


l6'2 

10-8 
8-2 

55 
42 


252 

16-9 

127 

8-6 

6-5 


363 
243 
183 

12*3 

93 


644 

431 

32-4 
217 

i6'4 


100-5 
672 

505 
33*8 
255 



The annexed formula will approximately give the nearest 
point/ which will appear in focus when the distance is accu- 
rately focussed, supposing the admissible disc of confusion to 
be '025 centimetres : — 

/ = -4l x/'xtf 
When/ s» the focal length of the lens in centimetres, 
a — the ratio of the aperture to the focal length, 

the result is in metres. 

In the application of the foregoing formula the stu- 
dent should note the advantage of using a lens of short 
focus in lieu of one of long focus, viz., that more of 
the foreground can be placed in the picture without any 
detriment to it through * fuzziness.' It can also be shown 
that an enlargement from a small negative is better than 
a picture of the same size taken direct as regards sharp- 
ness of detail. Suppose, for instance, we wish to com- 
pare for sharpness a picture taken with a lens 50 centimetres 
focus with an enlargement of the same size, from an original 
negative taken with a lens of only 10 centimetres focus, 
both having the same aperture ratio, say y^^. The negative 
in the last case would be only \ the size (linear) of the 
former. To compare the two the disc of confusion in this 
latter should only be '005 centimetres diameter, and thk 

p 



200 Lenses, 

Suppose the first measured line — 
A B to be 50 metres; b d, the 2nd line (the distance apart of 
the rods), to be 10 metres; a c to be 30 centimetres; and 
E c, the distance apart of the images of the bases of the two 
rods, to be 6 centimetres. 

Then bd + ce:cb::ce:cf, which is the equivalent 
focal distance. 

/. CF= ^^ ^- — =30 centimetres. 

iO'o6 

It is, therefore, this distance along its axis from the ground- 
glass of the camera to the optical centre of the lens. 

The student will readily devise tlie means of setting off 
the distance thus found on the brasswork. 

The relation of the conjugate foci to one another is ex- 
pressed by the following formula: — 

11 I 

V ~ f u' 

Where v is the distance of the optical centre of the lens from 
the ground-glass, u is the distance of the optical centre of 
the lens from the object to be photographed, /is the equiva- 
lent focal distance. From this it will be seen that if u is 

very great, then - is so small that it may be neglected, and 

there remains v -=/, That is, the image of an object at a 
great distance will be at the equivalent focal distance. 

Applying the above formula, suppose we have a lens 
where / = 30 centimetres and « = 40 centimetres : — 

I J I _ I 

V 30 40 " 120* 

That is z' = 120 centimetres, or the distance of the ground- 
glass from the centre of the lens must be 120 centimetres 
to bring it into focus. 

Let it he required that u should \>^ n xixci^ ^eater than 



On tlie Choice of a Lens, 201 



z/, which is the same as saying that' the image must be ^ 
the size of the object. 
Then— 



n 



or — 



I 

/ 


I I fl + I 

-- + __« ; 

V nv nv 




/{n+i) 

V = — • • 

n 



Suppose, as before, / = 30 centimetres, and it is required 
to diminish the image of an object to ^ of the size of the 
original : — 

V « - — — : = 37.5 centimetres, 

4 

2^ BE /^z/ = 4x 37.5 = 150 centimetres, 

or the ground-glass must be 37*5 centimetres and the object 
150 centimetres from the lens. 

By similar reasoning, if the object is to be enlarged 4 
limes, it will be found that the above distances must be 
reversed. 

In choosing a photographic lens the purpose for which 
it is required must be kept in view, for it will be evident 
that the requirements necessary may be different. In a 
lens for taking portraits we have, for instance, certain 
properties which are not essential, and even might be detri- 
mental in a lens for taking landscapes. With the former 
the objects to be photographed are generally within a few 
feet of it, and there are a variety of points situated in dif- 
ferent planes which ought to be impressed with sharpness 
on the photographic plate, and that ^-ithout any distor- 
tion. The last desideratum puts the employment of a 
single lens out of the question, and it is evident that a 
double lens must be used. Starting with this it is quite 
evident that the curves of the surfaces of portrait lenses 
must vary from those for landscape work, and must be so 
designed as to be capable of delinealm^ pom\s» vcv $c&^^'^^s>x 



202 



Lenses. 



Fig. 52. 



planes hot far from the lens itself. It will be found that this 
can be secured by combining lenses of the same or different 
focal lengths, separating the pairs by a long interval. This 
limits the extent of field and necessitates the emplo)anent of 
object glasses of wide diameter in order to cover a sufficient 
area. In practice the lenses are so far separated that the 
amount of surface of the photographic plate which can 

be utilised for some purposes, 
scarcely exceeds the diameter 
of the lens itself Again, rapid- 
ity is an essential quality of 
a good portrait lens, and the 
curves of the surfaces of the 
lenses, and their separation, 
must be so adapted that, with- 
out the use of any diaphragm, 
they shall give a fairly sharp 
image of a figure or part of 
a figure when placed at a 
reasonable distance. Spherical aberration is a positive ad- 
vantage for some of these requisites. Fig. 52 gives an idea of 

the curves and also the amount 
of separation which is given to 
the lenses of a Petzval portrait 
combination, on the pattern of 
which many of the modem ones 
are still constructed. The dark 
shaded portions show the crown 
glass, and the light shaded por- 
tions the flint glass lenses. 

In one of the beautiful por- 
trait lenses introduced by Dall- 
meyer we have a decided varia- 
tion from this model. The advantage of this lens, fig. 53, is 
that two components of the back combination are capable 




Fig. 53. 




Landscape Lenses, 203 . 

of being slightly separated, giving a greater depth (though a 
more diffused) focus than ordinarily obtainable. 

For landscape lenses it is not so necessary that points 
lying on different planes near the lens should be brought in 
focus on to the photographic plate, but that objects at a dis- 
tance from the camera, though lying in far different planes, 
should be sharply defined, and also that objects lying at 
a considerable angle from the axis of the lens should be 
in good focus. This latter requisite does not exist to 
nearly so lai^e an extent in a portrait combination; hence, 
evidently, the curvatures of the lenses must be different, as 
also the amount of separation between the two lenses, when 
a double combination is employed. For ordinary landscape 
work there is nothing to prevent the adoption of a single lens, 
since the distortion produced by it would pass unnoticed, 
though, as already pointed out, architectural subjects de- 
mand freedom from all distortion, and, therefore, a com- 
bination of lenses has to be resorted to. All single lenses, 
for certain optical reasons, have the meniscus form given to 
them, and fig. 54 gives an idea of the fonns adopted by 
some of the best makers. 

As already pointed out, the tenses are rendered achro- 
matic, theachromatismbeingadapted 
for the actinic rays more than for ^ ' ^ ' 

the visual rays Fig No i shows a 
meniscus flint lens cemented to two 
crown concavo convex lenses No 
2 has a crown double convex ce 
mented to a double concave Hint 
lens, whilst No 3 shows a crown 
concave convex lens cemented to a meniscus flint lens 

Of a combmation of lenses for arthttectural work we 
show three examples The first is of the 'rapid recti- 
linear' type, as made by Dallmejer, fig 55 It is formed 
by a symmetrical pair of lenses of flint and crown , the 
concave surfaces oi the lenses face eajii Qfe^t, "V^ -^t «&, 




204 



Lenses, 



Fig. 55. 



the focal lengths of the combination 10*5 in., the focal lengths 
of each lens will be found to be about 20, and the separa- 
tion between the two lenses to be 
about 2 inches. 

It may be useful to give a 
rule for ascertaining the focal 
length of any pair of lenses when 
combined. 

Multiply the focal length of 
one lens by that of the other, 
and divide by the sum of their 
focal lengths less the distance of 
separation. In the above case we 
have — 




/- 



20 X 20 



400 



= io'526. 



Fig. 56. 



40-2 38 

The diaphragms for this combination occupy a position 
half-way between the symmetrical lenses, and, therefore, 
give no distortion. This lens covers an angle of about 60°. 
The next lens, fig. 56, is what is known as a * wide angle ' 
doublet, in which the separation between the lenses is very 
small, and their foci considerably shorter, in proportion to 

the area of the circle that it is to cover. 
Some of these combinations are made so 
as to cover a circle whose diameter sub- 
tends an angle of 90° from the optical 
centre. The objection to these lenses is 
the unequal illumination and the small 
stop that is obliged to be employed with 
them, and their consequent slowness. 

The following diagram (fig. 57) shows 
a section of the * triplet lens,' in which 
the place ordinarily occupied by the dia- 
phragm is. replaced by a 3rd compound 
meniscus lens. There were certain advantages connected 
with this lens at the time wYvetv \X. v^^^ \tv\xo^mc^4, \i\it^* 




Flare Spots, 



205 



Fig. 57. 




since the manufacture of non -distorting doublets giving 
a fairly flat field has been perfected, they are compara- 
tively obsolete. It is, however, a good illustration of the 
ingenuity with which opticians 
aimed to meet the requirements 
of photographers. 

In the doublet lens the posi- 
tion of the diaphragm is important, 
otherwise — as can well be under- 
stood—the second lens will not 
correct the distortion of the first. 
In the case of a doublet in which 
both lenses are symmetrical, the 
diaphragm should naturally occupy * 
a position half-way between them. 

If the focal length of the front lens be different from that of 
the back, the diaphragm must occupy a position proportional 
to the focal length of the lenses. 

With certain classes of doublet lenses as formerly con- 
structed there was formed a fogged central patch on the 
exposed plate. This was due to what is called a * flare 
spot,' which is a circular patch of light seen on the 
ground-glass immediately in a line with the axis of the 
lens. It is, in reality, an image of the opening in the dia- 
phragni. If glass were perfectly transparent, such a defect 
could not exist ; but, owing to its reflecting light from its 
surfaces, it has 4 reality which is often very troublesome. 
The siuface of the lens reflects the aperture in the diaphragm 
and forms a distinct image of it, and if this image happen 
to coincide with the focal distance of the lens, the flare spot 
is sure to make its appearance. By slightly altering the 
position of the stop this defect is overcome. But as will have 
been noticed before, the position of the diaphragm in a 
doublet lens is of importance for eliminating distortion; 
hence by curing this defect distortion might be introduced. 
By previously altering the distatvee o^ >l\v^ ^^^^ax'a.\Ns>f^ oJv 



2o6 Lenses. 

the two lenses, both evils may be avoided. At the best it 
seems, however, that the flare spot is really only distributed 
over the entire area which the lens covers. This reflection 
from the surface seems to account in a measure for the veil 
on negatives, which is often apparent when using certain 
slow lenses where bright objects have been photographed, 
and the exposure prolonged to enable the details in dark 
shadow to be capable of development. The veil is pro- 
bably the photograph of the illuminated lens. 

We must again revert to the diaphragm, or *stop,' in 
order to give some further idea of its use, and also of 
the necessity which may exist for using one of large or 
small aperture. In the case of a single lens we have 
already shown that tho position of a stop affects the shape 
of the distortion, depending whether it be placed in front 
or rear of the lens. It may now be stated — and the 
reason will be apparent on examining the previous figures 
— that on the distance of the diaphragm from the lens 
is dependent the amount of distortion^ as is also the size 
of the picture which the lens is capable of defining; 
whilst at the same time the flatness of the field is also in 
a great measure due to a large distance being maintained 
between them. In constructing a lens, then, an optician 
has to hit a mean in order to give a satisfactory result. 
From these remarks it will be evident that a lens which 
embraces a wide angle should give least distortion, because 
the diaphragm must be necessarily closer to the lens than 
when the angle is curtailed. It is for this reason that the 
employment of a mde angle lens, with a plate of a size 
larger than that it was constructed to cover, is found to 
yield more satisfactory pictures than if a lens capable of 
embracing a less angle be employed. Thus a wide angle 
landscape lens intended to be used for a 40 x 30 centi- 
metre plate, gives more accurate pictures on a 20 x 16 
centimetre plate than does a lens embracing a more mode- 
rate angle when used for the same me^ ^\^\.^. 



Illuminatifig Power, 207 

When a diaphragm is used, with the ordinary landscape 
lens or a double combination of lenses, there is a certain 
inequality of illumination of the field. The aperture of the 
diaphragm is for obvious reasons circular, and when the 
rays of light strike this in any direction but axially, it is evi- 
dent that the admitted light must be diminished, varying in 
fact as the cosine of the angle the rays make with the axis of 
the lens. Thus the margins of the picture will on this 
account have less illumination than the centre. Another 
cause of the falling off of illumination is this : — If we have 
two equal, and equally bright and equidistant, objects, so placed 
that the image of one fa- Is on the margin of the plate and of 
the other at the centre, the area occupied by the first image 
will be greater than that occupied by the second, and conse- 
quently the marginal illumination will be less. Mr. Dallmeyer 
states in the first of two articles ^ which he has written on this 
subject, that * the diminution of light from the centre towards 
the margins of the pictures from both these causes increases 
rapidly with any increase of angle of view beyond 40°. At 
this obliquity the extreme margins only receive 80 per cent, 
of the light falling upon the centre, at 50° it is reduced to 
70 per cent., at 60** to 55 per cent, at 70° to 45 per cent, or 
less than one half. Therefore the larger the angle included 
in the picture the inore apparent becomes the defect' In the 
same article Mr. Dallmeyer insists that the aperture of a 
diaphragm should always be expressed in terms of the focal 
length. Thus an aperture of 5 centimetres when used with a 
lens of 50 centimetre focus, should be called -^ aperture, 
which is a means of expressing the intensity of a lens. The 
aperture of the diaphragm also determines the amount of 
depth of focus, and this increases as the diameter of the 
aperture diminishes. Any point which is out of focus is re- 
presented by a disc of confusion, and when such a disc does 
not exceed a certain diameter, the eye is unable to distinguish 
it from a point. In practice i minute of arc is taken as 
» Year Book of Photography y \V\^ ?ccA \Vn . 



2o8 Lenses. 

the limit. When the diameter of this disc, as viewed from 
an ordinary distance for examining a picture (40 to 50 
centimetres) subtends more than a minute of arc, the 
object will appear to be out of focus, whilst if less it will be 
in focus. Hence we may argue that the smaller the aperture 
of the diaphragm the greater the depth of focus there will be, 
since the focus of nearer objects and distant ones may all 
be made to fall within this limiting angle by diminishing it. 
A reference to fig. 43 will aid the student in comprehending 
this. Taking a disc of '25 millimetre diameter, which is about 
a minute of arc as seen from a distance of 50 centimetres, 
as the greatest admissible diameter of disc of confusion, a 
table is readily constructed of the nearest point which will 
be in focus when any aperture of diaphragm is employed. 
Suppose we know the equivalent focus of the lens in ques- 
tion to be 25 centimetre focus, and that we are to use an 
aperture of 2*5 centimetres : 
Taking the formula — 

V f u^ 

when the distance is in focus, the nearest part of the fore- 
ground which can be considered sharp will have a focus 
which is longer than the equivalent focus by -25 centimetre, 
for — 

Cent. Cent. Millimetre. 

2*5 '.25 :: -025 : x 

X — "25 centimetre ; 

• • 2/ "^ 25 25 +-25 25x25-25 

I 



2525 

/. V = 25-25 metres. 

That is to say, all parts of the picture lying beyond 25 
metres will appear to the eye to be in focus. The following 
table has been constructed on that basis : — 



Advantage of Short Foci, 



209 



T «. •- 


Relative 
Exposures 


Focal length of lenses in centimetres 


Intensity, 

or 

Aperture 

Ratio 


10 


15 


20 


25 


30 


40 


50 


Distance of nearest distinct objects in metres 


1 

40 

• 


I 
2-25 

4 

9 
16 


41 
27 

2*1 

1*4 
i-i 


91 
61 
4-6 

31 

-•4 


162 

IO-8 

8-2 

55 
42 


252 

1 6*9 

127 

8-6 

6-5 


36-3 

243 

183 
12-3 

93 


64-4 

431 
324 
217 

16-4 


100-5 
672 

505 
33-8 
25*5 



The annexed formula will approximately give the nearest 
point/ which will appear in focus when the distance is accu- 
rately focussed, supposing the admissible disc of confusion to 
be '025 centimetres : — 

Wheny a= the focal length of the lens in centimetres, 
a — the ratio of the aperture to the focal length, 

the result is in metres. 

In the application of the foregoing formula the stu- 
dent should note the advantage of using a lens of short 
focus in lieu of one of long focus, viz., that more of 
the foreground can be placed in the picture without any 
detriment to it through * fuzziness.' It can also be shown 
that an enlargement from a small negative is better than 
a picture of the same size taken direct as regards sharp- 
ness of detail. Suppose, for instance, we wish to com- 
pare for sharpness a picture taken with a lens 50 centimetres 
focus with an enlargement of the same size, from an original 
negative taken with a lens of only 10 centimetres focus, 
both having the same aperture ratio, say V^^. The negative 
in the last case would be only ^ the size (linear) of the 
former. To compare the two the disc of confusion in this 
latter should only be '005 centimetres diameter^ and t\\\s. 

p 



332 



On ihe Picture. 



balance the general lines, and no impression of insecurity is 
left. The general composition, too, of the picture should 
be noted. The lines forming the extremities of the spars 
fall on the body of the skiff, while a sense of support to the 
outer line of the lai^e sail is given by the post. The line 
forming the top of the post and the top of the pier also 
approximately passes through the cap of the man and the 
top of the mast of the skitT. The picture is then built, as it 
were, on diagonal lines. A slight change in the position of 
the camera would have altered all this. Again note that 



the general mass of light is opposed 1 



) the blauk hull of the 
boat, intensifying the 
interest with which 
the boat, evidently 
the principal object 
in the picture, is re- 
garded. 

The accompany- 
ing woodcut, fig. 75, 
taken from a photo- 
graph by Woodbury, 
well illustrates the 
treatment of an old 
watermill. Jn this 
case the angle of 
the wall, that is, the 
base of the comer 
of the most promi- 
nent piece of ma- 
sonry, is placed abou\ 
\ way up the picture. 
Had it been placed 
lower it would have been aggressive, whilst if placed 
higher it would not have given sufficient solidity to the 
mill. The water-wheel base, the object of interest, is 
placed nearly centrally in the teeadth of the picture, as 
from the subject there is no 6a.T\gH ol sfmnv^svj, -^fjiiii 




A Woodland Picture. 233 

is always distasteful. The shoot of water occupies a posi- 
tion central in both directions. Had it been placed much 
lower, there would have been a sense of a want of falling 
room. It will be noticed that the fall of the water naturally 
enhances the effect of the composition, and the light on 
it at once attracts the eye from the dark surroundings. 
If the picture be covered from the bottom to where the 
board is thrown across the stream, it will be seen how a 
slight variation in the position of the camera might have 
altered the general aspect of the picture. 

A favourite study with some photographers are forest 
scenes, and the next two examples shalltreat of them. In the 




first, fig. 76, we have an old oak surrounded by smaller trees, 
the foreground composed of bracken and ferns. The base 
of the tree is placed about ^ way up the picture, for if 
lower there would have been a feeling that there was not 
sufficient ground for ;'/, the principal object, to have taken 
firm root in, and there would have been a sense of unfitness 
of position. Had it been placed h\%'c\e,i "Ctv^ lQ\s.'©ci^OTfe. 
woald have been too prominent, ani ftve,fe.xW.\fe»-'^'^^ 



234 Oh the Picture. 

have been that the raison d^itre of the picture was simply 
the ferns in the foreground, for those at the foot of the 
picture would have been out of proportion to the oak to 
play the part of an accessory. The distance, or what might 
be called the horizon line, is drawn about ^ way up the 
picture, but being so broken by the shrubs and smaller 
trees it is invested with no importance, and consequently 
need not be dwelt upon as following any particular rule. 
The outside of the trunk of the oak is placed i way from the 
right-hand edge ; had it occupied a more central position 
the picture would have appeared cut in two by it As it 
is the dark foliage behind it fills in the side of the pic- 
ture, and there is no feeling that the oak is out of 
place. Had the foliage been light there would have been a 
danger that the eye might have been offended, but this is 
one of the cases in which the position of the camera must 
be made subservient to the operator. The whole force of 
the picture is given by the light, which breaks against the 
trunk of the oak; and as with the trunk, so with the branches, 
care has been taken to prevent any single bough cutting the 
picture into two divisions. Notice, too, the stability given 
by the straight stems of the trees, in the distance. 

In the next picture, fig. 77, we have the distance, or 
perhaps more strictly speaking, the middle distance as 
the point of interest. The horizon line is kept in the 
weakest part, the centre, of the pictiure. The trees in 
the foreground are so grouped that they frame the 
view with dark masses, relieved by the light foliage of some 
of the nearer bushes and shrubs. The foreground finishes 
at a distance of about \ from the bottom. More of it would 
take away from the value of the middle distance, as it 
would place it in the weakest part of the picture —viz., 
centrally j less of it would have rendered the picture bald, 
and have cut off part of the deeper shades which are 
so valuable in giving the effect of distance to the stream 
beyond. This picture wou\d \v^\fe \i^^Tv ?»^oilt had the 



camera been so placed as to give more top foUage, since the 

bough which now panially crosses the picture at about 

f the height would fq „ 

have caused an 

ugly division and 

also the tops of 

the distant trees 

and the sky would 

have appeared i 

This latter in 

\tews such as that | 

under criticism is 

objectionable, as 

patches of white 

give the eye 

an inclination to 

wanderoffto wards 

it, and it would j 

have been an in- 
sufficient precau- 
tion to have print- ' 

ed in clouds from j_ 

another negative, ^ 

owing to the difficulty that would exist in subduing at the 
same time the lights on the leaves of Che near trees. As it is, 
the picture is in pictorial focus. By placing the stream to 
the right or left, the balance would have been wanting, and 
its general direction would have been altered to such an 
extent as to have given a feeling that it was a subsidiary part 
of the picture instead of an essential. 

The next example, fig. 78, is intended to show a picture 
taken on the diagonal; not on an absolutely straight line, but 
one in which the general direction of the picture is on the 
diagonal. The point of interest is the extreme distance of 
the stream, and accordingly it is placed in one of the strong- 
est positions in the picture, vii., \ -vxj m Xiofc &s.*i'«s.'=>"^'!. 




236 Ok. /fc PkUtre. 

from the tE^rgins, and it contains the fastest ligfat. as 
Men in iht wsur. Tias bnHianl I^u is repealed in ibe 
t.lo'ids. and more Cundrsdll it is echoed in the rocks, where 
:: lakes approxfinatdj' the same fixm, ihoi^ it is repeated 
ID a lower toot: and of different dimoisions. The picture 
might easHr hai'c been sfoih br placing the distance in a 




'.entral [rosition, and by arranging that the dark moss-covered 
rocks in the for^round should have been shifted to the side. 
Had these dark masses been closely opposed to the highest 
light, the value of the distance would have been increased, 
though in their present position they are fairly well placed. 

In the succeeding photograph we have a capital example 
by Manners Gordon of a picture built up on purely artistic 
principles. The principal object of interest is the cottage, 
the value of which is enhanced by the admirable grouping 
of the sheep. The middle distance and background may be 
considered merely as accessories to support the subject of 
<:hief interest. The general direction of the picture is on 
tht; diagonal, being carried down from the chimney-top of 
ihc cottage along the banU-s\dc \.o iVe T\^v\\a.tii \icM.Qm 
<:orncr. The value of the com^?AUO'£i^^'>e^ ^i\«i^5SL-i"\si.'iw; 



In a Landscape. 237 

light on the two sheep in the centre group, which reflects 
itself as it were in the whitewashed cottage front. This 
may be seen by imagining the front to be of the same local 
colour as the gable end of the cottage, or by first hiding the 
sheep by the finger, and then contrasting the effect produced 
on the mind with that as shown. It may be remarked that 




the opposing lines of the clouds balance the lines of the 
landst^pe, which would not have been the case had the 
general contour of the clouds followed in any degree the 
contour of the sky-line In a woodcut it is impossible to 
give all the expression that is to be found in the photograph, 
but the student may gain a fair knowledge of the rules which 
have been followed. 

As regards the introduction of figures into a landscape, 
it may be necessary to say a few words. It should be clearly 
understood that the one must be made subsidiaty to the 
other; that is, if the portraits of the figures are required they 
must be made the principal objects, and the whole landscape 
must be made subservient to them. On the other hand, if 
a landscape is to be photographed, the figures, though pro- 
minent, yet should occupy such a position as to be subor- 
dinate to it, though they may enhance a.tvd ^n^ ^Vvt ' fcmR.' 
points Co the picture; and above aV\ \.Vin%% cast -kni^N*: 



238 On the Picture. 

taken that the figures compose as well with each other as 
n-ith the landscape. 

Robinson, in his ' Pictorial Fhott^iaphy,' ' a work which 
every photographer should possess, says : 'The figoie must 
be ^ the subject as well as » it, in order that unity may 
be preserved ; it must be used with a purpose to give life 
to a scene, or to supply an impoitant spot of light or 
ilark; to give balance, or to bring other parts into sub- 
ordination, by either being blacker or whiter than those 
parts ; and that what is to be avoided is the indiscrimiDate 
dragging tn of figures into scenes in which they have no 
business, and where they do nothing but mischief.' 

We have such an example in the cut below, taken from 
a photograph by H. P. Robinson, called 'Blackberry 




Gathering,' fig, 80. The landscape is one which is most 
unpromising in its aspect ; the sombre bank of blackbeny- 
biishes alone, would form a melancholy, gloomy, picture ; 
but by placing these figures, as they are, some \ way from 
the left-hand side, the contrast of light they offer to 
the deep shadow behind them at once attracts the eye, 
and leads it gradually up the Nfrndwi^ broken path beyond. 



Groups of Figures. 239 

The spot of light, in fact, atfords the exact balance required 
to what otherwise would be an uninteresting picture. It 
accentuates everything, as it were, and gives the ' forte ' 
point, which is such a desideratum. 

Again, in this study we have an example of the value 
that a sky gives to a picture. It should be noticed how the 
lines of the clouds balance the lines of the hill. If the lefi- 
hand dark cloud be covered up how wanting in vigour is 
the composition. It will be seen how Robinson, out of 
such unpromising materials as a blackberry bank, a couple 
of figures, and a good cloud negative, has been able to build 
up a picture which is technically perfect and full of interest 
and repose. 

In the next illustration, fig. 81, which is from a photograph 
by the same artist, we have a capital example of the correct 
grouping of figures to form a picture in which they are the 




objects of interest, and the landscape merely subsidiary, 
though essential. Like the typical picture by Sir David 
Wilkie's, ' The Blind Fiddler,' this, which is named ' Holiday 
in the Woods,' is built up on a series of pyramids, the base 
being curved. It should be noticed how one py^miA t\is.=> 
into soiothtT, each corner being difieTeTvft-j =.\i-§'^\'w.&-\ "i^-. 
((X example, the right-hand comex ot X\ie, \;v% vr^™i^S>- ^ 



240 On the Picture, 

supported by the basket, and the left-hand corner by the 
ami of the reclining lad. Tracing the composition all 
through, it will be seen the lines have been artistically kept 
in view and the figures posed accordingly. The straight 
lines of the distant trees contrast with the fall of the pyra- 
midal lines, and give a firmness which would otherwise be 
wanting. The nearest object, too, is made the most distinct, 
whilst the darkest object, which is the figure of the boy, 
cuts across the highest light, giving just sufficient contrast 
and no more. Had the lad's head been raised higher the 
effect would have been to form two patches of white, fi*om 
either of which the eye would have wandered by the 
attraction of the other. Again, leaving the main group, the 
two small figures lead the eye instinctively to the distant 
glade beyond, so that over every part of the picture we are 
led to some fresh beauty. 

Very different are the groups so often seen as posed by 
many photographers. Either the heads of the standing 
figures are placed nearly in a line or, if a pyramidal com- 
position is attempted, there is only one pyramid to satisfy a 
rule which was never intended to be rigid. In grouping 
there should be no uniformity; if possible, every set of figures 
should form a complete study by itself, blending themselves 
into the other sets, whilst the whole should be in harmonious 
lines. Again, the effect of deep shade in some figures 
should be made to contrast with the lighter parts of others, 
care being taken that no patches of light or deep shadow 
should be obtrusive. Above all things, in posing a»group, 
let it be remembered that each figure is animate, and should 
not be made to look as lifeless as a statue. Let every member 
of it have a definite purpose in the group, the apparent 
occupation of each being in keeping with that of the others. 

The next cut, fig. 82, is left for the student to find out 
the rules that have been followed in the composition, and 
also to note any improvement which might have been 
made in it 



Hinls as lo placing the Camera. 241 

We may now sum up a few of the principal rules that 
should be observed in the composition of a picture, though 
it must never be forgotten that a rigid adherence to them at 
al! times is impossible. The great point with the photo- 
grapher is to know when and how he may transgress with- 
out spoiling the treatment of his subject. 

I. If the object of interest he 00 the foreground, its 
base should occupy a position of &om ^ to ^ the height of 




the picture; if it be in the distance its base should be 
about ^ way up the picture. 

a. In a general landscape the horizon lines should 
occupy a position about ^ way from the top or the bottom 
of the picture ; with the latter a cloud negative will pro,- 
bably be required. 

3. It is advisable that the general line of a picture 
should run on a diagonal or take a pyramidal shape. 

4. A long obtrusive line should never be permitted 
to intersect the picture ; it should always be broken up as 
far as possible. 

5. A picture should never be cut in two by a dark 
object against a light background or by a light object 
against a dark' baclc^^und. 



242 On the Picture. 

6. If the general features of a picture have a wedge- 
like form, care should be taken that the wedge is sup- 
ported near the point, in order to give the idea of 
stability. 

7. The general lines of a picture should be balanced 
by opposing lines, for the same reason as that given in 6. 

8. A large patch of one approximately uniform tint 
is distasteful to the eye, and should be broken up, if 
possible. 

9. The object of interest should be pictorially focussed 
by a general sweep of light (if it be a dark object) or of 
shadow (if it be a light object), thus causing the eye to fall 
naturally upon it. 

10. Avoid monotony, whether in constant repetition of 
lines, lights, or shades, and never allow a picture to be 
symmetrical on the right and left of its centre, A repeti- 
tion of a high light once or twice in a lower tone is, however, 
much to be recommended. See figs. 78 and 79, 

As regards 8 and 9, it must be borne in mind that in 
the printing of the picture a great power is placed in the 
photographers hands; by a judicious masking of parts he can 
cause pictures which would be inartistic to become merely 
inoffensive, and he may give an atmospheric effect other- 
wise unattainable by remembering that shadows in the dis- 
tance tend to become lighter, whilst high lights tend to 
become darker. Tissue paper stretched on the back of a 
negative, and a limited use of the stump, will be found to 
be powerful aids to the production of an artistic picture. ' 

An artistically educated photographer instinctively sees 
the most favourable aspect of the subject he may wish 
to delineate. When he perceives that he is in a favour- 
able situation, the camera should be erected, and the 
minor details of the composition must be. attended to. 
Then it is that he must exercise to the full extent his artistic 
jLnowledge. Knowing the position his principal subject 
should occupy in the pVate, Vve xowsX TLOte the different 



Focussing the Picture, 243 

-subordinate objects that are also seen on the ground glass 
of the camera. If the subject require figures to be intro- 
duced to give * forte ' points he should note where, and how, 
they should be arranged. He should also note the most 
favourable time of day for taking the view, bearing in mind 
that one of the chief charms of a picture is a proper massing 
of light and shade, which as a rule can only be secured by 
sunlight falling across the picture, and not coming from 
behind, or from the front of, the camera. 

As regards the absolute manipulation of the camera there 
is not much to learn beyond following a few simple rules. 
After selecting the view, the angle which is to be taken in 
should be roughly measured, and the lens selected accord- 
ingly. When this is determined, the view should be brought 
approximately on to the ground glass of the camera. It 
requires a certain amount of practice to form a correct pic- 
torial estimate of an inverted image, and it is probable by 
turning the head in such a position that the line joining 
the eyes is nearly vertical, a more correct idea can be 
formed than by keeping it in the usual position. At first 
no diaphragm should be in the lens, as the general sweep of 
light and shade can be better studied. When this is satis- 
factory and the lines of the picture are the best that can be 
obtained, d diaphragm may be inserted with an aperture df 
the largest size which will admit of a good general focus 
being obtained. The object of interest must, however, be 
that which is most sharply defined on the ground glass, and 
it is sometimes advisable to sacrifice the sharpness of the 
other portion in order to attain this, and when the character 
of the sweeps of light and shade are not as good as could 
be desired, it may also sometimes be necessary to adopt this 
artifice to secure the proper attention of the eye to that 
point. It is not intended to imply that a picture out of 
focus is more artistic than one sharply defined. Though 
the eye sees only one portion of a landscape at a time ia 

K 2 



244 On the Picture. 

focus, the remaining portion being blurred, yet, be it re- 
membered, the photographic print, when properly viewed, 
occupies the position of the natural landscape, and the 
same difference of focus away from the object of interest 
takes place naturally ; and, in the photograjih, as in nature, 
the eye may wander to the points of lesser interest, and still 
find a charm in the minute details. 

One of the essential suppositions of perspective is, that the 
picture plane should be vertical and tlie line of sight horizontal 
Nevertheless, in focussing a landscape taken merely for pic- 
torial effect it usually does not signify whether the camera be 
tilted downwards or upwards, or whether the ground glass be 
vertical, so long as the top and bottom of the pictures are 
parallel with the horizon line, though in architectural subjects, 
as we shall presently see, these points cannot be neglected. 
For a simple landscape, then, it will be found that the power 
of obtaining a good focus is well within the hands of the 
operator. From what has been already said at p. 197, it is 
seen that the focus of near objects is longer than that of 
•more distant ; thus, without using a diaphragm, the focus of 
the foreground will be longer than that of the middle dis- 
tance, and this again than the distance. By using the swing- 
back, to cause the top of the ground glass to swing out- 
wards, this is often secured. Again, on one side of the pic- 
ture a near object may have to be represented ; by using 
the horizontal swing, it may often be brought into focus. 
By the use of the vertical and horizontal swings together, it 
is sometimes possible to employ a much larger diaphragm 
than could otherwise be done. In landscapes this is often 
important, as a large available aperture to the lens means 
short exposure ; and where the operator is exposed to the 
caprices of gusts of wind, the success of a picture is often 
dependent on the rapidity of exposure. As regards the tilt- 
ing of the camera, latitude is allowable in landscape work. 
WJien the swing-back is used it is better to have a tilt 
downwsLTds than to keep \l\e\t\, ^>q^ ^^ Aavcs^tKe plate is 



Archit€€titral Pictures. ;?4S 

kept more n;early vertical than would otherwise be the case. 
A tilt upwards exaggerates, the distortion, and it is better to 
raise the board carrying the lens of the camera than to give 
it a tilt in this direction. Raising the lens board means 
taking off a portion of the foreground. This has the same 
effect as tilting the camera, though without causing the direc- 
tion of the axis of the lens to be altered. As a general rule, 
however, tilting should be cautiously and sparingly used, 
otherwise it' is apt to suggest that there is something wrong 
about the picture. 

When the best general focus has been obtained by the 
above artifice, the diaphragm should be inserted so tfiat the 
necessary amount of sharpness may be obtained, recollecting 
that the brighter the image, the greater will be the vigour of 
tne resulting negative* When great contrasts in light and 
shade are in question the introduction of a small stop may 
sometimes be advisable, as will be evident by consulting the 
next chapter. 

In focussing architectural subjects, where it is of im- 
portance to preserve the parallelism of vertical lines, the 
sensitive plate must always be kept in a vertical plane. In 
case the axis of the lens has to be tilted the swing-back 
must be used till this is attained. When this vertical plane . 
is not adhered to we shall have the vertical lines which are 
parallel in nature converging in the picture. If we look at 
a cube lying on a horizontal plane the feeling to the eyes is 
that the vertical lines are parallel (perhaps because the^ 
most natural position for natural movement of the axes 
of the eyes is in a horizontal rather than in a vertical 
plane) ; if, however, we tilt the cube the impression at 
once vanishes, and they will seem to converge. Hence, 
to give the idea to the mind that an object is stand- 
ing on a horizontal plane, the vertical lines must ap- 
pear parallel to the eye, whether seen in nature or 
seen in a picture. Now, it can readily and easily be 
^tmonstTBted that by tilting the camera ^Sx^oxbX ax^csss^"^^ 



246 On the Picture. 

swing-back, vertical lines must converge, hence the resulting 
picture would be untrue. It will perhaps aid the student 
in regard to the use of the swing-back to remember that for 
theoretical purposes the lens, whether its axis be tilted or not, 
may be replaced by a pin-hole in an opaque card, and that 
the image received through the pin-hole must be theoretically 
correct when received on a vertical plane. 

As regards the exposure to be given to a picture there is 
one golden rule to follow : ' Expose for the shadows and let 
the lights take care of themselves,' that is, the detail in the 
shadows must be developable. By the judicious use of 
different developers, page 67, effects may be given which 
would be unattainable were one formula alone to follow. 
Thus, if the picture be full of great contrasts in lights and 
shades, a strong developer swept over the plate, carrying 
into the sink the greatest part of the free nitrate of silver, 
will lessen the difference between them, whilst a weak 
developer, of which none is allowed to flow over the plate, is 
most suitable when the contrasts are little. Again, even 
during exposure something may be done to give harmony to 
the negative by shading the lens with a piece of blackened 
card from those parts, such as the sky and clouds, which are 
most quickly impressed on the plate. 

As regards portraiture the variations in lighting that can 
be produced in a well-appointed studio are so various that it 
would be impossible to treat of them in a limited space. For 
outdoor portraiture an angle of a wall facing the north with 
a background formed by a blanket is suitable for producing 
pictures that can be vignetted. 

Indoor portraiture can be attempted where a window 
with a northerly light is available, and where white screens 
are at hand to lighten up that portion of the face which is 
in shadow. Ordinarily the principal light should make an 
angle of about 45° with the vertical and horizontal planes 
which intersect in the axis of the lens. 

The student should refer lo ' "PVcXoxvaX '^\vQ\si^ca:^\v^^' Cor 



Actinometry. 247 

a comprehensive description of what to do and what to 
avoid. The rules given for landscape photography, however, 
in a great measure apply, particularly paragraphs 3-5, 
7-10. 



CHAPTER XXXII. 

ACTINOMETRY. 

Amongst the earliest methods of comparing the chemical 
energy of different lights is that known as Bunsen and Ros- 
coe's, its value having been originally pointed out by Pro- 
fessor Draper, of New York. The process is dependent on 
the fact that combination takes place between hydrogen and 
chlorine when the mixed gases are exposed to the action of 
light. The two gases may be evolved by the electrolysis of 
hydrochloric acid, and then they are in the right proportions 
for recombination. Such a mixture of gases, when exposed 
to sunlight, combines with explosive violence, though in dif- 
fused light the recombination takes place gradually, and in 
proportion to the intensity of the light, and to the time during 
which they are exposed to it. This affords a method of 
securing a registration of the intensity of the light, for 
the hydrochloric acid formed may be collected in water, 
and the amount may be estimated by various chemical 
means. As for general use this method did not prove 
altogether satisfactory, and Bunsen and Roscoe abandoned 
it for one which will be described in detail. Professor Draper 
had also pointed out that ferric oxalate when exposed to 
light, gives out carbonic acid, and, in 1859, Mr. H. Draper, 
of New York, turned this fact to practical use by elaborat- 
ing a system of which the following *\^ axi ovaJCivcvfc- 



248 Actinometry, 

The chemical reaction on which the method is founded 
is this : — 

Ferric oxalate = Ferrous oxalate + Carbonic anhydride. 
Fe^aCp^ = FeQO^ + 2CO, 

The light vibrations are able to split up the ferric oxalate 
molecules, and for each molecule so shaken, one molecule 
of carbonic anhydride is liberated. Mr. H. Draper's appa- 
ratus consisted of 500 grains of a standard solution of ferric 
oxalate held in a glass cistern, rendered opaque by japan- 
ning, the light being admitted by leaving uncovered one 
square inch of the cistern. After exposure to the light for 
any desired time the amount of carbonic anhydride dis- 
engaged was known by the difference in weight before and 
after exposure, the loss due to evaporation being checked 
by comparison with a similar cistern containing distilled 
water. There are one or two objections to be noted as 
regards the accuracy of this method. The ferric oxalate 
being a coloured solution, it is uncertain to what depth the 
light penetrates into it, and it has yet to be proved that with 
ecjual intensities of light acting for the same time through 
the same aperture, double the same amount of chemical 
decomposition is produced after passing through two units 
of thickness, as is produced after passing through one. In 
all apparatus of this kind, too, the surface reflection has to 
be taken into consideration, as also the material of which 
the transparent parts of the cistern is constructed. Jf we 
could be assured that the value of the ultra-violet rays in- 
creased in the same ratio as the blue rays, the apparatus 
would suffice, but we have reason to think that this is not 
the case. Hence this construction must be taken as yield- 
ing an approximation to the true results rather than as the 
tnie results themselves. We consider that the best results 
would probably be attained by throwing certain definite 
l)ortions of the spectrum on some medium, and noticing 
the results of each. This would give a true idea of the 
relative amounts of photogTa\)\\\c ^xv«^ ^^\^<\tv%\\i ^^.^ 



Bunsefi and Roscoe's Aciinomcter, 24Cf' 

portion. It cannot be too deeply impressed on the student 
that these processes do not measure the aaua/ energy in the 
impinging rays of light ; this is altogether a different matter, 
into which we cannot enter here. 

Professors Bunsen and Roscoe conceived the first 
practicable method of measuring the actinism of day- 
light and sunlight, by the exposure of sensitive silver 
chloride paper to their action for certain lengths of time. 
After an elaborate investigation, they came to the conclu- 
sion ' that equal quantities of the intensity of light into the 
time of insolation (exposure) correspond, within very wide 
Hmits, to equal shades of darkness produced on chloride of 
silver paper of uniform sensitiveness.' Starting with this 
idea they carried out a laborious research into the prepara- 
tion of a paper that should be uniformly sensitive, and which 
might, therefore, be considered as a paper of standard 
sensitiveness. The following is a short resume of their work : 
Choosing sodium chloride as the soluble chloride, they 
found, I St, that a paper will not give uniform results when 
simply floated on the solution; but that it must be im- 
mersed in it. 2nd. That the stronger the solution the 
greater sensitiveness would be given to the paper. It was, 
therefore, necessary to fix some reasonable limit to the 
strength, and this they fixed at a 3 per cent, solution. 3rd, 
that using the sensitising solution of silver nitrate above a 
greater strength than 6 per cent, gave no difference in the 
results as regards sensitiveness, but that below that strength 
it rapidly diminished. 4th. That the presence of the salt 
resulting from the decomposition of the sodium chloride 
and silver nitrate had no effect on the sensitiveness, and 
that at ordinary temperature and moisture the sensitised 
paper would keep at least 15 hours unaltered. 5th. That 
the thickness of the paper employed had no material in- 
fluence on the result — an important point, as in comparing 
the darkening due to the light with a standard scale oC ss^-a.- 
duated tints it wds necessary that l\\e ^sc^ex ^Q.\\^\i^ '5.Nj5S\r 



250 Actinametry. 

ciently oi>a/|ue to cut oflf all shade from the tint beneath 
which might shine through the paper. 6th. It wa« found 
that it wais possible to impregnate 5 square metres of paper 
in a solution containing 60 granmies of sodium chloride 
without any danger of reducing the strength of the solution 
to such a degree as to cause any variation in sensitiveness. 
7th. 7*he time of sensitising, betH-een 15 seconds to 8 
minutes, also caused no alteration in the readings. 

It will l>e noticed that the above results give great 
latitude in preparing a paper, the only conditions abso- 
lutely necessary being that the paper shall be uniformly 
soaked with a 3 per cent solution, that the sensitis- 
ing bath of silver nitrate shall not be suffered to drop below 
a 6 per cent solution, and that the sensitising shall not be 
les5 than 15 seconds of time. With such a preparation we 
nave then a standard paper which will give uniform results 
-that is, we have a paper, which, when exposed for a 
certain time to a certain intensity and quality of light will 
always produce the same amount of darkening. The most 
perfect method of measuring intensity of daylight, then, 
would be to cause a strip of sensitive paper to pass gradually 
before an opening of a certain size, such opening to be ex- 
posed to the zenith. Unfortunately, this method is impracti- 
cable; for, as we shall find shortly, the darkening action in 
strong light, such as we have in sunshine when the sun is in 
the meridian, is exceedingly rapid; whilst in weak light, such as 
we have on a cloudy day, the darkening is exceedingly slow. 
It might be suggested that the opening should be wedge- 
shaped, so that the shade itself might be graduated and 
therefore be readable at some part, and at a very early date 
in the history of photography this plan was proposed by 
Mr. Jordan ; but practically it is found that the opening 
necessary for the production of a readable tint in full sun- 
shine, with paper passing slowly before it, is so small, that 
tht:rc are mechanical difficulties in the way of f^ecuring 
accuracy; and when it is cotvft\d(iTt^ \!cv^\. otvfc ccA q>^ the 



Roscoe's Actinomeler. 



25" 



aperture would have to be at least lOO times the width of 
the other, the impossibility of obtaining a proper gradation 
with a slip of paper of reasonable width is apparent To meet 
this difficulty a most ingenious instrument has been devised 
by Professor Roscoe, in which exposure takes place for cer- 
tain regulated times, at fixed intervals, during every hour. 

The registration is effected by the following contrivance : 
Fig. 83. 

Fig. 83. 




A long strip of paper is rolled upon f, and fastened to d. 
In the latter is a clock-work arrangement, the escapement, b, 
being placed as sho«-n. To move the clockwork, the arma- 
ture of an electro-magnet takes the place of a pendulum, 
and every time it is attracted and released by the magnet, 
a tooth of the wheel is released, and the paper is moved 
a small piece forward across the weak spring, e, which 
is seen on the top of d. The use of the spring is to cause the 
p3[>er to be in contact with a circular aperture of about j- 
inch in diameter, left in the cover of the instrument, and 
through which the exposure is given. The result of the 
exposure is thus to leave circles of more or less blactsw^'^a 
(the blackness vailing according U> ftic YiAeK3&.-i ^&. *^R. 



252 



Actinometry. 



light and the time of exposure) at intervals along the strip 
of paper. These intervals and durations of exposure are 
controlled by the accompanying piece of apparatus, which 
forms part of a clock. 



Fig. 84. 







tf is a toothed wheel belonging to a clock, revolving once 
every hour, in one part of which a peg, c^ is inserted, and 
so placed that the lever, d^ is in contact with it for about two 
minutes ; ^ is a disc with platinum points placed at differing 
intervals, and revolves once every two minutes. The lever, 
d^ is caused to press on these points by a weak spring ; / is 
an ebonite insulating block, to which the two levers, g and //, 
are attached as shown, which are in connection with the 
terminals of a battery. When the pin, r, presses against //, 
and when it is in contact with the pins on ^, a current tra- 
verses the circuit, in which is introduced the electro-magnet, 
c; of fig. 2>2i, By this arrangement when the armature of the 
electro-magnet is attracted and xeXt.^L'&^d Vj ^^t ^^-saL^e of 



Cylinder Actiiiometer. 253 

the pins on b across the end of d, the paper moves on a 
small portion, and the exposure takes place for the time cor- 
responding to the intervals between the pegs. When the peg, 
c, is not in contact with d, a lengthened exposure is given, 
and this marks the hour intervals. * is a cord attached 
to k, which can be used to cause exposures, at any time 
that may be desired. This is useful when adjusting the 
instrument, and for determining the interval of time which 
elapses between the passage of the several pins. 

Another instrument for attaining the same end was 
devised by the writer on a different plan : — 




A cylinder, a, round which is placed sensitive paper, is 
connected by a strap with a small French clock, fl, and the 
diameter of the pulley is so adjusted that the clock causes 
the cylinder to rotate in 25 hours. If it were absolutely 
certain that the paper would be removed at the same hour, 
it would be more convenient to cause the cylinder to revolve 
once in 24 hours; the extra hour is allowed for any irregu- 
larity that may occur in the removal' of the paper. A 
cover, c, fits accurately over the cylinder, and in it is a slit, 
D, \ inch broad, which is covered by a wedge, the pro- 
cess of graduation of which will be subsequently given. 
Qne end of the wedge allows nearly all the tight to pass 
through it, whilst the other is nearly opaque. Over one 
end of the cylinder is placed a cap, fitting over the paper, in 
which are slitii, corresponding to ft\e »^ VQ^s». "^"wisv. "Csa. 



'2 54 A ctinometry. 

cover, c, is placed over the cylinder, and the cover E 
over the clock (to prevent the access of rain or dust to the 
works), the apparatus is placed in the daylight, the wedge 
approximately running from east to west As the light acts 
on the sensitive paper, a graduated darkening takes place, 
and through the brass cap on the end of the cylinder the 
time at which any particular exposure takes place is noted by 
the light itself. When the wedge is properly graduated some 
part of the band is always readable. 

In the first instrument a wedge was made of nearly black 
glass, having, however, a slightly green tint. Had it been 
pure black, equal lengths of the wedge would have given 
equal lengths in the scale of blackness. For, if a be the 
intensity of the light impinging on the wedge into the time 
of exposure, and a' be the intensity of the light after pass- 
ing through the thickness of any part of the wedge into the 
time of exposure, x be the thickness of the wedge, and /u a 
co-efficient due to the absorption of light by the wedgfe that 

A' = A€-'*«^ 

£ being the base of hyperbolic logarithms. This was found 
not to hold good unless the light were monochromatic, the 
reason being evidently due to the different coefficients of 
absorption of the rays which passed through the wedge. To 
remedy this defect, recourse was had to photography. A 
dry plate was exposed to the action of light beneath the 
wedge and developed ; a black tint was secured by employ- 
ing a platinum salt as the toning agent, and the results thus 
obtained were satisfactory, any actinic light giving the same 
relative readings. In some cases the platinum was burnt 
into the glass, but no great gain was found in so doing. 

The following is the standard tint of grey adopted 

by Roscoe. He took i,ooo parts of zinc oxide and r 

part of lamp-black, and ground them thoroughly to- 

gether to such a point that no further grinding altered the 

tint This he found the mosX coxiNtm^ivX. xvcvX. fat comparison ; 



Registration of Tints. 255 

and, when carefully gummed on to paper, it was unaltered 
in shade. This mixture 'hen gave the shade from which all 
his meaam-ements were made, all other tints being referred 
to it. 

To obtain a graduated shade he applied what is known 
as the pendulum apparatus, which in general outline con- 
sists, of a pendulum swinging in front of sensitised paper 
in such^a manner as to give a gradation of exposure to it, 
and a consequent variation in tint. At each point of the 
paper the time of exposure was known, and the point 
was then found answering to the standard tint, and the 
relative values of the other portions of the gradations 
calculated. 

It may perhaps be found necessary hereafter to apply a 
correction to those readings taken in sunlight, as it may be 
found that the different integrations of the spectra formed 
by sky, cloud, and sunlight produce slightly different effects. 




Another method of securing uniformity in measurement 
lias been employed by the writer. It consists of a rapidly 
revolving cylinder, or drum, b, on which is attached a series 
of black and white sectors, as in the diagram. A convenient 
length for this drum has been found to be 6 inches. To 
the cylinder, b, is fixed a small pulley firmly attached to one 
end, over which is passed a corii co'KKmm\c3.'«wj,-«\^ '^^ 



256 



Actinamctry, 



wheel, A. These are of such rdative dimensioiis that 
the cylinder rotates at least 15 times in a second, when 
A is caused to rotate but once. Along the top, and 
nearly touching the cylinder, is a blackened brass support, 
c, with a slot in it, on each side of which is a scale of inches, 
dividing the length of 6 inches into 120 parts, that is, each 
inch into 20 parts. Monochromatic light is dirown verti- 
cally downwards, on the scale, and any tint to be compared 
is brought on to the scale, and moved till an exactly identical 
shade is found on the rotating cylinder. A series of 
6 readings is taken, beginning by moving the tint fh)m 
white to black, and next from black to white. It will 
nearly always be found that this is necessary, as the read- 
ings in one case would be as much too high as in the 
other they would be too low. A mean of the six gives 
very nearly the truth. The accompanying diagram gives 
the results of the reading of a strip of paper which had been 
exposed beneath an apparatus, giving an arithmetical pro- 
gression of exposure for each unit of length : — 



FlG> 87. 




The ordinates measure the amount of white, i*o being 
white, and o black. The absc\&^9& ^Vvoyi the exposure, with a 



On tfie False Effects of Photography. 257 

•fixed intensity of light. From a curve of this kind, when 
the tint is compared and known, and the time of exposure is 
also known, the intensity of the actinism can be judged. 
Roscoe*s standard unit has an ordinate of 76, that is, it is a 
visual combination of 24 parts of black with 76 of white : — 
This method of examining the gradations caused by 
different intensities is well worthy of more complete study, 
as it throws much light on the false effects which are pro- 
duced in photographs. If we prepare a rotating wheel with 
black spokes so cut that when rotating in front of a uni- 
formly lighted surface they give the exposure along the 
length of the spokes in arithmetical progression (thus : if the 
length of spoke be 10 inches, the exposure at that dis- 
tance is I, at 5 inches ^, at 2^ at ^, and so on), we shall 
find, on exposing a plate to the image of this rotating wheel 
as formed by a lens, or by causing the wheel to pass close to 
it, that the gradation of the negative obtained, when viewed 
by transmitted light, will not coincide for any length with 
the gradation as seen by the eye. The following figure 

Fig. 88. 




r 



Blade o '25. 'SO JTS ±00 TV5?ife 



shows the results of measurements obtained by the dia- 
phanometer,* from plates to which different exposures 

* See Zondofif Edinburgh^ and Dublin Phil. Mag. '^je^^. V^'W 

S 



2^8 Actiuomctry. 

have been given behind the rotating wheel, a, b, and c 
are the curves from simply developed negatives. The ordi- 
nates measure the amount of transparency, i being total 
transparency, and o total opacit)' ; the abscissae denote the 
relative times of exposure, or what is approximately equiva- 
lent to it, the relative intensities of light acting on the negative, 
supposing Roscoe's law referred to above to hold good. It 
will be seen that the curves have very nearly, that is, within 
the limits of error of observation, the same form. Thus, 
taking the exposure of a equal to '5 and i, or i to 2, the corre- 
sponding transparencies are 77 and 37-. Taking the same 
transparencies of b, the times of exposure are '34 to '66, or 
very nearly i to 2. The same will be found with c ; the 
dotted curves, e and d, show a portion of the negatives, b 
and c, intensified in the ordinary manner, and the same rela- 
tion to exposure still holds good. This is an important 
point, as it shows that the same relative intensities of light 
are maintained in a negative, -as the opacity is increased. 
The ordinates to the chain-dotted straight lines show 
the transparency that should result if photography gave 
perfect gradations. It will be seen that the tendency in all 
negatives is to cause a loss of gradation in the deep shadows 
as well as in the lights. This accounts for the loss of detail 
that is always seen in the extreme tints of a photograph. 
It is also worthy of remark that a thin negative seems to 
give a better gradation than one intensified. 

It must be distinctly understood that the above curves 
apply to negatives only under one class of development. 
Under others the curves would show considerable varia- 
tions. Amongst the most striking would be the form they 
take when near the parts in which total transparency is 
represented. A more detailed account of these will be 
found in the * Photographic News,' for July and August, 

1877. 

As an extension of the foregoing we may describe what 

happens in repfoducing a ive^twt Vcv ^^ caccwoa. or by 



Density of Transparent Positives, 



259 



contact. The first method is often not so absolutely true 
as the latter, owing to the defects that a lens of necessity 
introduces ; but as regards the shades the same results hold 
good. In the accompanying figure f and k are two curves, 
representing transparencies taken from the negatives d and 
B in fig. 88. These two negative curves have been selected as 
showing the results obtained from a simply developed and 
also an intensified negative. It will be noted that the curve 
K' follows the line representing the correct gradation more 

Fic;. 89. 




0: 



I- 



' 1^ WAite 



closely than the curve f. Had k been intensified it would 
have shown greater curvature, and could not have conformed 
so nearly as it does to the dotted line. Hence we may 
infer that a thin positive from a thin negative more closely 
represents true gradation than if either or both of them 
be intensified. The theoretical curves coincide tolcably 
with those obtained by measurement. It must be noted that 
in the curve k no portion is absolutely transparent. The 
exposure has been so prolonged as to cause a slight veil. It 
will be seen that for the reproduction of a negative this is no 
detriment In fact, it follows the usual practical precept laid 
down that a transparency should show little or no bate ^%*3., 
The next figure shows the piodweUOTv oS. ^tv^^'vxn^^^'^sv 

s 2 



26o Actinometry. 

tht transparencies represented by f and k in the previous 
figure. K| is a curve showing the simple reproduction of k 
by development alone. Kh shows the same intensified, so 
that it possesses opacity at one extremity, f, and f, , re- 
present F under the same conditions. The ordinates to the 
strong curves show the amount of transparency, whilst the 
ordinates to the chain-dotted lines, as in the tn'o previous 
figures, show the amount of white in the original gradation. 
If we compare b iinth k, and K],, and d with f, and F,,,the 
immense deterioration that has taken place in the truth of the 
gradation as represented by the reproduced negative will be 

Fig. 90. 




Black o 'Z6 SO J7S j^>o White 



api)aren t. Another reproduction from any of the curves k , k i j , 
Fi Fii, would still further increase the falsity, till after a few 
more reproductions the shades would have nearly entirely 
disappeared, and we should have a negative represented by 
a large portion of entire transparency with an abrupt change 
to total or nearly total opacity. 

It is thus evident that in the reproduction of negatives 

the greatest care is required to keep approximately true 

gradation in the transparent positive as well as in the 

original negative ; this can only be obtained by keeping 

hotli as thin as is practicaWy ^o^^^Xt. Tcv\^ W1V5 accounts 



Density of a Reproduced and Original Negative. 261 

for the loss of delicacy that is so often seen in repro- 
ductions and enlargements, and it seems impossible that 
the same harmony can exist in them as in the original, 
supposing the latter to be capable of giving a good print. 
It may be possible to obtain a fair reproduction of a ne 
gative if that * positive ' transparency be very thin, and 
there is no doubt that such reproduction can furnish excellent 
prints, but if placed side by side with prints furnished by 
a fairly intensified negative produced by the first operation, 
there will be a marked inferiority in the former. 

It may not be uninteresting to finish this portion of our 
subject with a diagram showing the character of the gradation 
of the prints that would be obtained from the negatives 
A, E, and Kj, ; in each case the hypothesis will be made that 
no perceptible tint is printed through the most opaque por- 
tion of the negatives. 

Fig. 9^1. 




For ordinary photographic purposes actinometers are 
required, but their use being confined to the purpose of 
approximately measuring the exposure to be given to either a 
sensitive plate, or a sensitive printing surface, their accuracy 
need not be so great as those already d^smb^d. It^-^^t^^^w;!^ 



262 A ctinovictry, 

carbon tissue, Chap. XXIV., the opaque and usually dark 
pigment effectually prevents the change effected in the 
chromated gelatine by its exposure to actinic light from being 
observed, and in this particular instance, as also in the photo- 
engraving, and its kindred processes, an actinometer is abso 
lutely necessary. The simplest form consists of a long slip of 
sensitive albuminised paper, coiled in a small tin box, in the 
top of which is cut a slit. On each side of the slit the tint 
which the paper assumes after a certain exposure is painted. 
When a carbon print is exposed the actinometer is placed 
side by side with the printing frame in the light, and when the 
sensitive paper in the former has assumed the colour of the 
painted tint, a fresh portion is exposed, and so on, till it is 
judged that sufficient depth of printing has been given to the 
carbon tissue ; the number of tints required of course vary 
ing with the intensity of the negative. 

Another form of actinometer is formed as follows : Strips 
of oiled silk or coloured paper, which are only partially trans- 
parent to the actinic rays, are cut into diminishing lengths. 
These are pasted one upon another, so that at one end there 
is only one thickness of material, at (say) a quarter of an 
inch from that only two thicknesses, and so on, till at the other 
end, there are a dozen thicknesses. The number of thick- 
nesses is painted in black on these * steps ' and the actino- 
meter is complete. If the light were uniformly cut off by 
each strip, an exposure giving the same tint beneath the 
gradually increasing thicknesses would be in a geometrical 
series ; as it is they progress in a very complex manner, but 
after a little experience, a good estimation of their value can 
be formed. To use the actinometer, sensitked albuminised 
paper is exposed beneath it, and the tint judged by noting 
the last figure which can be read on the exposed surface. 
An improvement on this actinometer might be made by 
taking a graduated strip of glass as described at page 254, 
crossed at intervals with black lines which should give an 
arithmetical series of exposMies. 



Photo- spectroscopy, 26^ 



CHAPTER XXXIII. 

PHOTO-SPECTROSCOPy. 

One of the branches of science into the service of which 
photography has been impressed is that of spectroscopy, 
and the aid it has given dates from nearly the early days of 
the daguerreotype. In the researches at present made with 
the spectroscope it plays such an important part, that a 
rather detailed description of the apparatus necessary and 
the methods employed will be given. 

Photo-spectroscopy, however, has two aspects : in one it 
is the study as to the sensitiveness of compounds to the in 
fluence of different portions of the spectrum ; in the other, 
the study of the spectrum itself. The first may be con- 
sidered an essential preliminary to the second, and will 
therefore be examined first. 

Becquere], Herschel, Draper, and Hunt are the names 
of physicists to whom is due a great part of the know- 
ledge possessed up to the present date as regards the different 
degrees and extent of impressibility which a variety of com- 
pounds show. In the loan collection of scientific apparatus 
was exhibited the instrument with which Herschel made 
his various researches. His experiments were undertaken 
before the days of the collodion process, and his modus 
operandi consisted in giving washes of one or more 
solutions to paper, and then submitting the sensitised paper 
to the solar spectrum. The accompanying figure (p. 264) 
gives an idea of the prismatic arrangement he adopted. 
A is a flint-glass prism, capable of turning on an axis, d ; b 
a lens of about 24-inch focal length \ c the screen on which 
the sensitive paper was placed. The sunlight was re- 
flected by a mirror into the prism, the image of the sun after 
passing through a was elongated mXo ^. ^^^'s^nxvskv^ "sxw^ 



264 Photo-Speclrouopy. 

brought to 3 focus on c by means of the lens & One 
portion of the spectrum was caused to fall on the line, 
E E ; the particular ray being found by examining the 
spectrum by means of a piece of cobalt glass, which cuts 
oiT all rays excepting two. The cage was covered with a black 
velvet cloth. It will be noticed that no slit was employed, but 
that the spectrum waS really formed by a series of over- 
lapping images of the sun. The spectnun was thus of 
necessity an impure one, and in the results obtained with 
it this has to be taken into consideration. For a de- 
tailed account of the experiments carried out by Sir John 
Herschel, the 'Philosophic Transactions of the Royal 
Society ' should be consulted, and also ' Hunt's Researches 
on Light.' The registration of the Fraunhofer lines of the 




solar spectrum was effected by Becquerel and Draper with 
an instrument similar in general principles to that which 
will presently be described. Both of these eminent physicists 
employed the Daguerrean process with the greatest success 
in these researches. The latter performed the feat of regis- 
tering the lines in the least refrangible portion of the 
spectrum by the reversing action of the red rays, of which 
a description will be given subsequently. 

An apparatus tliat will answer the purpose for a student 
will now be described. The great essentials are good prisms 
and a collimator of fairly long focus. These may be pur- 
chased separately from an instrument-maker, and fitted upby 
the operator, if he be at all handy with ordinary carpenter's 
tools. Thus the colVimatot wtot owj \>fe Ba^'jorted in a 



A rrangement of Apparatus, 



265 



cradle, and the prisms mounted on a block of smoothly- 
planed wood, or a slate slab, so arranged that the axis of 
the lens of the camera is of the same height as the centre 
of the prisms, as is also the axis of the collimator. 

The accompanying arrangement shows the manner in 
which a temporary photo-spectrum apparatus can be fitted up. 
A is a heliostat, throwing the sun's rays into the condenser, 
B, by which an image of the sun is formed on the slit of the 
collimator, c. The lens, d, of the collimator is so placed 
that its equivalent focus falls accurately on the exterior of 
the slit This should be obtained by trial, and it is very 
advisable that the slit arrangement should be attached 



Fig. 93. 




to an inner tube, which can slide in that to which the 
lens is attached. The rays proceeding from the illumi- 
nated slit travel in such a manner that when certain 
requirements, which will be entered into presently, are 
fulfilled, the lens of the collimator is perfectly filled, or at 
all events the rays of light form a central disc on the lens, d. 
The light then travels to the prisms, e, by which it is 
refracted and dispersed, and then it reaches the lens, f, of 
the camera, g, by which lens the spectrum is thrown on the 
plate, H, where it is focussed. As difficulties frequently 
arise at first in the adjustment, &c., of the apparatus, a few 
hints may not be out of place. In order to secure good 
results the foc2A length of the conderv^vcv^ \^ts&^Ti^^^tsK5s. 



266 Photo-Spectroscopy. 

divided by its diameter, should never exceed the length of 
the collimator when divided by the effective aperture of its 
lens. Should it do so, it will be seen that the coUimating 
lens will be more than filled, and reflections from the sides 
of the tubes might interfere with definition. Again, it is 
useless to have prisms which cannot receive all the rays 
proceeding from the coUimating lens. Their height should 
therefore, be at least equal to the effective aperture of the 
coUimating lens, and the faces should be longer, since they 
are placed obliquely. In all cases the centre of the mirror, 
the axis of the condensing lens, and of the collimator should 
be in one straight line. To effect this, it is better at first 
to remove the con4enser from the train of apparatus. The 
image of the sun, when thrown on the slit, should give a bright 
diffused line occupying the centre of the coUimating lens, d. 
The position and height of the heliostat must be changed 
till this is obtained. The condensing lens, b, may now be 
inserted and moved till the rays of light form a circular 
disc, filling the centre of the lens, d, when a sharp image 
of the sun is thrown on the slit When this is obtained 
the prisms may be placed in position. By the aid of 
an ordinary small telescope the angle of minimum devia- 
tion may be obtained. Suppose it is required to photo- 
graph the portion of the spectrum about the line g. The 
prism would be placed roughly in position, and that line 
would be observed. It would be found that by turning 
the prism in one direction, the line would appear at first 
to travel in one direction, but that when a certain point 
was reached it would begin to travel in the opposite direc- 
tion. The position the prism occupied when the change 
in direction of the apparent motion of the line took place 
would be the position the prism should occupy, in order 
for that particular ray to be refracted in the angle of 
♦minimum deviation. The reason why this angle is of 
consequence must be sought for in books specially devoted 
to spectroscopy. It is suffidetiV. X.o xvoXfc >i5wax. v\ any other 



Adjustment of tke Prisms. 267 

position the true breadth of the absorption lines would 
not be obtainable. The next prism may -be adjusted in 
a similar manner, and so on for the others. Finally, the 
camera and its lens should be so placed thjt the ray 
for which the prisms have been adjusted should occupy 
the centre of the focussing screen or sensitive plate. It 




may be noted that four prisms of 60" will generally cause the 
axis of the camera to cross the axis of the collimator. 

Another contrivance ' for always securing the minimum 
angle of deviation is shown in the accompanying diagram. 
Cut out, in stout card or brass, pieces having the form a a a, 
also D aiid K. Care must be taken that the bases of the 
triangles are of uniform lengths, and slightly longer than the 
base of the prisms, which should be of uniform angle, and 
preferably of equal size. D should be let into the board s Sj 
so that its top surface is flush with it, and there should be a 
groove cut beneath the slot, b, to allow a ^\ti , ot ^ feasMSKs. 
' TTie principle of it is due lo Mi. Bio-trwn^, *it ov^>ji'i-t^- 



268 Phota-Spectroscopy, 

equal to that of the slot, to travel along it. The slots aaa 
in AAA and k are also placed over the pin. The first 
triangle is attached to the board, s, by a pin at b. The 
remaining triangular portions, and also k, are attached to 
each other at c c c^ and are free to move over the board s. 
The axis of the collimator is placed at right angles to the slot, 
^, and, by touching k, the arms from the triangular portions 
move about the pin, the slots, a a a^ guiding the motion at 
the same time the parts move separately about ccc^ and the 
whole system turns about b. It will be seen by this that 
each base of the triangle moves through t^sdce the angle of 
the preceding one, as also does k. The direction of the line 
joining the point in k, answering to the middle point of 
the base of the triangle, and the middle point of the base 
of the adjacent triangle, determines the position of the axis 
of the lens, f, of the camera. Such a board may be used 
as a pattern by which to set the prisms for any particular 
part of the spectrum, or the prisms themselves may be set 
on the triangular portions, provided the board, s s, be per- 
fectly plane, and that precautions be taken to raise by ob- 
vious means the prisms to the same level, and to cause the 
triangular patterns to be so adjusted that there shall be no 
deflection owing to their arms being at different heights on 
the pin. It will be found that two prisms of 60° and two of 
45**, or three of 62°, will be the greatest dispersion that can 
be employed, unless special arrangements are made. As a 
guide to the length of spectrum that can be photographed 
at one time, it may be stated that with a lens to the camera 
of 120 centimetres focus, and using one ordinary flint prism 
of 60°, a photograph of about 10 centimetres is obtained ; 
and with the same camera and two prisms of 60° the length 
of spectrum is about double. 

To focus the lines accurately is somewhat difficult, for it 

will be found that the focus of the violet rays is shorter 

than that of the red. A fair general ftjcus can, however, 

he obtained by using witYv \\ve cam^i^ ^ N^'^cally-pivoted 



Focussing the Spectrum. 26g 

swing-back. It is usually prescribed that the focus should 
be obtained by placing a transparent glass plate in the place 
of the ground glass and viewing the spectrum by a high- 
power magnifying lens. This latter should be attached to a 
sliding tube, so that when the end of the tube is placed against 
one surface of the glass plate the other surface immediately 
opposed should be in the plane of its focus. By this means 
the rays which have to be focussed on the inner surface of 
the plate can be viewed by the magnifier, and the plate of 
glass moved backwards and forwards by the screw motion 
of the camera, till the lines appear sharply defined to the 
eye. Theoretically, this is a perfect method, but practically 
it fails if two portions of the spectrum which are far apart 
have to be photographed at one operation, for the necessary 
inclination of the surface of the glass away from a plane per- 
pendicular to the axis of the lens (the deviation being 
effected by the swing-back), is so great that the axis of the 
'magnifying lens is thrown completely out of the direction of 
the rays of hght, and there is necessarily no image observed. 
The writer has found great advantage in cutting an opening 
across the top of the camera, pasting highly-glazed white 
paper on a piece of plate-glass, occupying the position 
that the sensitive film has to do, and by then focussing on 
this white surface through the opening by means of a small 
telescope. 

When the focus is fairly obtained, the exposure of a few 
plates and a minute change in the focal distance will show 
which is the best position to choose for photographing any 
particular part of the spectrum. A good method of obtain- 
ing the correct distance of the collimating lens from the 
slit may here be indicated. Make an ink mark on a piece 
of glass, and focus it with a magnifying lens fitting in a 
draw-tube, taking care that the surface of the glass on 
which is the mark is next the draw-tube. Now place the 
magnifier against the plates forming the jaws of the slit 
(which should be rather widely o\>tTv^d\ ^xA nSrt^ ^^\sv^ 



270 Photo-Spectroscopy, 

distant object through the collimating lens. When this 
appears quite sharply defined, the definition being obtained 
by altering the distance of the slit firom the collimating lens, 
this adjustment is complete. 

In the case of a lens only partially corrected, the focus 
may vary. In one position the image may appear perfectly 
sharp but surrounded by a blue fringe, and in another by 
a red fringe. When the most refracted rays are to be 
photographed, that focus should be chosen in which the red 
fringe is seen ; whilst, if the least refracted, that in which the 
blue fringe is apparent. 

It must be recollected that the greatest accuracy in all 
these points is requisite in order to obtain the best results. 

The examination of sensitive compounds other than 
silver is not so readily undertaken by the above arrange- 
ment, as the time of exposure to obtain a sufficiently 
marked result would be very prolonged. 

A more convenient, though perhaps less exact, appa- 
ratus for^examining these, where more dispersion than that 
given by one prism may be required, is the ordinary direct- 
vision spectroscope. It can be inserted in a small camera 
which is adapted for a lens whose equivalent focus is some- 
where about 20 centimetres. With such an instrument 
there is only a collimating lens and no telescope for view- 
ing the spectrum. In using it for photography, the dis- 
tance between the lens and the slit can be altered, so 
that it may produce an accurate focus of the Fraunhofer 
lines on the focussing screen. It is better then to open 
the slit to such a degree that the lines broaden out. One 
or two distinguishing Hnes, such as the h and b lines, 
will still be traceable, and from these, and by a comparison 
\\nth a picture taken on a silver compound, with a more 
closed slit, the limits of the spectrum impressed on the 
sensitive salt under consideration may be accurately ob- 
tained. For obviously, when the slit is say a millimetre wide, 
there is but little impuiily *m \)cv^ ^-^^c^xvaxv. With this 



Helios tat. 271 

apparatus, as in that above, it is advisable to employ a 
condenser. 

For the student who studies this branch of photography 
a heliostat is almost a necessity, and the writer has found 
the form given in the accompanying figure, designed by 
Stoney, to answer perfectly when a little care is used, a is 
a small French clock, round the drum of which, and also 
round a small wheel fixed on the instrument, passes a cord, 
B is a mirror, held in position by a rod which slides in a 
pivoted socket at the end of c. c is attached to a part 

Fig. 95. 




which answers to the polar axis in an equatorially- mounted 
telescope, and it is to this polar axis that the driving-wheel 
alluded to is attached. It will be noticed that the clock 
stands on a board which is attached to the base board 
of the entire instrument. A level is fixed on this clock 
board, and the plane of the board can be caused to make a 
Certain small angle with the base board by means of a screw 
adjustment. The amount of * tilt ' is indicated by the arc 
D. When the clock stand is levelled, a certain diminution 
or increase of angle to the vertical can be given to the polar 
axis. The instrument is placed in position as feVVo^'s, \ — 
The poJar axis is made to point lo \h^ ^cAfc, ^xv^ ^\ssa^ $$\*5&sx- 



272 Sensitiveness of Different Salts, 

ence (say of 2 or 3 degrees) of latitude being adjusted by a 
small arc d. When the clock stand is levelled the polar axis 
will then occupy the required angle with the horizon. The 
method of securing a true north point for the axis will be 
apparent when the instrument is examined ; for by making 
E, which is a circular annulus graduated to houi^ into a sun- 
dial, a north point, approximately correct, can be found, c 
is then placed at such an angle that the image of a small 
round hole, bored in a brass disc attached to its top, is seen 
to shine on a small ivory screen attached at its lower part. 
The graduated annulus, to which b is also attached, is then 
moved till the sun shines in any required horizontal direction. 
Vertical motion is given to the beam of light by using the 
screw shown in the figure. 



CHAPTER XXXIV. 

SENSITIVENESS OF DIFFERENT SALTS. 

When the proper apparatus is in possession of the student, 
he should attempt to reproduce the results shown in the 
second, third, and fourth diagrams of fig. 96. 

Silver Iodide, — When the spectrum is allowed to fall on 
silver iodide formed by the ordinary collodion process, it 
will be found that the maximum intensity is situated about 
the line G, and terminates far beyond the violet at one end, 
and about the line * b * rather abruptly. 

Silver Bromo-iodide, —When silver bromo-iodide is sub- 
stituted for the silver iodide, the same kind of photograph is 
obtained by the collodion process. In this compound, and 
also when the silver iodide is employed as formed in the 
Dagaerrean process, similar results are to be looked for. 
With this last process, Y\o>Nevex, ^\^«^w\. ^«vw!oana are 



Diagram of Sensitive Silver Compounds, 273 







= 


ul 






s 




I 




S 3 

S 5 


g 




3 




i-4 


i 
















■i. 




7 


/ 






\ 








\ 


v 


\ 








f \ 


A 






«j-~ — 




H 




* — 1 


s — \ 


a. 






' 


















= 


« 




^ 



274 PhotO'Spectroscopy. • o :. '\ 

observed when the plate has received a short preliminary 
exposure to white light, or when diffused light is allowed 
access to the plate during exposure to the slpectrum. On 
developing such a plate it will be found that the red rays 
and those which are usually inactive on these salts, have 
exerted a negative or reversing action on the sensitive 
plate. On development, the diffused light will have imr 
pressed a lightish border to the whole of the spectrum, the 
blue and violet part of the spectrum will appear lighter than 
this border, whilst the least refrangible portion of the rays 
will have caused, what is apparently, an undoing of the work 
executed by the diffused light, leaving that part of the plate 
of its normal hue. Thus in the most refracted part of the 
spectrum the aSsorption lines due to the solar atmosphere 
will appear as grey on a lighter ground, whilst at the least 
refracted part they will be light on a grey ground. The pro- 
cess for producing these pictures has been given by Dr. H. 
Draper in the * Phil. Mag.' Feb. 1877. This reversing action 
of the red end of the spectrum is one of the most remark- 
able phenomena to be met with in photography* Some 
recent experiments ^ made by the writer probably may throw 
some light upon this subject ; though incomplete in some 
particulars, they yet tend to show that another action, 
entirely differing from that already described in the early 
chapters, may modify the capability of development of the 
photographic image. They show that the image can be 
rendered undevelopable by oxidation of the altered silver 
compound forming it. Thus if a sensitive film be exposed 
to light under proper conditions as to sensitiveness, the 
image becomes undevelopable by the oxidizing agency of 
potassium permanganate, chromic acid, and ozone, Chastaing 
has recently announced that he finds the red rays promote 
the rapidity of oxidation ; hence, taking this as definitive, it 
is easy to see that the sensitive salt of silver which had been 

> Lond. Edin, ^ Dub. Phil. Mag. Jan. 1878. Photographic Journal, 
Dec. 1877. 



Reversing Action of the Red Rays. 275 

altered in chemical composition by a slight exposure to light 
would become oxidized, where the red light acted upon it, 
whilst where the dark Fraunhofer lines fell the salt would 
remain unaffected. Now it is most probable that the oxidation 
would be naturally aided by the colour of the altered com- 
pound ; for from Chapter II. it will have been gathered that, 
in order to obtain a photographic action from any particular 
ray, it must be absorbed by the sensitive compound. Ap- 
plying this rule to the case in point we know that bromo- 
iodide and iodide of silver darken or become bluer through 
the continued action of light. The colouration must exist 
in the individual molecules of the silver salt altered by 
light, though it may be indistinguishable, owing to the 
superior number of unaltered molecules. 

With the bromide of silver the reversing action of the 
spectrum can also be made apparent. Waterhouse has done so 
in some of the spectra he has produced, by giving his collodio- 
bromide plates a slight preliminary exposure to diffused light. 
A more sensitive surface may be produced, however, by 
chemically producing a small proportion of silver sub-bromide 
in the collodion, and on exposure to the spectrum remarkable 
results are obtained, which as yet have only been partially 
examined. 

Hunt, in his * Researches on Light,' records a combina- 
tion of potassium ferro-cyanide ^^^th silver iodide, which 
offered a very remarkable exemplification of this revers- 
ing action. Potassium ferro-cyanide is brushed over 
iodised paper where free silver nitrate has been applied, 
and the spectrum allowed to fall upon it. The black- 
ening of the paper takes place with extreme rapidity, first 
in the violet rays, and then extending over the ultra-violet 
or invisible rays, and down as far as there is a visible spec- 
trum. If removed as soon as the first darkening takes 
place, a coloured spectrum will be found impressed, the red 
rays impressing a red colour, and the blue rays blue. If the 

exposure be continued, in a short time a bleaching action 

T 2 



276 Photo- spectroscopy, 

comes on the red, and extends upwards to the green. In 
the first action there is no evidence of any protective in- 
fluence in the extreme red, but when the bleaching effect is 
set up, the space occupied by the extreme red ray is main- 
tained perfectly dark. 

The increased sensibility of this paper appears to 
depend on the joint decomposition of the potassium ferro- 
cyanide and the silver iodide. It is well known that potas- 
sium ferro- cyanide is decomposed by prolonged exposure to 
the sun's rays, with the formation of Prussian blue. 

Similar results are obtained with silver chloride, though 
here the phenomena to be observed are still more 
marked. If paper be impregnated with sodium chloride, 
and be then floated on silver nitrate, and exposed to the 
action of the spectrum, it will be found that the visible 
impression made is that shown in the figure, page 273. If, 
however, the paper be exposed to light till it assumes a 
lavender-grey appearance, and be then exposed to the spec- 
trum, a change will appear. The part exposed to the red 
rays assumes a brick tint approaching to red, the green 
assumes a green ashen hue, whilst the violet and blue have 
the same tint as is usually seen. On taking such a print, 
and developing with gallic acid and silver nitrate, as in the 
calotype process (p. 131), evidence of molecular change in 
the part that has been exposed to the led r^ys will be ap- 
parent, the developable spectrum in this case extending as 
far as b. In the effect produced by white liquid on ordi- 
nary sensitised paper, this action of the red rays must 
be taken into account 

When chloride of silver is suspended in collodion, and 
used in an emulsion in the same manner as silver bromide, 
like results are obtained, only in a more marked degree, the 
tints assumed being of a much more brilliant hue. The 
subchloride may be altered in composition, as indicated 
ahowQ. The visible coloration is generally supposed to be 
due to the same cause a?^ \s v\\t eo\o>\i q^ \Xv\xv plates, but 



The Addition of Dyes to the Sensitive Compounds, 2TJ 

some preliminary experiments seem to show it may be 
due to the different stages of oxidation of the metallic salt; 
but this cannot be pronounced with certainty, till some of a 
more detailed and confirmatory character, which are at 
present in progress, have been completed. 

Becquerel produced coloured photographs of the spec- 
trum on silver subchloride, which he produced by chemical 
means, using silver plates as a support. By immersing a 
silver plate in hydrochloric acid, attached to the negative 
pole of a battery, and opposed to a platinum plate, at- 
tached to the positive pole, the subchloride was formed. 
The surface of subchloride was exposed to the spectrum, 
which was allowed to print itself on it. Some beautiful 
examples of these spectra have been exhibited at Paris, 
and also in London at the Loan Exhibition of Scien- 
tific Apparatus. Ni^pce de St. Victor further extended 
this method to obtain coloured photographs of dolls 
dressed in coloured clothes. 

A consideration of the atomic weights of the different 
sensitive salts, and the effect of the light-waves on them, 
would lead to the suspicion that any alteration in the atomic 
weights obtained by forming a new molecule might alter 
that part of the spectrum which had a maximum effect. 
This has been practically proved by Vogel, of the Berlin 
Industrial Museum, Waterhouse, and others, who added 
aniline and some ,other dyes to silver bromide. These 
additions certainly alter the place of maximum sensibility, see 
fig. 96, but the writer has not been able to ascertain that, with 
this change, a corresponding lowering of the limit of sensibility 
has taken place, unless a double silver compound be formed. 
It has been assumed by Vogel that, because certain dyes 
absorb certain rays, therefore a sensitive film which is stained 
with such dyes should be more sensitive to that part of the 
spectrum in which the absorbed rays lie, than it would 
be were a sensitive film left unstained. If we admitted this, 
we must also admit that the periods oi o^^c^^N^a^c^^ixA'^'^ 



zys 



Photo- spectroscopy. 



paths described by, the sensitive molecules must be altered* 
This seems unlikely to be the case unless the molecules are 
altered in weight by the dye ; in which case we should have 
a combination between the dye and the molecule. In 
some cases this may occur, and probably does ; for Vogel 
notes that, in order to secure the alteration in the place of 
maximum sensitiveness, it was necessary to have some free 
silver nitrate left in the silver before applying the dye. An 
examination of the dyes he employed to obtain the effects 
he describes, shows that they are capable of forming an 
unstable compound with silver, and this organic silver 
salt probably combines with the silver haloid, weighting 
the molecule as already suggested. It is somewhat remark- 
able that the dyes principally effective are fluorescent, and 
the effect of fluorescence might be to lower the limit of sen- 
sitiveness. The writer has further found that the addition 
of certain resins, albumen, and other organic bodies, when 
combined with silver, tend to lower the limit of the impressible 
spectrum, and the place of maximum sensibility ; so much 
so, indeed, that it is possible to obtain an unreversed im- 



FiG. 97- 




pression of the thermal spectrum. The accompanying 
figure gives an idea of this. A beam of light was allowed to 
pass through flashed ruby glass, and the spectrum was then 
thrown on a resinised plate in the ordinary manner, a 
being the limit of the visible spectrum, it will be seen how 
much lower the photograph extends. 

In repeating the experiment of adding ' organic matter 
to silver bromide, Vogel of Potsdam obtained a revereed 
image of the thermal spectrum without any preliminary 
exposure. This might be accovitvted for by the fact that 



• S.cattered Rays, 279 

organic matter becomes oxidized in the least refracted rays ; 
and unless it be minute in quantity and consequently in close 
contact with the silver salt, it would not have the power, by 
its oxidation,.of reducing even a small portion of the latter 
lo the metallic state, which could act as a nucleus for de- 
velopihe^n't. When the quantity of organic matter is large 
and: not in close contact with a silver salt, the organic 
matter when oxidized probably prevents the access of the 
developer to the sensitive film protected by it, and conse 
quently the parts which are less protected by the unoxidized 
matter are reduced first, and thus determine the position 
of the further reduction. The above might also explain the 
action of dyes on the sensitive film, particularly when they 
are readily oxidizable. 

It also must not be forgotten that the light transmitted 
through a film containing a compound in a fine state of divi- 
sion, may be very different to the light that the compound 
itself will allow to be transmitted. In the one case we may 
have the effect due to particular scattering, in the other we 
probably have the colour due to the substance itself. Thus 
silver bromide, when fused, is a transparent body of an 
orange colour; in an emulsion it may be so formed as to 
transmit either orange and red rays to the exclusion of 
the blue, or by different manipulation may be made to cut 
off much of the former, and transmit the whole of the blue 
and the violet rays. 

The spectra produced upon other metallic compounds 
than those of silver have been studied by Herschel and 
others, and the student is recommended to consult Hunt's 
* Researches on Light * for the particulars. The salts of iron 
are especially worthy of attention, some of these being sensi- 
tive to the least refrangible portion of the spectrum. 



?8o Phcio-Sp€Cir0U9pj. 



CHAPTER XXXV. 
Solar, Sidlar, and MeUOHc Sfetirm, 

In regard to the examination of spectra, phoCogiaphj is play- 
ing a roost important part, and we cannot do better than 
refer to the work first instigated by LodEjer, and miw beitig 
carried oat by himself^ H. C Yugid of Potsdam, Roscoe, 
and others. This is a preparation of a map of die solar 
spectrum, in which, as far as possible, every dark line is 
referred to the absorption due to some metallk: or other 
vapour existing in the solar photo^here or the earth's at- 
mosphere. Though Fraunhofer and others have already pro- 
duced similar maps 'from ocular measurements, yet it was felt 
that, if photography could be enlisted into the service as a 
rq2;istrar, p^eater accuracy would be obtained and the ultra- 
violet {xxrtion could also be ma]^)ed. In order to carry into 
effect this project, Lockyer has fitted up a spectroscope with 
the necessary photographic apparatus as sketched in the 
figure. A is a vertical slit, whose exterior fece is covered by a 
plate, capable of moving horizontally, in which two or more 
apertures are cut en echelon^ the top of the lower one being in 
accurate continuation of the bottom of the upper one, and 
80 on. When the top aperture of the movable plate is in 
front of the slit, only the top part of the slit is uncovered, 
and when the bottom apertiu-e is in fi-ont of the slit the bottom 
part of the slit is uncovered. At b is a rack and pinion, 
which is used to adjust the distance of the slit from the col- 
limating lens, inserted at the other extremity of the tube c. 
At D is a train of prisms (the number in which can be 
altered at pleasure), set to the angle of minimum deviation 
for the mean of the rays which it may be desirable to 
examine, e is a camera, some 6 feet long, furnished with 
a, lens of that focus, and the usual means of focussing. 
At r is inserted the daik sV\d^, ca.^25c^^ <2>1 c^wtain- 



Lockyer's Apparatus. 281 




282 Photo- Spectroscopy. ' 

ing a plate 6x2 inches. The spectroscope and camera are 
rigidly connected one with another, the prisms and collimator^ 
c, being fastened to an iron plate, t, supported on a solid 
j)illar, s. This completes the photo- spectroscopic arrange- 
ment. In order to compare the spectrum of a metal with 
that of the sun, Lockyer adopted the arrangement shown. 
K is an electric lamp, between the points of which the metal 
to be examined is volatilised by means of the electric cur- 
rent passing between them. The points are so placed that 
the interval between them lies in a continuation of the axis 
of the collimator c. At h is a small lens, the distance 
between a, h, and p being so arranged that a and P are 
conjugate foci of h. In some cases the place of the lamp is 
occupied by a Ruhmkorff coil, and the metal volatilised by 
the heat of the spark, g is another condensing lens on to 
which the solar rays are thrown by means of a heliostat, and 
placed at such a distance from a that its principal focus — 
the focus for parallel rays — is at p. By this arrangement 
the lens h will throw a perfect image either of the sun, or 
of the electric arc, on a. 

Fig. 99 is a different view of the apparatus as used by 
Lockyer, giving an idea of the arrangement of the prisms 
and heliostat. 

When the portion of the solar spectrum required has been 
accurately focussed on the plane to be occupied by the sensi- 
tive plate at f, fig. 98, the top half of the slit is uncovered, the 
metal to be examined brought between the carbon points, p, 
and the current caused to pass between them. The spec- 
trum of the volatilised metal falls on the sensitive plate, 
and impresses itself in from half a minute to half an hour, 
the time varying according to the portion of the spectrum 
worked with. When sufficient exposure has been given, the 
carbon points of the electric lamp are separated, the bottom 
half of the slit uncovered (the top half being at the same 
time of necessity shielded from the light), and an image 
of the sun allowed to faW otv \\.. Ks evw3>i\\xN!^ Temains 



Quantitative Estiination of Metals. 2S3 




284 Photo^ spectroscopy. 

in the same position, excepting the part of the slit left 
open, the corresponding portion of the solar spectrum 
falls on the sensitive plate, but in this case immediately 
above that of the metal After development the coinci- 
dence on the negative of opaque lines which are due to 
the bright lines of the metallic spectrum, with the trans- 
parent lines due to the absorption caused by the same 
vapour in the solar photosphere, can be at once determined, 
the one being in continuation of the other. It is not 
necessary to enter into the details of the results obtained 
from this method of observation ; it is sufficient to say that 
hitherto metals which were supposed to be quite free from 
all impurity have been found to be contaminated with other 
metals. For a detailed account of these researches the 
student is referred to the * Proceedings of the Royal Society * 
and * Nature ' for the last four years. It will be seen that it 
is quite possible to compare the solar spectrum with two 
metals by increasing the number of apertures en khelott in 
the sliding plate to three, if the solar spectrum be taken with 
the middle of the slit. Two examples of the photographs 
obtained by Lockyer are annexed, fig. 100. The first 
shows the coincidence of some of the bright lines (near h) 
of the spectrum of iron, with the absorption lines in the 
solar spectrum, and the second shows a similar comparison 
between calcium, aluminum, and the sun. 

The distances between the Fraunhofer lines are mea- 
sured by a micrometer, and are then mapped to a scale of 
wave-lengths. The wave-lengths of certain lines have been 
definitely determined by Cornu and others, and the 
measured distances of the lines are interpolated between 
these, and the lines due to the different metallic vapours 
referred to this normal spectrum. 

From the photographs so obtained Lockyer has been able 
to obtain other important results, and is able to estimate 
quantitatively the different proportions of metals existing in 



Lockyer's Apparatus, 



2SS 



an alloy, by observing the disappearance of some lines from 
1 metallic vapour spectrum, and the retention of others. 

More recently Lockyer has been working with a diffrac- 
tion grating of over 17,000 lines ruled on a linear inch. 
By employing the spectrum of the third order produced by it, 
he has obtained the wave-lengths in definite measure, which 




he only secured previously by interpolation. It should be 
noted that the earliest published photograph of a diffraction 
spectrum, at all events of any scientific value, was due to 
r^r. H. Draper. He first published it in England in the 
'Phil. Mag.' for Dec. 1873. The lines are not absolutely 
defined, but still sufficiently so to be of value. 

A more recent application of photography to the sijec- 
ttoscope is to the securing recorc's oi s\a.i ^^w-Wii- 



286 Photo-Spectroscopy, 

Id 1874, the ycurgcr Draper of New York ccm- 
menced photographing spectra, of some of the stars of 
the a Lyrae and a Aquibe group,' and he also directed 
his attention to Venus. These spectra were taken with his 
28-inch reflector and his 12-inch refractor. On Dec 14, 
1876, Dr, Huggins exhibited to the Royal Society a photo- 
graph of the spectra of a Lyrae. This physicist's method of 
working is described in the ' Proceedings of the Royal Society.' 
Briefly it may be stated that he used an 18-inch reflecting 
telescope, and that the reflector caused an image of a star 
to ^11 on a slit placed in front of an Iceland spar prism, 
through which it passed, and finally the spectrum was 
focussed on a miniature sensitive plate. The image of a 
star is always a very small disc ; hence, when the image was 
motionless as regards the slit, owing to accurate adjustment 
of the driving clock of the equatorially mounted instrument, 
the spectrum, as photographed, would only present a thin 
line, broken here and there by black dots. Huggins got over 
this difliculty, however, by slightly altering the instrument in 
declination. The image of the star now travelled along the 
slit, and hence caused the line of altered sensitive compounds 
to broaden into a band, which was more easily comparable 
with other spectra impressed upon the same plate. The 
plate with the undeveloped image was left till daylight, when 
a spectrum of light from the sky was impressed on the 
plate immediately beneath the star spectrum. 

Here may be mentioned the researches by the late Dr. 
W. A. Miller, on the absorption spectra of different transpa- 
rent solids, liquids, and gases. This work was undertaken 
at an early date in the history of photography, but even 
now the results are useful when it is desired to ascertain the 
best material to employ for prisms, or object-glasses, with 
which to photograph any particular part of the spectrum. 
The following are some of the results obtained by Miller. 

The line B reads 84 in the scale. The line H is 100. 

* Nature^ Jan. 1877, 



Absorption Spectra. 



287 



In every case the commencement of the photograph was 
at 96*15 on the scale, silver iodide being the sensitive salt 
empiloyed. 



Name of Substance 


Thickness in 
Inches 


Termina- 
tion of 


Relative 
lengths of 


Remarks 




Spectrum 


Spectra 




Ice . . 


About '5 


1705 


740 


1 


Diamond 


•032 


155*5 


590 


i 

1 


>» • • • 


•017 


159-5 


62-0 


1 
1 


Quartz 


•16 


170-5 


740 


1 

i 


Fluorspar 


•17 


170-5 


74-0 




Rocksalt , 


•75 


159*5 


63-0 


1 


Silver nitrate . 


•75 


io6'o 


9*5 


Saturated so- 
lution 


Iceland spar . 


•35 


160 


63-5 




Faraday's optical glass 


•54 


ioi*5 


l-o 


Pale yellow 


Flint glass 


•68 


105-5 


90 


1 


Window sheet glass . 


•07 


112*5 


i6-o 


1 


Hard Bohemian glass 


•18 


114-5 


180 




Plate glass 


•22 


III-5 


15-0 




Crown glass 


•74 


1065 


lo-o 


Greenish 



The spectrum apparatus was fitted with a quartz lens 
and a quartz prism, hence no estimate can be formed of the 
extent to which it is possible to photograph the spectrum 
apart from the absorption due to that material. 

The lower limit of the extent of the spectrum has not as 
yet been ascertained. In a paper read before the British 
Association, at Plymouth, Lord Rayleigh showed what 
should be the theoretical lower limit of the prismatic spec- 
trum, but this is not necessarily the limit with a diffraction 
spectrum. In Sir John Herschel's experiments with the 
thermal spectrum, the apparatus described in the last 
chapter seems to have been employed ; and, according to 
the recorded results, the limit does not agree with those 
obtained from similar experiments carried out by Lord Ray- 
leigh, and which apparently agree with the theory. The 
principle adopted by Herschel in these experiments was the 
drying of paper moistened with alcohol when exposed to the 
heat rays. A paper was coated at the back with lampblack, 



288 Celestial Photography, 

brushed over in front with alcohol, and immediately ex- 
posed to the. spectrum. The paper, which when moist was 
translucent, became opaque when the heat had caused the 
alcohol to evaporate, and thus the thermal region was indi* 
cated When ferro-cyanide of potassium and ferric oxalate 
are brushed over paper together they do not immediately 
combine and form Prussian blue, but exposed to heat they 
pass through an intermediate stage of combination, forming 
a brown compound. In Lord Rayleigh's experiments paper 
treated by such methods gave the same limit to the pris- 
matic spectrum, which differed from that obtained by Her- 
schel. Further investigation is still required. 



CHAPTER XXXVI. 

CELESTIAL PHOTOGRAPHY. 

Physicists have turned photography to account in their 
study of the heavenly bodies, most of which, in one way or 
another, have been made to impress their image on sensitive 
plates. The student who may ta' e a landscape with the sun 
shining direct into the lens will soon satisfy himself that 
the exposure necessary to obtain a good photograph of our 
luminary, when unclouded, is very small, so short, indeed, 
that solarisation is frequently induced, though the landscape 
itself may be capable of proper development. With the 
ordinary camera and lens an image of the sun is practically 
useless, since a lens of short focus is only capable of 
giving a very small image, and one on which none of the , 
markings which characterises his surface can be seen, even 
with the aid of a magnifier ; and since the prime object 
of solar photography is to enable the surface of the sun to 
be studied, it is evident that other means must be adopted 



The Siderostat, ' 289 

in order that it may be delineated on a sufficiently large scale. 
An ordinary lens or object-glass gives an image of the sun 
of a diameter of about ^V ^^ ^^ ^^^^ ^^ ^ ^^^'^ o^ i^s focal 
length. It is, therefore, evident that in order to secure a 
photograph of it of 4 inches (about 10 centimetres) diameter, 
the lens employed must have about 40 feet focal length. 
Now, 4 inches has been proved by experience to be about 
the least diameter for a solar image in which sunspots can 
be effectually studied. Hence, for a direct photograph 
taken at the principal focus of the lens, the focal length 
should not be less than 40 feet. Before the introduction 
of Foucault*s siderostat a telescope would have had to be 
mounted equatorially, and a clock motion would probably 
have been necessary, since the motion of the earth, even with 
the short exposure necessary, would have marred the defi- 
nition to a certain extent. Since siderostats have been 
classed amongst available instruments, the difficulty at- 
tendant on the mounting of such an enormous length of 
telescope has disappeared, and a lens of great length 
can be employed, mounted on a less heavy tube, placed in 
any convenient position, and supported in its length, if 
necessary, along the ground. A siderostat belonging to the 
Royal Society, made by Cooke on Foucault*s model, is given 
in the accompanying fig. loi. Its principle is the same as 
that of the heliostat, already described at p. 271, and shown 
at ii%, 95. A is a mirror, silvered on the external surface, 
which has been worked to a perfect plane. It is suspended 
on two axes, x x, working a U -piece, s s, pivoted at the 
base, and therefore capable of moving the mirror so as to 
face any given direction, p is the polar axis, set so as to 
point to the pole of the heavens ; the inclination being 
regulated by a movement along an arc, affixed to the prin- 
cipal supporting pillar of the instrument. Attached to the 
polar axis is the declination circle, e, to which the ordinary 
movement is given by the clockwork, g, which communi- 
cates its motion by the connecting rod, f. To the lower 

u 



290 



Celestial Photography. 



extremity of the polar axis is attached, a movable ann, 
which can be clamped, so as to form any angle «-ith it At 
the bottom of this arm is a socket joint, pivoted at b, in 
which c, a rigid and perfectly true rod, is capable of sliding. 
When using the siderostat, it should be set nith the polar 
axis in the meridian. The beam of light can then be 
caused to be projected in any given horizontal direction by 




the motion of s s, whilst its vertical direction is adjusted by 
the movement of the ann b. k k are cords which can tuin 
F, and consequently E, and hence the motion of A in the 
horizontal plane can be adjusted without interfering with the 
movement of the clock, h h are cords working on the 



Position of Exposing Apparatus, 2<j i 

movable arm, to which b is attached ; a vertical adjustment 
can therefore be given to the reflected beam. 

The following method can be employed for obtaining a 
solar image with the very long focussed lenses by the aid 
of the siderostat. The lens with its tube is placed in a posi- 
tion such that the direction of their axes cuts approximately 
the centre of the mirror. Since the mirror is supposed 
to be a perfect plane, it is manifest that an image of the 
sun should be formed at the principal focus of the lens, as 
perfect as if the axis of the lens itself were pointed to the 
luminary. It is needless to describe the camera, which, 
in fact, instead of being attached to the tube, may remain 
detached so long as the plane of the sensitive plate is kept 
accurately perpendicular to the axis of the lens, and so long 
as all light, except that admitted through the lens, be excluded. 
This is, perhaps, better than rigidly attaching it to the body 
of the tube, as it gives facilities for exposing the plate very 
close to the principal focus. It has been considered most 
important that such a position for the exposure should be 
obtained. The reason of this will be evident when it is re- 
membered that the only means of giving the exposure is by* 
causing an opaque screen, in which a slit is cut, to pass 
across the beam of light. Were such a screen passed in . 
fh)nt of the lens, or at any part of the telescope other thaa 
the principal focus, the impression of the image might con-^ 
tinue during the entire exposure. When the exposure^ 
however, takes place at the principal focus of the lens, dur- 
ing each portion of the exposure a definite portion of the 
image alone is impressed. To secure good detail in the 
representation of the sun^s surface such a method of im- 
pressing the image is necessary, since, however excellent 
may be the workmanship of an instrument, there is always 
some small tremour in the movements, and conse- 
quently a risk of an imperfection in the image. There is 
much to be said in favour of this method of solar photo- 
graphy, and something to be said against it, and it seems a 

u 2 



292 Celestial Photography. 

point which has yet to be decided as to whether this or the 
plan next to be described is likely to give the most accurate 
results. 

The instrument which was first adopted for solar photo- 
graphy was one designed by De la Rue, and known as the 
Photo-heliograph. The accompanying figure shows the latest 
pattern, and is taken from one of those which was lately em- 
ployed by the expeditions for observing the transit of Venus. 
At ^ is a lens of about 4 feet focus, having a cell on which 
is cut a very fine screw, so fine and accurate, indeed, that 
the. lens can be caused to advance or recede from b by the 
TIT w*^ part of an inch by turning the cell through a portion 
of a turn. About /is the principal focus of the lens, at which 
point are placed cross wires or a ruled grating ; the focus of 
which can be accurately obtained by a slow-motion screw 
turned by the handle, h. This moves an inner tube in which 
the diaphragm holding the wires is inserted. Immediately 
in front of/, and running in a pair of grooves, is the exposing 
screen, in which there is an adjustable opening or slit. At 
^ is a spiral spring, which tends to keep the slit below the 
point where the image is formed, whilst at ^ is a little 
pulley, over which runs a thread attached to the top of the 
exposing diaphragm, and terminating by a loop. The pre- 
liminaries to exposure are to draw the diaphragm up to e 
by the thread, and then to place the loop over a pin (not 
shown in the figure) ; this brings the slit above the place 
where the image is formed. The exposure is given by cut- 
ting the thread ; the spring, g^ pulls the diaphragm towards 
it, and the slit traverses the image. The duration of expo- 
sure can be regulated between ^\jth and yJiyth part of a 
second, a margin sufficiently wide to suit the sun as seen 
through almost any condition of the atmosphere. 

Below / is placed a magnifying lens, which takes the 
form known as * the rapid rectilinear.' Its function is the 
same as that of an eye-piece in a telescope, and by altering 
.the distance between its optical centre. and the focus of the 
object-glass any size of image can be produced. In the 



The Photo- Heliogr^Ju 




294 Celestial Photography, 

instrument under consideration the diameter of the sun's 
image has been fixed approximately at 4 inches, and con- 
sequently the adjustments of the secondary lens are made 
so that there cannot be much variation from those dimen- 
sions. B is the holder in which the slide carrying the sensi- 
tive plate is placed. Some of the means of adjustment 
have already been pointed out ; a further one is that of the 
secondary magnifier, which by a slow-motion screw can be 
caused to recede or advance along the axis of the telescope. 
It will be seen that every means of securing a sharp image 
of the sun together with that of the cross-wires or ruled 
gratings is to be found in the instrument. The telescope is 
mounted equatorially, d being the polar axis, c and e the 
declination and right ascension circles, and f the clock 
movement. By means of g a motion can be given to the 
tube in right ascension, and by a corresponding handle 
attached to the tube (and not shown in the figure) a motion 
in declination. The greatest danger to the accuracy of this 
instrument is distortion, through the multiplication of lenses, 
and the risk that exists of these not being properly 
centred. When attention has been paid to this, as it has been 
by the eminent optician who has constructed them, they 
leave little to be desired. 

It is not probable that the student will be in possession 
of either of the two instruments which have been described. 
A very fair substitute for them can, however, be made by 
anyone who possesses a telescope of fair defining power, a 
doublet lens of say 20 to 25 centimetres equivalent focus, 
and a perfectly plane mirror, silvered on the exterior surface 
and mounted on a stand. The eye-piece should be removed 
from the telescope, and the tube of the latter mounted on a 
cradle of such a height that the axis of a photographic lens, 
when in the camera, coincides with the axis of the object- 
glass. For this purpose it is advisable that the distance of 
tht lens from the ground-glass screen of the camera should 
be capable of adjustmeivt b^ ^ ^xv^ i^cV ^wd \iinion motion 



Exposing Apparatus, 295 

governing the lens itself. Supposing it be required to secure 
a photograph of the sun, having a diameter of about 20 
centimetres, the lens must be roughly placed at the distance 
which will give that size, and afterwards any small alteration 
in focus made by moving the lens in the draw-tube, or 
by using the adjustment at the back of the camera. As 
before stated, the former method is the better, as then the 
camera may be fixed rigidly in position, and nothing altered 
excepting the lens itself. In the interval which there will 
be between the lens and the eye end of the telescope will be 
the principal focus of the telescope, and at that point may 
be placed an apparatus for giving the necessary exposure. 
This may be entirely separate from the apparatus, and may 
be conveniently made by causing a diaphragm with slit to 
drop in front of the instrument by means of a falHng weight 
or a spring. One can be constructed of cardboard and 
wood, which will answer every purpose. First, a board, 
A, some \* centimetre thick, was placed on a stand, s. 
On each side of a were j5xed a couple of small l^ shaped 
battens, which formed a groove sufficiently large for a stout 
Bristol board to slide freely in. At f was an opening (shown 
by the dotted circle) which was at the principal focus of the 
telescope. The cardboard, c c, was cut with an opening, e, 
which could be widened or narrowed, as might be re- 
quired, by a small card, d, likewise running in grooves 
placed over it. Attached by a loop to the bottom of the 
card and to the stand is an India-rubber spring e. The 
card c is held in position by a small pin, n, covering 
this A, and worked by a trigger, m. To expose, the card is 
first brought into the position shown, and the trigger pressed. 
The slit passes over the opening d, and the movement is 
finished when the card comes in contact with g g, two 
projecting pieces, as shown. If the card be blackened, and 
a black cloth be lightly thrown over this apparatus, the end 
of the telescope, and the camera, there is no danger of any 
extraneous light affecting the p\aX^, ^\q.n\^<^^ ^<^\iS^Kt\3fc 



296 



Ceiestial P/totography. 



capped immediately before and iromediately after exposure. 
The plane mirror, of course, must be adjusted so that the 
beam of light from its centre falls along the axis of the 
object glass. 




Some of the best solar photographs that the writer has 
seen were taken by an uncorrected lens of long focus; and 
it certainly is as good to have one totally uncorrected as 
one only approximately achromatic, if a collodion be used in 
which there is very little bromide. The reason of this will 
be apparent when it is considered where the maximum 
chemical activity of light of the spectrum is situated in 
regard to silver iodide. When sufficiently short exposure is 
given to the pbte, only that particular part of the spectrum 
is effective in forming the image. Now the foci of the dif- 
ferent rays of the spectrum vary enormously when an uncor- 
rected lens of long focus is employed; if therefore a plate be 
exposed at that point vjVveie x\ie ta-'j^ til TOa.-*.\\flam effect are 



Janssen's Revolving Plate, 297 

brought to a focus, it is manifest that a perfectly sharp image 
can be secured. It is perfectly feasible by this expedient 
to secure photographs of a fair size without having to resort 
to a magnifying arrangement, and the expense of fitting up 
such an apparatus would be, comparatively speaking, small. 

The latest development of solar photography is due to 
Janssen. He employs a revolving plate, and causes exposure 
to be made automatically on different portions of it at certain 
fixed intervals. By this means he is enabled to secure a 
series of pictures with the greatest comfort, and can examine 
any changes that may occur in every few hours on the sun's 
surface. There seems to be a promise of good results being 
derived from this procedure, and it remains to be seen if 
new inferences may not be drawn by the comparison of ob- 
servations simultaneously made at different parts of the earth. 

The next question that arises is as to the process of 
photography which is to be adopted. This depends en- 
tirely on the purpose for which the sun-picture is to be 
made. To study the sun's surface^ unquestionably the 
process should be employed which will give the greatest 
roundness ' to the picture, and this is to be found in the wet 
process, using the pyrogallic acid developer, with a nearly 
neutral bath, as given at p. 89. The following formulae 
for dry plates are used by Janssen, and the pictures, 30 
centimetres in diameter, leave nothing to be desired. The 
collodion is made as follows : — 

Pyroxyline 8 grammes 

Alcohol 4CX) cc. 

Ether 600 cc. 

Ammonium iodide .4 grammes 

Cadmium iodide 3 grammes 

Potassium iodide 2 grammes 

Ammonium bromide I gramme 

Cadmium bromide I gramme 

' There is a variation in the intensity of light at the limb and at the 
centre of the sun's disc: that of the latter is between four and six times 
that of the former. 



298 Celestial Photography, 

The pyroxyline is of a peculiar character, being exces- 
sively soluble, and is probably prepared at a high temperature. 
For ordinary samples the quantity might well be nearly 
doubled. The plate is sensitised in any ordinary bath, and a 
solution of tannin of about i per cent is floated over the plate 
after it has been carefully freed from all free silver nitrate. The 
development is conducted by plain pyrogallic acid till a faint 
image is brought out, after which intensity is given by the 
application of pyrogallic acid, acetic acid, and silver nitrate. 
Janssen states that alkaline development caused a loss of 
roundness and relief in the image, and the same may certainly 
be said of development of a wet plate by iron solution unless 
that solution be very weak. The reason of this is that, in order 
to cause the smallest differences in the chemical activity of 
light to be apparent, the reduction of silver should take place 
veryslo7vly (read p. 65). Plain pyrogallic acid is a much less 
energetic reducer than alkaline pyrogallate or .than ferrous 
sulphate ; hence the roundness of an image is lost by em- 
ploying the two latter. 

When it is necessary that the limb of the sun should 
be exceedingly sharp and defined, as it was in photograph- 
ing the transit of Venus across the sun's disc, so that 
measurements of the distance of the planet's limb from 
the solar limb might be taken, the wet method employing 
the iron developer is effective, or a dry plate process 
with strong alkaline development will be efficacious. For 
the English expedition sent out during the last transit the 
process given at p. 109 was employed, and from the measure- 
ments from different plates proving fairly accordant it 
is to be supposed that it is suitable for the purpose. 

It seems that in the earliest days of the discovery of 
photography by Daguerre impressions of the solar image 
were made, and it would require a somewhat long list to 
record the names of those who have successfully adapted the 
art to astronomical purposes. For the registration of the 
phenomena connected with the total eclipses of the sun the 



Solar Eclipses, 299 

same difficulties as to names of the workers would arise. The 
first recorded endeavour to employ photography for this work 
dates back to 185 1, when Berkowsky obtained a daguerreotype 
of the solar prominences during the total eclipse. From 
that date nearly every total solar eclipse, the observa- 
tion of which was possible to European observers, has been 
studied by its aid, and has tended to the solution of some 
of the problems which arose concerning the solar physics. 
In i860 the first regularly planned attack on the problem 
by means of photography was mstde by De la Rue and Seech i, 
and in subsequent eclipses it has been continued. In 1875, 
in addition to photographing the corona, attempts were made 
to photograph its spectrum. To what extent success was 
obtained in this is not yet officially known, as the report of 
the observers has not as yet been pubhshed. 

As regards photographing the corona the general opinion 
seems to be that it is better to employ an ordinary photo- 
graphic lens of a focal length of some 80 centimetres with 
the camera mounted equatorially, than to employ the ordi- 
nary telescopic objective. The coronal light during the 
eclipse is faint, and in order to get full effect it is necessary 
that the ratio of the aperture to the focal length should be as 
great as possible. It is for this reason that success with a 
photoheliograph, where the image is enlarged, is more than 
problematical, unless a process which is very much more 
sensitive than any of those at present known can be brought 
into operation. 

There is no great speciaHty in the methods of manipula- 
tion which need be referred to, discipline, regular drill, and 
absolute cleanliness being the chief essentials when the 
atmospheric conditions render success possible. 

Lunar photography has occupied the attention of various 
physicists from time to time, and when Daguerre's process 
was first enunciated, Arago proposed that the lunar sur- 
face should be studied by means of the photographically 
produced images. In 1840, Dr. Draper succeeded in im- 



300 Celestial Pliotography. 

pressing a daguerreotype plate with a lunar image, by the 
aid of a 5'inch telescope. The earliest lunar photo« 
graphs, however, shown in England were due to Pro- 
fessor Bond, of the United States. These he exhibited at 
the Great Exhibition of 185 1. Dancer, the optician, of 
Manchester, was, perhaps, the first EngUshman who se- 
cured lunar images, but they were of small size. After 
these might be mentioned many names, but it is unneces- 
sary to refer to any before that of Crookes, who took the 
next step in the matter. The instrument that Crookes em- 
ployed was an S-inch refractor, belonging to the Liverpool 
Observatory, which had a focal length of about \2\ feet The 
diameter of the moon was therefore about 5 centimetres. 
Crookes afhxed a small camera to the telescope and focussed 
the actinic rays by trial, there being found a great deviation 
between their focus and that of the visual rays. The motion of 
the moon not being capable of being followed in the telescope 
by means of the ordinary equatorial arrangement driven by 
clock-work, the necessary accuracy was obtained by me- 
chanically following it by means of the slow-motion screws 
attached to the declination and right ascension circles. 
The cross wires in the finder were kept on one point of the 
image of the lunar surface, a high magnifying power being 
used in the eye-piece. Crookes found that with different 
telescopes the necessary exposure varied between 4 seconds 
and 6 minutes. 

In 1852, De la Rue began experimenting in lunar 
photography. He employed a reflector oi some 10 feet 
focal length, and about 13 inches diameter. An abstract of 
a paper read before the British Association appeared in the 
* British Journal of Photography.' In it is given a very com- 
plete account of the methods he adopted 

In the first part of the paper De la Rue points out that if 
the image of a bright star is allowed to traverse a photogra- 
phic plate, the result is not, as one would expect, a straight 
line, but one which is broken up and disturbed, and which 



Photograpfis of the Moon, 301 

consists of an immense number of points crowded together 
in some parts, and scattered in others. These disturbances 
being due to our atmosphere, it follows that if the tele- 
scope be made to follow the motion of a heavenly body, an 
exposure other than instantaneous must, to a greater or less 
extent, render every point of it a confused disc, and that, 
therefore, a photographic image will never be so perfect as 
the optical image given by the same telescope until instan- 
taneity be secured. 

' Notwithstanding, however, the disadvantages under 
which a photographer labours, I have obtained pictures of 
celestial objects showing details which occupy a space less 
than two seconds in each dimension, I might, I think, 
say even one second. Now i second = ^-J^ of an inch on 
the collodion plate, a second on the lunar surface, at the 
moon's mean distance being about i mile. The lunar 
picture in the focus of my telescope is about i-j^ inch 
diameter, but this varies of course with the distance of our 
satellite from the earth.' .... 

De la Rue then stated that he considered a magnify- 
ing arrangement attached to the telescope as impracticable 
to secure good pictures, owing to the increase of ex- 
posure that would be necessary, and the consequent defects 
due to atmospheric disturbances. He considered that the 
enlargement ought to take place after the negative is 
taken. 

He then describes the adjustment of the motion of his 
telescope to the lunar motion, which he effected by altering 
the length of a conical pendulum or friction governor, 
which altered the time of its rotation (or double beat). 
He proposed to effect the same alteration by another plan, 
which he subsequently adopted. 

De la Rue at first obtained his lunar pictures in his 
13-inch reflector, by placing the sensitised plate at the side 
of the tube opposite the diagonal reflector, the light being 
thus twice reflected. Subsequently he obtained pictures 



302 Celestial Photography, 

directly at the focus of the mirror, which did not give him 
that increased rapidity of exposure which he had conjec- 
tured would result He states: *I am inclined to infer 
that Steinheil's result, as to the loss by reflection of the 
luminous ray, does not hold good as regards the actinic 
ray.' 

He next compares the advantages of the reflector over 
the refractor, the principal one being that the foci of the 
actinic and visual rays are coincident. 

* The time occupied in taking lunar pictures varies con- 
siderably. It depends on the sensitiveness. of the collodion, 
on the altitude of the moon, and the phase. I have 
recently produced an instantaneous picture of the full moon, 
and usually get strong pictures of the fiill moon in from 2 

to 5 seconds The moon, as a crescent, under like 

circumstances, would require about 20 to 30 seconds in 
order to obtain a picture of all the parts visible at the dark 
Ihnb.' 

* Portraits of the moon equally bright optically, are by no 
means equally bright chemically ; hence the light and shade 
in the photograph do not correspond with the light and 
shade in the picture ; and hence the photograph frequently 
renders visible details which escape optically. Those por- 
tions of the moon near the dark limb are copied photo- 
graphically with great difficulty, and it frequently required 
an exposure 5 or 6 times as long to bring out those portions 
illuminated by a very oblique ray, as others apparently not 
more bright when more favourably illuminated.* 

In the practical instruction for the photography, De k 
Rue lays down that the silver bath must be as nearly neutral 
as possible, that cadmium iodide is the best iodiser to use 
with the collodion, and that the pyrogallic acid developer 
should be employed. For lunar photographs there can be 
no doubt that if they are required to be enlarged, iron de- 
velopment should not be attempted, since the deposit be- 
comes too granular ; but we are inclined to think that the 



Suitable Condition of the Atmosphere. 303 

rapid bromide emulsion plates developed by the alkaline 
method will furnish pictures which are equal to those pro- 
duced by the wet method as described above, and certainly 
give a great decrease in exposure. 

. Mr. Rutherfurd at a later date having tried an i i^-inch 
refractor of the ordinary type, and also a 13-inch reflector, 
finally constructed a refracting telescope in which correct 
tion was made only for the chemical rays, and with this in- 
strument he has produced some of the finest pictures of the 
moon which have ever been taken. With the great Mel- 
bourne reflector, however, photographs which are nearly 
perfection have been obtained, and there seems even yet to 
be a balance of opinion in favour of the reflector as against 
the refractor for this kind of work. Undoubtedly, where 
absolute coincidence of foci of all rays can be secured, all 
other conditions being the same, the best photographs 
ought to be obtained. In lunar photography an unfavour- 
able condition of the atmosphere is undoubtedly the greatest 
difficulty to be encountered. In a climate like England the 
air is rarely steady enough for the purpose. In countries 
which are more favourably situated as regards hygrometric 
conditions the difficulty is much reduced In 1874-5, 
whilst the writer was in Egypt, Colonel A. Campbell and 
himself had an opportunity of taking some lunar photo- 
graphs with a refracting telescope of 7 -inch aperture belongs 
ing to Mr. W. Spottiswoode. On the nights that the experir 
ments were made really excellent negatives were obtained, 
which bore enlarging to 1 2- inch diameter. The apparatus 
employed was extemporised, and therefore of a rather rude 
description, but quite sufficiently true to give an idea of the 
excellent pictures that might be taken in such a climate 
with the appliances usually adopted for such work. 

Photography as applied to delineating the planets or 
stars has not as yet yielded much that is satisfactory. Of the 
former Jupiter and Saturn, Venus and Mars, have all been 
photographed, but without increasing the knowledge that 



304 Celestial Photography. 

already exists regarding them. Photographing the stars is 
more a feat of photography than of practical utility in the 
present state of our knowledge, though at some future time 
it may be possible to map the heavens more thoroughly by 
its aid than has at present been done. 

Rutherfurd has been the most successful in this branch, 
applying it to the measurement of the distances of double 
stars, and it may not be uninteresting to point out the method 
which he adopted. With the refractor already referred to, 
the proper chemical focus having been ascertained, the 
stars which it is desired to photograph are brought into the 
field and an exposure given for the time that may be con- 
sidered necessary ; stars of the 9th magnitude requiring an 
exposure of 8 minutes. The telescope, of course, must be 
made to follow very accurately the apparent motion of the 
star, and perhaps it is the uncertainty of this that is likely to 
cause the greatest difficulty in the whole of the operations. 
After an exposure is given, the clock is put out of gear for 
some seconds ; the axis of the telescope now occupies a 
slightly different position in regard to the stars in the field 
of view, and a fresh exposure of the plate is given. On de- 
velopment a double series of stars appears, an artifice 
adopted to prevent any mistake being made betiu'een the image 
of a star and an accidental blemish on the film. Finally, 
to determine the true position of the stars as regards the 
north point, a bright star is brought on to the left edge of the 
plate and the telescope allowed to remain stationary. The 
track made by the star determines the true east and west 
points for the picture. 



PJwtography with the Microscope, 305 



CHAPTER XXXVII. 

PHOTOGRAPHY WITH THE MICROSCOPE. 

Photography from a very early period of its existence 
has been utilised for securing accurate drawings in mono- 
chrome of what the eye can see in the microscope. This 
bmnch of the art is excessively fascinating, and can be 
worked in any leisure moments, either by day or night, 
when the enlargement is limited to say 50 diameters; 
but in order to secure images of greater dimensions it is 
always advisable to employ sun-light. The apparatus 
required is not very extensive. An ordinary microscope 
with say :^-inch and i-inch objectives and an A eye- piece is 
all that is necessary as far as the instrument itself is con- 
cerned. If the objects to be photographed are mounted on 
a slide, and not merely placed in a cell for examination, 
any ordinary camera may be attached to the microscope, as 
the tube can then be brought into a horizontal position. It 
has often been recommended to employ a camera as much 
as 6 feet in length, ir* order to secure great increase in the 
size. of the object, but in the writer's experience it is unwise 
to go beyond 18 inches, a length just sufficient to enable 
the operator to grasp the slow-motion focussing-screw, whilst 
bis eye can be directed to the focussing-screen. When the 
longer camera is employed, the operation of focussing has to 
be conducted by an assistant, and, however intelligent the 
latter may be, it will always be found that greater accuracy 
will be obtained by the operator's own hand, for it must 
be recollected that the difference of -p^^ of an inch in 
length of focus may determine whether the definition is 
good or bad. 

The camera and the microscope should not be attached 
rigidly to one another. It is far better that each should be 

X 



5o6 Photography with the Microscope 

free to move independently, though care should be taken, 
when an accurate focus is obtained, that each shall occupy a 
perfectly unalterable position during exposure. Perhaps the 
most simple way of attaining this object is to substitute for 
the ordinary photographic lens used with the camera a short 
brass tube, which screws into the flange. A piece of velvet 
should then be formed into a cylindrical bag, <^>en at both 
ends, and a little longer than the brass tube above referred to. 
If each opening of the bag is provided with an dastic band, 
a perfectly light- tight junction between the tube and the 
body of the microscope may be made. 

Some operators prefer to use the eye-piece as a magni- 
fier ; it seems better, however, simply to employ the objec- 
tive. If the objective only be used, it is wise to unscrew 
the tube of the microscope, in order to secure a larger field, 
which otherwise the diameter of the tube would limit It 
must not be inferred that the use of the eye-piece as a 
magnifier will cause indifferent pictures in every case. In 
the instrument used by the writer the definition given by it 
was certainly bad. 

The student is recommended to oHnmence with a 
comparatively low-power objective. The i-inch will be 
suitable in every way; and whenever he has obtained 
mastery over the manipulations with it, he may venture on 
the \ or ^-inch. A higher power than these can seldom be 
recommended; probably the ^-inch is the highest power which 
can be worked with ease. The tube of the microscope 
should be placed in an accurately horizontal position, as 
should also the camera ; and care should be taken that the 
axis of the tube fixed in the latter should be in the exact 
continuation of the axis of the lens. This can only be effected 
by very careful arrangements. As a rule it will be found 
that when the body of the microscope is in a horizontal 
position the friction on the axis on which it turns is 
sufficient to cause it to remain in the position in which 
it is placed ; if not, obvious precautions must be taken to 



Focussing the Image, 307 

prevent any movement between the time of focussing and 
exposing the plate. Supposing that sunlight is to be em- 
ployed for the purpose of illuminating the object, the next 
operation is to throw the image of the sun by a con- 
denser on the object, in such a manner that the axis of 
the condenser and that of the objective may be in a line 
with one another. This may readily be ascertained by 
noticing the illumination when no object intervenes 
between the rays emerging from the condenser. It is 
advisable, first of all, however, to place the heliostat (the 
one described at p. 271 answers the purpose) in position. 
This can be done with sufficient accuracy by rough obser- 
vation with the eye, and noting that the centre of the mirror 
is about the same height, and in the same horizontal 
line as the tube of the microscope. The condenser is 
then brought into the reflected rays, and an image of the 
sun brought to a focus on the object In some cases the heat 
rays have to be cut off, otherwise injury to it ensues. A 
glass cell with parallel sides containing a solution of alum 
is foimd to subdue the heat sufficiently when placed in the 
path of the beam. The focussing is now proceeded 
with, and is best performed by removing the ordinary 
ground glass, and substituting for it a plate of ordinary 
patent plate, viewing the image by a focussing glass, as 
described in photo-spectroscopy, page 269. The portion 
of the object to be photographed should be brought into 
the centre of the field, and when nearly in position the 
slide should be clipped on to the stage by a couple of wire 
springs, and the adjustment effected in the usual manner. 
The absolute focussing should next be taken in hand. A 
rough approximation is first obtained by the rack and 
pinion motion, and the final focus obtained by the slow 
motion screw, which is attached to every good micros<iope. 
When viewing the image through the focussing glass it will 
be found that in no position is the object quite free from 
colour. In one focus it will appear sharply defined, though 

x 2 



3o8 Pliotography with the Microscope, 

surrounded by a red band, whilst the definition will appear 
equally good when in a different focus, when surrounded by 
a blue halo. These colours are due to a want of achroma- 
tism in the objective, and the former position should be 
chosen to obtain a sharp photc^raph ; for since the blue rays 
are as a rule the most active in causing the photographic 
image to be formed, it is evident, if the latter focus, which 
is most accurate for the red rays, be chosen, the resulting 
picture will be blurred. 

Monochromatic Light, — ^The fact that coloured fringes are 
sure to border the image shows at once that the objective 
is not properly corrected, and there would evidently be an 
advantage were it possible to work with monochromatic 
light. This can be accomplished in the following manner, 
and it is believed the arrangement is somewhat novel : cer- 
FiG. 104, tainly the photographs obtained by 

this plan are far superior to any ob 
tained by the writer in which white 
light is employed. 

A is the heliostat, throwing the 
sunlight on b, a condenser of 4 feet 
focus. Such gives an image of the 
sun on a slit about a quarter of an 
inch in width. The lens d, of a focus 
of about 12 inches, takes the place of 
the collimating lens, throwing parallel 
rays on a prism e. This may be a 
hollow prism filled with carbon disul- 
phide, such as is used for the exhibit 
tion of spectra with the electric . light. 
The rays of light after being refracted are received ' by 
a lens f. This may be of varying length, according to 
the power of the objective employed. Should it be a 
long focussed objective and a correspondingly large object 
which has to be photographed, the focal length of f 
should be about 18 inches. With a ^-inch a 9-inch focal 




Monochromatic Light 309 

length will be found sufficiently powerful. The reason 
of this difference is that the spectrum is thrown on the slide 
containing the object, and the part of it to be photographed 
should be illuminated with rays of the same colour, and 
for this reason also the slit should be widely opened. 
The light, of course, will not be absolutely pure, but it will 
be sufficiently so to prevent any appreciable difference in 
the colour of the rays transmitted through the objective. 
The same object may be obtained by throwing the direct rays 
of the sun on the prism and then collecting them by means 
of condensers of variable focal lengths, the length being de- 
termined as given above. The spectrum thus produced is 
focussed on the plate as before. There is a slight danger in 
this method of getting the spectrum rather too impure. If 
either plan be adopted, any portion of the spectrum may be 
made to illuminate the object, by slightly shifting the lens f 
in a direction at right angles to the axis of the microscope 
but care should be taken that as far as possible the rays are 
in the direction of the axis itself To fit up an apparatus as 
above described takes a little ingenuity, but after a trial or 
two it is easily accomplished. 

The best rays to employ are the indigo rays immediately 
following the blue. It is a mistake to employ the violet, 
as the exposure then becomes unnecessarily long. It may be 
noted that by using a monochromatic light for eye observa- 
tions the definition is wonderfully improved Many details 
which are altogether wanting with white light are brought 
into view by it. It may be found that for studying objects 
of different colours different rays are the most suitable for 
illuminating them. 

The photographic delineation of opaque objects is much 
more difficult to accomplish than that of those which can be 
examined by transmitted light. The difficulties will be 
found to increase rapidly if any endeavour be made to use 
a higher power than a :J-inch. The same arrangement 
as that indicated may be employed, causing, however, the- 



310 



Photography with the Microscope. 



rays to fall on the object This gives a very feeble illu- 
mination, and with great magnifying power the difficulty of 
focussing is excessive. When once a focus is obtained all 
difficulty vanishes, and by the use of dry plates any amount 
of exposure may be given without any deterioration of the 
image. 

The annexed figure shows an arrangement by which a 
microscope may be employed in its ordinary vertical positioiL 
The instrument was in the Loan Collection of Scientific 




Instruments at South Kensington, and is of German make. 
The form of the camera might advantageously be altered 
to a bellows shape. A is the microscope ; b a right-angled 
prism reflecting the rays of light which pass through the 
objective into c, a brass tube fixed to the camera d^ The 
length of the camera is supported by two iron rails G. It 
will be noticed that the same method of illumination can- be 
applied in this position of the microscope as when it is used 
horizontally, for the rays of light can be reflected by the 



Sunlight and A rtificial L ight, 311 

• 

Hitherto we have only supposed the student to be work- 
ing with sunlight, but as may already have been inferred, 
artificial light may be employed. The great desideratum with 
the latter is that it should be steady, and shojild proceed as 
nearly as possible firom a point The light from a magnesium 
lamp has been recommended, but in the writer's experience 
it is most unsatisfactory. The electric light is good, but 
is somewhat difficult to manage, though, owing to the inten^ 
sity of the illumination, the time necessary to keep the points 
in proper adjustment is not very great. The lime-light, 
perhaps, possesses the greatest advantage over all lights, 
since it is so perfectly steady. With an oxyhydrogen 
source of illumination an object can be well enlarged up 
to 50 diameters, though with the spectrum apparatus the 
illumination is very feeble. 

As already stated, the wet or dry processes may be em- 
ployed in photography with the microscope. The former 
is perhaps the most satisfactory, if the exposure only 
reaches to reasonable limits. There is a danger of the 
sharpness of the image being blurred by irradiation, and the 
writer has found that by using the Wamerke tissue this has 
been entirely overcome. Another great obstacle to the 
attainment of good pictures consists in the diffraction images 
of parts of the object. This, of course, is dependent on 
the relative sizes of the object and of the aperture of the 
objective.* It is partly for this reason that the J-inch gives 
superior photographs to the jj^. This defect may, however, 
be almost totally remedied by throwing the light from a 
source of illumination on to a translucent screen, and making 
this surface the source of illumination of the object. The 
exposure is of necessity much prolonged by adopting this 
plan. 

* See Airy's * Undulatory Theory of Optics.* Macmillan and Co, 



3 12 Miscellaneous AppliccUums of Photography, 



CHAPTER XXXVIIT. 

MISCELLANEOUS APPLICATIONS OF PHOTOGRAPHY. 

In a short work, limited as to size, it would be naturally im- 
possible to give even an outline of all the many and varied 
applications to which photography can be and has been 
applied. In this chapter it is proposed to give a few of 
them, more for the sake of informing the student what has 
been done, than for teaching him the practical method of 
working them. The method of securing the automatic 
registration of barometers, thermometers, and magneto- 
meters should command our attention first. It will be 
necessary to divide these into two classes which require 
different treatment A mercurial thermometer may be taken 
as the representative of the first class. 

Supposing we have a darkened chamber, in the side of 
which is a slit of just sufficient dimensions to allow the bore 
of the capillary tube to fill it, and that light can only have access 
to that chamber after passing through that slit when so closed, 
it is manifest that if a strip of sensitive paper be caused to 
pass gradually behind such a thermometer tube the different 
height of the mercury will be registered, owing to the opacity 
of that fluid to light. If the supply of paper be properly 
regulated it is also manifest that the height of the mercury at 
any particular instant will be known. Since daylight is not 
always available, resort must be had to artificial light to 
impress the sensitive paper, and a suitable process of deve- 
lopment employed. 

Such a method exists for registering the movements of 
this class of instruments, the details of apparatus and mani- 
pulation being altered to suit each individual case. There 
are, however, other instruments to which such would be 
totally inapplicable. As an example, we may take the 
magnetometer. The oscillations of the suspended magnet 
as used, for measuring the horizontal or vertical components 



Registration of Small Deflections. 313 

of the earth*s magnetism are very minute, so minute indeed 
that they can scarcely be perceived by the eye. If to one of 
these magnetometers, however, we attach a very small and 
light mirror, the plane of which is at right angles to the 
axis of the magnet, and cause a beam, proceeding from a 
source of light, to pass through a small ap)erture, thence to a 
fixed lens on to the mirror, which reflects the beam of light 
on to a screen so placed that the image of the aperture is 
in the focus of the lens, any small deviation of the mag- 
netometer will cause the beam of light to deflect on the 
screen. The amount of the deflection will be dep)endent 
on the focal length of the lens, and the distance of the 
aperture and screen from the mirror. Suppose the screen 
to be opaque, and that a slit is cut in it in the direction that 
the deviation of the beam would take, and lying in the same 
plane as the deviation, and that a strip of sensitive i>aper 
moves behind that slit in a direction at right angles to 
its length, then at each instant the position of the beam of 
light will be registered on the paper. On developing the 
image we shall have a sinuous line corresponding to the 
deflections of the magnetometer at every time of day and 
night, the reading of the time being dependent on the rate 
at which the paper travels. This method is capable of 
application to any instrument in which the scale is dependent 
on the oscillations of a bar, needle, or surface. To the same 
class belongs the most recent application of Stein, when he 
shows the pulsation of, and also the effect of the human voice 
on, a stretched membrane. 

For meteorological purposes we may also hope that 
photography will be more utilised than it has hitherto been. 
Mr. A. Mallock, at the meeting of the British Association 
at Plymouth, has shown a way in which it may be madei 
subservient to ascertaining the heights of clouds.* 

In military science it is only necessary to call to mind] 
the service that the pigeon post performed during the siege; 
of Paris. A large series of letters were printed on one sheef, 

* Photographic News^ Sept. 1877. 



314 Miscellaneous Applications of Photography, 

and then photographed to a very small scale on collodion pel- 
licle. Such pellicles, measuring about 6x2 centimetres, were 
tied to pigeons, which when liberated carried the despatch to 
Paris, where they had been trained. On arrival the collodion 
pellicle was detached from the pigeon, placed in a lantern, 
and the letters transcribed and sent to the various addresses. 
Of so much use was this pigeon post that the German 
military authorities have established a regular service of 
pigeons in the chief fortresses of the Empire, which would 
be used in case of investment or siege by a hostile army. 

During the investigation of the action of torpedoes the 
use of photography was also largely brought into requi- 
sition by the writer in order to ascertain the work that was 
expended by different charges of gun-cotton. The method 
adopted was roughly this : — A mine having been laid down 
at a known depth and position in water, a scale was 
placed over it, and photographed from the position the 
camera was to occupy. On the explosion of the gun-cotton 
or powder an instantaneous exposure was given to a specially 
sensitive plate, and ihe height, breadth, and general form of 
the resulting column of water was obtained from the photo- 
graph after comparing it with the photographic scale. 

At Shoeburyness, again, a regular staff of photographers 
is kept in order to photograph all the experimental work 
carried on by the artillery against iron shields, &c., and the 
series of such pictures has been able to convey more to the 
minds of committees than the most elaborate drawings 
could do. 

We cannot conclude these applications of photography 
without recalling the fact that it has proved exceedingly useful 
in the repression of crime. The portrait of every convict is 
taken by an authorised photographer in each convict estab- 
lishment, and when necessity arises prints from such nega- 
tives are produced by the hundred and distributed, in order 
that the various police authorities may be enabled to 
(identify a criminal who may have happened previously to 
i?e placed under their surveillance. 



APPENDIX. 



■ ef AhsiduU Alcohol by Wtighi in too parts of Spirit of 
difftrtnt Specific Gravities atdcfV. 
(Fewnts, ' Phil. Trans.' 1847.} 



AI^~S 


Z^ 


Akohol 


l>«ific 


Alcohol Specific 


Akohol 


pcciSc 


paoem, gi 


,avi.y 


ptrdftii. 


ra-rily 


.percfDL gravity 


percent. 


ravUy 


0- 1 


0000 


25 


9652 


5' 


9160 


76 


SsSi 


OS 


999' 


26 


9638 


52 


913s 


77 


8557 




9981 


27 


96^3 


S3 


9"3 


78 


8533 




9965 


38 


9609 


, 54 


9090 


79 


S508 


5 


9947 


29 


9593 


> ss 


9069 


80 


8483 


4 


9930 


30 


9578 


; 56 


9047 


81 


8459 


5 


9914 


3' 


9560 


57 


902s 


82 


8434 


6 


9898 


32 


9544 


5^ 


9001 


83 


l'^ 


7 


98S4 


33 


9Sa8 


59 


8979 


84 


S382 


8 


9869 


34 


95" 


60 


8956 


85 


8357 


9 


9855 


35 


9490 


1 61 


8931 


86 


833 > 




9841 


36 


947° 


1 62 


8908 




830s 




982S 


37 


9452 


1 63 


8886 




S279 




9815 


38 


9434 


. 64 


8863 


89 


8254 


13 


9802 


39 


9416 


, 6s 


8840 


90 


8228 


14 


9789 


40 


9396 


66 


SS:6 


91 


8.99 


IS 


9778 




9376 


67 


8793 


92 


8172 


16 


9766 


4* 


9356 


6S 


8769 


93 


i:;i 


17 


9753 


43 


9335 


69 


874S 


94 


18 


974' 


44 


93'4 


1 70 


8721 


95 


8085 


19 


9728 


45 


9292 


71 


8696 


96 


8061 




9716 


46 


9270 


72 


8672 


97 


803. 




9704 


47 


9249 


73 


8649 


98 


800, 


12 


9691 


48 


9228 


74 


S625 


99 


7969 


"3 


9678 


49 


9206 


75 


8603 




7938 


^4 


9665 


50 


9.84 











3»^> 



Appendix, 



shinning thf ptrtmtnfif ampuni 0/ NUrU Aeid (IfNO^ ctmimntd in 

AgHfPUf Spluthnt p/variout SpeHAc GravUia. 

(Kolb. Ann. (Jh. I1iy». (4) 136,) 






00 'H4* 

007^* 
oo'5a» 

06'fjri 
0$'27* 

Ot'(Ht 

H7'45* 
H4'fKi 

79'rxj 
7 7 '66 
76 '00 
75 '00 

74'0i* 
7V00 



hj»«fft< 


(ifftvily 






Hp^fk 






iUiMfUC- 


HNO 








lUfft 


(Hir (uisrtt. 




Al«^ 


Al i-s" 






Alo* 


» '550 


« 'sy* 


o*orjoo 


64*00 


1*415 


i'552* 


I'SK 


0'rxxj4 


6359 


I'4I3 


«'55^* 


1 -JIO* 


0*fJfJ|O 


62*00 


r404 


»'5S7» 


f'5-io* 


0'(XjI4 


6f*2l* 


1*400* 


»'55»* 


I'Sa.!* 


o'rj(j65 


6ot>o 


f'393 


f'S4H 


1*520 


o'(xyxj 


12:r 


I 391* 


»'544 


1*516 


0'0I20 


f'387 


i'54»» 


I*SI4* 


0*0142 
0*0182 


5S'oo 


l'3«3 


»'5J7 


X'SiH) 


5700 


I '376 


«'5,U» 


I'V^j* 


()'020H 


$(i'U)^ 


1*371* 


f'jao 


I'VM 


o'f>a4a 


55*00 


i'3^ 


i'$a6 


f '40<i 


0*0272 


5400 


«'i59 


i'5^2^ 


I •4'i5^ 


o'oyii 


53»« 


t'358 


l'52l* 


1 •404» 
l*4MH 


0*0315 


53*00 


«'353 


i'5i4 


''•<'354 


52 '33* 


«'349* 


»'5»3* 


I*4)i6» 


O'fj^rK; 


50*99* 


»'34»*' 


1 •5o7» 


l'4H2 


0'ri404 


40 '97 


\1^ 


f503 


i'47» 


<^»''H33 


49*00 


I'W) 


I '474 


00459 

o*o4*;j 

0*0508 


4800 


1*321 


i'4'i5 


1*470 


47'i»* 


1*315* 


1*402 


1 *467 


46*64 


1*312 


f4MH» 


1 '463* 


0*0531 


45'«> 


1*300 


f4K4 


1 '460 


0*0556 
0*0580 


43'53* 


1*291* 


i'4Ki 


1 '45^' 


42*00 


1'j8o 


I '47^' 


1*451 


0'o6io 


41*00 


1*274 


I '469 


I '445 


0*0643 


40*00 


1*267 


1465 


1*442 


o*o666 


39*00 


1*260 


1*462* 


l'43H* 


O'ijdm 


37 '95* 


1*253* 


»'457^ 


I '435 


0*0708 


3600 


1*240 


^'455* 


I '43a* 


0*0732 


35 '2?^ 


I '234 




1 •429» 


0*074f> 


33*86* 


I *226* 


^444 


I 'An 


0*0760 


3200 


1*214 


^pM 


1*419* 


0*0771 


31*00 


1*207 


IM5 


1*414 


0*0784 


30*00 


1*300 


Hmo 


1*410 


0*0796 
o*o8rj6 


29*00 


1*194^ 


Hpe 


1*405 


28*00* 


1*187* 


^^^^^^^^ ^ 


1*400* 


o*o8i8 


27*00 


1*180 



Mt^ ' 



395 
393 

381* 

374 

368 

3^>3 

35» 

353* 

34<'> 

34' 

339 

33$ 

33'* 

323* 

3n 

312 

304 
298* 

295 
284 

274* 
264 

257 
251 
244 

237* 



225 

318 

311* 

198 

193 

185 

179. 

173* 

166 



Ufm 



0'<^33 

0'o85o 
O'0^54 

O'o86i 
0'o864 
0'o868 
0*0870 
0*0874 
0*0875 
0*0875 
00875 
0-0875 
0*0872 
0*0867 
0*0862 
0*0856 
0*0850 
0*0848 
0*0835 
0*0820 
O'o8o8 
0*0796 
0*0786 
0*0755 
0*0762 
0*0740 
0*0729 
0*0718 
0*0692 
00678 
0*0664 
0*0650 
0*0635 
0*0616 



Appendix. 

Table II.- 



HNO 

percent. 


Specific Gnvily 


ConlrK- 


HNO 

per ceni. 


Specific Gravity 




Alo" 


A,„» 


A.O- 


At,s= 


"°" 


13-00 

I7-47* 
15-00 
13-00 


'■•S3 

iu5» 
1099 

I -08s 


Mr 

1077 


0-0593 
0-0510 
0-0483 

00422 
0-0336 
0-0316 


4-00 


I -075 
1050 
1-026 
I -013 


I ■045' 
0-999 


0-0296 
0-OZ06 

o-ooss 



■(■ The Bumbeis nuiked ■ are tbe resuks of diieu obsecvati 
obuiiKd by IntcipolatiorL 

Table III. 

Shewing tht Ptrcaitaff. ef Sulphuric Acitl {H^O^ 11 
titmi ef varieHS Sfteific Gravitia. 
(Bineau; OUa Temp, is".) ■ 



SEiS 


«:;. 


o^vi^; 


I".. 


SSeif? 


Per 


^% 


F« 


i84s6 


100 


1-675 


75 


1-398 


SO 


i-iSz 




1-842 




.■663 




1-3886 








1-8406 


<« 


1-651 


73 


1-379 


48 


I -167 


23 




97 




72 


1-370 


47 


I -'59 




1-8384 


<w 


1-627 


71 


.-361 


46 


i-.S'6 


















20 


'■8356 


94 


1-604 






44 


1-136 


19 




93 


1-592 




I 333 


43 


1-129 




.83. 




I -580 


67 


1-324 


42 










1-568 








1-1136 


t6 




90 


I -557 


6S 


1-306 


40 


1-106 


15 










1-2976 


39 








88 




63 




18 










1-523 






37 


1083 




1-794 




1-512 




1-272 


16 


l-^" 




1-7S6 


«S 




60 


1264 














1-256 








1-767 


Ml 


1-480 




12476 


13 


10536 




1756 




1-469 








,-0464 




1745 




I-4S86 


so 


1-231 


31 


1-039 




1734 






55 


1-223 


30 


1031 






79 




54 


1-215 


29 


1-0256 






78 




53 


1-2066 


28 


I -019 




1-698 




1-4.8 




1-198 










70 


1-40S 


5' 


i-igo 




lt»64 





3i8 



Appendix. 



Names. 
Aluminium 
Antimony 
Arsenic 
Barium 
Bismuth 
Boron . 
Bromine 
Cadmium 
Caesium 
Calcium 
Carbon 
Cerium 
Chlorine 
Chromium 
Cobalt 
Copper 
Didymium 
Erbium 
Fluorine 
Glucinium 
Gold . 
Hydrogen 
Indium 
Iodine . 
Iridium 

• 

Iron 
Lanthanum 

Lead . 

Lithium 

Magnesium 

Manganese 

Mercury 

Molybdenum 

Nickel . 

Niobium 

Nitrogen 

Osmium 

Oxygen 

Palladium 

Phosphorus 

Platinum 

Potassium 

Rhodium 

Rubidium 

Ruthenium 

Selenium 

Silver . 

Silicon . 

Sodium 

Strontium 





List of Elements. 








Symbols. Combining Weights. 


, . * ^Vl • • • • 


. 27-4 




. Sb . 








122 




. As . 








75 




. Ba . 








. 137 




. Bi . 








2IO 




. B . 








II 




. Br . 








8o 




. Cd . 








112 




. Cs . 








133 




. Ca . 








. 40 




. C . 








12 




. Ce . 








. 92 




. CI . 








35 5 




. Cr . 




• 




52*2 




. Co . 








. 587 




. Cu . 








. 635 




. D . 








95 




. E . 








112*6 




. F . 








19" 




. Gl . 








9'3 




. Au . 




* 




197 




. H . 








I 




. In . 








37-8 




. I . 








127 




. Ir . 








198 




. Fe . = 








56 




. La . 








92 




. Pb . 








207 




. Li . 








7 




. Mg. 








24 




. Mn. 








55 




. Hg. 








2CX> 




. Mo . 








96 




. Ni . 






• 


587 




. Nb. 








94 




. N . 








14 




. Os . 








199-2 




. O . 








16 




. Pd . 






r 


106 -6 




. P . 








31 




. Pt . 








197-5 




. K . 








. 391 




. Rh . 








104-4 




. Rb . 








. 85-4 




. Ru . 








. 104-4 




. Se . 








79*5 




. Ag. 








. 108 




. S: . 








28 




. Na . 






« 


23 


• < 


. Sr . 








87-5 



Appendix. 



319 



List of Elements — continued. 



Names. 


Symbols. 






Combining Weights. 


Sulphur 


• . . . • . • 32 


Tantalum 


. Ta . 






. 182 


Tellurium 


. Te . 






. 128 


Thallium 


. Tl . 






. 204 


Thorium 


. Th . 






• "57 


Tin . 


. Sn . 






. 118 


Titanium 


. Ti . 






50 


Tungsten 


. W . 






. 184 


Uranium 


. U . 






. 120 


Vanadium 


. V . 






• S^'Z 


Yttrium 


. Y . 






. 616 


Zinc 


. Zn . 






65-2 


Zirconium 


. Zr . 




■ 


. 896 



COMPARISON OF THE METRICAL WITH THE 

COMMON MEASURES. 

From Dr. Warren ue la Rue's Tables. 



Measures of Length^ 



Millimetre 

Centimetre 

Decimetre 

Metre 

Decametre 

Hectometre 

Kilometre 

Myriometre 



In English 
Inches. 



0-03937 

0-39371 
3-93708 

39-37079 

393-70790 

3937-07900 

3937079000 

393707-90000 



In English Feet 
3= 12 Inches. 



0*0032809 

0*0328090 

0*3280899 

3*2808992 

32*8089920 

328*0899200 

3280*8992000 

32808*9920000 



I Inch = 2*539954 Centimetres. I Yard = 0*91438348 Metre. 
I Foot = 3-0479449 Decimetres. I Mile = 1*6093149 Kilo- 
metre. 



Measures of Surface. 



Centiare, or sq. metre 
Are, or 100 sq. metres 
Hectare, or 10,000 sq. metres 



In English 
Square Feet 



10*7642993 

1076*4299342 

IO76429934183 



In English 

Sq. Yards 

^ 9 Square 

Feet 



1-1960333 

119*60^3260 

11960*3326020 



I Square Inch 
I Square Foot 
I Square Yard 
I Acre 



6*4513669 Square Centimetres. 
9-2899683 Square Decimetres. 
0*83609715 Square Metre, or Centiare. 
0-40467 1021 Hectare. 



3>o 



Photography with the Microscope. 



rays to fall on the object This gives a very feeble illu- 
mination, and with great magnifying power the difficulty of 
focussing is excessive. When once a focus is obtained all 
difficulty vanishes, and by the use of dry plates any amount 
of exposure may be given without any deterioration of the 
image. 

The annexed figure shows an arrangement by which a 
microscope may be employed in its ordinary vertical position. 
The instrument was in the Loan Collection of Scientific 




Instruments at South Kensington, and is of German make. 
The form of the camera might advantageously be altered 
to a bellows shape, a is the microscope ; b a right-angled 
prism reflecting the rays of light which pass through the 
objective into c, a brass tube fixed to the camera d. The 
length of the camera is supported by two iron rails g. It 
will be noticed that the same method of illumination can- be 
applied in this position of the microscope as when it is used 
horizontally, for the rays of light can be reflected by the 



Sunlight and A rtificial L ight. 311 

• 

Hitherto we have only supposed the student to be work- 
ing with sunlight, but as may already have been inferred, 
artificial light may be employed. The great desideratum with 
the latter is that it should be steady, and shojuld proceed as 
nearly as possible from a point. The light from a magnesium 
lamp has been recommended, but in the writer's experience 
it is most unsatisfactory. The electric light is good, but 
is somewhat difficult to manage, though, owing to the inten^ 
sity of the illumination, the time necessary to keep the points 
in proper adjustment is not very great. The lime-light, 
perhaps, possesses the greatest advantage over all lights, 
since it is so perfectly steady. With an oxyhydrogen 
source of illumination an object can be well enlarged up 
to 50 diameters, though with the spectrum apparatus the 
illumination is very feeble. 

As already stated, the wet or dry processes may be em- 
ployed in photography with the microscope. The former 
is perhaps the most satisfactory, if the exposure only 
reaches to reasonable limits. There is a danger of the 
sharpness of the image being blurred by irradiation, and the 
writer has found that by using the Warnerke tissue this has 
been entirely overcome. Another great obstacle to the 
attainment of good pictures consists in the diffraction images 
of parts of the object. This, of course, is dependent on 
the relative sizes of the object and of the aperture of the 
objective.^ It is partly for this reason that the :J-inch gives 
superior photographs to the ^. This defect may, however, 
be almost totally remedied by throwing the light from a 
source of illumination on to a translucent screen, and making 
this surface the source of illumination of the object. The 
exposure is of necessity much prolonged by adopting this 
plan. 

* See Airy*s * Undulatory Theory of Optics,' Macmillan and Co. 






Index. 



CAK 

Carb.-)ii tissue, sensitising, i68 
Changing box (Hare's) for dry plates, 216 
Charles's claims to Wedgwood process, 2 
(Chloride in an emulsion, 114 
Chlorine and hydrogen, action of light on 

a mixture of, 33 
Chromic acid and alcohol, 112 
Chromium salts, action of fight on, 31 

— - printing with, 160 
Cleaning glass plates, zs~n 

Clouds, heights of, determined by photo- 
graphy, 313 
Coating the plate with collodion, 78 
Collfxlio-chloride process, 153 
emulsion, washed, 15^ 
fuming, with ammonia, 154 
fixing, 154 
(Jollcxlion, 42 

and India-rubljcr as a support, 3^ 

- - considered as a vehicle for sensitive 
salts, 37 

— action of solvents employed in, 49 
formula for plain, 50 

- - discussion as lo iodtsers in, 51 

effect of alkalis in, 52 
- formula: for negative iodised, 53 
testing plain, 54 
limpid and viscous, 55 
reticulation in, 55 
■ for positives, 93 
for dry plates, 93 
for albumen Ixter process, 109 
coatinK the plate with, 78 
formu& for oromised, 119 
Colloidal bodies, addition of, to develo- 
pers, 67 
Colour aod density of the de|p<jsit, 82 
Coufier bromide intensifying solution, 
tlieory of the, 70 
sulphate in developers. 67 
Corona, photographing the, 299 
Crime, jmolography and, ji^j 
Cyanide, potassium, as a nxiiig agent, 74 



DAGUKkRF/S discoveries, j 
Daguerreaii image, devclojimeiit of 
the, 40 

intensifying the, 41 ' 

fixing, 41 
I )aguerrcotype, 38 

plate, manipulations in the preparation 
of the, 39 

— etching by Crove's method, 42 
J>aguerreotyi>es. reproduction of by 

cTeCtrolypy, 41 
I)allnieycr, ms portrait lcn», 202 
I)ark-n>imi, 220 
Dark-tent, 222 
])efects in a negative, 85 

— - weak images, 85 

pin-holes, 85 
olack specks, 85 
• coniet-fike siM>tH, 85 
transparent spots, 85 



KXi' 

Defects in a negative, scum on the film, 85 
- markings like watered silk, 86 
- - transparent markings, 86 
- - want of sharpness in the 
image, 86 

. . blurring, 87 

Defects in gum galnc plates, 108 

■ - silver prints, is2 

De la Rue's I/Unar photographs, 301 
Density and colour of a deposit, 82 
Deposition of silver by the developer, 64 
Detergents, formula for, 57 
Developer, explanation of the term, 20 
Developers absorb oxyeen, 64 

— theory of strong ancTweak, 65 
- - viscous, 66 

for negative pictures, formula; for, 67 
• - copper sulphate in, 67 

■ - for positive pictures, formula: for, <fi 

formula: for dry plate, 103, 104 
alkaline, 10^ 
Development, methods of, explained, 19 
of wet plates, 80 
of dry plates, 96 
of the photographic image, 63 
theory of alkaline, 96 
of gum gallic plates, 108 
of albumen beer process, 1 10 
of the calotype picture, 130 
■ of iron prints, 156 
Diamond's, Dr. Hugh, connection with 

the collodion process, 5 
Diaphragms, uses of, 195-206 
Dichromates and organic matter, 31 
Diffraction gratings, photography with, 
283 

— images with the microscope, 311 
Dippers, 79 

Disc of confusion, 207 
Dispersion of light by prisms, 193 
Distortion caused by single lenses, 197 
Draining rack, 84 

— box, 84 
Drying the plate, 95 
Drying wasned emulsion, 122 

— washed emulsion plates, 124 

Dry plate processes with the bath, 91 
Dry platc.^, giving intensity to the image 
on, 105 



ED(1IN(), or substratum for dry plate», 

Kmulsion processes, 112 

— experiments with bromide, 112 

— forming an, 116 
Emulsions, washed, 119 
Energy, transference of, 10 

— expended not necessarily shown by 
^ number of atoms decomposed, 1 1 

Enlarged photographs, comparison of, with 

originals, 209 
Experiments with light^ 6 

— on organic salts of silver, 27 
with silver chloride, 27 



SuttliglU and Artificial Light 311 

Hitherto we have only supposed the student to be work- 
ing with sunlight, but as may already have been inferred, 
artificial light may be employed. The great desideratum witii 
the latter is that it should be steady, and should proceed as 
nearly as possible from a point The light from a magnesium 
lamp has been recommended, but in the writer's exjwrience 
it is most unsatisfactory. The electric light is good, but 
is somewhat difficult to manage, though, owing to the inten- 
sity of the illumination, the time necessary tokeepthe |X)ii)ts 
in proper adjustment is not very great. The lin)e-ligUt. 
perhaps, possesses the greatest advantage over all lights, 
since it is so perfectly steady. With an oxyhydrogen 
source of illumination an object can be well enbrgod up 
to 50 diameters, though with the spectnnn apparatus the 
illumination is very feeble. 

As already stated, the wet or dry processes may l»e em- 
ployed in photography with the microscope. 'ITie former 
is perhaps the most satisfactory, if the exiXKure only 
reaches to reasonable limits. There is a danger of the 
sharpness of the image being blurred by irradiation, and tht 
writer has found that by using the Wamerke tissue ih». has- 
been entirely overcome. Another great obstacle Xi- th-, 
inment of good pictures consists in the difbaction noac^ 
s of the object. This, of course, is dcpender- r.- 
tative sizes of the object and of the apcrtUT" r' f 
It is partly for this reason that the r^-rncr- r— ■ 
fKor photographs to the ^. This defect mav K>^- 
* almost totally remedied by tiirowiiig the bcT"' "" 
iitce of illumination on toairaBBlHceot sereer.?""- ■ 
g^the source of illiiniioanoti of th" "?*"■ 
£ necessity nuLch fwotanfed *n" ? «■ 



324 



Index. 



LEN 

Lens, aperture of a, 207 

Lenses, on the choice of, 208 

— forms of, 198 

~ - landscape, 203 

Iu;;ht, action of, on various compounds, 

21 

- on silver chloride, 21 
- - iodide, 22 

- bromide, 26 

- - - organic salts of silver, 27 

- ferric salts, 29 

- uranic salts, 30 
vanadic salts, 30 

chromium salts, 31 

- oreanic bodies, 33 

• - - - chlorine and hydrogen, 33 

• antimoniuretted hydrogen and 
stilphur, 34 

- - - - ferric oxalate, 248 
Lithographic press, 179 
Lunar photography, 300 



MAGNETO-ELECTRIC light, in- 
creasing intensity of the, 16 
Magnifying lenses, use of, for focussing, 269 
MaUock, his method of obtaining the 

heights of clouds, 313 
Manipulations in wet plate photography, 

77 

the preparation of photographic 
transfers, 177 
Mercuric chloride intensifier, theory of 

the, 70 
Methods of development explained, 19 
Methyl, compounds of, 19 

in alcohol, 75 

Microscope, camera for the, 305 

photography with the, 305 

- - process to be employed in, 31 1 
Microscopic examination of a developed 

image, 66 
Military science and photography, 314 
Miscellaneous applications of photography, 

312 • 

Monochromatic light for the microscope, 

308 
Moon, photographs of the, 300 



NEGATIVE, intensifying the, 69-81 
Negatives, over-exposed and under- 
exposed, 81 

defects in, 85 
- reproduced, compared with originals, 
260 
Niepce (Nicephore de), his process, 2 
Nitric acid, action of anhydrous, or or- 
ganic matter, 43 



ON the picture, 227 
Oscillation upon another oscillation . 
effect of one, 14 



PYR 

Oscillation, effect of well-^med applic 

of force on an, 17 
Oxyen absorbers, as developers, 64 

PAPER as a support and substratum 
35 

— for the calotype process, 128 

— albuminising, 144 

— plain salted, 144-145 

— resinised, 1^5 

— for photo-lithographic transfers, 176 
Photo-collotype processes, 186 

process by Albert, 187 

Photo-engraving and relief processes, 181 
Photogenic drawing, 4 
Photo-lithograph, 293 
Photo-lithographic transfers, 176 
Photo-spectroscopic arrangements, 265 
Photo-spectroscopy, 263 

Herschel's expennients in, 264 

Picture, printing the, 147 

— on tne,' 227 
Pigeon post, 314 

Plate-cleaning solution, formula for, 57 

— glass, cleaning^ the, 57-77 

— coating the, with coUoaion, 78 

— drying the, 95 

Platinum process (Willis's), 157 

Points, the nearest, of a landscape in 

focus, 209^ 

Poitevin, his improvements on Ponton's 
process, 6 

process, 158 ^ 

Ponton (Mungo), his dichromate process, 5 

Positive and negative, example of a. pic- 
ture, 4 

— pictures and their support, 36 

by the wet process, 89 

Potassium cyanide as a fixing agent. 74 
and nitric acid, dangerous effects of, 

57, 
Powder process, 173 

Preservative, definition of, 94 

j - applying the, 04 

Printing, manipulations in silver, 144 

— sensitising baths for silver, 146 

— frames, 147 

— the picture, 147 

Prints, colour of developed, 143 

— developed, 143 

— silver, fixing, 149 

— washing silver J 150 
Prismatic dispersion of light, 192 
Prisms, method of placing, at the angle of 

minimum deviation, 267 
Pure water for washing dry plates, the 

necessity of, 94 
Pyroxline, manufacture of, 45 

— effects of high temperatures in the 
manufacture of, 45 

— effect of diluted acids in the manu&c- 
ture of, 45 

— formula for preparation of, 45 

— Wamcrke, nis fomula for {M-eparation 
of, 48 



Index. 



32s 



REA 

READE, his discovery of the use of 
gallic acid, 5 
Red end of the spectrum, photographs. of 

the, 27 . 
Reflectors, coincidence of visual and chemi- 
cal rays, 303 
Refraction, laws of, 191 
Registration of tints, 255 
apparatus for, 255 

— of the height of the barometer and 
thermometer by photography, 312 

— of the deflection of the magnetic needle 
by photography, 313 

Rehef blocks by photography, 184 

Reproduced negatives, errors existing in, 
256 

Resinised paper, 145 

Reticulation m collodion, 55 

Ritter, his researches, i 

Room, the dark, 226 

Roscoe, his actinometer, 25 

Rouch, his dark tint, 224 

Rules to be observed in choosing a land- 
scape subject, 241 

Rutherford, his lunar photographs, 303 

stellar photographs, 304 



SCHEELE, his researches, i 
Sensitised paper, preservation of, 146 
Sensitising bath, 58, 61 

keeping in order the, 62 

- '— over-iodised, 62 

for dry plates, 93 

formulse for, 61 

— the paper for the calotype process, 129 
Sensitive compounds, theory of, 1 1 
Sensitiveness of salts of silver prepared 

from double decomposition of various 

metallic salts, 52 

comparative, of different salts, 272 
Shoeburyness, photography at, 314 
Siderostat, 289 
Silver acetate in the sensitising bath, 62 

— chloride, experiments with, 27 

— iodide, action of light on, 22 

— bromide, action of light on, 26 

— chloride, action of light on, 21 

— organic salts of, action of light on, 27 

— iodide dissolved by silver nitrate, 60 
sensitiveness of, when exposed to 

the spectrum, 272 

— bromo-iodide, do.. 272 

— bromide, do., 274 

— chloride, do., 276 

— bromide, reversing action of red rays 
on, 275 

— iodide, do., 274 

— nitrate, purity of, 60 

silver iodide dissolved by, 60 

— or^^anic salts of, experiments with, 27 

— pnntin§. 133. 

manipulations in, 144 

sensitising baths for, 146 

Slide, adjustment of the dark, 219 



VEH 

Slide, Wamerke, his roller. 217 

— dry plate (double back), 216 
Sodium acetate in the toning bath, 141 

— hyposulphite and iodine, 142 

— tetrathionate, its formation, 142 

— hyposulphite testing for, 151 

— hyposulphite aS a fixing agent, 74 
Solar photography, 288 

Solvents, action of the, employed for collo- 
dion, 49 

Spectra, photographic, in natural colours, 
276-277 

— absorption, photographs of, 287 
Spectrum, lower limit of the prismatic, 287 

— apparatus, Lockyer's, 280 

— simple means of forming a, 8 
Stains due to intensifying, 83 
Star-spectra, photographs of, 284 
Starch as size for paper, 136 
Stellar photography, 304 
Substitute for groimd glass, 219 
Substratum or edging for dry plates, 92 
Sulphur and antimoniuretted hydrogen, 

action of light on, 3^ 
Sulphuric acid, action of, on organic 

matter, 42 
Support and substratum, 35 
Swan, his double transfer carbon process, 

i6i 
Swing back, uses of the, 196 
the natural position of, 245 



TABLE of elements and their combining 
weights 318 

strength of solutions of nitric 

acid, 318 

sulphuric acid, 317 

alcohol, 317 

Talbot, his discoveries, 1 1 
Tent, dark, Howard's, 222 

Rouch's, 224 

Testing plain collodion, 54 

— for sodium hj'posulphite, 151 
Thermometer, registration of the height of 

the, 312 
Toning prints, theory of, 137 

— solutions, 149 

Torpedo explosions, use of photography 

in registering the dimensions, 314 
Transfer, double, of carbon prints, 162, 163 
Transfers, photo-Iithographic, 176 

URANIC salts, action of light on, 30 
- — printing with, 155-159 

VANADIC salts, action of light on, 30 
Varnish, formula for, 76 
- intensity given by, 76 
Varnishing the film, 75-84 
Vehicle, collodion as a, 38 _ 
Vehicles for holding sensitive salts in situ, 
36 

— defects of gelatine and albumen as, 92 



Registration of Small Deflections. 3 1 3 

of the earth's magnetism are very minute, so minute indeed 

that they can scarcely be perceived by the eye. If to one of 

these magnetometers, however, we attach a very small and' 

Ugbt minor, the plane of which is at right angles to the 

.«fM of the magnet, and cause a beam, proceeding from a 

Pjfijurce of light, lo fiass throngh a small aperture, thence to a 

Mted lens on to the mirror, which reflects the beam of light 

B to a screen so placed that the image of the aperture is 

fa the focus of the lens, any small deviation of the mag- 

tetometer will cause the beam of light to deflect on the 

The amount of the deflection will be dependent 

( the focal length of the lens, and the distance of the 

ind screen from the mirror. Suppose the screen 

9 be opaque, and that a slit is cut in it in the direction that 

"e deviation of the beam would lake, and lying in the same 

s the deviation, and that a strip of sensitive paper 

ives behind that slit in a direction at right angles to 

^)f»igth, then at each instant the position of the beam of 

on the paper. On developing the 

sinuous line corresponding to the 

icter at every time of day and 

!iim.' being dependent on the rale 

cl-. This method is capable of 

■ ]fiu in which the scale is dependent 

ii ir, iR'L'dii;, or surface. To the same 

■ ■'.Liii a]iplication of Stein, when he 

iid albo the effect of the human voice 



AC win be 




purposes we may also hope that 

lore utilised than it has hitherto been, 

lUie meeting of the British Association 

^owu a way in which it may be madei 

^^ertaining the heights of clouds.' 

only necessary to call to mind. 
■igcon ]iost performed during the siege 
es of letters were printed on one ^|^^ 

Phategrafkit A'ruii, Sept. 1877. 



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