CD
PANCHROMATIC
NEGATIVE PLATE
COLOUR SCREENS
DUPLICATING METHOD
PMSET PRIZE PLATE C?
WATFOHO. CNCM.ANO,
Open only in comnlete^ darkness or Green Safelight.
PHOTOGRAPHY
IN COLOURS
SOME USEFUL BOOKS.
HANDBOOK OF PHOTOMICRO-
GRAPHY. By H. LLOYD HIND, B.Sc.,
F.I.C., and W. BROUGH HANDLES, B.Sc.
With 44 plates, comprising 8 three-colour
reproductions of direct colour-photomicro-
graphs, and 85 half-tone reproductions of
photomicrographs, and 71 text illustrations.
8vo, cloth, Js. 6d. net.
HOW TO USE A CAMERA. By CLIVE
HOLLAND. A Practical and Up-to-date
Manual for the beginner. Fully Illustrated.
Crown 8vo, is. (postage 3^.).
TELEPHOTOGRAPHY. A Simple and Prac
tical Manual on the Theory and Practice of
this important branch of Photographic
Art. By CYRIL F. LAN-DAVIS, F.R.P.S.
With 16 plates and 7 diagrams. Crown 8vo,
2s. net (postage $d.).
"A useful and easily understood handbook." — Daily
Telegraph.
PHOTOGRAPHY IN
COLOURS
BY
GEORGE LINDSAY JOHNSON
M.A., M.D., B.S., F.R.C.S.
FELLOW OF THE ROYAL SOCIETY, ITALY (MODENA)
FELLOW OF THE ROYAL PHOTOGRAPHIC SOCIETY ; LATE EXAMINER
IN PHOTOGRAPHY AND THEORETICAL AND APPLIED OPTICS TO THE SPECTACLE
MAKERS' COMPANY, LONDON ; ETC., ETC.
WITH FOURTEEN FULL-PAGE PLATES (FIVE IN COLOUR)
AND NUMEROUS ILLUSTRATIONS IN THE TEXT
THIRD (REVISED] EDITION
LONDON
GEORGE ROUTLEDGE & SONS, LTD.
NEW YORK: E. P. BUTTON & Co.
1916
Copyright Registered
All Rights of Translation reserved
PREFACE
TO THE THIBD EDITION
THE writer has not found it necessary to make many altera-
tions in this edition. In fact he has only corrected a few
errors in the text, and rewritten page 91 (§ 43) which
answers the question " How the appearance of White is pro-
duced on a Colour plate." He regrets that he did not give
sufficient credit to Professor Forster for his interesting dis-
covery of the reason why white can be photographed as
white on a colour plate, and he has, therefore, rewritten the
paragraph relating to it.
But if few alterations have been found necessary, he has
been obliged to enlarge the book somewhat, owing to the
progress which has been made since the last edition was
written. A full description of the Eaydex process has been
added, as well as Gaumont's new method of Cinematography
in Colours, and Carrara's method of reproducing Auto-
chromes on paper.
A chapter has been added on Art in Colour Photography,
and also a further chapter on Photomicrography in Colour.
The writer desires to express his sincere thanks to Mr.
Walter Severn, A.B.C.Sc., F.C.S., for revising the latter.
His great experience in this department of microphoto-
graphy as Assistant Government Bacteriologist hi Cape Town
is a guarantee of its accuracy. He desires to add that the
apparatus arranged for taking microphotographs by com-
bining a vertical microscope with a horizontal camera is
entirely Mr. Severn's own invention, as are the special
methods of staining the slides.
CASTLE MANSIONS,
JOHANNESBURG, 1916.
PREFACE
TO THE FIRST EDITION
SINCE the publication of my previous work, " Photographic
Optics and Colour Photography," the advances made in
Photography in Colours have been so great that, rather
than revise the former section on Colour, it was thought
desirable to write an entirely new work on the subject,
which should embody all the latest methods. This I have
endeavoured to do, and it is hoped that the Amateur will
find all the information he requires in this work to practise
any of the recognised colour methods with success. There
is a general impression among Amateurs that both single -
plate and three -plate colour photography present great
difficulties to the beginner, but I can assure the reader that
it is not the case. Single-plate colour processes at any rate
are quite as easy to manipulate as ordinary dry-plates, and
occupy even less time than the latter.
The striking analogy which exists between the physio-
logical perception of colours and the phenomena associated
with colour photography has convinced me that both the
ophthalmic surgeon and the physiologist who have taken up
the study of colour blindness and colour vision, will find that
the serious study of this fascinating science will illuminate
many obscure phenomena connected with the physiology of
vision and colour blindness, and will well repay them for
the time spent in acquiring a practical knowledge of at least
one of the leading processes described herein.
I beg to offer my thanks to those who have assisted me
in revising the work, and also to the makers of the
" Autochrome," the " Dioptichrome," the " Omnicolore,"
and the " Thames " colour plates, for revising the sections
devoted to their processes, as well as for a large number of
practical hints scattered throughout the work.
Lastly, my best thanks are due to Dr. Kenneth Mees for
elucidating several points of difficulty, as well as to the
Editors of the British Journal of Photography for the loan
of the Photomicrographs of the various " colour-screens."
G. LINDSAY JOHNSON.
CONTENTS
o
CHAPTER I
THE NATURE OF LIGHT AND COLOUR
PAGE
1. Sources of Light 1
2. Nature of Light 1
2A. Ether 3
3. Nature of Ether Waves 4
4. Brief Outline of the Wave Theory 5
5. Electro-magnetic Theory of Light 7
6. Nature of White Light 9
7. Cause of Sensation of Colour 11
8. How Colour is produced — Colour of Froth, Powders, and
Ice — Cause of Whiteness in Clouds — Cirrhus and Rain
Clouds 12
9. Coefficient of Absorption and Transmission of Light — Lam-
bert's Law — Dichromatism — Colour of Pigments . . 15
10. Surface Colours 18
11. Saturation . 19
12. Transparency and Trauslucency 20
13. Reflection . 21
14. Shadows 22
15. Coloured Shadows 25
16. Shadows cast by Lenses 25
17. Colour of the Sky— Tyndall's Experiments. ... 26
CHAPTER II
§ 18. THE EVOLUTION OF COLOUR PHOTOGRAPHY
Goethe — Zenker — Wiener — Lippmann — Young — Clerk
Maxwell — Helmholtz — Ducos du Hauron — Ives —
Lumiere Freres— Dr. J. H. Smith— Urban and Smith . 29
X CONTENTS
CHAPTER III
THE SENSATION OF COLOUK
PAGE
§ 19. Description of the Eye— The Retina and Fundus Macula—
Fovea — The Eye compared with a Camera, and the Retina
with a Colour Plate 34
§ 20. Reason why the Yellow Spot is Yellow .... 44
§ 21. Remarkable Similarity between the Autochrome Colour
Screen and the Colour Screen in certain Birds and
Reptiles 45
§ 22. Colour Vision and Colour Blindness 47
§ 22A. The Visual Purple .53
§ 23. The Meaning of the Sensation called Black .... 54
CHAPTER IV
§ 24. THE SENSITIVENESS OF A PHOTOGRAPHIC PLATE AS COMPARED
WITH THE EYE TO DIFFERENT PARTS OF THE SPECTRUM
Curves of Sensitivity — -Panchromatic and Orthochromatic
Plates 56
§ 25. Purkinje Phenomenon . . . . . . .59
CHAPTER V
METHODS OF OBTAINING PHOTOGRAPHS IN COLOUR
§ 26. Lippmann's Interference Method— Newton's Rings — Other
Interference Colours 63
§ 27. Theory of Colour Formation —Making a Three-Colour
Transparency 67
CHAPTER VI
SINGLE-PLATE COLOUR PROCESSES
§ 28. Joly's Ruled-Line Screen Process 71
§ 29. Comparison between the various Screen-plates — Table
giving characteristic features of each .... 73
§ 30. Parallax 75
§ 31. Jougla's " Omnicolore" Plate 76
§ 32. Dufay's " Dioptichrorne" Plate 76
| 33. The Thames Screen-plate 77
§ 34. Combined and Separate Screen-plates compared . . 78
§ 35. Paget Plate 79
§ 36. Development of Colour Plates 81
§ 37. Combined Paget Plate 84
§ 38. Lumiere "Autochrome" Plate 84
§ 39. Relative Speeds of Colour Plates 85
CONTENTS XI
CHAPTER VII
SINGLE-PLATE PROCESSES DIAGRAMMATICALLY EXPLAINED
PAGE
40. Von Hiibl's Diagram 86
41. First Black Condition 89
42. Second Black Condition 90
43. How the Appearance of White is produced on a Colour
Plate 90
CHAPTER VIII
PRACTICAL DETAILS OF THE WORKING OF SINGLE COLOUR-SCREEN
PLATES
§ 44. Choice of a Plate . . . . . . .93
§ 45. Apparatus and Manipulation required for making a. Single
Colour-plate Picture 95
45—1. The Camera 96
46—2. The Lens 97
47 — 3. Choosing the Subject — Exposure 99
48—4. Insertion of Plate into the Slide 100
49—5. Colour Filters .101
50—6. Focussing 103
51—7. Use of Hood 104
52—8. The Exposure 105
53—9. Dark-room Lamp or Safelight 106
54 — 10. Formation of the Coloured Positive .... 108
55—11. First Development . 108
56. Rules for Development 109
57—12. Reversal of Image Ill
58—13. Second Development Ill
59—14. Clearing 112
60— 14A. Hardening 112
61—15. Intensification 113
62. Explanation of the Process of Intensification . . .113
63—16. Reduction 117
64— 17. Drying the Positive 118
65—18. Varnishing 119
66—19. Covering the Positive 119
67 — 20. Final Improvement of the Tones of the Image . .121
68 — 21. Binding up the Colour-screen and Transparency . .122
69—22. Defects in Colour-Plate Positives 123
70—23. Copying Colour Plates 129
71—24. Indoor Portraiture 133
72—25. Lantern Projection in -Natural Colours . . .134
73—26. Resensitizing Colour-screen Plates . . . .135
74—27. Preparing Light Filters 137
75. Stereoscopic Effect of Colour Pictures . . . .138
76. Colour-screen Filters for Monochromatic Light . . . 138
Xll CONTENTS
CHAPTER IX
THREE-PLATE AND TWO-PLATE COLOUR PHOTOGRAPHY
PAGE
77. Theory of Three-colour Photography .... 141
78. Ives' Kromskop 142
79. Colour Filters 145
80. Testing of Three-plate Filters 148
81. Making Three-plate Negatives 148
82. Butler's Three-plate Camera 149
83. Two-plate Colour Photography 152
CHAPTER X
THREE-PLATE PHOTOGRAPHIC COLOUR PRINTING
84. Colour Prints 154
85. Practical Details for working the Three-Plate Method
with Butler's Camera 154
86. Three-colour Half-tone Process 158
87. Collotype Colour Process 161
88. Sanger-Shepherd's Imbibition Process .... 163
89. Pinatype Process . .166
90. Colour Carbon and other Processes ..... 170
90A. Raydex Colour Process . . . . . . .171
CHAPTER XI
COLOUR PRINTING FROM SINGLE-PLATE TRANSPARENCIES
91. Uto-color Printing 179
92. Theory of Bleach-out Process 180
93. Permanent and Fugitive Colours .... 182
94. Details of Bleach-out Process 186
95. Nature of Dyes 189
96. Uto-color Paper 192
97. Practical Details of Printing on Rapid Uto Paper . 195
98. Methods of Improving the Print .... 198
99. Fixing the Uto-color Prints 199
100. Uto-color Stripping Paper 200
101. Uto Lantern Slides 202
CHAPTER XII
KINEMATOGRAPHY BY MEANS OF COLOURED LIGHTS
102. Projection of Kiuematograph Pictures in Colour . . 203
103. The Urban-Smith " Kinemacolor" Process . . .203
104. The Kinemacolor Camera . ~. 208
105. The Kinemacolor Projector 209
105.\. Gaumont's Method of Colour Cinematography . . 212
CONTENTS Xlll
CHAPTER XIII
COLOUR PHOTOMICROGRAPHY
PAGE
106. Discussion as to the Advantages of Different Methods . 214
107. Low Power Photomicrography 215
108. Illumination 218
109. Methods for Finding the Position of Object and Image-
Magnification 218
110. Exposure 221
111. Factors which Influence Exposure 222
112. High-power Photomicrography . . . . . . 225
CHAPTER XIV
ART IN COLOUR PHOTOGRAPHY
113. What constitutes Art ? 241
114. Primary, Secondary, and Tertiary Colours . . .241
115. Hue, Tint, and Shade 244
116. How to produce Shadows in Colours 245
117. General Hints as to Colour 247
118. Shadows 250
119. Choice of Subjects 251
120. Portrait Photography 254
121. Backgrounds 256
APPENDIX
Theories of Colour Perception 259
1. Table of Exposures for Separate and Combined Colour Plates 263
2. „ ,, Sunset Exposures 265
3. „ ,, Additive Colour Effects (Colour Synthesis) . . 266
4. „ „ Relative Brightness of Spectrum . . . .266
5. „ ,, Slowest Exposures for Sharpness .... 267
6. „ „ Factor Numbers ... .... 267
7. Instructions for developing Autochrome Plates . . . 270
8. Lumiere's Improved Formula, 1908 271
9. „ Graduated Developer, 1910 272
9A. Professor Namias's Method of Developing Autochromes . 274
10. Other Developers 275
11. Intensification Formulae 276
12. Reduction Formulae 277
13. Instructions for Developing Omnicolore Plates . . . 278
14. „ „ „ Dufay Plates .... 280
15. „ „ „ Paget Plates .... 282
16. Elimination of Green Spots 283
17. Devices for protecting Slide from Heat 284
XIV CONTENTS
PAGE
18. Sensitizing Colour Plates 284
19. Colour Screen Filters for Monochromatic Light . . .285
20. A. B. Hitchin's Developer .286
21. Metric Equivalent Tables 287
22. English and Foreign Sizes of Plates 290
23. Comparative Plate Speeds . 290
24. Wave Lengths of Visible Spectrum 291
25. Thermometric Scales 292
26. List of Names mentioned in Text 293
INDEX .295
LIST OF PLATES
PLATE
I. JULY AT HAMPTON COURT, REPRODUCED FROM A
RAYDEX PHOTOGRAPH .... Frontispiece
FACING PAGE
II. NORMAL FUNDUS (BACKGROUND OF THE EYE) OF A
MAN FORTY YEARS OLD 43
III. COLOURED OIL GLOBULES IN THE RETINA OF A
TORTOISE, DOMESTIC FOWL, AND PIGEON, COMPARED
WITH APPEARANCE OF THE STARCH GRAIN LAYER
IV.
V.
VI.
VII.
VIII.
IX.
X.
XI.
SPECTRUM OF TOTAL COLOUR-BLINDNESS
SECTION OF A LIPPMANN PHOTOGRAPHIC FILM .
CHARTS OF ADDITIVE LIGHTS AND SUBTRACTIVE
51
65
68
76
78
84
86
208
"THAMES" AND " OMNICOLORE " SCREENS X 100
" DlOPTICHROME " AND " KRAYN " SCREENS X 100
STARCH GRAINS OF AUTOCHROME MAGNIFIED
"AUTOCHROME" AND "WARNER POWRIE " SCREENS
X 100
KlNEMACOLOR NEGATIVE AND POSITIVE FlLMS .
XII. THE KINEMACOLOR PROJECTOR, SHOWING COLOUR
FILTER IN POSITION 210
XIII. APPARATUS ARRANGED FOR KINEMACOLOR PROJECTION 212
XIV. ANOTHER VIEW OF THE CENTRAL PART OF THE
APPARATUS FOR PHOTOMICROGRAPHY . . . 228
PHOTOGRAPHY IN COLOURS
CHAPTER I
THE NATURE OF LIGHT AND COLOUR
§ 1. Sources of Light. — When a body is raised to a
high temperature it becomes incandescent, and sets up
vibrations or waves in all directions in the ether, which
waves, impinging on the back of our eyes, give rise to
the sensation of light. With the exception of Fluore-
scence, Phosphorescence, and a few other sources of
energy, which need not concern us here, all light has
its source in bodies which are in a condition of white
heat or incandescence. Thus we see either by reason of
the white hot matter of the sun and stars, or by means
of artificial sources, such as the incandescent particles
of carbon in a candle or gas- flame, or the white-hot fila-
ments in the electric bulb. It is by no means necessary
that the source of light should be seen. Thus most of
the objects around us, such as the moon, trees, houses,
etc., are rendered visible by the light reflected from
them, which light can invariably be traced to one of
the sources just named.
§ 2. Nature of Light. — The way in which objects
are seen, and what constitutes light, are problems that
have occupied the mind of man ever since he became
B
2 PHOTOGRAPHY IN COLOURS
a thinking animal. The primeval savage must have
observed that the gilding of the sky, and the creeping
darkness, accompanied the setting sun. He must have
wondered why his spear appeared bent when he thrust
it into the pool, and why his shadow became shorter
as the sun rose in the heavens.
The Greeks, who wasted far too much of their time
in fruitless speculation without testing their theories
by experiment, imagined that light was something
that passed from the eye to the object seen, in the
form of some invisible tentacle or emanation. This
theory was abandoned for the one that sight was due
to the impact of an infinite number of minute luminous
corpuscles which stream out from every visible object
at an immense velocity, entering the eye and giving
rise to vision by their impact against the retina, much
in the same way that the particles or corpuscles are
seen to dart out of a speck of Radium when ob-
served through a Spinthariscope. The fact that light
could pass through glass and other transparent bodies,
was accounted for by the supposition that these cor-
puscles were so minute that they could pass between
the molecules of any transparent substance. This
view, which is known as the Corpuscular Theory, was
held by Newton ; but Huyghens, his contemporary,
was the first to propound the wave theory of light.
Unfortunately the wave theory was overshadowed by
the great name of Newton, and it was not generally
accepted until the experiments of Thomas Young and
Fresnel established it on a solid basis at the beginning
of the nineteenth century.
In order to understand the wave theory, we are
THE NATURE OF LIGHT AND COLOUR 3
obliged to assume the existence of an omnipresent
medium which is called the Ether.
§ 2A. Ether. — This is supposed to be a perfectly
elastic, frictionless, and imponderable medium which
occupies the entire space throughout the visible universe,
surrounding and penetrating the molecules and atoms
of which all matter is composed. It is further supposed
to be absolutely motionless, and also to possess enor-
mous rigidity, so that no amount of force is able to
rend or displace it. Its chief property is that of pro-
pagating vibrations or waves which travel transversely
to the line of force. These waves radiate in every
direction like the circumference of an ever-expanding
globe when considered as a whole, although any single
wave propagated from the source of origin will continue
to generate waves in a straight line to an infinite dis-
tance without any appreciable loss of energy. It is
further supposed that gravity, or the action of one body
on another at a distance, together with the various
forms of energy, such as light, radiant heat, electricity,
magnetism, Hertzian waves, etc., are all manifestations
of the same wave motion, and that these waves are
due to stresses in the ether, which are propagated
through that medium with immense velocity. These
light waves are supposed to cause movements in the
ether, which are so small and so rapid that the latter
behaves like an elastic solid. The familiar illustration
of waves formed in a still pond may help the reader to
grasp the idea. If the surface of a smooth pond be
strewn with corks, and a stone be thrown into it, a
series of concentric circular waves will be generated
which will spread further and further to the shore, but
4 PHOTOGRAPHY IN COLOURS
the water itself will not travel, as will be shown by the
corks, which, although they bob up and down, never-
theless remain where they were originally placed. In
like manner the ether remains unmoved, save that
minute stresses are caused which propagate impulses
with the velocity of light in all directions, and which
follow one another with inconceivable rapidity, the
frequency amounting to many billions of waves per
second.
§ 3. Nature of Ether Waves. — The energy engen-
dered by the source of light sets up vibrations transversely
to the direction of motion. These waves vary enor-
mously in length, i.e. in the distance between the crests
of successive waves. Thus many kinds of electrical
vibrations, especially Hertzian waves, vary from two
or three mm. to several miles in length, whereas the
waves which we perceive with our eyes vary between
768 fjifj, (million ths of a mm.) for the dark red rays, to
375 fjLfi (or slightly less than half the former length)
for the violet rays. By passing the light through a
fluor spar prism, waves of only 200 pp, can be rendered
visible, and by the additional help of photography and
the use of a vacuum camera, waves of a still shorter
length, viz. 100 /*/*, can be registered on the photo-
graphic plate. Waves which exceed 768 pp in length
are invisible to the unaided eye, and are called the
infra-red or heat-rays of the spectum. These, although
they cannot be seen by the eye, may be regarded by
•means of an instrument called a Bolometer. Those
rays which are shorter than 375 pp belong to the
ultra-violet end of the spectrum, and are likewise
invisible.
THE NATURE OF LIGHT AND COLOUR 5
§ 4. Brief Outline of the Wave Theory.— Every
part of a source of light generates a wave, which travels
in every direction. Let one of these parts L be an in-
candescent point of light. This will form the centre of
a minute sphere whose diameter is equal to a wave
length (A.). Every point in the circumference of this
sphere will at once form a fresh centre of disturbance
and will generate a new sphere, and this again another,
and so on. Now every one of these tiny spheres may
be supposed to lie side by side, each overlapping its
fellow, forming tangents to points on their combined
circumference, which points are situated on imaginary
radii from the original point of light.
Since each fresh sphere generated from the smaller
sphere behind it, has for its centre a point on the same
radius, tangents to points on their
combined circumference will, if
taken collectively, form a wave
front (a, I, c, d, e). Now, as the
centre of each sphere lies on the
wave front of the sphere behind
it, the diameter of each sphere is
clearly equal to a wave-length.
Each successive wave front (X^ X2) pIG< it
(Fig. 1) may thus be considered
as the crest of a wave, and the space between it and
the next wave front (A.^, Xu) as the trough of a wave.
Thus light waves advance to form an ever enlarging
sphere, and the tangent to this sphere represents the
wave front. Since the ceritre of each tiny sphere lies
on a common radius, light waves may be said to
advance in straight lines from the source, just as waves
6 PHOTOGRAPHY IN COLOURS
may be said to travel along a stretched rope when it is
shaken. Such a straight line of force is termed a ray.
A small collection of such rays is termed a pencil, and
a larger one a bundle, or if divergent from a point of
light a cone of rays. For some purposes it will be
found more convenient to refer to light in terms of its
wave front, but for most purposes, and more especially
in geometrical optics, the direction, and not the wave
front, has to be considered, and may conveniently be
represented by a straight line.
We must also distinguish between a train of waves
and a single wave. If we take the familiar example of
the stone dropped into the pond, we notice that the
front wave gets feebler and feebler as it spreads out
towards the shore, while the main body or train of the
waves moves on undiminished. In free ether, as there
is no resistance, both the train and the single waves
travel unimpaired to infinity, but in a refracting
medium such as glass or water, the single waves
become rapidly exhausted. Thus a single wave cannot
pass through a thin microscope cover-slip, much less
through a plate of glass. It has been found that in
Bisulphide of Carbon a wave loses ± of its amplitude
for each wave-length, and that after 14 or 15 wave-
lengths the wave has quite died out. The train also
suffers in a dense medium, not only by becoming
slower in direct proportion to its refractive index, but
by becoming exhausted as well. The train of waves
lose about ~ of their amplitude for each fathom of
water penetrated, so that light cannot pass through a
thickness of 50 or 60 fathoms. Objects below that
depth being in absolute darkness. Of course a much
THE NATURE OF LIGHT AND COLOUR- 7
less thickness of glass would suffice to obstruct all light.
The difference between trains of waves and single
waves may be illustrated by a regiment of soldiers who
are taking a fortress by assault. The front men drop
down one by one, but the main body of men, which
represents the train of waves, rushes on unhampered to
the goal.
§ 5. Electro -magnetic Theory of Light.— Al-
though it is generally accepted that light is due to
transverse vibrations set up in the ether, it is extremely
doubtful if these vibrations are propagated by succes-
sive portions of the ether which set each other in motion,
since this force can be resolved into two components,
one of which will be found to be in the direction of the
wave-front, and will consequently set up longitudinal
vibrations. But there is no evidence whatever to
prove that longitudinal waves can be transmitted in
ether at all.
Clerk Maxwell, in his electro-magnetic theory of
light, got over this theory by supposing that the
waves do not necessitate any change in the position of
the ether particles, but that they cause periodic and
almost instantaneous changes in their electro-magnetic
condition. Faraday showed that the ether was a
perfect dielectric (non-conductor) which, like all
dielectrics, is capable of polarisation. He traced all
electric, magnetic, and optical phenomena to stresses
in the ether. In fact we can classify these ether
stresses according to the size of the waves and their
method of formation, so that we are able to group them
into light waves, heat waves, and electric waves. By
means of the electro- magnetic theory Clerk Maxwell
8 • PHOTOGRAPHY IN COLOURS
established the close relationship between light and
electricity on a mathematical basis, and it remained
for Hertz, the brilliant pupil of Helmholtz, to prove
experimentally that electric waves were not only propa-
gated in straight lines, but were capable of reflection,
refraction, and polarisation. Further, the velocity of
electric waves was measured and found to be identical
with that of light. In a word Hertz demonstrated
by experiment the truth of Clerk Maxwell's assumption,
that light was nothing more nor less than an electro-
magnetic effect in the dielectric and polarisable ether.
Whenever electric waves pass through a dielectric we
get a displacement at right angles to the direction of
the wave, exactly as we find to be the case with light-
waves in the ether.
The phenomena of electrolysis, kathode rays,
Becquerel rays, and especially the Zeeman and the
Kerr phenomena, all go to prove that electricity is
merely another condition of light ; and physicists seem
to be more or less agreed that the electrical phenomena
are due to the propulsion of electrons, which are the
smallest particle of any body we know of, and which
appear to be a kind of transition between matter and
ether. Whenever an electron is set free, it is ac-
companied by an electric current which sets up a
stress in the ether, and generates a series of intensely
rapid alternating positive and negative electric polari-
sations or displacements whereby the electro-magnetic
waves which exhibit the phenomena of light are
produced.
The reader must dismiss the popular idea that
electricity is a fluid which passes through, or is dis-
THE NATURE OF LIGHT AND COLOUR 9
tributed around, a conductor or wire. Electricity (or
light) charges the surrounding ether with energy
which manifests itself in the form of waves. When-
ever an electric conductor is said to be charged, it is not
the conductor or wire which is charged, but the ether around
it, and a flow of electricity merely implies a flow or
distribution of energy through the electric or electro-
magnetic field.
§ 6. The Nature of White Light.— The classical
experiments of Newton apparently showed that white
light consisted of a mixture of all the colours, and that
a prism merely separates them out into the seven
primaries. We find this stated as a fact in the majority
of text-books even to-day. If, however, we examine
the question a little more closely, we find the solution
is by no means so simple. It is evident that white
light consists of an infinite number of trains of waves
each having a different wave-length. Hence the
stresses set up in the ether must be the resultant of all
these numberless trains, and consequently they can
have no regularity of sequence. On the other hand,
we know that a prism or grating will separate white
light into a number of distinct colours, and a pure
colour can only be produced by a regularity in the
sequence of the train waves. Hence arises the question,
does the prism or grating separate out the waves into
orderly trains of sequence, or were they originally
present in the white light ? As Professor Wood rightly
puts it, if the regular wave trains were manufactured
by the prism, was Newton's discovery really a discovery
after all ?
The answer to this question is an extremely
10 PHOTOGRAPHY IN COLOURS
complicated one, and it is very difficult in an elementary
text-book to make the reasoning intelligible to the ordi-
nary non-mathematical reader. Many Physicists insist
that white light from a source consists of regular trains
of waves, because interference fringes can be obtained
from it, either by employing two slits, or by means of
Fresnel's mirrors (i.e. by reflection from two mirrors
placed at an extremely wide angle to one another).
But Schuster has shown that the interference is due to
a physiological peculiarity of the eye. We may consider
that all the retinal rods and cones are toned to the
three primary colours in the same way that musical
instruments or gas flames enclosed in glass tubes
respond to certain vibrations or multiples of them.
This conception is quite a different thing from Helm-
holtz's idea that the cones were divided into three
groups, each one responding to one of the three primary
colours, and not to the other two. According to
Schuster's theory, the retinal elements have a period
of their own, so that they respond to certain wave-
lengths and not to others. Now suppose the white
light to consist of a series of pulses which result from
an immense number of simple harmonic waves differing
from one another by insensible gradations. When
two pulses strike a cone one after the other the effect
will depend on the interval between the two impacts.
If the first impact sets the cone vibrating and the
second shock arrives before the first has died out, in-
terference will obviously take place, the second impact
either intensifying or annulling the first one, according
to whether the waves are in the same phase or a
different one. In this way interference fringes can
THE NATURE OF LIGHT AND COLOUR II
readily be seen. The molecules in minute particles of
silver compounds in the film likewise have their periods
of resonance and can be set vibrating, and Schuster
has even succeeded in photographing the fringes
formed.
We may perhaps explain the whole matter in a very
simple way :— Imagine a post bag filled with letters
from London and forwarded to Liverpool. The letters
are all mixed up anyhow. This represents our waves
of white light. On arriving at the G.P.O. in Liverpool
some are sorted for the United States, some for local
distribution, some for the Docks, and so on. This sort-
ing corresponds to the effect of a prism or grating
which manufactures the colours by sorting out the
wave-lengths into regular periods corresponding to their
respective wave-lengths. But suppose that the United
States letters were held back at the post-office, and
then added to others going in the same direction so
that they could all arrive at New York in a number of
big sacks. This would represent the way in which the
pulses follow on in the eye by the fusion of the periods
of resonance due to the first pulse with that of the
next arrival.
§ 7. The Cause of the Sensation of Colour,— -The
proof that colour is due to the frequency of the vibrations,
and not to their wave-lengths, can readily be shown by
their analogy to sound waves. Our sensations of light
and colour are due to an enormously rapid succession
of waves, or taps, on the ends of the rods and cones of
the retina, which we can perceive provided their
frequencies lie between certain limits. Exactly the
same thing happens to the ear in the case of sound
12 PHOTOGRAPHY IN COLOURS
waves. These latter vibrations are termed the pitch of
the note, and we can hear them, provided they amount
to not less than 39 or more than 36,000 vibrations a
second, when they tap the hair cells of our auditory
nerve, which is situated inside the inner ear. Now,
owing to the slow rate of our auditory vibrations, it is
quite easy to show that the pitch or musical colour is
determined by the frequency of its vibrations, and not
by the note, or musical wave-lengths. From this we
may infer that colour is due to the frequency of the
waves and not to their wave-length.
Again, we know that the velocity of light is retarded
when passing from air through a denser medium such
as water or glass. If, therefore, we allow a colour of
known wave-length, such as sodium light, to pass
through the slit of the spectroscope, and place in its
path a plate of glass, it should cause an alteration in
the colour towards the blue end of the spectrum, owing
to the reduction of the speed. But no such change can
be observed. Hence, in the equation V = A/, where
V = velocity of light, A = wave-length, and / = fre-
quency, if we reduce V in causing the light to pass
through the glass plate, we must either reduce \ or /to
satisfy the equation. But, as we have just seen, colour
does not change when light passes through a denser
medium, therefore colour must be due to frequency and
not to wave-length.
§ 8. How Colour is Produced. — The chief source
of colour in objects is selective absorption of portions
of the constituents of white light reaching it, and the
scattering of the residue, which latter causes the colour
of the objects which we perceive.
THE NATURE OF LIGHT AND COLOUR 13
In the same way, when white light passes through any
substance some of its components are absorbed, while
the rest of the light which emerges is coloured in
consequence. An object is only colourless when white
light passes through it unchanged, the portion ab-
sorbed consisting of trains of waves of every refrangi-
bility.
When light reaches the surface of any non-trans-
parent object, it either penetrates a greater or less
distance beneath the surface, where it is reflected and
scattered by the layers and the surface, and thus
reaches the eye; or it rebounds directly from the
surface from whence the light is scattered by its in-
equalities. In the latter case the light remains
unchanged, but in the former case some of the con-
stituents of the light are absorbed, the remainder being
coloured light which gives the characteristic hue or
colour to the substance. For example, a rose appears
red because the white light falling on it penetrates the
cells of the petals which absorb the green and some of
the blue constituents of the light, while all the red,
some of the blue, and some of the unchanged light
is reflected back and scattered at, or beneath, the
surface, giving the impression to the eye of a pink
or red flower. It is because substances with few
exceptions are not homogeneous (that is of uniform
density throughout) that internal reflections and re-
fractions occur which project the unabsorbed light into
space, and thus give rise to colours. To this action,
according to T. E. Goodall, the luminous shadows
which appear on the human face are due. When a
faint shadow is thrown upon the face, a portion of the
14 PHOTOGRAPHY IN COLOURS
light penetrates the skin and is reflected back by the
reddish tinge of the layers beneath. It is to this that
the warmth of tone and transparency of the shadows
are due. In the portraits by the old masters the painter
seized upon this effect which gave such life to the por-
traits. To those who use face powders this reflective
action of the skin is prevented, hence the corpse-like
shadows which are seen instead of the beautiful pearly
luminosity of a fresh untouched face.
To say, as some text-books do, that bodies and pig-
ment particles reflect certain colours more strongly
than others is incorrect. Light reflected from a homo-
geneous medium is always white.
The difference between a liquid and its froth is a
striking example of internal reflections and scattering
of the light. Thus, a block of ice has a pale cobalt-
blue colour, but when powdered it is quite white. In
the same way the crest of a wave blown about by the
wind, or the froth of beer is white. Almost any crys-
talline substance, whatever its colour, will become
white, or nearly white, if sufficiently finely powdered,
or condensed in the form of snow. This can be readily
shown in the case of sulphate of copper, or bichromate
of potassium crystals. In all the above cases the light
which falls on the liquid or powders suffers reflection
or refraction at the surface of each particle or bubble
on which it falls. These rays undergo innumerable
reflections, and the light is scattered in every direction.
As very little light is absorbed all the colours are
equally reflected, and at the same time the intensity of
the intromitted rays is almost unimpaired ; the result
being that the surface of the body appears dazzling
THE NATURE OF LIGHT AND COLOUR 15
white. Thus a cloud when at a great elevation consists
of minute crystalline ice-particles (cirrhus clouds) which
particles are so small in comparison with their areas,
that gravity has little or no effect on them, and they
form brilliant white clouds. If the pressure of the
atmosphere diminishes the clouds sink, and the par-
ticles melt and run together. This causes the light to
become more absorbed, and they form cumuli. But
since the light is not selectively absorbed they appear
grey. If the clouds condense still more, the light
is largely absorbed, and they form dark nimbi or rain
clouds.
If in the case of bodies or liquids the substance has
a selective power of absorption, the emittent rays are
deprived of some of their constituents, and the body
appears coloured. In some cases — as, for instance,
the powders above mentioned — the light is reflected
very close to the surface, and does not pass through
enough of the substance to allow of any selective
absorption.
§ 9. Coefficient of Absorption and Transmission
of Light.
Lambert's Law. — Lambert found that the amount
of light which passed through a coloured filter or glass
plate diminished in geometrical progression as the
thickness increased in arithmetical progression.
Thus, if we superpose a number of layers of, say,
one millimetre thick, as the thickness of the layers
increases in the ratio of 1, 2, 3, 4, 5, etc., the intensity
of the emergent light will diminish in the ratio of 1, J,
Ji |» TG, etc., so that if we place a series of identical
coloured glasses of the same thickness one after another
1 6 PHOTOGRAPHY IN COLOURS
in front of the lens in a camera, the exposure would be
in the ratio of 1, 2, 4, 8, 16 times, as the thickness
traversed by the light was increased from 1 mm. to
2, 3, 4, and 5 mm. ; or, to put it in another way, the
exposure varies as the logarithm of the thickness. Thus,
let I = intensity of the incident light, and I# = intensity
after transmission, through a unit of thickness, (t) being
the coefficient of transmission. Then for one unit of
thickness (or t) we have an intensity = la, for 2£ an
intensity la . a = la2, for 3t la . a . a = la3 . . . and for
xt an intensity of la*.
As a rule, the colour merely increases in depth with
the thickness of the material, but there are exceptions
to this. For example, the well-known liqueur Creme
de Menthe, which is sold in Florence flask-shaped
bottles, when held up in front of a small source of
light, such as an incandescent globe, appears green in
the neck, but red in the spherical part, and a yellow
transition at one spot between the two. The explana-
tion of this is as follows : — Let 1^, Ir be the initial in-
tensities for the green and red colours, and ag, ar their
coefficients of transmission. Then, from what we have
just stated, the intensities of the colours after trans-
mission will be I^ J and Irarx. Now, if the thickness
of the plate is small, there will not be much difference
between ag and ar, and if Iff is much greater than Ir,
and ar only slightly greater than aff, then 1^ (that is, the
green rays) will greatly predominate, and the liquid
will appear green. If we increase the thickness of
the layer it is obvious that ar will greatly exceed ag
until \a>g = IA.*> when the two intensities coincide,
and the result will be yellow. Thus, suppose \ = 100,
THE NATURE OF LIGHT AND COLOUR 1 7
Ir = 50, ag = 5 and ar = 8, then, since I^/ = I^/, by
changing it into logarithms we may write
log ar - log ag
log 100 - log 50 _ 2 - 1,7 _ 0,3^
log 8 - log 5 ~ 0,9 - 0,7 "~ 0,2
Therefore x = 1, thus determining the thickness at
which the two intensities are equal and the liquid
appears yellow. If the thickness be still further in-
creased, Ira* will be greater than Iga*, and the liquid
will appear red when held up to the light.
A still better example of di-chromatism can be made
by dissolving " Brilliant Green " and Naphthaline Yel-
low in hot Canada Balsam, and, when cool, squeezing
the mixture between two glass plates ground in the
form of a thin prism. The thin end of the wedge will
then appear green, and the thick edge red, and some-
where between the two ends where the transmission
is the same for both, yellow.
When a white flower is placed in coloured light, it
assumes the colour of the light thrown on it, since
it does not absorb any special colour, but reflects all
light equally. In the same way most coloured flowers
will assume the colour of the light in which they are
placed, but some flowers absorb so much of the incident
light that they appear nearly or quite black. Thus a
red poppy or a red tulip will appear a brilliant red in
red light, but nearly black in green or blue light, since
the cells of the petals are filled with a substance which
absorbs the green and blue light. On the other hand,
a red rose or a carnation will appear a brilliant blue in
c
1 8 PHOTOGRAPHY IN COLOURS
blue or greenish-blue light, or if seen through spectrum-
blue glasses, but appears unaltered in a red or white
light.
If a dilute coloured fluid be placed in a white basin,
the white light which penetrates the fluid will be re-
flected from the interior of the basin, and after being
deprived of certain constituents will appear of its proper
colour to the eye. If the same fluid be placed in a
black basin all the light will be absorbed, and the
liquid will appear black. If, however, some white
powder be sprinkled into the basin, the white particles
will reflect and scatter the light so that the original
colour is restored to the liquid.
The colour of pigments is entirely due to their power
of absorption. The entering light penetrates a slight
distance into and between the particles, and is reflected
back after losing all those colours which do not con-
tribute to its proper colour. Thus a mixture of Prussian
Blue and Gamboge appears green to the eye because
the blue paint absorbs all the red, orange, and yellow
rays, and transmits the green and blue ; while the
yellow paint absorbs the violet, indigo, and blue rays.
Hence the green rays are the only ones left to be
reflected by both, and therefore the mixture appears
green.
§ 10. Surface Colours. — Some bodies possess the
peculiar property of reflecting one colour and transmit-
ting another. Thus if an alcoholic solution of Aniline
be allowed to evaporate on a glass slide, it will appear of
one colour when seen by reflected light, and its com-
plementary colour when seen by transmitted light. It
is well known that gold-leaf transmits green light, and
THE NATURE OF LIGHT AND COLOUR 19
reflects yellow rays. Cyanine, again, is purple by
transmitted light, but of a peculiar plum colour by
reflected light. This is due to a very dark absorption
band in the green which absorbs this colour, and hence
only the blue and red rays reach the eye.
§ 11. Saturation. — A colour is said to be saturated
when it is not combined with white. If a beam of white
light is made to pass through two coloured glasses of
the complementary colours they appear black, since
each absorbs all the colours except the one it transmits.
Thus a pure red glass only lets red light through, and
a pure green glass only transmits green light. Hence
no light at all passes, and therefore when superposed
they appear black when held up to the light. By this
means a strong light may be reduced to any desired
degree. In the same way, if a body of any colour be
viewed through a coloured glass which absorbs the
rays reflected by that body, the latter appears black.
For example, a green body appears black when viewed
through a red glass of the proper shade, since the green
rays which are reflected are all absorbed by the red
glass. On the other hand, a body viewed through a
glass of the same colour appears of the same tint
and hue as a sheet of white paper placed beside it.
Hence the apparent colour of a body varies with the
light by which it is viewed. Thus blues and greens
are nearly indistinguishable in a yellow artificial light,
such as a candle or gas light.
Yellow, as we shall prove later (p. 46), is not a primary
colour, and it is formed in. the spectrum by the overlap-
ping of the red and green ; but in the eye it appears to
be a distinct sensation, since if one blinds one's eyes
20 PHOTOGRAPHY IN COLOURS
temporarily by gazing for a long time at a bright Sodium
flame through a spectroscope, one will observe that
the red and green run into each other with no trace
of yellow. This shows a remarkable fact, viz. that
with ourselves yellow is a distinct colour, and not
merely a mixture of red and green, such as is evolved
by the fusion of those colours in Helmholtz' experi-
ments. Furthermore, yellow light does not fatigue
the eye for either red or green.
§12. Transparency and Translucency. — A body is
said to be transparent when light passes freely through
it with a minimum of absorption or reflection. It is
said to be translucent when it only transmits a portion
of the light, or scatters some of it. Thus frosted glass,
horn, and tortoise-shell are said to be translucent.
Some of the light is transmitted, but much is absorbed
or scattered, so that objects are seen dimly through
them. The substance through which the light passes,
whether it be gaseous, liquid, or solid, is termed the
Medium. If it be uniform in all respects, it is termed
homogeneous ; if not, it is called heterogeneous.
Transparency and translucency are only relative
terms, since no substance is absolutely transparent or
absolutely opaque. Below sixty fathoms the sea would
appear pitch dark to the eye. Opaque substances, if
sufficiently thin, will permit of some light passing
through. Thus gold-leaf will transmit green light when
held in front of a lamp, and direct sunlight will pene-
trate through the wood of any dark slide in sufficient
quantity to fog a photographic plate if the sun be allowed
to shine on the slide for a quarter of an hour, or even
less. Iron, silver, and copper in very thin layers will
THE NATURE OF LIGHT AND COLOUR 21
transmit some trace of light. An opaque body may
be rendered invisible, and sometimes even transparent,
by making it incapable of reflection. Thus, if a drop of
Canada Balsam be placed on the ground-glass surface
of a camera focussing-screen and a microscope cover-
glass be gently pressed over it, the inequalities of the
surface are filled up by the balsam. As this latter is
of the same index of refraction as the glass, the screen
becomes quite transparent and indistinguishable from
a piece of clear glass at that spot. By this means the
most minute details can be readily focussed-up by a
magnifying glass. In the same way a piece of paper can
be made translucent by smearing it with oil or grease.
The air spaces and fibres of which the paper is composed
have a different refraction from the intervening par-
ticles, so that, when the whole is saturated with oil,
the indices of the parts are nearly equalised, and, the
paper being rendered homogeneous, very little light is
scattered. Again, if a glass rod be placed in a test-
tube, the rod can be clearly seen ; but if the tube be
filled with Cedar oil, which has nearly the same re-
fractive index as the rod, the part of the rod which lies
in the oil becomes at once invisible. The absence of
colour becomes of the highest importance in the case
of photographic lenses. Thus a very faint tinge of
yellow in the glass which is quite invisible when held
up to the light, and only perceptible when placed on a
sheet of white paper, will increase the exposure as
much as 20 per cent, or 30 per cent, owing to the
absorption of the blue-violet and ultra-violet rays.
§ 13. Reflection is the rebound of light-waves from
the surface on which they are incident, into the original
22 PHOTOGRAPHY IN COLOURS
medium. If the surface is polished and uniform the
reflection is said to be regular, and the image of the
source of light can be seen. If roughened or uneven
the reflection is irregular, in which case the surface can
be seen, but the image of the source cannot be seen.
Mercury and polished silver reflect about 90 per cent, of
the light, and a silvered mirror about 80 per cent.
Fresh fallen snow is a good example of irregular
reflection or scattering of light in which nearly all the
light is reflected.
§ 14. Shadows. — When an opaque body interrupts a
portion of the light within an illuminated area, it casts
a shadow on the surface in front of it. A region which
is screened off from a radiation or vibration of any kind
is also termed a shadow, although the latter may be in-
visible to the eye. Thus we talk of " sound-shadows "
and " electric-shadows " whenever an obstruction cuts
off, or even diminishes, a series of vibrations. This
may be observed the moment when one suddenly turns
a corner from a noisy to a quiet street. The sound of
the traffic in the street becomes at once lessened. One
is in a " sound-shadow."
A shadow cannot produce an image, but only a
silhouette of the object which obstructs the light from
the source. In nature the source is never entirely a
point of light, but consists of a very large reflecting
surface which causes interposed objects to cast greatly
extended half-shadows. These produce the well-known
half-tones which so greatly add to the beauty of land-
scapes. Usually the sources are multiple, so that each
half-shadow overlaps others, and thus breaks up the
tones still further, so that when a surface is illuminated
THE NATURE OF LIGHT AND COLOUR 23
from two sources the shadow becomes split up into
four parts. First, two distinct penumbrse (half-
shadows), one from each source, then a darker combined
penumbra, and lastly, a dark umbra where no light
reaches it is seen. These facts are of the greatest impor-
tance to artists, and deserve careful study. A some-
what similar phenomenon to the formation of shadows
occurs when direct sunlight passes through a hole an
inch or so in diameter. If the opening be square or
irregular in shape, and the screen be perpendicular to
the line of light the image will be square or irregular
and sharply denned — provided the screen be not distant
many times the diameter of the opening. On the
screen being removed further back, the image will
become larger, the margins less distinct, and the corners
rounded off, until at length the image forms an im-
perfectly defined disc. If the screen is tilted to the axis
of the rays the disc becomes oval. This explains why
the images made by the irregular spaces between the
leaves of a tree form circular patches of light on the
ground when the sun is vertical, and become more and
more elliptical as the sun descends. We may therefore
say that when the screen is perpendicular to the axis of
the sunlight through the aperture, and the distance of
the screen not many times greater than the size of the
opening, the image will be an exact copy of the
aperture ; but if the distance of the screen be large
compared with the aperture, then the image becomes
circular or elliptical, according to the angle at which
the screen is placed.
In photography a knowledge of these facts plays a
very important part. If one takes a group or portrait
24 PHOTOGRAPHY IN COLOURS
out-of-doors, and there are trees in the immediate back-
ground, the apertures between the leaves will appear of
their normal irregular shape if the lens be focussed on
them. But — as is always the case when a group or
figure forms the principal subject— the lens is focussed
on the group, the trees will be more or less out of focus,
and in consequence the apertures between the leaves
will appear as round white discs in the print or positive.
The larger the aperture used, the bigger will be the
discs, and consequently they will become very obtrusive
and spoil the effect of the picture. To avoid this it is
necessary either to focus for a point behind the group
so as to bring the trees into sharper focus, or to stop
down the lens to a small aperture (which must never
be greater than F/16 in any case), but F/32 is much
to be preferred and will generally suffice to prevent the
discs being noticed at all.
It is generally stated that the brighter the light the
blacker will be the shadow cast. This is not strictly
true, because if the light be very bright, such as
direct sunlight for example, the greater part of the light
is reflected from the sky and surrounding objects, and
some of this will be thrown on to the shadow and
adjacent parts, thus greatly reducing the contrast.
The same thing happens when a photographic plate is
over-exposed or fogged by light getting into the camera.
The developed image will appear flat, and will give a
dull picture. Now the light of the full moon is only
the cooooo' part of that of the sun at noon, so that the
reflected light is very feeble, and has very little effect on
the shadow cast on objects by the moon. Hence the
contrast between the shadow and the surrounding
THE NATURE OF LIGHT AND COLOUR 2$
surface is very marked, and the shadow appears inky
black, which is never the case with a sun-shadow out-
of-doors.
§ 15. Coloured Shadows.— If a sheet of white paper
be pinned against the wall and illuminated by a light
held a foot or so away, and a slip of red glass be held
in front so as to throw a red image on the paper, the
shadow cast by any opaque object (such as one's
finger) placed between the red glass and the paper at
the margin of the red light, will appear of the comple-
mentary colour, viz. green. In the same way a yellow
glass will cause a blue shadow. Again, if in the twilight
the light from a blue sky is allowed to fall into a room
illuminated by a candle, and one holds an opaque rod
in front of the latter so as to cast a shadow on a white
surface, while another rod be held so as to cast a
shadow from the blue sky on to the same paper, the
first shadow will appear blue, and the second yellow,
while the approximation of the two shadows will only
intensify the contrast.
§ 16. Shadows cast by Lenses. — If a concave lens
be held between a light and a screen in a darkened
room, it will cast a dark central shadow just as if it were
an opaque body. The transmitted rays being divergent
they will be turned away from the spot around the axis
of the lens and will be concentrated around the margin
of the cone, thus forming a highly illuminated ring.
The further the lens is removed from the screen, the
larger and fainter will be the ring of light. A convex
lens, on the other hand, if held at its focal distance
from the screen will throw a bright central image
(focus) on it, which is surrounded by a dark shadow,
26 PHOTOGRAPHY IN COLOURS
since the rays are refracted towards the axis of the
lens. Very little light therefore will reach any
portion of the screen behind the lens, except around
the centre.
A prism acts much in the same way as a convex
lens. It throws a dark shadow behind it, since the
light is deviated towards the base of the prism, outside
which it forms a bright patch, which can be made to
travel right away from the shadow if the prism be
withdrawn further from the screen.
§ 17. The Colour of the Sky. — In the case of ordin-
ary obstacles to 'the passage of light their size is enor-
mously greater than the reflected waves of visible light
(which vary from 0,3 ^ to 0,7 /x, a " p " being the —^ part
of a millimetre), so that the ordinary laws of reflection
bear no reference to wave-lengths when a wave is in-
tercepted by such a large obstacle. The screening
action is perfect, save for a small diffraction-band
around its edge. Now it is obvious that a small obstacle
will not be so effective as a screen for the long (red)
waves, as for the shorter (blue) ones. When the
source of light is low down, such as near the time of
sunset, the long waves will be more freely transmitted
than the short ones, and consequently as the sun sinks
to the horizon, the white light from it will pass to
yellow and then into orange and red. In the same way
the snow on a distant mountain at sunset, will appear
of a reddish tint. But as the long waves are the most
freely transmitted the short ones will be most freely
reflected and scattered. Hence, as we look at the sky,
the light gradually becomes bluer towards the zenith,
because the blue rays from that part of the sky are
THE NATURE OF LIGHT AND COLOUR 27
reflected and scattered in all directions. This is the
reason why the light from the sky appears blue, unless
it is intercepted by clouds or mists.
In the same way we can explain the cause of the
yellow tinge in fogs, especially if the sources of
obstruction are due to large particles such as soot. Of
course a great deal of the light is both stopped and
modified by these obstructive particles in its path to
our eyes. During the transmission, the rays of high
refrangibility (i.e. the blue-violet rays) are stopped or
hindered, while those of low refrangibility (i.e. the red-
orange rays) are destroyed by scattering, hence the
light which reaches the eye must be weak in rays from
both ends of the spectrum. In other words the light
is chiefly made up of blue and greenish rays. When
the particles reach a certain size they will affect all
waves equally, with the result that the light will appear
white. In the low atmosphere the obstructive particles
are largely due to dust and aqueous vapour, but in the
high atmosphere they are probably due to extremely
minute particles of matter such as very finely divided
vapour or ice particles, or possibly to gas molecules
(Eayleigh). The blue tints of a distant mist, the smoke
from a cigar or a wood fire are all due to the same
scattering of the light by exceedingly minute particles.
Tyndall formed an artificial cloud by precipitating
the vapour of iodide of allyl through the action of light.
He found that the particles gradually increased in size,
and as they did so, the blue colour disappeared and the
scattered light appeared white, as we have already
stated above. If at this stage, the light which has now
become polarised, be observed through a NICOL prism
28 PHOTOGRAPHY IN COLOURS
held vertically, viz. in the position in which ordinary
scattered light would be extinguished, the blue colour
will appear again much richer than before. This deep
blue was termed by Tyndall the " residual blue." Lord
Eayleigh confirmed Tyndall's experiment, using for his
dust- cloud a precipitate of sulphur made by adding a
little very dilute sulphuric acid to a weak solution of
hyposulphite of soda.
CHAPTER II
§ 18. The Evolution of Colour Photography
COLOUR photography may be said to date back to the
time of the TjV\rberi1p.hrf> nf (^r>fti-,hfi in 1810. It is there
stated that "if a spectrum produced by a prism is
thrown on to moist chloride of silver paper, if the
printing be continued for fifteen minutes, I observe the
following : in the violet the chloride is a reddish-brown
(sometimes more violet, sometimes more blue), and
this coloration extends well beyond the limit of the
violet ; in the blue, the chloride takes a clear blue tint
which fades away, becoming lighter in the green. In
the yellow, I usually found the chloride unaltered ;
sometimes, however, it had a slight yellow tint. In the
red, or beyond the red, it looks a rose or lilac tint.
The image of the spectrum shows beyond the red and
the violet, a region more or less light and uncoloured.
Beyond the brown band which was produced in the
violet, the silver chloride was coloured a grey-violet for
a distance of several inches. In proportion as the
distance from the violet increased, the tint became
lighter. Beyond the red, on the contrary, the chloride
took a feeble red tint for a considerable distance."
It was not until 1868 that any satisfactory explana-
tion of these phenomena was forthcoming. In that
30 PHOTOGRAPHY IN COLOURS
year Zenker showed that colours could be produced by
stationary light-waves formed in the layer of silver
chloride. It was further found that many body
(pigment) colours were highly sensitive to light, be-
coming bleached by its action. Wiener showed that
a light-sensitive substance can only be bleached by
those rays which are absorbed by it, all other rays
being either transmitted or reflected. In fact, a sub-
stance is called red because it reflects red rays and
absorbs green and blue ones, and so for other colours.
If, therefore, any substance happens to be sensitive to
the action of different wave-lengths of light, it must be
due to rays of such colours as will be absorbed by the
body. Quite recently Dr. Smith has made use of this
principle in his Uto-color bleach-out paper (see Chapter
XL), and Szczepanik, Neuhaus, Liesegang, and others
are still working in the same direction.
In 1890, or 22 years later, Lippmann confirmed
Zenker's discovery, and succeeded in producing inter-
ference pictures by placing a very thin chloride
emulsion plate in contact with a layer of mercury
which acted as a reflector. By this means the light-
waves from the object which pass through the film
meet previous waves reflected by the mercury mirror,
and so produce the interference phenomena. Thus the
silver chloride is changed at the spot where the crest
is increased, but unaltered at the spot where the wave
is neutralised.
Meanwhile Vogel, Obernetter, Eder, and Abney were
experimenting to increase the range of the collodion plate
affected by the action of coloured lights. By bathing
a plate in Eosin, Vogel found that he could extend the
EVOLUTION OF COLOUR PHOTOGRAPHY 31
sensitivity of the plate from the yellow-green as far as
the orange, and Abney, going a step further, extended
it to the red by means of Cyanin blue. This was soon
applied to the gelatine plate, and so orthochromatic
plates came into vogue. Later on Aethyl red was
employed by Miethe, and finally Pinacyanol was
applied, which enabled plate-makers to produce a
panchromatic plate, i.e. one sensitive to all the colours
of the visible spectrum, or roughly speaking corre-
sponding to wave-lengths from 400 to 700 pp (micro-
millimetres).
The discovery of the power of certain dyes to render
the photographic plate sensitive to all visible colours
was the one step needed to render the methods of
colour photography and colour printing possible. The
early Daguerreotype plates were only sensitive to the
violet, the blue, and a little of the green. The gelatine
plates, before the action of dyes was known, were
sensitive to the violet, blue-green, and a little of the
yellow. Eosin and Erythrosin dyes extended their
action through the yellow and orange, while Aethyl
red, Cyanine and Pinacyanol brought the sensitivity
nearly up to the end of the visible red.
Gradually our ideas of colour were built up. The
early suggestions of Thomas Young (1773-1829),
elaborated by Helmholtz in Heidelberg, established
the new theory which has been called the Young-
Helmholtz theory of colour vision. Clerk Maxwell in
Cambridge further exemplified this theory and showed
that every possible colour and shade of colour was
either a blue-violet, a green, or a red, or else a mixture
of two or of all three of these colours. Ducos du
32 PHOTOGRAPHY IN COLOURS
Hauron in Prance, and Ives in America, applied these
facts in practice by making three separate negatives of
the coloured object through a red, green, and blue
glass respectively. Then positives were made and the
three projected on to a screen through the same glasses
and made to coincide. Ives went further, and took
stereoscopic pictures in the same way and combined
the chromograms by means of his Kromskop instru-
ment. In this way the picture was seen, when
brilliantly illuminated from behind, in all the original
colours and in stereoscopic relief. Later, relief blocks
were made on bichromated colloids taken through the
same coloured glasses, and impressions made by means
of the complementary colours either from inks or dyes,
and prints made one over the other on the same sup-
port, or on separate thin films which were superposed.
In this way facsimiles in colour were produced which
could be viewed by reflected light, or bound up as trans-
parencies to be seen by transmitted light. In the same
way all the beautiful coloured "process" prints are
obtained. This fundamental method of producing prints
has been elaborated in various ways by Dr. E. Konig,
and by Sanger- Shepherd; also by the Rotary Company's
stripping pigment process, the Carbon, Collotype and
Baydex methods. In the year 1904 Messrs. Lumiere
obtaineddirecb tran sparencies in colour by means of
screens coated with starch grains dyed in the three
primary colours, and coated with a panchromatic emul-
sion. This method has been followed by others, based
on the same principle ; but instead of covering the glass
with coloured starch grains, the makers have ruled the
glass with fine lines or dots, or a combination of both,
EVOLUTION OF COLOUR PHOTOGRAPHY 33
in the three primary colours. Such plates yield exceed-
ingly brilliant pictures and very pure and intense colours,
but they do not possess the softness or range of hues
and tints that the autochrome plates do, nor are the
colours so true to nature.
At length, after several years of experimenting and
many disappointing failures, bleaching-out papers have
been produced by Szczepanik, and especially by Dr.
J. H. Smith, by which copies of colour positives can
be printed in direct sunlight, which may be fixed and
mounted like ordinary photographic prints. Such
copies present the great advantage of being viewed
by reflected light, which permits of their being hung
on the wall, or pasted into an album. It cannot be
denied that these pictures are far from perfect, the
colours are impure and the whites never come out as
pure whites, but the process is being improved every
day, and we have no reason to doubt that in a com-
paratively short time such pictures will take a promi-
nent position on the walls of our photographic ex-
hibitions.
Finally we must mention the achievement of Messrs.
Urban and Smith, and later by Gaumont, by which
kinematograph pictures in colour can be projected on
to the screen, whereby the illusion of objects in motion
is greatly increased. This method will be found
described in detail in Chapter XII.
CHAPTER III
§ 19. The Eye compared with a Camera, and the
Retina with a Colour Plate
THE human eye is a spherical ball almost exactly one
inch in diameter, closely resembling a plum, the stalk
of which represents the optic nerve. This latter is a
round hard cord measuring J of an inch in diameter,
made up of an immense number of bundles of nerve
fibres, surrounded by a tough sheath. If traced back-
wards it will be found to pass through a hole in the
back of the orbit where it enters the skull, and imme-
diately to cross obliquely towards the middle line,
where it becomes intimately united with the nerve of
the opposite side. Here it again separates, the halves of
each nerve fusing together to form a flat band. This
band passes along the under surface of the brain, into
which it soon enters, and becomes lost in the great
optic nerve ganglia which are situated in its substance.
There the fibres become intimately connected with the
brain-cells, which interpret the visual impulses pro-
jected from the eyeball, and enable the person to
perceive mentally the images formed at the back of
the eye. The eyeball consists of three coats. 1st. An
outer thick tough fibrous coat, the Sclerotic, which
serves to keep the eye in shape, and protect the deli-
cate coats inside it. 2nd. A dark layer, made up almost
THE EYE COMPARED WITH A CAMERA 35
entirely of a network of blood-vessels, which serve to
nourish the parts around. And 3rdly the Ketina. This
is a highly complicated nervous layer formed by the
spreading out of the fibres of the optic nerve over the
entire surface of the back half of the eye. It forms
FIG. 2. — Semi-diagrammatic vertical section of retina showing
the layers and systems.
the receptive layer on which the images of external
objects are perpetually being formed. Each individual
fibre is connected with a large cell called a ganglion-
cell, from which one or more fibres pass directly back-
wards in a very complicated manner. These ultimate
nerve fibrils divide up into a number of fine short
36 PHOTOGRAPHY IN COLOURS
branches, or arborisations as they are called, which
are connected with other branches from a second set
of nerve fibrils, and which terminate in a series of
peculiar bodies called rods and cones. These form
the terminals of the retina. Seen from above, they
FIG. 3. FIG. 4. FIG. 5.
FIG. 3. — Cross section of the rods and cones in the retina of a
human eye, taken midway between the periphery of the retina
and the yellow spot. The small discs are the rods, the large
ones the cones. Observe that the dots in the centre of each
circle (which represents a cross section of a rod or cone) are
kept in the middle of the surrounding insulating substance
by radiating fibres. Each dot is the cross section of an axis
cylinder (terminal nerve fibril). From a photograph of a
microscopic section prepared by the Author.
FIG. 4. — Bepresents a magnified fragment of a Lumiere screen.
The black discs represent red grains, the shaded green-
coloured grains, and the white discs blue- violet grains.
FIG. 5. — Cross section of the cones at the Fovea (centre of the
yellow spot) ; cones surrounded by a single layer of rods are
to be seen towards the circumference. From a microscopic
section made by the Author.
form a mosaic pattern which is made up of large discs
surrounded by one, two, or more rows of small discs.
These are closely packed together, and are so numerous
that they amount to about 5,000,000 in all. The ends
of the rods and cones lie in contact with a dark layer
of six-sided cells, which are filled with minute granules,
THE EYE COMPARED WITH A CAMERA 37
and have a network filled with fine oat-shaped crystals
which finds its way under the action of light between
the ends of the rods, and retracts again in darkness.
Their function seems to be to prevent halation, while
the function of the six-sided cells is to secrete the
retinal purple— a remarkable chamois or purplish-
coloured substance in which the ends of the rods are
bathed. Its use is not known, except that it is in
some way connected with vision, and that it bleaches
when exposed to light.
Exactly in the centre of the retina, and on the line
of the visual axis at the posterior pole (back of the
eye), is a dark reddish spot, about 2 mm. or 2,5
mm. in diameter — the macula. This is the area
of most distinct vision, and here the rods become
greatly reduced in number, until, at the centre where
there is a tiny pit, there are no rods at all, but only
cones. These are not cones in the ordinary sense of
the word, such as are to be found over every other
part of the retina, but long narrow cylinders closely
packed together, so as to get as many as possible in a
small space. By this means the sensitivity of the spot
is largely increased, since the image of an object will
cover far more cones than if it were to be formed any-
where else on the retina. We therefore only see dis-
tinctly over the macula area a roundish patch about
two mm. across, and which subtends an angle of
about 5 degrees in the field of vision. Critical vision
is confined to the pit itself, which is a much smaller
area still ; in fact, it occupies in the field of vision an
angle of less than 1 degree (about 8"). So that it may
literally be said that we read with our maculas only,
38 PHOTOGRAPHY IN COLOURS
and not with the whole of our retinae, although we can
get a general impression of objects over the whole
retinal field. As a matter of fact we only see one
object at a time, but by revolving the eye very rapidly
both up and down and horizontally, everything can be
seen satisfactorily. It is to our advantage that we do
not see everything sharply at the same moment, other-
wise we should see such an overwhelming mass of
detail, that we should be quite unable to take the view
in, or pay attention to anything. As it is we see one
point after another in succession, and by combining
the sum of all the points looked at, we receive the
impression of the whole view sharply denned. In all
the lower mammals below the monkeys we find no trace
of a macula, but, on the other hand, we find an extended
sensitive area. So that most of the animals do not see
so well as we do over a small area, but they see much
better over a larger one. This is the reason why no
animal (or, more strictly speaking, no mammal) below
the monkeys habitually moves its eye, but if it wishes
to look at an object in another direction it almost in-
variably turns its head, but not its eye. Any one who
takes the trouble to watch the eyes of any animal at
any Zoological Gardens can prove that fact for him-
self.
If you look at Fig. 2 you will observe that each cone
has a nerve fibre exclusively belonging to it, which al-
though twice interrupted in its course by arborisations,
nevertheless permits the current to pass along the con-
tinuation of the fibre direct to the sensorium. Moreover,
the nerve fibril is furnished with a relay ganglion
(e, e, e, e, Fig. 2). The rods, on the other hand, have not
THE EYE COMPARED WITH A CAMERA 39
each a separate nerve which connects them with .the
brain. If you examine the figure carefully you will
notice that from 2 to 10 or more rod fibrils are
embraced by a single arborisation, so that the square
of these numbers will represent the number of rods
which have united to convey a single impression to the
brain. Hence 100 or more rods will only convey the
impression of the same amount of detail that a single
cone will effect.
It is the cones, therefore, which carry the image-
forming vibrations to the sensorium. The function of
the rods appears to be twofold. 1st. They act as
dampers to the acuity of vision outside the macula
area, so as to allow the mind to be concentrated on the
area of fixation. 2nd. By occupying very little room,
and being very numerous, the rods, although they do
not help the definition of objects, enable the eye to
perceive dimly lighted objects, which the scattered
cones alone would only perceive very imperfectly, or
not at all. (See the Purkinje Phenomenon, § 25.)
Occupying the whole of the front of the eye is a
clear transparent modification of the sclerotic coat
known as the Cornea. Inside this is a coloured mem-
branous ring — the Iris, having a black circular aperture
in the centre — the Pupil. Behind the Iris, and resting
against it, is a highly refractive curved body — the Lens.
Between this lens and the cornea is a space filled with
a clear watery fluid — the Aqueous. The remainder of
the eye is filled with a clear, colourless, thin jelly
known as the Vitreous Humour. The eye thus forms
a spherical camera similar in many respects to a pho-
tographic camera. In the eye the lens system is
40 PHOTOGRAPHY IN COLOURS
formed by two lenses: Istly, the cornea, which is a
spherically curved shell of dense transparent tissue;
and 2ndly, the crystalline lens, which is a strongly
curved biconvex lens placed a little distance behind
it, and separated from it by a watery fluid — the
Aqueous (see Fig. 6). Between the two lenses lies the
Iris. This forms the variously coloured membrane
which contributes so largely to the beauty of the eye.
It forms a perfect iris diaphragm, having a circular
aperture in the centre, which appears black to the
observer. This contracts or dilates automatically with
the increase or diminution of the light. At its full
opening, it measures from 6 to 8 mm., or in some
people even 10 mm. across. The focal length of
the lens combination in situ is 15,5 mm., or about
3/5 in.,1 so that at its full opening it works at F/2,5
or F/2, and in some cases even F/1,5, and in a very
bright light at about F/8. This fraction is understood
to mean the ratio-aperture of the lens, i.e. the ratio
between the focal length of the lens and the diameter
of the stop or aperture. Thus, supposing the lens to
have a focal length of 8 inches, and the diameter of
the stop to be 2 inches, the ratio aperture would be as
8:2, and we should speak of the lens as working at
1 This is the case if we measure the focal length from the
nodal point (situated at the posterior pole of the crystalline lens)
to the retina. If we take the focal length to be the distance from
the posterior principal plane to the retina (as is done when work-
ing out the measurements of the eye), then the focal length will
be approximately 20 mm. But the former distance is the most
convenient for our purpose, since the distance between the nodal
point and the retina is the one used for determining the magnify-
ing power of the eye and the size of the retinal image.
THE EYE COMPARED WITH A CAMERA 4!
F/4, and the same applies to every other focal length
and stop.
The camera lens usually consists of two lenses (or
rather of two cemented combinations), and there is
also an iris diaphragm working between them. The
ratio-aperture varies from F/3 in a very rapid portrait
lens, or F/8 the full aperture of a rapid rectilinear
lens, to F/45 its smallest aperture. Hence the eye
has a great advantage over all manufactured lenses as
regards rapidity. Moreover, a camera lens rarely em-
braces an angle of over 100°, whereas the eye embraces
an angle of 160°, or about 170° when both eyes are
open. The eye camera is a rigid one, and it is adjusted
for various distances by altering the curve of the front
surface of the lens. This is effected by means of a
circular band of muscular fibres embedded in the coats
of the eye, and surrounding the edge of the lens. When
this circular muscle contracts, it relaxes its tension on
a ligament which presses against the front of the lens,
thereby allowing it to bulge forward, and thus increasing
its refractive power. Hence if the eye were previously
focussed for infinity, it would now be adapted for
nearer and nearer objects in direct proportion to the
amount of contraction of the ciliary muscle, and con-
sequently to the amount of bulging forward of the front
surface of the lens. In the photographic camera the
same result is obtained by racking the lens further
away from the screen.
In the camera the rays come to a focus, and thus
form a picture on the ground-glass screen, or on the
sensitive plate when that replaces it. In the same
way the rays are brought to a focus on the retina
42 PHOTOGRAPHY IN COLOURS
(B, Fig. 6) at the back of the eye by means of the lens
system, the image falling on a fine mosaic consisting
.ScL
'ON
FIG. 6. — Horizontal section of the human eye through the
Macula and optic nerve : magnified x 2.
Cj. Conjunctiva or Membrane which spreads over the eye to the
rim of the Orbit.
Sol. Sclerotic or fibrous capsule of the eye.
Vi. Vitreous humour (or jelly) filling the cavity behind the lens.
Ch. Choroid or vascular layer which secretes the visual purple
and also nourishment for the rods and cones.
R. Retina, or expansion of the optic nerve adapted to receive
impressions of light. This is a complex nervous structure,
the fibres of which terminate in minute rods and cone-
shaped bodies. They are closely packed together, forming
a mosaic when viewed on the surface.
M. Macula (yellow spot), the highly sensitive area of the retina.
F. Fovea or centre of the Macula.
ON. Optic Nerve (cut short) leading to the brain.
of a layer of rod and cone terminals. Behind this
mosaic is a layer of dark pigment cells which secrete
PLATE II.
Normal Fundus (background of the eye) of a man
forty years old.
BIJ permission of Prof. Dimmer, of Grutz.
The oval patch on the left is the disc or head of the Optic
Nerve. A little to the right of the centre is a dark round
patch. This is the most sensitive part of the eye, and
comprises the macula (yellow spot) area. In the centre of
this patch is a depression or pit known as the fovea. The
fixation point or region of perfect vision is only to be found
at the fovea. The dark lines are due to blood-vessels,
arteries, and veins, which enter the eye near the centre of
the disc and are distributed over the greater part of the
retinal background.
To face p. 43.
THE EYE COMPARED WITH A CAMERA 43
the visual purple, and behind these cells is a dense
layer of blood-vessels held together by tissue, and
known as the choroid (Ch). The analogy between the
way in which light acts on the retina and on a Lipp-
mann film is very close. In the Lippmann camera the
light passes through the lens and sensitive chloride-of-
silver film, and is reflected from the mercury mirror in
contact with it behind. In the case of the eye the light
passes through the lens and retina and is reflected
from the concave spherical mirror which constitutes
the choroid, directly on to the ends of the rods and
cones, thus giving rise to image-forming vibrations
which pass along the fibres of the optic nerve (ON)
to the brain, where the psychic transformation into a
visual image takes place. It is conceivable that if
reflected rays meet the entering rays half a wave-
length later in a different phase, interference phenomena
will be produced in the eye.
In colour photography we place a colour screen
with red, green, and blue-violet dots, patches, or lines
in front of the sensitive film. In the eyes of mammals
there are screens1 with zones of brilliant colours —
emerald green, gold, vermilion, scarlet, ruby, orange,
yellow, brown, blue, violet, and purple, in a similar
way. In man and many 6ther mammals, birds, and
reptiles we find a monochromatic vermilion or scarlet
colour-screen. In a few animals (e.g. albinos) we find
a creamy white fundus,2 interspersed with patches of
1 These coloured screens are situated in the front part of the
choroid, immediately behind the retina, and in some animals
constitute what is known as the Tapetum Lucidum (Brilliant
layer).
2 Or background of the eye (see Plate II.).
44 PHOTOGRAPHY IN COLOURS
vermilion, which reflects all colours, and in a few we
find a chocolate-coloured screen, but in no animal is
there a black screen to be found, which ought to be
an essential requisite if the usually accepted theory
were true that the image of external objects is pro-
jected on to the layer of rods and cones, instead of
being formed on the mirror and reflected back again
on to their ends as we have stated.
The photographic plate is coated with a thin layer
of gelatine and rendered sensitive to light by being
impregnated with Bromiodide of Silver. In the eye
we have a mosaic made up of the ends of the retinal
nerve fibres on which the image of the object seen is
focussed, and which is rendered more sensitive to light
by the presence of the visual purple which is constantly
being secreted and in which these nerve terminals are
bathed.
§ 20. Reason why the Yellow Spot is Yellow.— In
taking a photograph through any kind of colour-screen
plate, it is necessary to restrain the intense action of the
blue-violet rays by placing a yellow colour-filter in the
path of the rays in front of the screen. In our own
eyes, and in those of all other vertebrates, we have a
yellow colour-filter interposed for the same purpose
throughout the entire extent of the retina. This occurs
in the narrow-meshed plexus (Ch) of the capillary
vessels which lies immediately in front of the sensitive
layer. The only exception is afr' the f ovea, i.e. the tiny pit
at the centre of the macula (M) or yellow spot as it is
called. Here there are no blood-vessels, and nature
therefore has placed at this spot a yellow pigment behind
the sensitive layer which serves the same purpose even
PLATE III.
A.
H.
FIG. I.
A. Appearance of the coloured oil globules in the retina of a tortoise's eye,
as they would appear if seen from above looking towards the choroid, x 700.
G. L. Johnson.
B. Do. do. Appearance of the oil globules in the retina of a domestic fowl,
X 500. G. L. Johnson.
C. Appearance of the starch-grain layer in an Autochrome plate, x 140. G. L.
Johnson.
FIG. II.
Vertical section of a pigeon's retina, showing appearance and position of the
coloured oil globules, x 870. (After Prof. v. Greeff' s monograph in the Graefe-
Saemisch Handbuch der Gesamten Augenlik.)
To face p. 45.
THE EYE COMPARED WITH A CAMERA 45
more effectively.1 If there were no yellow pigment at
the macula, when looking at a white sky or white
surface we should see a blue-violet disc projected in
the line of regard in front of our eyes, corresponding
to the non-vascular area in the centre of the visual pit.2
The amount of blue-violet absorbed by the capillary
screen is very considerable, extending from G to K
(4300 to 3920), and resembling in its colour and
action that of a Lumiere screen.
§ 21. Remarkable Similarity between the Auto-
chrome Colour Screen and the Colour Screen
in certain Birds and Reptiles. — If we examine
a Lumiere Autochrome or a Thames colour plate
with a magnifying glass, after having stripped off
a piece of the sensitive film, we notice the whole
surface is covered with a mosaic of red, green, and
blue-violet dots. Now all the birds and most of the
reptiles show no trace of vessels which nourish the
retina, which we find so highly developed in most of
the mammals. In the majority of the mammals we
find the retina nourished by a rich supply of blood-
vessels and capillaries, but in all the birds and most of
the reptiles this blood-supply is completely wanting,
the nourishment being derived by osmosis, or, in other
words, by a soaking-through or percolation of choroidal
lymph-plasma through the layers of the retina. Con-
sequently they have neither a brightly coloured fundus
nor any tapetum lucidum (i.e. a reflecting colour-layer) of
1 The Author ventures to offer this theory as the true explana-
tion why the fovea is yellow. As far as he knows, it is here
advanced for the first time.
2 See the Author's work, " Pocket Atlas and Test-book of the
Fundus Oculi," Adland & Son, London, 1910.
46 PHOTOGRAPHY IN COLOURS
any kind such as we find in most of the mammals ; but
instead, some of them are provided with a mosaic of
oil globules. These latter are tiny droplets of a highly
refracting coloured substance, and they are situated
just where the cones penetrate the external limitating
membrane, and consequently immediately in front of
the sensitive layer (i.e. tips of the cones) where they
receive the light impression. Their position will be
seen to be almost in contact with the pigment-laden
filaments which pass between the separate nerve ends.
See Plate III.
It will also be noticed that these coloured drops form
four rows. First a yellow, then a red, then another
yellow, and lastly, a green row. In the tortoises we
find in addition a blue-violet row. Thus Lumiere's
discovery has been forestalled by the reptiles and the
birds. But why should there be a yellow layer of dots ?
Are not the three primary colours enough? In
coloured light mixing by spectroscopic colours, Clerk
Maxwell, Helmholtz, and Abney successively showed
that the three primary colours, red, green, and blue-
violet were sufficient to produce all the different shades
of colour ; and in colour printing likewise every colour
could be produced by these three primaries if only a
shade of grey were added, as Mr. Handel Lucas has so
ably pointed out (see " British Journal of Phot.," supple-
ment, 1912, pages 43 and 55) ; although I agree with
Messrs. Newton and Lucas that in this latter case the
use of grey is only necessary on account of the imper-
fection of our printing inks. But the perception of
colour by the eye differs in one respect from the purely
physical production of colour. In the latter case
THE EYE COMPARED WITH A CAMERA 47
yellow is produced by the mixture of green and red in
certain proportions. In the eye, however, yellow is a
distinct colour sensation. This fact has been made use
of to make a special form of test for colour blindness,
by which a pure yellow comprising the D line is trans-
mitted through the slit of a spectroscope, and a second
yellow is made to match by employing two other slits,
one transmitting a red beam, and the other a green
beam. The candidate is then required to adjust the
green slit until by overlapping, the red together with
the green forms a perfect match with the uncombined
yellow. If the observer were colour-blind to red or
green, the match could not be made. The writer has
repeatedly suggested that a screen -plate of four colours
would give a more perfect rendering of nature than
the usual three colours.1 Perhaps some colour-plate
makers will carry out the experiment.
§ 22. Colour Vision and Colour Blindness.— In
order to understand the rationale of three-colour photo-
graphy, it may be useful to some of our readers if we try to
explain the nature of colour vision and colour blindness.
The light impression gives rise to three sensations
which are quite distinct — a light sense, a form sense,
and a colour sense.
The first is the faculty of distinguishing illumination
and its degrees of intensity. This is effected in the
most simple case by the presence of pigment spots in
the cuticle of an animal or plant, and forms the most
rudimentary of all forms of eyes.
1 According to Hering's theory of colour visions these four
colour sensations must be present together with white and black,
thus forming three opposing groups, viz. red and green, yellow
and blue, and white and black (see Appendix, p. 261).
48 PHOTOGRAPHY IN COLOURS
The form sense is a higher development of the sight
faculty, and needs a transparent refracting body to
form a real image, and nerve terminals to convey the
collected impression to the animal's brain. This image
may be quite independent of colour.
The colour sense constitutes a still further develop-
ment of vision, which we will now discuss.
As was first shown by Newton, white sunlight can be
resolved, by means of a prism, into six distinct colours,
viz. violet, blue, green, yellow, orange, and red. These
are the only pure spectrum colours which most of us
can perceive, although some people can see indigo as a
distinct colour, thus making seven colours in all. We
have reason to believe some animals can see other
colours beyond the range of this spectrum. We have
stated that all the colours in nature can be formed by
suitable admixtures of blue-violet, green and red, while
white is formed by the action of these three colours
together. Black is not a colour at all, but is caused by
the absence of all colour sensation.1 Thus, if these
three primary spectrum colours be projected on to a
screen by separate lanterns and then superposed, the
result is a white disc of light, the colours being added
together (additive method). If, now, you put a red glass
in front of a lantern emitting white light, on the top of
that a blue, and finally a green glass, since each glass
absorbs all the colours except its own, no light at all
will reach the screen, and the result will be a black
patch, the colours being subtracted (subtractive method).
But three coloured lights are not really necessary to
produce black, since any two complementary coloured
1 See end of this Chapter.
THE EYE COMPARED WITH A CAMERA 49
lights or glasses if superposed will effect the same
purpose by subtraction, that is to say, each colour
will absorb (subtract) all the colours of the spectrum
except its own, and in the same way any two com-
plementary coloured lights if mixed will produce white
(by addition), as can be proved by the reader for
himself.
If you look steadily for a minute at the red part
of the spectrum in a spectroscope illuminated by
a very intense light, in which the rest of the
spectrum is cut off, and then look at the entire
spectrum through another spectroscope moderately
illuminated, you will see the blue-violet and green
bands; but the green runs right into the yellow
as far as the C line, where it suddenly ends. The
yellow will be found to have entirely vanished — it is all
green. Now rest your eyes for about ten minutes and
repeat the experiment with the green part of the spec-
trum, and you will notice that you can again see three
colours, but they are changed to violet, blue, and red.
The green has quite gone and the blue runs straight
into the red. In the same way you can blind your eyes
to blue, and the green and violet will be seen to run
into each other. Again, you may blind your eyes to
the violet. This is more difficult, as it requires a longer
gaze and a very intense light. The blue will still be
visible, but it ceases abruptly at the violet end. Lastly,
if you blind your eyes to the yellow by gazing for a long
time at a bright sodium flame, you will observe that
the red and green will run into one another. This
forms yet another proof of what we stated in the pre-
vious paragraph, viz. that yellow is a distinct colour
50 PHOTOGRAPHY IN COLOURS
and not merely a mixture of red and green, as is evolved
by the fusion of those colours in Helmholtz' experiments.
Furthermore, yellow light does not fatigue the eye for
either red or green.
To sum up. A red-blind person sees violet, blue, and
green. A green-blind sees red, blue, and violet; a
violet-blue-blind sees a little blue, all the green, and a
little red, but no yellow, since the green and red have
met together.
"Unfortunately, it has been impossible up to the
present to imitate a pure spectrum violet, or to find
it in natural colours. According to Dr. Edridge-Green,
the corn-flower and some varieties of Lobelia nearly
approach to it, so that we have to content ourselves
with a blue-violet dye, i.e. a mixture of violet and
blue, which is the closest imitation of violet that we
can procure. It is quite possible that some dye will
yet be found that will give us a pure violet, as well as
a pure blue. The other colours are much easier to find.
Thus, ruby glass and the purple of Cassius (oxystannate
of gold) form fairly pure reds. Sulphur and bichromate
of potash make good yellows, and a saturated solution
of ammoniacal sulphate of copper makes a nearly pure
blue. But the reader must not go away with the
idea that these primary colours stop abruptly. They
each run on far beyond the point at which they
appear to stop in the spectrum. " In other words, the
red and green sensations overlap, as do the blue and
green and also the violet and blue, so that we must
take the middle point of the combined overlapping as
the natural boundary between the adjacent sensations "
(Burch).
PLATE IV.
"I .llf
rt "cT rt- £ c
>^ o >^ «
|1 S a a
& - £ § 0,
8 I § :s -s
2 -o w > x
r; ° o <u J3
S1-
THE EYE COMPARED WITH A CAMERA 51
As to how we see colours we are quite ignorant.
Most physiologists assign the perception of colour to
the cones, leaving to the rods the function of seeing
feeble luminosities. Helmholtz' theory, that certain
cones respond to the stimulus of red undulations, others
to green, and others to blue-violet, will not bear
close investigation. Some physiologists assign the
sense of colour as well as perception of form to the
action of the visual purple. This, again, is open to
objection that there is no visual purple at the fovea,
or in certain animals, e.g. bats, but this difficulty is
got over by assuming that other secretions, such as
visual white, visual green, or visual yellow, take on
the same function. The author found all these bodies
in the retinae of animals. Edridge-Green has sug-
gested that the visual purple flows into the fovea
during the act of vision. This theory explains certain
phenomena, but is not accepted by many physiologists.
Again, we have reasons for believing that there is a
colour centre in the brain. A remarkable case bearing
on this point occurred in the author's practice. The
patient was suffering from a form of creeping paralysis
which gradually affected the limbs of the left side. At
the same time, as more and more of the muscles became
paralysed, the sense of colour slowly vanished in the
corresponding eye, until ultimately the patient could
see no colour at all, everything appearing black, grey,
and white, like an engraving. This was tested by
getting the patient, who was a good water-colour
painter, to make a coloured drawing of the spectrum,
first with the sound eye and then with the colour-blind
one. Notwithstanding the absence of all sense of
52 PHOTOGRAPHY IN COLOURS
colour, vision was hardly affected at all, and the colour
sense remained perfect in the right eye, while that of
the left eye never returned (see Plate IV.).
Colour blindness, which affects about four out of every
hundred people one meets, may be due to a deficiency in
light perception of a portion of the spectrum, usually in
that part which lies at the red end. Dr. Edridge- Green
has classified colour-blind people according to whether
they can perceive six colours, five, four, three, two,
or only one colour. Thus, one who distinguishes
all six colours, or a hexachromatic person, may be con-
sidered as normal. One who distinguishes five of the
spectrum colours confuses orange-red and red, orange-
yellow and yellow, rose-red and red, purple-violet and
violet, bluish-green and green. A tetrachromatic person
sees only four distinct colours. He confuses red, orange
and rose-red, greenish-blue and green, and pure blue and
violet. A trichromatic person sees only three distinct
colours. He confuses red, orange and rose, most of the
yellows, and blue, violet, and purple. He also confuses
rose, grey, and green, and many browns, as well as
bluish-greens and greens. A dichromatic person sees
only two colours. Thus, red, orange, yellow, and green,
all seem alike, as do all blues and violets. He also con-
fuses blue-green, purple, and greys. A monochromatic
person sees no colour at all. Everything appears as
impure whites, blacks, or greys. One- and two-colour
cases are exceedingly rare.
The Young-Helmholtz theory fits in better than any
other with the phenomena relating to colour photo-
graphy, but it by no means harmonises with all the
facts connected with colour vision. Thus, a dichromatic
THE EYE COMPARED WITH A CAMERA 53
red-blind person ought to see green best, whereas he
sees yellow most distinctly. Again, the phenomena of
after-images cannot be explained by this theory. Nor
does it account for the additive and subtractive forma-
tion of white and black sensations in persons possessing
only two or three units of colour perception. Further-
more, a pencil of coloured light, e.g. light which has
passed through a coloured glass focussed on a very
minute area of the macula, will produce the sensation
of white, whereas it ought to give rise to a very decided
sensation of the colour.
Again, as we have already pointed out, yellow is a
distinct colour and not merely a combination of red
and green coloured lights.
Lastly, if it were true that the retina consisted
entirely of three groups of fibres, corresponding to
red, green, and blue-violet sensations, how can
persons blind to all these colours have nearly normal
vision ? and how can they see white objects as white ? 1
§ 22 A. The Visual Purple. — We are still in doubt as
to the function of the substance secreted by the outer-
most layer of the retinal (hexagonal cell) layer, called by
Boll the " visual purple." According to some writers,
who have made it a special study, vision depends entirely
on its decomposition. The author's theory, which he
thinks harmonises best with the facts, is briefly as follows :
We know that this visual purple is rapidly decomposed
in the presence of bright daylight and at the same time
is continually being re-formed. Now, in bright sunshine
this visual purple is used up as fast as it is secreted,
so that if one steps into a dark room, the purple having
1 See Appendix, p. 259, " Theories of Colour Vision "
54 PHOTOGRAPHY IN COLOURS
been nearly all used up, one cannot see anything, and
one has to wait a minute or two until the purple
accumulates, which it quickly does. As the amount
increases the vision improves, or, to use a familiar
expression, the eyes get accustomed to the dim light.
If, now, one steps out of the room into the bright
sunshine the amount of accumulated purple generates
so much visual energy that one is dazzled and almost
blinded for the moment until the superfluous store of
purple is decomposed. It may be objected that bats,
which can certainly see in a very dim light, have no
visual purple at all, but then they possess a buff-grey
visual substance which answers the same purpose.
§ 23. On the Meaning of the Sensation called
Black. — We have stated on page 48 that black is not
a colour at all, but is caused by the absence of all
colour sensation. This requires some qualification.
We must distinguish between the absence of all light
stimulus on a portion of the active normal retina, capable
of conveying a colour sensation to the brain, and a gap
in the field of vision produced by a part of the back of
the eye not adapted for conducting a sensation to the
brain.
The former gives rise to the appearance or sensation
of black, whereas the latter does not give rise to any
sensation whatever, so that the gap is not perceived.
For example, our visual field is not bounded by black ;
on the contrary, it fades away imperceptibly into
nothingness in all directions. Again, the head of the
optic nerve or papilla (ON, Fig. 6) occupies a space in
the field of vision about the same size as that of the
macula area (see bottom of page 37). It is known as
THE EYE COMPARED WITH A CAMERA 55
the blind spot, and is situated a little to the outer side
of the line of regard. Although light of every colour
reaches this spot at the back of the eye, it does not
appear as a black or white disc in space, since we are
unconscious of any defect there. If, however, we shut
the Right Eye, and hold our right finger about a foot
away, exactly in the line of vision, and then place our
left forefinger close to it, and move it slowly outwards
to the left-hand side (without moving the head or line
of regard), the top of the finger will suddenly disappear
and then reappear again. This is due to the gap in
the field of vision due to the blind spot. Another
simple experiment which shows the same thing is to
draw a black cross and a dot on a sheet of paper,
about the distance between the pupils apart (61 mm.
or 2f in. apart).
FIG. 7.
Close the right eye and hold the paper so that the
round spot is in the line of vision of the left eye, about
12 inches away. If now the paper is slowly with-
drawn or approached towards the eye a position will
be found at which the cross becomes invisible. On
withdrawing or approaching the paper from this
position the cross will again become visible. By this
means the area of the blind spot can be calculated or
mapped out. It will be found to be about 2-25 mm.
in diameter.
CHAPTER IV
§ 24. The Sensitiveness of the Photographic Plate
as compared with the Eye to different
Parts of the Spectrum
IN order to demonstrate the effect of different colours
of the spectrum on a sensitive plate, we may draw a
Ked. Yellow. Green. Blue. Violet.
Fraunhofer lines
in the solar
spectrum.
Prism dispersion.
The luminosity
of the spectrum
expressed as a
print from a
perfect negative.
Same as No. 2
expressed as a
curved line.
FIG. 8.
curve on squared paper in which the height of the curve
(ordinate) represents the intensity of the light, while
THE SENSITIVENESS OF THE PLATE 57
the distance it. is projected horizontally (abscissa)
represents the scale of wave-lengths of the different
colours of the spectrum.
In Figs. 8 and 9, a curve is shown representing the
sensitivity of the eye to different parts of the spectrum
of sunlight. It will be seen that the curve rises rapidly
from the red towards the yellow, and slopes very
gradually on the blue side, there being hardly any
intensity at the blue end.
Red. Orange. Yellow. Green.
Blue. Violet. Ultra-violet.
65O
6OO
500
400
JffO
JOO
FIG. 9. — Luminosity Curve of Eye (dotted line). Ordinary Non-
colour Sensitive Plate (plain line). The numbers express
the wave-length of light in micromillimetres (^u).1
Now, when a photograph of the solar spectrum is
taken with an ordinary plate, we shall find the curve,
which is altogether wanting at the red end, rises to a
maximum in the blue, where it is prolonged far beyond
the range of visibility of the human eye (Fig. 9).
By bathing the plate with one of the isocyanine
dyes (pinacynanol) it has been found possible to extend
the curve of sensitivity into the red (Fig. 10), and pro-
duce what is known as a panchromatic plate, which
1 A micromillimetre or p.^ = 1,000,000th part of a millimetre.
Some writers express the wave-length in terms of Angstrom
Unit (A.U.), which is the ten-millionth part of a millimetre.
Thus, 500 w may be written 5000 A.U. See Appendix, p. 285.
PHOTOGRAPHY IN COLOURS
must be used for all methods of colour photography
(excepting the two-plate method).
A plate dyed with eosin or erythrosin (and thereby
rendered more sensitive to green and yellow rays) will
Red. Orange. Yellow. Green. Blue. Violet. Ultra-violet.
JOO 650 GOO 55O 50O 450 40O 36O 300
FIG. 10. — The Spectrum Curve of a Panchromatic Plate.
render colours sufficiently true to nature for ordinary
purposes. Such a plate is termed iso-chromatic or
ortho- chromatic (Fig. 11).
Red. Orange. Yellow. Green.
Blue. Violet. Ultra-violet.
/^
^
\
/"
^-^
"^_
x
\
00 ff$0 600 SSO SOO 450 400 35O SO
FIG. 11. — The Spectrum of an Isochromatic or Orthochromatic
Plate.1
In order to render these two kinds of plates more
effective, it is advisable to restrain the excessive action
1 As it is rather difficult for the beginner to fix in his mind
the numbers which correspond to the various colours, Dr. Mees,
to whom photographers owe so much for his lucid pamphlets on
this subject, has suggested an admirable mnemonic, which is as
follows : —
The wave-lengths for Blue-violet lie between 400 and 500 p/t
„ „ Green „ 500 and 600 ^
„ „ Bed „ 600 and 700 /*/*
THE SENSITIVENESS OF THE PLATE 59
of the blue-violet rays by the use of a yellow or
green colour-filter. When these means are employed
the curve of sensitivity of the plate will approximate
more nearly to that of the human eye.
§ 25. Purkinje Phenomenon. — The intensity of a
coloured light is largely the result of the admixture of
mixed (or white) light with it, so that the more intense
the coloured light, the whiter the colour becomes to the
eye; and conversely, the feebler the light, the more
does it approach black in shade. But this latter
change is by no means equal for the three primary
colours. If two different parts of the spectrum, say
the blue and the red, be illuminated so that the bright-
ness of each is the same, and then the common light
is gradually reduced, the brightness of the two colours
will no longer be equal. The long red waves become
invisible much sooner than the blue- violet ones. As
the light gets more and more feeble the spectrum
becomes less and less visible towards the red end, so
that the violet-blue and greenish-blue are the last
colours to disappear, so that one can recognise objects
of these colours by their bright shimmer or sheen,
whereas the red, orange, and other colours appear
dark purple, grey, or black. This change of colour is
known as the Purkinje Phenomenon, from its dis-
coverer. It shows us why the sky still appears blue
in the twilight after all other colours have died away.
If one goes into a picture gallery at dusk all the red
and orange colours appear nearly black, while the blue
colours, although " washed out," appear whitish and
light in colour. If, on the other hand, the light be
increased more and more, the reds and yellows will
60 PHOTOGRAPHY IN COLOURS
become gradually more visible until they overstep the
greens and blues, and ultimately become the brightest
of all the colours.
Mr. T. E. Goodall has pointed out to me that a
convolvulus growing on the western wall of a house is
a rich blue in the morning, because it receives only
diffused light, but, as the sun works round to the west,
the colour ultimately attains a magenta-red just before
the sun goes down.
The probable explanation of the Purkinje phe-
nomenon lies in the nature of the rods and cones of
the retina itself. As we have stated in § 19, the sensitive
layer of the retina consists of an immense number of
rods and cones packed together, so as to form a kind
of mosaic when seen in cross-sections under the
microscope. At the foveal pit (i.e. the centre of the
macula) there are only cones, but the further one
recedes from this spot the fewer are the cones, and the
greater the number of rods. Now there are several
ways of showing that the cones are most sensitive to
the green, yellow, and orange rays (i.e. between 526/x/x
and 566/x/x), whereas the rods are most sensitive to the
blue-green rays (round about 570/x/x.). When one fixes
one's eyes on an object as in direct vision, it is the
cones of the fovea which receive the sharp impression.
This constitutes central vision, or cone vision. On the
other hand, all surrounding objects not fixed with tho
eye, and which in consequence are only imperfectly
seen, are observed by indirect vision, or rod vision, since
it is the rods that greatly predominate. The following
experiment will go far to prove the above statements.
First of all one must have a projection lantern fitted
THE SENSITIVENESS OF THE PLATE 6 1
with an arc light L, and a condenser C (Fig. 12). In
front of this is a dispersing (negative) lens which
renders the convergent rays parallel. The parallel beam
then passes through the two Nicols Nt and N2. Then
the light passes through a round opening in the
FIG. 12.
diaphragm S, and finally through the condensing
lens L, and the direct spectroscope P. In this way a
pure spectrum can be projected on to the screen Q.
The upper half of the screen is covered with two sheets
of red-coloured tissue paper, and the lower half with
two sheets of blue-green paper of such tints that the
red when illuminated is perceptibly brighter than the
blue-green. If now one of the Nicols is rotated so that
the light is gradually reduced, the blue-green field will
become brighter than the red one, until at length the
red field becomes invisible ; while the blue-green field
degenerates into a colourless whitish sheen. On
covering the big sheet with black velvet and cutting a
small square hole out of it in the
centre (see Fig. 13) a small piece
of the blue-green and red is to be
seen through it. If now, while the
Nicols are arranged so that the
red is just a little brighter than FIG. 13.
the blue-green, and while one is
gazing at the hole, a second person suddenly lifts off
62 PHOTOGRAPHY IN COLOURS
the black velvet sheet, the red sheet will appear quite
black, while the white of the upper (blue-green) half
of the sheet will appear quite colourless, but wonder-
fully bright and glistening like silver.
This effect applies to the photographic plate as well,
for it will be found extremely difficult if not impossible
to photograph reds, yellows, and yellow-greens in their
natural colours in a dull light. This explains why
such a disproportionately short exposure is necessary
when photographing with colour plates in bright sun-
shine, and such a surprisingly long exposure (compared
with the proportionate exposure for ordinary black and
white pictures), when photographing with colour plates
in a very dull light (see exposure table in the Appendix
in proof of this).
CHAPTER V
METHODS OP OBTAINING PHOTOGRAPHS IN COLOUR
§ 26. Lippmann's Interference Method.— Among the
various methods that have been tried to produce
photographs having the natural colours of the original,
none has excited more scientific interest than that of
Prof. Lippmann, first shown in 1890, for no pigments
of any kind are employed, but the colours are produced
by the interference of light reflected by thin films. A
very fine-grained, translucent plate is employed, which
is placed, glass side towards the lens, in a special dark
slide. This is filled with mercury, so as to form a
mirror in contact with the film.
Let us suppose (Fig. 14) a series of parallel (plane)
waves to be refracted through a lens on to such a plate
in the camera. The waves will pass through the film
and be reflected at the surface of the mirror.
These waves will, on their return, engender a number
of stationary waves, owing to their neutralising the
opposing waves. Consider a point P in the film, which
is at a distance x from the mirror. Then the waves
about to proceed to O from P will encounter the waves
which have been to and are returning from O, so that
at P we have two sets of waves, the direct waves and
the reflected waves. They both started in the same
64
PHOTOGRAPHY IN COLOURS
phase at P, but the reflected waves have travelled a
distance equal to 2# further. Moreover, on reaching
the mirror, they are found to differ in phase by half a
wave-length, because the reflection took place at the
surface of the denser medium, and, by a well-known
Gelatine emulsion film very greatly magnified.
Q
r
FIG. 14.
law, reflection at the surface of a denser medium causes
a retardation = A/2 in the wave.
Hence the path traversed by the returning wave
= 2x -f- A/2. Now this must be an odd number of
half-wave lengths, provided that PO = wA/2, n being
an even number, since we have the extra A/2 to make
the total an odd one. If this is so, there will be
PLATE V.
Px Tjo; P..X
[Cttpyriyht.
Section of a Lippmann Photographic Film, made through the red end
of the spectrum of arc light ( x 11,000). Prepared and photographed
by Edgar Senior, Esq., and reproduced here by his permission.
Zeiss' Apoch. Obj. 3 mm. Oc. 2; yellow screen. The dark deposits
of silver P, Px, P2, etc., occur at half wave-length intervals x, x, x.
Tofacep, 65.
OBTAINING PHOTOGRAPHS IN COLOUR 65
interference throughout the planes P and Plf because
P! is separated by the distance x from P, and so for
the other planes P2, P3, etc., throughout the film. The
distance between these planes is equal to half a given
wave-length of light for a definite line of the spectrum, so
that these planes will vary for each colour, being closer
together towards the violet end, and wider towards the
red. Wherever the reflected wave meets an incident
wave in the opposite phase, the trough of the wave
will be filled up, so there will be a calm region, and
it will have no effect on the silver bromide at that
spot ; but where it meets its opponent in the same
phase, the wave will be intensified and have a strong
action on the silver bromide particles. If, therefore,
the plate is developed and fixed, there will be a series of
planes of darkened silver particles at regular intervals,
the width being in strict proportion to the length of
the wave, while in between these will be other planes
corresponding to other parts of the spectrum. Dr.
Neuhaus first succeeded in stripping off a piece of
such a film, and making very thin sections, which,
when highly magnified, showed an appearance similar
to Plate V.
The dark lines are due to the reduced silver particles,
the bright lines to planes where no action occurred. If,
therefore, the film, when fixed and dried, be turned so
that the eye sees the film at or near the angle of
reflection,1 the colours corresponding to those of the
1 The angle of reflection is the angle which the reflected ray
makes with a perpendicular line drawn from the surface at the
point of impact. It is in the same plane with, and equal to the
angle made by, the incident ray with the normal (perpendicular
line) above mentioned.
F
66 PHOTOGRAPHY IN COLOURS
original object photographed will be distinctly perceived.
By fixing a shallow-angled prism behind the plate surface
reflections will be got rid of, and the colours will be
brought out much more vividly. In this way, not only
can the solar spectrum be reproduced, but, under favour-
able circumstances, landscapes, flowers, butterflies, and
other brightly coloured objects may be photographed.
Many of the old Daguerreotypes showed traces of the
natural colours, as can be seen in specimens at the
present day if observed at the proper angle, since
the polished silver backing takes the place of the
mercury trough, but it remained for Zenker and Lipp-
mann to give the correct explanation of the phenomenon.
Colours produced by interference were observed as
far back as Sir Isaac Newton, who described them as
occurring when two glass plates are separated by a
very thin film of liquid or air. Thus, if a slightly
convex surface of glass be placed on a perfectly flat
surface, the thin film around the point of contact will
give rise to coloured circles, which are known as
Newton's rings. By measuring the diameters of the
rings, the curvature of the glass may be calculated, or,
knowing the curvature, the thickness of the film can
be found, and if monochromatic light be used, its wave-
length may be calculated also. The colours in the
plumage of many butterflies and birds and the bodies
of beetles, as well as the exquisite tints of mother-of-
pearl and the soap-bubble, are due to interference
phenomena and not to actual pigments. This may be
seen by regarding these structures at different angles,
when the colours will be seen to vary. A still more
familiar example may be observed when tar or petrol
OBTAINING PHOTOGRAPHS IN COLOUR 67
is spilled and spreads over the road, especially if it is
wet, so that the liquid expands in a thin film over
the water.
§ 27. Theory of Colour Formation. — If we pro-
ject transparencies from a set of three-colour negatives
on to a screen by means of three lanterns, we must
place in front of each lantern slide a coloured glass
similar to that used for the corresponding negative, i.e.
we must illuminate the transparency from the red filter
negative with red light, the green with green light, and
the blue with blue light, but when we superpose the
transparent prints, made from each of the negatives,
we must first colour each of these prints not in the
colours used for their respective filters, but in the
complementary colours to these, i.e. in colours which
transmit the other two colours which, added to the
filter, made up white light. Thus the negative taken
through the red filter is printed in a colour transmitting
green and blue, these being the other two colours
which, with red, form white light. This colour is
cyan-blue, the complementary colour to red. It is,
moreover, a light greenish- blue, quite different from
spectrum deep blue. The green filter negative is
printed in the complementary colour to green, viz. a
magenta-pink, and the blue filter negative is printed in
the complementary colour to blue, viz. canary- coloured
yellow.
The reason is as follows : — In the former case discs
of red, green, and blue lights are overlapped, so that
coloured lights are added to coloured lights (additive
method), but in superposing one print over another we
are adding not lights to lights, but opacities to opacities,
68
PHOTOGRAPHY IN COLOURS
since each additional print abstracts part of the light
transmitted by the first one. Thus, if one paints a
patch of red pigment on a piece of white paper the latter
reflects light of all colours and consequently appears
white, but the patch of red pigment absorbs the green
and blue and only reflects the red light, and therefore the
patch appears darker than the white. If we now paint a
patch of green and another of blue over the red patch,
the latter will appear black and not white, whereas if
light is transmitted through transparent discs of three
primary colours in their correct proportions and super-
posed on a screen, the result will be a white disc.
Fig. 15 represents the additive effect of overlapping
the coloured discs of red, green, and blue lights, the
result being a white, or nearly white patch on the
screen, whereas Fig. 16 represents the subtractive
FIG. 15. — Diagram showing
the effects of additive
lights.
FIG. 16. — Diagram showing
the effects of subtractive
colours.
effect of superimposing discs of gelatine films or glass
stained with the complementary colours to red, green,
and blue, the result being a black patch. The coloured
illustration (Plate VI.) shows the effect of the super-
position of the colours, to which the above figures
furnish the key.
PLATE VI.
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OBTAINING PHOTOGRAPHS IN COLOUR 69
In order to make a three-colour transparency, we
must therefore proceed as follows : — A red filter negative
is taken and a contact transparency made by exposing
a plate behind the negative, and developed in the usual
way; the grey-black image of reduced silver is now
replaced by ferrocyanide of iron, the metallic silver
deposit acting as a mordant, the result being a greenish-
blue colour, which fortunately happens to be the exact
complementary colour of the red filter. Two thin
transparent celluloid films are now coated with a
soluble gelatine film containing a trace of bromide of
silver and sensitized like carbon tissue with bichro-
mate of potash. This renders the parts affected by the
light insoluble in water. One of these is now placed
celluloid side down in contact with the green filter
negative, and after the details of the image are quite
visible the exposure is stopped. The film is then
washed in warm water to remove all the unaffected
gelatine, fixed in " hypo," washed and dried. It is
then dipped in a crimson -pink dye bath, so as to get
a pink print, the complement of the green filter.
Lastly, we obtain a print of the blue filter negative
with the remaining film celluloid side down, and, after
treatment in the same way, it is stained a bright yellow,
which is the complementary colour of the blue ; when
these two prints are dry they are mounted together in
correct register. The pink print is cemented on to
the greenish-blue transparency, and the yellow print
on the top of all, the films being placed face downwards.
This is necessary, since the greenish-blue print on the
glass is a direct print made by placing the sensitive
side in contact with the film side of the negative,
70 PHOTOGRAPHY IN COLOURS
whereas the two celluloid films are printed through
the back by placing the celluloid side next to the film
of the negative, and both are turned round on finally
placing them together. This not only secures the
prints being all turned the right way, but the two most
important components, viz. the greenish-blue and pink
are mounted in actual contact, while the third (yellow)
print is only separated by the thickness of one film of
celluloid, which does not affect the results. The three
pictures are then mounted behind glass and used as a
lantern slide, or framed and hung in a window. If
the proper values have been given to the colours, the
final result, whether seen on the screen or examined
by reflected light, is strikingly effective and realistic.
The difficulties attendant on three-colour photo-
graphy, and especially on making all three exposures
at one time and of equal gradation has led to attempts
being made with two colours. Gurtner has invented
and patented a very simple process, which, while
ignoring the red element, still enables one to produce
pictures of natural scenery.1
1 See § 83, pp. 152-153.
CHAPTER VI
SINGLE-PLATE COLOUR PROCESSES
§ 28. Joly's Ruled -Line Screen Process.— This is
essentially a three-colour process, invented by Professor
Joly of Dublin, in 1897, but which for various reasons
has not been taken up commercially. The method is
as follows : —
A glass plate is ruled with a series of orange, blue-
green, and blue lines, about ~so m- apart, and repeated
in the above order across the plate. This triple-
coloured glass is placed just in front of a sensitized
plate, and a photograph of a coloured object is taken
in the camera and developed. The negative may
therefore be considered as composed of three parts,
each corresponding to its particular line. A trans-
parency Is now made by contact, and another plate,
ruled with the same number of lines, is placed in
contact with it, only, instead of the coloured lines
being orange, blue-green and blue, they are now ruled
red, green, and blue-violet, thus corresponding to the
three-colour sensations. The red lines are adjusted
to fall on the image formed behind the orange lines,
the green on the blue-green, and the blue-violet on the
image formed behind the blue image. It is of prime
72 PHOTOGRAPHY IN COLOURS
importance that the lines are in exact register, other-
wise the whole aspect of the picture will be changed.
Therefore the lines on the negative which were behind
the orange lines of the screen, must, when viewed
through the positive transparency, be exactly in register
with the red lines of the second screen, and so for the
other two colours.
The positive and second screen can be placed in
register, and thrown on to a sheet by an optical lantern,
and a facsimile in colours of the original object may be
seen by an audience on the sheet. It is necessary that
the sheet should be at some distance from the audience,
otherwise the lines, being highly magnified, would be
seen. At a little distance away the lines blend, and a
remarkably faithful and brilliant image is seen. If such
a slide be placed in front of a window the colours can
still be seen, but they vary according to whether the
slide is looked at in front or from either side. Thus the
colours of a dress may appear of a rose colour when
observed obliquely from the right-hand side, but a
greenish-blue when seen from the left side of the
picture. This is due to the fact that the positive and
second screen have their corresponding lines in register
when seen from the front, but when looked at obliquely,
parallax is set up, so that on the one side the blue-
green lines predominate, while on the other side the
red are most seen. Since the red and green lines
together produce yellow or orange, the green and blue-
violet, blue, and the blue-violet and red, crimson, it will
be seen that all shades of colour can be reproduced,
although the mixing of lights and the mixing of pigments
will not produce the same results.
SINGLE-PLATE COLOUR PROCESSES 73
§ 29. Comparison between the Various Screen
Plates. — We may divide all the single-plate colour
processes into three classes according to the nature of the
colour-screen. It is unnecessary to describe the methods
of manufacture as they are extremely complicated, and
what the reader really wants to know is how the various
screens affect the appearance of the picture. All the
screens, however, have one feature in common : they
all consist of a pattern composed of the three primary
colours, red, green, and blue-violet, which transmit
their respective coloured lights. By drawing up a table
we can best review all the screen plates that have so
far appeared, omitting only those that have been
abandoned.
It will be seen from the accompanying table that the
plates in Class I. are distinguished from those in Classes
II. and III. by being " regular " in texture, whereas the
latter are " irregular."
A " regular " plate is one which is ruled or impressed
with a regular series of discs, lines, or squares, of which
the Paget and Omnicolore are good examples.
An " irregular " plate is one in which the coloured
particles are strewn haphazard over the plate. The
Autochrome is the best known example of this class.
The " regular " plates all possess extreme softness
and brilliancy of colouring. The colours are very rich
and vivid, and produce a woven silk-like appearance
due to the symmetry of the pattern. If a suitable sub-
ject be selected and the plate correctly exposed and
developed, the result is remarkably fine, and cannot be
excelled (if equalled) by any other kind of plate. On
the other hand, the range of colours, and especially the
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SINGLE-PLATE COLOUR PROCESSES 75
tones and shades, are more restricted, and the colours
less true to nature than in the case of an Autochrome
similarly treated. This is partly due to the fact that
all " regular " plates partake more or less of the nature
of a diffraction grating, and are liable to give rise to
imperfectly formed and impure spectra which interfere
seriously with the general appearance of the picture.
The conspicuousness of these spectra is in direct pro-
portion to the fineness of the rulings. (As discs do not,
like rulings, give rise to spectra, the Thames plate is
free from this defect.) Another drawback is that the
colours tend to change into their complementaries or
into mixtures of one or more of the adjacent colours
when the picture is viewed sideways. This is owing to
parallax, and is specially noticeable in those plates in
which the screen is separated from the sensitive film.
§ 30. Parallax — may be defined as the displacement
of one object with respect to another when viewed from
different positions. If, for example, you place a red, a
green and a blue marble to represent the three coloured
discs side by side on the table at right angles to the
line of view, and put a white marble a couple of inches
in front of the centre (green) one and then look along
the table at it a couple of feet away, the white marble
will hide the green one. If you move a little to the
right it will hide the red one. If you move to the left
it will hide the blue marble. This is due to parallax,
and the amount of parallax increases directly with the
distance between the two planes. It is this parallactic
displacement which enables astronomers to measure
approximately the distance of some of the stars, when
the earth is first at one end and six months afterwards
76 PHOTOGRAPHY IN COLOURS
at the other end of its elliptical orbit as it travels
round the sun.
The Autochrome plate gives rise to a purer and wider
range of colours. As the film is very thin there is no
parallax, and, like all other "irregular plates," cannot give
rise to diffraction spectra even when the transparency
is illuminated by the arc or clear bulb electric light.
The grain is so fine that complex colours such as white,
greys, gold, silver, flesh, ivory, shagreen, etc., can be very
faithfully reproduced, and these moat of the " regular "
plates fail in rendering properly.
§ 31. Jougla's " Omnicolore ' 'Plate.— The colour-
screen consists of a series of parallel blue rulings or
stripes, the intervening spaces being filled by green colour
surrounding red squares (Plate VII.). The breadth of
the blue stripes = 0-04 mm. The breadth of the green
spaces 0-07 mm., and that of each red square 0-05 mm.
Hence, if we divide up the blue stripes into as many
areas as there are red squares or green intervals, we
shall find the ratio of the red, green, and blue areas to
be as 3 : 4 : 5, the blue areas being nearly twice the size
of the red ones, and the green intermediate between the
two. The relative sizes correspond to the apparent
brightness of each of these colours to the eye. This
difference is only apparent, as it is corrected by the
filter, which is of a light canary yellow colour, and is
slightly more rapid than the Autochrome one. The
film is tough.
§ 32. Dufay's " Dioptichrome " Plate closely
resembles the Omnicolore both in appearance, method
of developing, and in results. The " First black con-
dition " (see § 25, p. 59) is very perfectly fulfilled. The
PLATE VII.
Thames " Screen x 100.
Omnicolore." Screen x 100.
To face p. 76.
SINGLE-PLATE COLOUR PROCESSES 77
colours are therefore remarkably true to nature. The
colour- screen consists of a series of parallel green
rulings, the intervening spaces being filled by alternating
red and blue squares (Plate VIII.). The breadth of
the green stripes = 0-06 mm. That of the blue squares
= 0*06 mm., and the red 0*07 mm. Like the Omni-
colore and Thames, the colours, if strong and vivid, have
a charmingly artistic texture as if painted on woven
silk. The plates are said to be more rapid l than the
Autochrome in the proportion of 5" to 6J (see § 39), and
the film will bear rougher handling than the Autochrome,
but less than the Omnicolore or Thames plates. The
colour filters of the Omnicolore and Dufay are practi-
cally the same, and therefore interchangeable, but they
will not be correct for either the Autochrome or Thames
plates, the former having a pinker tint and the latter
a pale yellow one. The positive, owing to its great
transparency, is admirably suited to lantern projection.
§ 33. Thames Screen Plate. — This plate, invented
and made in London by the Thames Colour Plate
Company, has a screen impressed with rows of alter-
nating red and green circular discs. The interstices
are filled in with a violet-blue dye, so that the entire
plate is covered with the three colours, and presents no
blank spaces. See Plate VII.
The colour-filter is a pale canary yellow. Since the
colours are much more transparent than the starch
cells of the Autochrome plate, the rapidity is corre-
spondingly increased. Also a lighter colour-filter being
used, shutter exposures as- short as ^ sec. can be made
1 According to the author's experience they are somewhat less
rapid (see Appendix table).
78 PHOTOGRAPHY IN COLOURS
under favourable conditions with a stop of F/5'6,
whereby objects in motion can be photographed.
§ 34. Combined and Separate Screen Plates
compared. — The Thames Colour Plate Co.1 issue
plates of both kinds, and each has its advocates.
The combined plate in which the sensitive film is
permanently attached to the screen is easier to work,
as the positive is obtained direct by immersion into a
dissolving agent such as acid bichromate. Trouble
from parallax, etc., as described above, does not arise
in the combined plate.
The separate, method (i.e. one in which the colour-
screen is on a separate glass from the panchromatic film)
is perhaps a little more complicated in the working,
since after taking the negative a positive must be
made from it, and carefully adjusted to a screen.
The registration of the latter requires a consider-
able amount of care to perform correctly. Owing to
the slight separation of the screen and film, parallax
readily appears; nevertheless the separate method
has many points in its favour. If the negative is
defective we may substitute another panchromatic
plate and make a second exposure at a very much
smaller cost. Also we may use the negative to print
off on P.O.P. any number of copies, and further, as the
screen causes the entire picture to be broken up by its
almost invisible pattern, the prints possess a peculiar
charm of their own. Enlargements in monochrome
may be made from them, or they can be reproduced in
1 This company no longer issues plates. But an improved
plate is now sold by the Paget Prize Plate Co., Ltd. (see Paget
plate).
PLATE VIII.
" Dioptichrome " Screen x 100.
" Krayn 'r Screen x 100.
To face p. 7 8.
SINGLE-PLATE COLOUR PROCESSES 79
colour, as will be described later. It is obvious that
" irregular" plates can never be worked by the separate
method, as it is impossible to readjust the positive and
screen into register again. In the separate method
one can always use the negative for printing pur-
poses in the same way as any other negative, and
then make a transparency on a slow process or lantern
plate for the purpose of binding up with a screen for a
colour transparency. As the screens are exactly alike
any one of them may be used. The uncombined or
" separate plates " are twice as rapid as the combined
ones. In both cases the films will stand fairly rough
usage compared with the Autochrome.
Quite recently a method of getting rid of all parallax
has been devised by the makers of the Thames plate.
Instead of making a positive transparency and binding
it up in register with a colour-screen, they coat the
colour-screen with a transparent (and consequently
extremely thin) and very slow emulsion — in other
words, a lantern-slide emulsion. One of these is placed
film to film with the original negative and carefully
registered by candle-light in the dark room, a ground
glass being placed in front of the candle to distribute
the light. The registration must le made in the comple-
mentary colours of the subject, e.g. yellow flowers must be
registered as blue, and green leaves as red, etc. An
exposure is then made to strong light and the colour-
screen plate developed and fixed in the ordinary
manner. In this way picture and screen are combined
in a single transparency. • These coated colour-screens
are about to be placed on the market by the Company.
§ 35. Paget Plate. — This make of colour-plate is
80 PHOTOGRAPHY IN COLOURS
produced exactly on the same lines as the Thames plate
already described, in fact it is merely an improvement
on the latter, and like it is issued in two forms, the
separate and the combined plate.
In the former case the panchromatic sensitive plate
and the colour-screen are issued separately and are
placed with their films in contact in the dark slide as
tightly as possible, so that the two films have nowhere
any air space between them, otherwise parallax will be
set up and the result will not be satisfactory. For this
purpose a hinged back is the best, and care should be
taken that the spring which holds the plate down is a
strong one. The glass side of the colour-screen must,
of course, be placed facing the lens. After exposure,
the plate is developed in the ordinary way, fixed, and
dried, and a positive made from it in a printing-frame,
having the glass side of the screen facing the light
exactly as in the former case. After development,
fixing, and drying, the positive is adjusted carefully
behind a viewing colour-screen and bound up like a
lantern slide.
The other form of plate has the screen coated with a
panchromatic emulsion in the same way as an Auto-
chrome or Dufay plate.
The chief differences between the Paget and the
Thames plates are; (1st), the screen is formed of small
coloured squares instead of circles which are slightly
smaller than the latter, being each about 3^ inch in
diameter instead of ^ inch. (2nd) Two screens are
employed, one for taking the negative (taking-screen),
the other for binding up with the positive copy (viewing
screen). These two screens are identical, save that the
SINGLE-PLATE COLOUR PROCESSES 8 1
colours of the squares are not quite the same. The
taking- screen is of a pale indigo colour, with a faint
trace of green, the other also a pale indigo, but in-
clining very slightly towards a brown shade. But
they are very nearly alike.
The filter used is cut from a thin sheet of gelatine
stained with Aurantia, or some such yellow dye. In
cutting out a disc to place between the lenses of the
combination it is advisable to cut it out with a pair of
scissors, holding the gelatine between two pieces of
paper in order to prevent the fingers from marking it,
as the gelatine is very easily smeared by the fingers,
and the marks cannot be removed afterwards. If the
inner flange of the lens be pressed on a piece of paper,
it is quite easy to cut round inside the ring so formed
with a pair of scissors.
The exposure (see Tables) is about Jth or Jth that
of a Lumiere Autochrorne, and corresponds to 15
Watkins speed number; or F24 Wynne, with plate-
screen in position. For an open landscape in good
light at F/8, \ second exposure is enough. For an
open-air portrait (entire figure) in sunlight, 1 second
with F/8, or 3 seconds with head and shoulders only,
in diffused light.
As there is a dip in the spectrum-curve of the plate,
viz. in the region of the green, a developing-lamp may
be used if the light be screened by three sheets of
yellow and three of green Virida paper as for other
single colour-plates, but on no account must a RED
light be used.
§ 36. Development. — Any developer which gives
rich black tones may be employed, but the makers
G
82 PHOTOGRAPHY IN COLOURS
recommend 1 : 30 Eodinal. Development should be
complete in two minutes. It is advisable to cover the
dish over with a card during development, only exposing
the plate to the light for a second (or less) after the
expiration of 15 seconds from pouring on the developer,
so as to know the instant the picture begins to appear.
Once this is recognised, the time can be multiplied
by the factor-number, and the development completed
in darkness. The great point is to obtain a clean,
brilliant (plucky) negative entirely free from fog,
and1 with clear shadows. Otherwise a dull positive
will result, and weak or disappointing colour- effects
when the positive is bound up with the viewing-
screen.
When the negative is dried, a transparency copy is
made by contact in a printing-frame with any Ordinary
plate. The Paget Company issue a special fine-grain
slow plate for the purpose, which has a very thin film
similar to that of a lantern plate. This is developed
in the ordinary way, using a red light. A developer
which gives a dense black image is the best. For this
purpose Metol or Metol - hydroquinone is recom-
mended. The exposure averages about 15 seconds
with an ordinary candle at 1 foot, or 5 seconds with
a 16 candlepower electric light at 3 feet.
It is best to use a hinge double back to hold the
two plates, since there is more room than in the case
of a solid slide ; and, besides this, the metal division
is furnished with a spring which presses the plates
together. If the slides have a very shallow rebate,
or if metal sheaths are used to hold the plates, it is
necessary to procure extra thin taking- screens. These
SINGLE-PLATE COLOUR PROCESSES 83
can now be obtained from the Paget Company in the
place of the thick ones.
The essential point is to get absolute contact between
the two plates, and for this reason a strong spring to
press them together is a sine qua non. If they are
not firmly squeezed together you will hardly get any colour
at all i/i the finished picture.
The makers do not recommend either intensification
or reduction of the negative, so that any modification
in the density must be effected by altering the exposure
or development of the positive.
For fixing, use hypo 6 oz., metabisulphate of potash
J oz., and water 20 oz.
When finally binding up the positives a sheet of
fine-ground glass is recommended. Some trans-
parencies, however, are better without it. When the
viewing-screen is placed behind the transparency, a
coloured moir6 pattern will be noticed. As the screen
is shifted, this pattern will be seen to grow larger and
larger, until it finally disappears. This is -the moment
when the colours are correct.
Notwithstanding all that has been written to the
contrary, there can be no doubt that combined
plates are much easier to work, and give more satis-
factory results than the separate method. Moreover,
they give richer colours with more body, and run no
risk of getting out of register, a fault which is very
difficult to adjust afterwards.
On the other hand, it cannot be denied that the
separate plates possess three distinct points in their
favour. (1) They are three times as rapid as a Lumiere
plate, which fact allows of a moving object being taken
84 PHOTOGRAPHY IN COLOURS
with full aperture. (2) Being very transparent, they
are specially suitable for lantern projection. (3) Any
number of copies can be taken by making fresh
transparency copies and binding them up with new
viewing-screens.
§ 37. Combined Paget Plate.1 — This requires no
special description. It is exposed and developed pre-
cisely in the same way as an Autochrome or Dufay
plate, the picture being reversed in an acid bichromate
solution afterwards, and then redeveloped in full day-
light (see instructions for developing Autochrome
plates).
As the grating is broken up by the ruling which
forms microscopic squares, the spectra are barely
perceptible even with artificial light.
§ 38. The Lumiere "Autochrome" Plate.— This
beautiful process, patented by A. Lumiere et Fils
of Lyons in 1904, depends on a colour- screen built up
of starch grains dyed with the three primary colours,
red, green, and blue-violet, and overlaid with a thin pan-
chromatic sensitized film. The grains are of ordinary
potato starch, varying between O'Ol and 0-02 mm. The
smallest cells are about the • size of a white blood-
corpuscle, so that the grains are just within the limit of
perception. There are about four million grains per
square inch. When an Autochrome slide is projected
on to a lantern sheet the coloured grains are invisible
to the audience a few yards away (see Plates IX.
and X.).
After dyeing, the starch cells are mixed together very
intimately (so as to avoid " clumping ") in the proportion
1 This plate is not yet on the market (November, 1915).
PLATE IX.
Dyed-starch-grain Filter. Three-colour print, copied from a Lumiere Autochroi
plate, showing the dyed grains ; magnified 700 diameters.
To face
SINGLE-PLATE COLOUR PROCESSES 85
of four green to three red and two blue. The layer of
cells is flattened by rollers to fill up the interstices, and
then varnished. Since the three colours combine to
give the effect of white to the eye, an Autochrome plate
should resemble an ordinary dry plate. As a matter of
fact, the screen has a pale salmon-pink colour when
held up to the light.
§ 39. Relative Speeds.— The following table by
Mees and Pledge gives the relative speeds of emulsion
and screens, and the exposure speeds behind their
respective filters for the respective plates : —
Autochrome.
Paget.
Omnicolore.
Dufay.
Emulsion speed (Watkins)
Screen factor ,,
35
12
120
8
22
7
13
5
Filter factor
2
1|
1*
1£
Effective speed
»> •
1|
4
2*
2
so that if the Paget plate requires 1 sec. exposure, the
Autochrome requires about 3 sees., the Omnicolore
plate 2 sees., and the Dufay a little less.
CHAPTEE VII
SINGLE-PLATE PROCESSES DIAGRAMMATICALLY
EXPLAINED
§ 40. Von Hiibl's Diagram. — As some readers can
understand the subject better by means of a diagram
than by words alone, Fig. 26, reproduced from a
diagram by Von Hiibl, shows in diagrammatic section
the action of light passing through the glass and
falling on the three-colour layer of the screen
plate. This diagram may be applied when con-
sidering the case of any of the classes of colour-
screen plates, which, though differing in construction,
are all identical in principle. They all agree in
having lines, patches, or spots of red, green, and blue
in more or less equal proportions over the entire
plate (r. gr. bl., Fig. 17). If such a plate is now
exposed in a camera, with the glass side turned towards
a coloured object, we shall obtain the following result :
A cinnabar red object which absorbs green and blue will
only emit red rays ; these will pass through the red
elements of the screen, and will act on the particles of
silver bromide behind them, and the film at that spot
will become blackened in the course of development.
The red rays will be absorbed by the green and blue
elements of the screen, and so no change will be seen
in the film behind them. If the plate be now developed
PLATE X.
" Autochrome " Screen x 100.
" Warner Powrie " Screen x 100.
To face p. 86.
SINGLE-PLATE PROCESSES EXPLAINED 87
FIG. 17.
88 PHOTOGRAPHY IN COLOURS
and fixed we shall find all the red elements covered
with blackened silver, but the blue and green ones will be
transparent, and seen together will give rise to a bluish-
green colour (see right-hand diagram). If the object
is green, the rays will be absorbed by the red and blue
elements, and the film behind the green elements will
become blackened, so that after development and fixa-
tion the plate will show a purple-red colour. In the
same way, a yellow object (which appears yellow
because it absorbs all the blue rays) emits green and
red rays, and these will be absorbed by the blue
elements and will pass through the red and green ones.
The result will be that the silver behind the red and
green elements will be blackened by the developer, and
the plate will appear blue at that spot. If the object is
a brown one (see bottom space), it will emit a large
number of red rays, a small number of green rays, and
hardly any blue at all, so that the red elements will
allow about half the light to go through, the green
about a quarter, • and the blue perhaps an eighth.
After development and fixing, the film behind the red
elements will appear dark grey, that behind the green
elements a light grey, and that behind the blue hardly
changed at all, with the result that after development
and fixing, the negative will show a grey yellowish-green
colour. The fixed negative, therefore, will always
appear in the complementary colour to the objects
photographed. If after development but without
fixing the negative be placed in a bath of acid
permanganate of potash, or acid bichromate of soda,
the whole of the blackened silver deposit is dis-
solved away. If now the plate is exposed to daylight
SINGLE-PLATE PROCESSES EXPLAINED 89
and redeveloped, the whole of the silver bromide
which has not been acted on originally, will be reduced
to blackened silver, so that the image will now be
reversed, and all those parts which were blackened
during the first development will be now more or less
transparent. In this way the picture will appear in
the colours of the objects photographed. This can be
readily seen by comparing the right and left diagrams
of Fig. 17. A black object will reflect hardly any
light, so that the developed negative will be nearly
transparent in the region of the image if the plate is
fixed. If it is not fixed but exposed to light and
redeveloped, then all the bromide of silver will be
reduced to blackened silver, and the positive will show
a black image. If, on the other hand, a white object
is photographed, the light will pass through all three
colours, red, green, and blue, with the result that the
image will appear quite black on development. In the
reversal bath this image will be nearly completely dis-
solved, so that on redeveloping there will be next to
nothing left to develop, and the image will appear
white. In fact, we may say generally that after the
second development there is nothing left to fix in the
hypo.
§ 41. The First Black Condition. — McDonough
has stated that the perfect screen plate is one in which
the colours are so perfectly balanced that when viewed
as a transparency it is entirely free from colour. This
he calls the first black condition. There is no plate
which entirely comes up to this standard, but the
Dufay and the Lumiere line screens are practically
neutral, while the Paget is a pale greenish-indigo.
QO PHOTOGRAPHY IN COLOURS
Some varieties of colour plates fall lamentably below
this standard, and cannot even reproduce the spectrum
colours so that they can be recognised. Such plates
are worse than useless, and tend to bring the art into
discredit.
§ 42. The Second Black Condition.— If the first
condition be fulfilled, it is further necessary, in order
to produce a perfectly compensated plate, that it should
comply with the second black condition, viz. that the
colour-filter should have such power of absorption that
when a grey object is photographed an identical silver
deposit should be produced behind each of the three
coloured elements. Several makes of plates are satis-
factory in this respect.
§ 43. How the Appearance of White is produced
on an Autochrome Colour- Plate. — It may be ob-
served by any one that if a piece of sensitive film be
stripped off a colour-plate leaving the coloured starch-
grains on the plate, and the latter be held up to the
light and examined, the stripped portion does not
appear white (as one would expect) owing to the
combined sensation produced by the three primary
colours; but, instead, a pale pink, grey, or greenish
tint is seen. Why, then, does a snow-mountain, a
white shirt-front, or a collar, come out a pure white
in the transparency? The reason appears to be as
follows : — If an Autochrome positive be examined under
a high-power microscope, and if the grains be carefully
focussed up, they will be seen as perfectly clear grains
wherever a white object has left its image on the
positive.
If, now, the objective be very gently racked away
SINGLE-PLATE PROCESSES EXPLAINED 91
from the grains by means of the fine adjustment, an
immense number of very fine black reduced silver
particles will be seen covering the field. It is this veil
of fine pigment particles which causes the sensation of
white, and the finer the silver particles, and the more
evenly distributed they are, the whiter will the image
appear. This interesting discovery was first made by
Professor A. Forster, of Berne, in 1910. There can
be no doubt that this is the cause of the impression of
white to the eye. The professor ascribes the white
appearance to the black particles forming a grating
(Easter), but after a careful examination of a great
number of specimens under the microscope, the writer
came to the conclusion that a grating cannot possibly
be formed by these pigment particles, and the action
of the particles in producing the sensation of white
light requires some other explanation. This the writer
has attempted to do as follows : if we take a plate
from which the gelatine layer has been stripped off, we
shall find that the light passes through the red, green,
and blue starch grains in the form of train waves
having regular periods. When, however, the light of
a white object has acted on the silver emulsion of
a plate exposed in the camera, it causes a fine pre-
cipitate of black reduced silver particles over each of
the starch grains. In the transparency, these particles
break up and scatter the light, so that it no longer
reaches the eye as periodic waves, but in the form of
scattered waves of red, green, and blue, which, mixing
together, leave on the eye the impression of white.
This, in the opinion of the writer, is why a white object
can be photographed as white (see § 6 in Chapter I.,
92 PHOTOGRAPHY IN COLOURS
entitled " On the Nature of White Light "). For a com-
plete account of the microscopic appearance see Prof.
Forster's very interesting paper, entitled " Wie ensteht
das Weiss auf Dr. Lumiere's Autochromplatten."
Zeitschrift fur wissenschaftliche Photographic, Band
IX., Heft 9, 1911, Leipsig.
CHAPTER VIII
PRACTICAL DETAILS OF THE WORKING OF SINGLE
COLOUR-SCREEN PLATES
§ 44. Choice of a Plate.— Whatever make of colour
plate is selected, it is advisable to use as fresh a packet
as possible.
Messrs. Lumiere now pack the plates face to face
between black cards, and guarantee the plates good for
at least six months after the date stamped on the box.
My experience in South Africa does not coincide with
this. I find that plates begin to deteriorate even
before the date stamped on the box has expired. This
remark applies to all single colour plates. I have
shown in another place (§ 73) how stale plates may
be revived. Autochrome and Dufay plates are now
packed in boxes bearing the date up to which they may
be used, and it would be well if other makers were to
adopt the same course. Plates used after the date
stamped on the box are found to be less sensitive,
requiring two or three times the usual exposure, and
moreover, the colours are somewhat dull. Still older
plates show other marked signs of deterioration, and
are apt to become veiled, with fog at times.
Each make of colour-plates has certain advantages
and certain disadvantages, and the reader must judge
94 PHOTOGRAPHY IN COLOURS
for himself which to get. We may say, however, that
the Autochrome plate gives the greatest and the truest
range of colours. The Dufay and the Jougla plates are
the most brilliant in the colours, while the Paget
separate plates are by far the most rapid. The latter
make of plates, however, present considerable difficulties
in manipulation, and the reader must not be surprised
if he should fail in getting a good result. The writer
had many failures before he finally succeeded in pro-
ducing a fairly satisfactory picture. Lined ruled plates
are open to the great objection that they form a screen
and thus give rise to diffraction spectra. These spectra
are very noticeable when held up in front of an artificial
light, and the brighter the light and the narrower the
rulings the more conspicuous are the spectra. In the
Dufay Dioptichrome plate the lines are broken up by
squares, and so there is hardly a trace of spectrum
bands, for they cannot be perceived in daylight, but in
the Warner-Powrie positives the spectra are so marked
in daylight that the pictures are practically worthless.
In the Jougla positives the spectra are very conspicuous
by artificial light, but are only slightly noticeable by
daylight. Of course the Lumiere plates exhibit no
trace of spectra since there is only a mosaic of dots.
For the same reason the Krayn plates are free from
spectra, but the squares are very large, and hence the
definition is coarse.
There can be no question that for all-round work
the Lumiere plates give the most satisfaction and are
the easiest to manipulate, but the Dufay plates are
capable of producing the finest results of any plate in
the market. Some of the positives are exquisite in their
SINGLE COLOUR-SCREEN PLATES 95
brilliancy, richness of colour, and sharpness of outline,
while all colours except blue are perfectly rendered ;
but the blues have much too green a tinge, especially
the skies. On the other hand, red, orange, yellow and
greens of all shades are superb in their brilliancy and
delicacy of tones.
§ 45. Processes concerned in making- a Single
Colour- Plate Picture. — The following 27 headings
should be carefully studied by the Amateur who wishes
to obtain a perfect finished picture : —
1. The camera.
2. The lens.
3. Choosing the subject.
4. The insertion of the plate in the slide.
5. The colour filter.
6. Focussing.
7. Use of a hood.
8. The exposure.
9. The dark-room lamp.
10. Processes concerned in the formation of the
colour positive.
11. First development.
12. Ke versing the image.
13. Second development.
14. Clearing and hardening.
15. Intensification.
16. Eeduction.
17. Drying the positive.
18. Varnishing.
19. Covering the positive.
20. Final improvements of the tones of the image.
21. Binding up the colour screen.
22. Defects in colour positives, causes and remedies.
96 PHOTOGRAPHY IN COLOURS
23. Copying of colour plates.
24. Indoor colour-plate portraiture.
25. Lantern projections in natural colours.
26. Resensitising colour screen plates.
27. Repairing light-filters for colour photography.
§ 45 — 1. The Camera. — Any camera which takes
plates, and not films only, may be used, since a colour
plate is exposed exactly in the same way as any ordinary
plate, the only difference being that in colour photo-
graphy a yellow filter is essential, whereas in ordinary
photography it is optional. In the same way any dark
slide may be used, but it is as well to use one that has
plenty of room inside, for it must be borne in mind
that the plate is reversed (glass side towards the lens),
and as the film is exceptionally thin, and liable to be
scratched or torn, nothing should be allowed to touch
it except velvet or smooth tissue paper. If the makers
would only construct dark slides, so that the film side
of the plate merely rested along the edges of a rebate,
enabling the surface of the plate merely to face, without
actually touching, the partition between the two plates,
it would prevent those flaws and green spots from
appearing after development, which so greatly mar the
picture. If you cannot get a slide made like this, it
will do equally well to protect the film by a frame of
cardboard |th of an inch wide, on which the edges of the
plate rest, and then remove the spring of the partition
flap. This is Grant's device, and answers admirably
if the slide is a folding one. , If you have a solid dark
slide, such as Boss and many others provide, it is a
good plan either to cover each side of the card partition
with black velvet, or to gum the edges of two or three
SINGLE COLOUR-SCREEN PLATES 97
sheets of fine tissue paper round the margins of the
partition. In the case of Paget separate plates, the
screen and sensitive plate must be kept tightly in
contact by means of a strong spring, or else the colours
will not be properly formed in the positive.
§46 — 2. The Lens. — One may take a picture in colour
with any lens, but in order to get the finest definition,
a lens should be chosen which will bring not only the
yellow, green and blue, but the orange and red rays to
a common focus. In photography, with ordinary
plates it is not necessary to bring the orange and red
rays to a focus, since the film is not affected by them,
but in all colour work, and in ordinary photography
when isochromatic or panchromatic plates are used,
three rays must be brought to a focus, viz. the red and
the blue and the yellow. There are a large number of
lenses which will do this in a satisfactory manner, in
fact, almost any anastigmat lens will accomplish it,
especially if made by a first-class firm. At the present
day three principal types of lenses are in common use.
The Petzval portrait lens, the Aplanat, which is a cheap
but very efficient lens for ordinary photography, and
the Anastigmat, which is the lens par excellence for all
purposes. It is a much more expensive lens than the
other two, but owing to its superb definition, wide
aperture, great covering power, and to the fact that all
the rays of the visible spectrum are practically brought
to a common focus, it forms an ideal lens for colour
photography. Such lenses are made by all the best
firms — Zeiss, Goerz, Dallmeyer, Boss, Watson, Cooke
(Taylor & Hobson), Beck, Stayley, Aldis, Voigtlander,
Busch, Lacour-Berthiot (Paris), Salmoiraghi (Milan),
98 PHOTOGRAPHY IN COLOURS
and many others. The reader will not make a mistake
if he procures an Anastigmat from any of the above-
mentioned firms.
Which is the best focal length of lens to use ? For
ordinary photography a lens having a focal length
equal to the diagonal of the plate is generally recom-
mended. Thus for a quarter-plate a 5J in. or 6 in.
lens, for a 5x4 plate, a 6J in. or 7 in. lens, and
for a half-plate an 8-in. lens will be found to give
the best all-round results. In colour photography,
however, a lens having a much longer focal length will
be found to give more artistic results. In fact, the
lens suitable for the next larger size of plate should be
used. Professor Miethe goes even further, and recom-
mends a lens of 14 to 17 cm. (5J in. to 7 in.) for a
lantern slide, one of 7 in. to 8 in. for a quarter-plate,
one of 8 in. or 9 in. for a 5 x 4, and one of 10 in.
or 12 in. focal length for a half-plate. The reasons for
this are several. First, there is always a danger of
overcrowding too much on the plate. With a black
and white picture this does not matter so much, as it
can always be enlarged, and this is always done if the
photographer intends to put a lot of work on the print
afterwards. With a colour positive, however, an
enlargement is not only a more difficult and tedious
business, but the picture loses a great deal more by
enlargement than an ordinary negative, for reasons
which we have given elsewhere. Secondly, what one
wants ijj a colour picture are large surfaces of one
colour, rather than a number of tiny patches of different
colours. With a short focus lens this is generally the
case, since the magnification is so small, and one is
SINGLE COLOUR-SCREEN PLATES 99
apt to get a profusion of small coloured objects which
carry the eye all over the picture, instead of the eye
being arrested by the principal subject, as it ought to
be, and thus the general effect is spoilt. Again, with
a short focus lens the background will appear too
small in comparison with the foreground, an effect
which will always destroy the balance and harmony of
the whole. Of course these remarks apply equally well
to every kind of colour photography.
§ 47 — 3. Choosing the Subject. — In choosing a
subject, avoid too much contrast in the lighting. In
ordinary photography deep shadows and brilliant high-
lights often contribute largely towards forming a
harmonious picture, but in colour photography extremes
of light and shade when in large masses and in the
same picture are very difficult to develop properly, the
reason being that the high-lights have to be greatly
over-exposed to bring out detail in the dark parts
and shadows. Now, as we shall see, a greatly over-
exposed subject will be eaten away in the reversing
bath, and leave a thin detail-less picture, while the
under-exposed dark subject will be dull, heavy, and
opaque. For example, if a portrait group be taken
against a very dark laurel or holly bush, the flesh tints,
and white costumes especially, will appear washed-out
and thin after reversal, if development be carried far
enough to bring out the details of the dark leaves. On
the other hand, if the figures be rightly developed the
leaves will come out dense and without detail in the
reversing bath. Of course, in many cases, this latter
adds to the pictorial effect. If, however, detail every-
where is wanted, the only thing to be done is to take
100 PHOTOGRAPHY IN COLOURS
the portrait over again, either with a much lighter back-
ground, or if that is impossible, by using a concentrated
developer to secure detail in the shadows, and long
before the development is finished to pour it off,
rinse the plate, and renew development with a dilute
developer, so as to allow the image to become
strengthened very slowly. In the same way, it is
almost impossible to photograph the interior of a
church and at the same time get good colours in the
windows. This can only be done either by using the
modified developer just mentioned, or else by brushing
over the windows, as soon as development has begun,
with a solution of Bromide of Potassium, so as to
restrain the action of the developer locally. Above
all, it is essential to give the correct exposure for the
dark parts and shadows.
The correct exposure is the key to the
whole position, and the greater the con-
trasts the more necessary it becomes to
get it absolutely right. Then the modifica-
tions in development above described will secure the
desired result.
Groups and portraits often come out better if taken
when fully illuminated by sunshine. This, as the
reader is well aware, is not the case with ordinary
photography, but in colour work, the colours come out
much more brilliantly in full sunlight, and the result
is not a flat picture, as one would expect, but often a
most pleasing effect is thereby attained without any
appearance of flatness.
§ 48—4. Insertion of the Plate into the Slide.—
The plate should be inserted into the slide several feet
SINGLE COLOUR-SCREEN PLATES IOI
away from a very feeble yellow-green or " Virida " light
(see § 53), taking care to have the glass surface facing
the lens, in order that the light may pass through the
starch grain filter before reaching the film. The bright
reflection of the lamp will show at once which is the
glass side, as the coated side does not reflect the light
at all. Most plates are packed film to film, with two
pieces of thin brownish paper between them. These
need not be removed, but a black card * should be placed
on the top of the paper, and then the plate covered
with both, should be placed in the slide. In this way
the film is doubly protected both from dust and
friction, as well as from any stray light. A better plan
still is to have the slides reconstructed so that the
whole of the film except the extreme edges rests against
air only, since the card, pressed by the spring, may act
on the plate injuriously. This prevents contact of the
film, except along the edges, so that there is no chance
of abrasions whereby the water may soak in and give
rise to green or other pigment stains during develop-
ment.
§ 49—5. The Colour -Filters.— In order to give the
correct values to the three colours during exposure,
it is imperative to put a colour-filter either in front of
or behind the objective, so as to reduce the excessive
1 A white card will do just as well, and if placed in contact
with the film will reduce the exposure to about f ths, i.e. if using
no card at all, or a black card, the exposure required is 10
seconds, with a white glazed card the exposure will be 6 seconds.
Be careful not to expose the glazed card to bright sunlight as
the glaze is often fluorescent and the luminous exhalations are
apt to fog the plate slightly. Interference phenomena have no
effect on the image.
102 PHOTOGRAPHY IN COLOURS
action of the blue end of the spectrum. If no filter
at all is used, or unfiltered white light gets access
to the plate, the final image will appear throughout
of a violet-blue colour. Moreover, any ordinary yellow
screen will not give correct values ; in fact, the attain-
ment of the correct colour is one of the difficulties
which all makers of screen-plates have had to overcome,
since not only must the ultra-violet rays be absorbed,
but the correct proportion of the spectrum colours must
be arrived at, so as to get the right balance of colours.
The filter which Lumiere finally adopted (auto-filter)
consists of a piece of glass coated with gelatine, and
stained a delicate rose orange-yellow colour, and pro-
tected by a second piece of optically worked glass. The
Lumiere filters are issued in five sizes, in a square
form. This is an awkward shape, as it takes up too
much room and requires a box adaptor to attach it to
the lens. Both Boss and Sanger Shepherd & Co. under-
take to trim and grind these square screens to circles,
which is the most convenient form, as they can be
fitted either into split rings or solid rings, which screw
on to the flange of the lens, or let into the lens cap with
the front removed. These latter methods allow of the
filter being left on the lens, and one runs no risk of
leaving it behind, or forgetting to put it on before ex-
posure. It is quite easy to shape the square to a circle
one's self. It is only necessary to mark the circle in
ink on one of the surfaces, and then crunch off small
particles of glass through both thicknesses at once with
an optician's edging shanks, until the circle is reached.
A couple of minutes' work on a grindstone will round
it off to a smooth edge. The glass will break if a
SINGLE COLOUR-SCREEN PLATES 103
diamond be used, because the two faces are cemented
together.
Wratten's Kl Filter does fairly well for Thames
plates, but is useless for Autochromes. It is, how-
ever, necessary to supplement the ordinary filter when
exposing upon snow or ice by a second filter, owing
to the great excess of blue-violet light in these cases.
When copying oil-paintings, especially in a gallery,
a lighter tint filter gives better results than Lumiere's
auto-filter, which we have referred to above. Lumiere
supplies one under the name of " Auto-JM filter."
§ 50 — 6. Focussing-.— As the film surface of the plate
is turned away from the lens, it is advisable to reverse
the focussing ground glass (or both ground glasses in a
reflex camera). Then the plane of the film will corre-
spond with that of the image surface of the ground glass,
and whether you put the colour-filter in front or behind
the lens it will make no difference, because the correc-
tion due to the filter is made with the eye when focus-
sing. If no ground glass is used, and the focus adjusted
by the scale, when the filter is placed behind the lens
this will just correct the error, because the filter is
usually 3 mm. thick, and the colour plate 1-5 mm.
Now, the effect of a filter behind a lens is to lengthen
out the focus one-third of the thickness of the filter.
But in this case the light passes through two plates of
glass (viz. the colour filter 3 mm. thick and the sensi-
tive plate 1- 5 mm. thick), the displacement will therefore
be one-third of (3 mm. + 1*5 mm.), or 1'5 mm. behind
the front (or glass) surface of the plate. Therefore, this
displacement of the focus just coincides with the thick-
ness of the sensitive plate (since it is reversed and the
104 PHOTOGRAPHY IN COLOURS
film is now behind), and no adjustment will be needed,
unless a thinner plate be used, say of 1 mm. thick, when
it will still be necessary to rack out about 0'3 mm. If
the filter is in front of the lens it will have no effect on
the focus if the object is at some distance, i.e. if the
lens is in focus near the infinity mark. In this case
the lens must be racked in an amount equal to the
thickness of the sensitive plate (1*5 mm.). If very small-
sized plates are used, a movement of 1 mm. in will suffice.
§51 — 7. Use of a Hood and Effect of Shadows
from Coloured Objects. — It will often be found ad-
vantageous to use some form of hood in front of the
lens, so as to screen off all the light not actually used in
forming the picture. For this purpose one with an
oblong opening, the size and shape of the picture, gives
the best results, as all rays are cut off which do not enter
into forming the picture, and which would interfere
with the purity of the image. In ordinary work this
is of little consequence, but in colour work all reflected
light spoils the purity of the colours, either by shedding
white light (which is a mixture of all the colours), or
one particular colour, on to them. Thus, a green
object casting a shadow on a red object will give rise to
an orange-yellow, or a bright silver teapot on a red
tablecloth will appear splashed with red or pink owing
to the red light reflected from the cloth on to the
teapot. Hence in portraiture it is not uncommon to
see the neck of the sitter appear of a greenish-yellow
colour owing to the reflected light from an adjacent
mass of green leaves being thrown across it, but it
is only right to add that under-exposure is often
responsible for this. Another point is to regulate the
SINGLE COLOUR-SCREEN PLATES 105
exposure so as to give the correct exposure to both sky
and foreground in the picture. This can be done by
means of a flap-shutter, a guillotine (up and down)
shutter, or a black card moved up down by the hand.
You may get a blue sky and clouds with correctly
exposed landscape in several ways. First, you may
give a shorter exposure to the sky, by means of a
flap-shutter or a screen. Secondly, you may over-
expose the whole subject about twice and considerably
shorten the time of development. Or, lastly, you may
lift the plate out of the developer and then pour a little
water containing 1 % Bromide of Potassium over
the sky area of the plate, returning it immediately
to the developer, repeating the process three or four
times if necessary. In fact, generally speaking, the
plate may be treated, under special circumstances, in
much the same manner as in dealing with a similar
subject on an ordinary plate.
§ 52 — 8. The Exposure. — The right exposure is a
matter of great difficulty. This is the more to be de-
plored as the success of thefinishedpicture
depends largely on the correct exposure.
This is so, to a far greater extent than is the case with
ordinary plates. As a rough guide, it may be said
that one should under-expose for subjects
taken in sunlight and at midday in sum-
mer, and considerably over-expose (two or two
and a half times the calculated time) in
dull light, or towards evening, or for
objects in the shade. You may use either
Wynne's or Watkins' or the Imperial plate Actino-
meter, which is calculated on Hurter and Driffield's
106 PHOTOGRAPHY IN COLOURS
numbers,1 or else Wellcome's exposure record. The
latter gives you the Autochrome plate speed with
colour-filter Indoors as 24, Outdoors 12, which speed
we may compare with Imperial Special Eapid or Flash-
light. The Autochrome filter increases the exposure
about five times; the starch grain backing about six
times, while the film is about the same speed as an
Imperial Eapid. If, therefore, you take an Ilford
" Zenith," an Imperial " Sovereign " plate, or a
" Premo Filmpack," and use any one of them with-
out a filter in a good light and give minutes for
seconds, you will get about the right exposure under
similar conditions for a Lumiere Autochrome plate
with filter. Whatever you do, beware of under-ex-
posure. An under-exposed plate can never yield a
perfect picture ; an over-exposed plate, even up to three
times the correct exposure, can be turned into a splen-
did positive if carefully restrained during development
by bromide of potassium, and by diluting the developer
with water. In photographing sunsets rather under-
develop than overdevelop; overdevelopment ruins
sunset effects.
§ 53—9. The Dark-room Lamp or Safelight.—
Since the film of an Autochrome, Omnicolore, or any
other three-colour plate, must of necessity be a Pan-
chromatic one, it is obvious that the same light should
be used for all red-sensitive plates. Formerly the writer
1 H. and D. is equivalent to 1J Watkins, so that to convert H.
and D. into Watkins, add one-half ; or take one-third off Watkins
to obtain H. and D. equivalent. See Appendix, Table 1 for correct
exposures. For Autochromes out of doors use H. and D. 2,
Watkins' Meter 3, Wynne's Meter 11. For Paget plates use H.
and D. 5, Watkins' 3, and Wynne's 18.
SINGLE COLOUR-SCREEN PLATES 107
used a very suppressed dark red safelight, consisting
of two sheets of ruby glass, one sheet of orange-red
wrapping paper, and two of canary yellow fabric, but
a bluish-yellow-green light is unquestionably safer and
less irritating to the eye than a deep ruby red. Be-
sides, as Purkinje first pointed out, blue and blue-green
colours are more readily perceived in a dim light than
red or yellow (see § 25). An admirably safe coloured
paper is now sold, in packets containing yellow and
green sheets, by Messrs. Lumiere, under the name of
" Virida " Paper. For use take three yellow, and _two_
bluish-green papers. Cut thenT to" the size of the
lantern window and place all jdxjogether between two
sheets of plain glass, and slide them into the groove of
the lamp in the place of the usual red filter. The three
yellow papers are recommended to be placed next the
light. The writer uses an extra bluish-green paper
gummed on to a plate of glass, which he places in front
of the lantern until development is half completed.
A developing lamp is now issued by Messrs. Wratten
and Wain wright which has some excellent features.
It is so constructed that only diffused light is seen, the
direct rays from the gaslight or lamp being entirely
blocked out, but those rays which spread backwards
are reflected by a bent sheet of white enamelled metal
through the yellow-green screen. Thus the light is
not only safer, but is much softer and more uniformly
distributed. Wratten's screen consists of a sheet of
glass coated with a bright yellow gelatine film, one
coated with a bright green film, and a thick sheet of
specially tested green paper between the two.
Some operators soak the plate in the developer for
108 PHOTOGRAPHY IN COLOURS
two minutes in total darkness, before adding the alkali
accelerator, which starts the development. This de-
sensitises the plate sufficiently to allow of an ordinary
deep red light being used. Others soak it in a 2 %
solution of soda bisulphite or metabisulphite to which
a little Bromide of Potassium has been added. This
effects the same purpose.
§ 54 — 10. Processes concerned in the Forma-
tion of the Coloured Positive. — The treatment of the
exposed plate, in order to obtain the complete picture,
appears complicated, and is apt to frighten the beginner,
but it is really an exceedingly simple process, and only
occupies about fifteen minutes from the time the
plate is put into the first developer to the completion
of the positive. If you omit intensification, clearing,
and varnishing (none of which are essential), everything
necessary can be done with two solutions, viz. a
developer and a reversing solution, as the same bath
serves for both first and second development.
§ 55 — 11. First Development. — This, as well as all
subsequent operations, excepting the reversal process,
are exactly the same as with other plates. Pyro-
ammonia, Pyro-soda, Metoquinone, Quinomet, and
Eodinal all give excellent results.
The chief thing to remember is that when working
with Autochromes you must cut down the time of the
various washings as much as possible, as, owing to the
extreme thinness and delicacy of the film, it will not
stand long immersion in any liquid, nor will the film
stand a jet of water, such as would have no effect
whatever on an ordinary negative film. With other
colour plates it does not so much matter. The
SINGLE COLOUR-SCREEN PLATES . 109
following method, recommended by Lumiere, is reliable,
and if the tyro follows it carefully he will certainly
correct his under- or over-exposures very materially.
§ 56. Rules for Development. — Clean two white
porcelain or glass dishes. See that the water and
developer are of the right temperature, between 55° F.
and 65° F. This is of the utmost importance. If it is
too cold you will not get the image out properly, if it
is too warm you will get frilling and over-development.
For a quarter-plate or 9 X 12 cm. plate put 40 c.c.
(1 oz. 2 drms.) of distilled water (by preference) into a
cup. Add to it 2-5 c.c. (42 m.) of concentrated Quino-
met developer. Place it by the lamp.
Into a second smaller measure put 7'5 c.c. (2 drms.
8 m.) of same developer. Place it next to the other
measure.
Adjust your watch or clock so that the minute hand
is at a full minute when the second hand reaches
zero. Put the watch in the best light possible. Place
the Lumiere development time-table near the lamp, or,
better still, write it in Indian ink on the outermost
paper of the lamp (see Appendix, Table 9, p. 272).
In a separate dish place the Acid Permanganate
or Acid Bichromate solution. Put it anywhere away
from the lamp/ Place the dark slide in a dry place
away from the light. Light the green lamp, let your
eye get accustomed to the green light, and then note
the position of everything you want before making the
room dark.
When all is ready remove the plate from the
slide carefully. Put the plate film side (lighter side)
upwards in the dish, in almost total darkness, and holding
110 PHOTOGRAPHY IN COLOURS
it in shadow near the lamp; wait until the second
hand is about to reach zero, noting at the same time
the position of the minute hand. Then pour the
developer over one end of the plate, cover the dish
with a card, and rock well, so as to cover up instantly
any islands of film that may form. After 12 seconds
hold the dish nearly at right angles to the light, so
as not to allow much light to fall on the surface,
uncover the dish and watch for the first indications of
an image other than sky. The moment the image
begins to be visible you can let more light fall on the
plate. Then note the position of the second hand, and
immediately pour the 7*5 c.c. Quinomet solution over
the plate while rocking. Put a cover over the dish and
gently rock the dish until the time is up according to
the time-table. If the image fails to appear after 40
seconds, add 22 c.c. of Quinomet. If n,o image appears
at the end of 60 seconds, it will be hopelessly
under-exposed. The only chance is to add an ounce
of water and leave it for another minute. If the
image begins to appear, prepare some fresh developer
(33 c.c. of Quinomet to 1J oz. of water), cover up
the dish and wait until the image is dark enough,
examining it for an instant from time to time, until,
on holding the plate horizontally against the light, the
details of the subject can be clearly perceived. These
details need 'not be nearly as dark as
they should be in an ordinary nega-
tive. Then wash well in a dish, pouring the water
off and filling up again about five times.
If you prefer a Sodo-Pyro or Hydroquinone and
Metol developer you can pour over the full strength
SINGLE COLOUR-SCREEN PLATES III
solution right away, watch for the appearance of the
first trace of image, neglecting the sky, multiply the
time by the factor number, and cover up the dish with
a card until the calculated time is up, then rinse well
in the dark and place in several baths. In developing
the plate there are three marked stages. First, a
gradual strengthening of the image until it reaches
a certain maximum; second, the image gradually
loses strength until it has almost disappeared, owing
to the loss of opacity of the white emulsion. The
darker parts of the image remain, but they appear
transparent ; third, if the development be pushed be-
yond this stage, the image in the clear parts gradually
change into a positive. It is at the second or trans-
parent stage when development must be stopped to
get the finest result.
§ 57 — 12. Reversal of Image. — The negative (still in
the dark) is now put in a clean dish and the acid per-
manganate solution (or acid bichromate solution)
poured over. Eock well for a few seconds. Cover
up the dish for half a minute and then turn up the
gaslight. After three or four minutes remove the
negative (now a positive) and wash in four or five
quick changes of water as before. The image should
be examined from time to time in daylight or gaslight,
and reversal stopped the moment all the details are
fully out, otherwise the high lights will be eaten away
and detail lost in them.
§ 58 — 13. Second Development. — Expose the posi-
tive to bright daylight for a few seconds, or, if at night,
burn a foot or two of magnesium six inches from the film,
holding the plate upright to avoid any ash falling on
112 PHOTOGRAPHY IN COLOURS
the plate, or hold it close to an incandescent burner or
Osram lamp for a minute or two. (If you have no
ribbon, or are called away, shake off the superfluous
water and leave it to dry anywhere in subdued light.
It may then be developed at your leisure.) Then put
the plate in the first developing bath in bright daylight
and leave it there until the positive becomes uniformly
dark, in fact nearly or quite black, by reflected light,
which occurs after three or four minutes. This process
must be very thorough if intensification is required
afterwards, because the necessary hypo bath afterwards
will cause the image to fade if the second development
is incomplete. Einse well under the tap.
§ 59—14. Clearing.— If the image looks dull and
somewhat brownish it may be cleared and rendered
bright by soaking in a weak 1 % solution of Sodium Bi-
sulphite, or the old reversal solution diluted to a pale
colour, about one part to 20 or 30 of water. The
plate should not be left in the bath more than 30
seconds.
§ 60 — 14A. Hardening. — This is optional, but useful,
as it brightens up the picture and hardens the film. Put
the plate in a bath, made by dissolving about half an
ounce of chrome alum to a quart of tap water. It may
be used over again a great many times, or you may
use 1 part of alum, 2 parts citric acid, and 100 parts
of tap-water. This is a favourite bath of the writer's ;
it may be used over and over again. Formalin solution,
1 %, is also recommended. It is essential to use one
of them in hot weather, or if the temperature of the
water or developer exceeds 65° F.
Wash the positive in four or five quick changes of
SINGLE COLOUR-SCREEN PLATES 113
water (3 or 4 sees, between each change). Shake off
superfluous water and let it dry in an upright position.
The positive is now finished, provided it has been
correctly exposed and developed.
§ 61 — 15. Intensification.— Often the colours are
not bright enough, or they are too thin and watery, or
lastly, they are dull, heavy and dense. For the two
former cases intensification will probably put matters
right. In the latter case reduction will increase the
transparency and brightness, giving the transparency
more " pluck." It must then be intensified to bring up
the colours. We will first give the explanation and
then the practice of Intensification.
§ 62. Explanation of the Process of Intensifica-
tion.— If the positive looks weak and the colours
are not as bright as they should be, it should
be intensified. The following diagram shows what
happens. We have seen that in the positive the com-
plementary colours are hidden behind a deposit of
blackened silver. When the positive looks weak the
deposit is too thin, and the contrast between the trans-
parent parts and the veiled parts is not sufficiently
prominent. If, therefore, we intensify the deposit by
adding another coat of blackened deposit to it, the
contrast between the two will evidently become more
vivid. This, in a word, is the rationale, of intensification.
Suppose a bright orange object which consists of a
full measure of red, some green and a trace of blue, be
photographed, the positive, if correctly exposed in the
first instance, will show -in section an appearance
similar to Fig. 18. Here all the bromide of silver
behind the red grains, about half the green and a
i
PHOTOGRAPHY IN COLOURS
quarter of the blue, will have been acted upon and
dissolved. If the plate is under-exposed, the proportions
will be altered. Only half the silver bromide behind
the red, a quarter of the green, and a mere trace of
the blue (Fig. 18) will be reduced.
If now we reduce this deposit by a dilute solution of
acid permanganate we can take a layer of deposit off
the whole of the film, so that we may have about three-
before
after
Intensification
Correct
Under
Over
A.
Exposure
B. C.
FIG. 18.
i).
quarters clear glass behind the red, nearly half behind
the green, and a quarter behind the blue. In this way
we can improve the picture. If the plate be over-
exposed, it is obvious that a much larger quantity of
bromide of silver would be acted on by the light and
reduced to silver by the developer, and as this would
all be dissolved away by the acid permanganate, we
SINGLE COLOUR-SCREEN PLATES 115
should get nearly clear glass behind the red and green,
so that there would be very little left to redevelop, and
a section would resemble C (Mg. 18). If this layer be
intensified by any method, we should again get an
appearance more like D by increasing the deposit over
the green, but much more in proportion over the blue.
In this way the contrasts will bo heightened and the
colours made more brilliant, since the blue which is
the opposing colour is nearly blocked out, and so
allows the red and green to shine unopposed by
their complementary blue. As, however, the deposits
behind the red would also be increased, whereas in the
correctly exposed positive there was none at all, we
should change a dull yellow into a brilliant yellow
instead of a brilliant orange, as it ought to be. This
also shows us how the slightest error in any one colour
will always upset the proper proportions of the other
colours, which will be still more disturbed on any
attempt to freshen up the colours by intensification or
reduction, although either may greatly increase the
brilliancy and beauty of the image.
In the same way any alteration in the intensity of
illumination of the object will modify the colours,
since we have reasons for believing that Purkinje's
Phenomenon holds good for the plate as well as for
the eye. On the other hand, if the total amount of
light reaching the plate is the same in two cases, no
matter how it is portioned out, the result will be the
same. Thus, if you photograph a well-lighted grey-
coloured object with a lens working at F/4, exposing
it for two seconds, and photograph again with the lens
stopped down to F/8 and give four times the exposure,
Il6 PHOTOGRAPHY IN COLOURS
the two plates, if developed together, should give
identical results.
Intensification is best done immediately before, or
after removal from the clearing bath. Either Lu-
miere's Pyro and Silver, or the Mercury and Sulphate
formula gives excellent results; the latter is some-
what more intense but less under control than the
former. Both may be used one after the other if
great intensification is desired. Lumiere's formula
(indicated by the letters F and G in his original
instructions for developing Autochromes) is: F, Pyro
1 gm., Citric Acid l.gm., distilled water 30 c.c. ; G,
Silver Nitrate 1-65 gm., distilled water 30 c.c. (Note,
add G to the water before F, or you may get a pre-
cipitate.) For use take 5 c.c. of each and pour into 55
c.c. of distilled water. Pour over the plate immediately
the solution is made (or the solution will turn black),
rock and examine from time to time until sufficiently
intense, or until the bath becomes turbid. (See Ap-
pendix, Table 11.)
The mercury and sulphite intensifier (see Appendix,
Table 11) is a very safe and reliable one. It may be
varied in several ways after the first bath. You may
use either Sulphite of Sodium or Sulphite of Po-
tassium 5 % solution in water, or you may re-develop
with any developer you choose, except Pyro (which
has too much colour). Ferrous oxalate1 produces a
beautiful blue-black deposit which, by quite obscuring
the complementary colours, will give very vivid tones.
1 Ferrous sulphate, saturated solution 1 part.
Oxalate of potassium do. do. 4 parts.
Leave to settle, and pour lear liquid for use.
SINGLE COLOUR-SCREEN PLATES 117
In this latter case it is well to soak the plate im-
mediately afterwards in a dilute solution of Citric Acid,
to dissolve away any oxalate of lime which may be
formed by the oxalate of Potassium uniting with the
lime in the tap water.
Lastly, Piper and Carnegie's Chromium intensifier
may be recommended.1 The author does not advise
a Uranium intensifier, as it upsets the balance of
colours and results in a mess. After intensification
it is occasionally advisable to use a clearing bath of
neutral Permanganate 1 % solution, which will at
once remove the yellow stains which creep over the
whites during prolonged intensification. If the colours
are not impure the clearing solution may be omitted,
but the fixing bath must be used if the plate has been
intensified by the Pyro-silver method. This, according
to Lumiere, is essential if the plate has been intensified
at all, otherwise it may be omitted.
§ 63—16. Reduction. — This is often needed when
the shadows are too dark and obstruct the colours, or
the high lights are obscured ; also in portraiture when
the hands and face are too brown. It may be effected
in several ways. (See Appendix, Table 12.)
1st. By immersion in the Acid Permanganate or the
Add Bichromate bath, diluted 1 to 10 parts of water.2
The image must be constantly examined and plate
flooded with water the moment the reduction is suffi-
cient.
1 This is sold in tabloid form by Burroughs Wellcome & Co.,
Holborn Viaduct, and all Photographic Chemists.
2 The undiluted bath must not be used for this purpose, or it
will take the whole image away.
Il8 PHOTOGRAPHY IN COLOURS
2nd. By using the Ammonia Persulphate reducer.
3rd. Farmer's solution. — This solution, so valuable
with ordinary negatives, is apt seriously to reduce the
brilliancy of the colours owing to the Hypo dissolving
the bromide of silver unacted on by the 2nd developer
(unless the redevelopment has been very thorough).
If this precaution is attended to the result is most
satisfactory.
The mischief can usually be remedied by reintensifi-
cation. Whatever reducer is used, the action must be
very carefully watched, and it is well to begin with a
weak one.
The positive is then washed and allowed to dry.
§ 64—17. Drying the Positive.— This maybe done
by fixing it on a whirler or in front of an electric-driven
fan, which dries it very rapidly. The plate may, how-
ever, be dried just as well but more slowly by shaking
it a few times, drying the back and placing it verti-
cally against a support. If after a few minutes
any large drops have collected on the
surface they must be shaken off, since,
owing to the restricted washing imperative with these
plates, the surf ace water still contains traces of impurities,
which on evaporation will give rise to faint marks.
One must never attempt to hasten
drying by holding the negative to the
fi r e. If there are any drops of water on the film, these
will become heated and melt the film beneath, leaving
a nearly transparent spot or a blurring of the image.
In any case the experiment is a risky one. Drying
with blotting-paper is also dangerous. Soaking in
alcohol to hasten evaporation may cause the colours
SINGLE COLOUR-SCREEN PLATES 1 19
to run. Hence, natural evaporation is the only safe
procedure. At a temperature of 65° an Autochrome
plate should be dry in 30 to 40 minutes. Omnicolore
and Thames plates will take considerably longer,
owing to the thickness of the films.
§ 65 — 18. Varnishing. — As I have stated, varnishing
is not necessary, but it brightens the image and makes
the colours more vivid, and prevents the heat of the
projection lantern from injuring the film. It requires
an experienced hand to do it properly without leaving
lines and ridges of varnish, which, unfortunately,
are not entirely transparent. If they do occur, the
best plan is to lay the plate in a bath of benzine, when
the ridges will dissolve and disappear. As a rule the
difficulty of varnishing arises from the varnish being
too thick. In this case thin it down with some benzole,
Mclntosh recommends pouring the varnish over the
positive while on a whirler. He. commences the whirl-
ing before it has had time to set. The author believes
that an alum and citric, or chrome alum bath will
clear and harden the film quite effectively, and the
heat of the lantern will not crack the film more than
a varnished non-hardened one. Spotting out should
be done by means of transparent colours, otherwise
whatever colour you use will appear black when you
view the positive as a transparency. You may mix
the paint with a drop of gum or oxgall, to allow of
its biting the glass. The finest sable-hair brush pro-
curable should be used.
§ 66—19. Covering the Positive.— The film should
finally be protected by a plate of glass, and the two plates
bound together with plaster strapping. The ordinary
120 PHOTOGRAPHY IN COLOURS
paper binding strips are not to be recommended.
They take a lot of time to put on. They do not
keep the dust or damp out, which creeps in at the
corners, and they are in many other respects un-
satisfactory. By far the best method is as follows : —
Procure a long strip of black or yellow rubber self-
sticking surgical plaster, which can be obtained at any
chemist's shop. It is sold on flat reels, each holding
about ten yards, and about fin. (1 cm.) wide. In order
to bind, pull out enough plaster to go round all four
sides of the plate. Lay it flat on the table, sticky side
uppermost. Take the positive covered with the pro-
tecting glass in both hands. Place one side carefully
midway between the edges of the plaster. Then lift up
the reel, keeping the plaster on the stretch, and rotate
the positive with its cover-glass over each edge in
turn, keeping the plaster taut all the time, until you
arrive at the point where you began. Cut the tape
close to the glass, and then press the two sides of the
tape against the sides of the glass. In this way
two plates become hermetically sealed, and neither
dust nor damp can get in. The corners can be pressed
down at once, and do not need to be trimmed in any
way. The whole process takes about twenty seconds
to perform, and the result is admirable.
Of course, the same method will do equally well in
the case of lantern slides. Seabury and Johnson make
a good plaster, which is known as Mead's plaster. A
German firm make an excellent black rubber plaster
known as Gummi-Pflaster, which is even better than
Mead's.
A mask of black, brown, or olive-green paper with
SINGLE COLOUR-SCREEN PLATES 121
an opening of any desired shape may often be found
useful to hide any defects or superfluities in the picture.
It is cut to fit the size of plate, and placed over the
film before covering with the protecting glass. When
finished, the picture may be rendered much more
effective by a suitable frame of metal, gilt, or dark
wood, or by one of the many devices now on the
market for screening the picture from outside light, or
examining it by reflection in a mirror.
§ 67 — 20. Final Improvement of the Tones of
the Image.— The protecting cover-glass affords a
means of correcting the general tone of the picture.
Thus, if there is an excess of blue in the foliage, it may
be corrected by using the palest tinted yellow glass. If
the red tone is redundant, a very pale shade of green
(or of cobalt-blue glass if orange), may be used for the
cover. The former will strengthen the yellows, the
latter the blue of the sky. Another good plan is to coat
a separate sheet of glass with a thin layer of gelatine,
which you can stain any colour you please. Thus, for
a yellow tint use a pale-tinted bath of Porrier's Orange
II. For a pink tint, use Carmine. For a blue, Victoria
blue or Turkey blue. For a greenish-blue, Methylene
blue. The aim is always to select the complementary
colour to the one you wish to counteract or modify.
Often charming and totally unexpected results can be
achieved in this way. Thus, if the transparency is too
blue, a little yellow in the coating will produce a
greenish tint, and an orange red will help a sunset,
and so on.
Dyeing the film of the transparency itself has been
recommended by Von Hiibl, Konig, and others, but it
122 PHOTOGRAPHY IN COLOURS
is open to great objections. Thus, you may easily
overstain the film, or the stain may penetrate into
the starch grains with disastrous results, or the effect
may not be pleasing, and cannot be altered.
§ 68 — 21. Binding up the Colour- Screen and
Transparency of Thames or Paget Separate
Plates. — This is easily learnt, but requires a little man-
03uvring. If you place two screens in contact, coloured
side to coloured side, and hold them up to the light,
you will notice the discs of the screens (if you are
using Thames screens) will form a coloured moire
pattern which, as you rotate the one or the other
grows rapidly larger and larger until the pattern loses
all shape and vanishes. If you continue the rotation,
the pattern will again become rapidly smaller. Now
substitute the transparency for one of the screens,
placing the film against the coloured side of the screen
as before, and move it until the pattern just vanishes,
Fix the two with a spring letter clip and hold them to
the light. If the colours are not quite right you must
shift the positive ever so little one way or the other
until, when holding the pair squarely in front, the
colours are correct. Now fix three out of the four
sides with clips and proceed to bind the remaining side
with black adhesive binder. Then place a fourth clip
over the binding and carefully remove the opposite
clip. In this way registration may be secured without
any shifting.
The effect of the picture is greatly enhanced by
having it mounted in a wooden frame of plain black
moulding, about 2J to 3J ins. in diameter, and curved
so as to throw the picture into a recess, as it were. It
SINGLE COLOUR-SCREEN PLATES 123
makes more difference to the appearance of the picture,
than even a heavy broad gilt frame does to an oil painting.
§ 69—22. Defects in Colour- Plate Positives—
their Causes and Remedies.— (1) Under-exposure.
— This may be partly guessed by the length of time which
has elapsed before the image begins to appear. After
development, the image appears incomplete, hard and
without details, buried as it were. After reversal the
positive appears dark, dull, and heavy, and there is
absence of all detail in the shadows. The remedy is
to reduce with weak Acid Permanganate, Farmer's
Solution, or Ammonium Persulphate.
(2) Over-exposure. — The image flashes up instead
of slowly becoming visible here and there and gathering
strength gradually over the plate. When the image is
examined by reflected light after it is put in the
reversing bath, it appears loaded with details and very
dark. After reversal, owing to so much of the silver
being dissolved away, the image appears weak, then
transparent, and much of the detail eaten away.
Remedy. — The moment you find the image coming
up too quickly in the first developer, or the picture
appearing at once, flood the plate with water, and
prepare a freshly diluted developer with 5 to 20 drops
to the ounce of a 10 % solution of Bromide of
Potassium, and renew development, watching the
image carefully. Stop as soon as detail appears in
the shadows.
(3) Yellow Stains and Dichroic Fog.— If the
development be prolonged unduly, or too strong a
developer used, the whites may become stained yellow.
Remedy. — Bathe the plate freely in 1/1000 Neutral
124 PHOTOGRAPHY IN COLOURS
(i.e. non-acid) Permanganate of Potash, followed by
half a minute in a fresh Hypo bath containing Bisulphite
of Soda.
(4) Brown Stain. — Cause. — (a) Immersing plate in
a bath of neutral Permanganate of Potash before all
the developer has been washed out. This is especially
liable if Pyro has been used as the developer. It can
never be entirely got rid of. (b) The picture has been
intensified by the silver method before the developer
has been washed out, or the positive has been left too
long in the Pyro-silver bath. Remedy — Wash thoroughly
and immerse in 10% sulphite of soda. If that fails,
place in the reversal bath diluted 1 to 10 with water
and then into Alum and Citric Acid bath.
(5) Black Spots. — Cause. — Insufficient action of per-
manganate reversing solution, by which small collec-
tions of reduced blackened silver particles remain
undissolved. To avoid. — Examine the transparency
carefully before redeveloping, and if you see any black
spots put the plate back in the permanganate bath for
a moment. Remedy.— Sponge the surface of the film
over, both in the first bath and permanganate bath,
with a soft cotton -wool pad dipped in water. Pick
the spots off lightly with a needle, or, better still,
a fine-pointed penknife. Lumiere advises them to
be dissolved out by means of a fine camel-hair
pencil dipped in strong acid permanganate solution or
in a mixture made up of Potassium Iodide 3 gms.,
Iodine 1 gm., water 50 c.c. This is too delicate an
operation for most people, as the liquid is likely to
spread a little, leaving a washed-out spot much larger
than the black speck. In any case the camel-hair
SINGLE COLOUR-SCREEN PLATES 125
pencil should be made of as few hairs and as fine as
possible. The former method is quite easy, safe, and
effective, therefore why run any risks ?
(6) White Spots may be due to minute thickened
specks of unsensitised emulsion. Remedy. — Scrape off
with the point of a penknife. If due to bubbles, fill in
with colour after varnishing.
(7) Green Spots. — Game. — Abrasions in the film and
access of water, which dissolves out the green colouring
matter and spreads in an irregular circle round the
crack. Also washing the plate for too long a time, by
which the water has soaked through to the green stain
and caused it to run. Remedy.— Cut the green spot
clean out and then paint over space with a drop of
solution of gelatine and let it dry; or put a smear of
gum on the area removed, and fit in a bit of an old
film of the right colour; or, lastly, varnish the plate
after the patches are cut or scraped out. Then fill in
the desired tint with any transparent colour. (N.B. —
Chinese white is opaque to light and appears black in
the transparency.) To prevent green spots use alum
bath or 1/1000 pure Formaldehyde (or 1/400 Shering's
Formalin, which equals 40 % of pure Formaldehyde).
Either of these baths may be used before or imme-
diately after first development. Also shorten the
washing processes as much as possible, see that the
water is not warm, and above all never put off intensifi-
cation or reduction until after the positive is dry, if you do
you are sure to get green spots (see Appendix 16).
(8) Red Spots frequently occur in Omnicolore plates.
Remedy. — Pick them off with a fine-pointed penknife
or needle.
126 PHOTOGRAPHY IN COLOURS
(9) General Violet Tone — Causes. — (i) Access of
traces of white light to the plate which have not passed
through the lens filter, or through the lens itself, e.g.
a pinhole in the bellows, or chink in the slide, or space
between the slide and back of camera, or minute traces
of white light reaching the plate before or after exposure,
or in the dark room.
(ii) The yellow filter is too small, and allows light
to creep in round the edges when placed in front of
the lens.
(iii) The yellow filter may have been forgotten,
(10) General Blue Tone. — (i) Under-exposure ; (ii)
Under-development. This latter may be used locally
to give a blue sky and aerial misty effect.
(11) Veiled Fog.— (i) May be due to any of the
causes of general violet tone ; or to (ii) too much light
in the dark room, or plate exposed too much to the
light. If the light reaches the film side first, it pro-
duces general grey fog in the first developer. If it
reaches the film through the starch grains, a fog, the
colour of the light which reached the plate, will appear.
Thus, if light from a red lamp reached the film through
the starch grains screen, only the red rays would pass
through and the fog would be red. In the same way
a green light would cause green fog. Remedy. — These
all disappear in the Permanganate Bath, but it has a
tendency to thin the plate down and to degrade the
colours. A good plan is to bathe plate for 1-2 minutes
in the following solution —
Bichromate of Potassium . 1-2 gms. (12-20 grs.)
Hydrochloric Acid .... 0*35 c.c. (6 minims)
Water 100 c.c. (3£ ozs.)
SINGLE COLOUR-SCREEN PLATES 127
If the positive shows the colours too weak, intensify
by one or other of the methods given in the Appendix.
(12) If the Brightness of the Colours disappears
the moment the Plate is put in the Fixing Bath. —
Cause. — The second development was either too short,
or else the developer was too weak or too cold (below
55°), the result being that some of the Bromide of
Silver which ought to have been reduced by the second
developer has not been acted on, and becomes dissolved
away by the Hypo, leaving a flat, weak image. It is
well, therefore, if you intend to use a Hypo bath, to
leave the plate in the second developer at least three
or four minutes.
(13) Frilling or Blisters.— Cause.— (i) The use of
water at too high a temperature (above 65° F.); (ii)
Careless handling; (iii) Differences in temperature of
baths. The water may be cool enough, but the dark
room much too hot. Remedy. — Chrome alum, or
Formalin solution after first development. Also ensure
that the baths are all about the same temperature.
Autochrome plates are much better coated than when
they were first issued. They rarely frill or blister now,
unless the water or room is above 68° F., and green
spots are seldom met with.
(14) There may be a General Reddish Tint.—
Causes. — (i) Over-exposure; (ii) Too prolonged wash-
ing (should never exceed five minutes) ; (iii) The red
light getting access to the film through the starch
grains or grating. Remedy. — Bind up with a pale
greenish-blue cover-glass..
(15) If the Film is Scratched or Broken.—
Probable cause. — Friction of springs or card against the
128 PHOTOGRAPHY IN COLOURS
film. Preventative. — Do not put the plates into the
slide until absolutely necessary. Glue a raised border
round the card, or place a piece of brown or dark
coloured tissue paper between the film and the card.
(16) If the Background of a Portrait or Group
is of a Dirty Brown or Grey Colour (or the
Picture appears clogged up or opaque). — Cause. —
Under-exposure or over-development. Development
has been forced with too strong a developer, or pro-
longed to bring this detail out, resulting in a dirty,
heavy, impure drab. Remedy. — Take the portrait over
again with more exposure and weaker developer.
(17) If the Face of the Portrait appears Thin
and Eaten away. — Cause. — Over-development
brought about by trying to bring out details in a dark
background. Remedy. — Nothing can be done to improve
the positive. Expose another plate with a lighter back-
ground or one better illuminated.
(18) The Picture looks Thin and suffers from
Want of Detail, especially in the High Lights.
— Cause. — (i) Over-exposure; (ii) Over-development
in first bath. Remedy.- — Take the picture over again,
with less exposure.
(19) The Picture looks Dull and Opaque.— Cause.
— (i) Under-exposure', (ii) Under- development in first
bath. Remedy. — First reduce with Acid Permanganate,
or the Persulphate, or Farmer's reducing bath (see for-
mulae in the Appendix), then intensify with Lurniere's
Silver and Pyro bath (F. and G.), and repeat two or three
times until the image is sufficiently dense. Bleaching,
followed by Bisulphite of Soda or Quinomet, is very
effective, but it is somewhat risky, as it is liable to
SINGLE COLOUR-SCREEN PLATES 129
alter the tone of the colours and the contrasts. For
the same reason Farmer's reducer might upset the
balance of colours by forming a Silver Ferricyanide,
which is somewhat irregular in its action. Lumiere
wisely recommends dilute Acid Permanganate for
Autochrome plates, but Farmer's solution is better for
thick films (e.g. Omnicolore, Thames, and Dufay).
(20) The Colour has nearly all disappeared in
Places, and the Positive resembles an Ordinary
Positive.— Cause. — (i) The film has contracted through
the heat of an illuminating lamp, and slightly shifted
the register of the picture on the starch grains. The
plate has been dipped in an acid bath which has
decolorised the starch grains. Too strong Citric Acid
or too long immersion in it will take out the colour.
Remedy. — (ii) None. Take the photograph over again.
(21) The Positive after Varnishing shows a
Number of Red=orange Spots. — Cause. — Action of
varnish on traces of developer left on the positive when
the film, though apparently dry, still contains moisture.
Remedy. — Dissolve off the varnish with benzole and
immerse plate in 1 : 1000 neutral Permanganate of
Potash. To avoid. — Soak the plate in the above so-
lution before applying the varnish.
§ 70—23. Copying- Colour Plates, i.e. of colour
plates in which the sensitive emulsion film is attached
to the colour-screens.
The reproduction of these plates is carried out in
almost exactly the same way as lantern slides from a
negative. There are two ways of doing it. 1st. The
colour transparency may be placed in front of a
camera and copied through a lens. This allows of
K
130 PHOTOGRAPHY IN COLOURS
the size of the copy being varied. If you have a dark
room and a window facing the daylight which can be
blocked out by a shutter, so much the better. The
author has a large square hole cut out of the shutter,
into which he can -fit plate carriers of various sizes.
The transparency is fitted into a carrier suitable to
the plate, and the latter is kept in position by two
buttons. On the daylight side of the transparency he
has a large board inclined at 45° and covered with a
sheet of white paper. This reflects the light of the
sky through the transparency. On a table facing
the shutter is a long extension camera with an anastig-
mat of large aperture. To copy the same size, the lens
is placed midway between the transparency and the
plate, the two being separated by four times the focal
length of the lens. A colour-filter is placed imme-
diately in front or behind the lens, or between the
combination if made of gelatine. The transparency to
be copied should have the film side facing the ground
glass, otherwise the picture will be reversed ; and of
course the copy must be placed with the glass side
facing the light. The after-treatment of the plate is,
in all respects, similar to that of the original. The
other way is —
By Contact — This requires a very much shorter
exposure than the previous method, but it is open to
the objection that the film of the copy cannot be
brought in contact with that of the original, but
requires to be turned round so that the light traverses
the colour-screen of the copy before it reaches the
film. This may be obviated by using a small but
bright source of illumination. Lumiere recommends
SINGLE COLOUR-SCREEN PLATES 131
a box (ABCD) (Fig. 19). The transparency (0) and
copy-plate (P) are placed in the frame (HI). The film
of the transparency is placed in contact with the glass
side of the copy-plate. The source of illumination is
a piece of magnesium ribbon (M) 2-5 mm. in width and
10 cm. to 20 cm. in length, according to the density
of the transparency. This is folded double and pushed
inside an iron wire spiral (S) (made by winding the wire
round a penholder and then stretching it until each
spiral is separated by a centimetre from the next)
(Fig. 19). The end of the spiral is fixed by a screw
0
tt-P
i?
D A
FIG. 19. — Autochrome Copying Camera.
into the support (G). According to Lumiere the object
of the magnesium in the principal axis of the camera is
to prevent parallax owing to the separation of the film
of the copy from the transparency by the thickness of
the glass support. It is necessary that no other light
should enter the camera, except that of the magnesium.
E is a colour-filter, and V a movable slide.
In the case of a Paget plate in which the screen
and film are on separate 'glasses, the copy is made by
contact with the original negative taken through the
screen. This gives a positive which may be bound up
132 PHOTOGRAPHY IN COLOURS
with a new screen to give the colour, or used as a
monochrome transparency.
In any case, copying a colour-plate from a colour-
plate is not altogether satisfactory, because a third of
all the light only goes through each coloured line or
grain in the transparency to be copied, and of this only
a third of the light, or \ of the initial light, gets through
the second screen, since each coloured bundle of rays
is stopped by two out of the three colours of the screen.
Moreover, in the case of an Autoehrome, whenever a
coloured ray meets a clump of the complementary colour,
the light is all absorbed, and the result is a black dot in
the copy at this spot after reversal.
M. Gimpel first pointed out that whenever an Auto-
chrome made in daylight is copied under the same
conditions with the same light filter, the reproduction
always possesses a predominant tint, which is usually
yellow. This he overcomes by the use of a pale violet
filter. But the difficulty may be overcome in another
way. Instead of reversing and making a positive copy,
the original negative may be fixed in hypo without
reversal. In this case the inevitable predominant tint
is reproduced in the copy in the complementary colour,
and if made under the same conditions, the two will
correct each other.
§ 70A.— Achilla Carrara's Method.— The Auto-
chrome is placed in a Lumiere copying camera (see
Fig. 19), in contact with a panchromatic plate. Instead
of the Lumiere filter he uses a set of analysis filters.
In front of the filters he puts a sheet of fine ground
glass (a fixed-out matt emulsion plate does well). The
luminant preferred is a single filament of a Nernst
SINGLE COLOUR-SCREEN PLATES 133
lamp, 5 inches in front of the filter. He first makes an
exposure through the red filter, An average density
Autochrome requires 30 sees, exposure with the red
filter, and 90 sees, with the green or the blue filter.
The negatives are all developed together with Eodinal
solution 1'22 for five minutes, preferably with the
addition of a few drops of 10 per cent, bromide. Prints
can be made by any three-colour process such as
Eaydex, Autotype, or Sanger-Shepherd's methods.
Sensitising is best done by his quick-drying Bichromate
of Ammonia solution 1-5 of water, of which he takes
5 c.c. and adds to it 35 c.c. of alcohol for a 2 per cent,
bath, 20 c.c. for a 4 per cent, bath, and 15 c.c. for a
5 per cent. bath. This he brushes rapidly over the
tissue, which he pins on to a board, three times in
succession, and then it is hung up to dry. In twenty
minutes the tissue is ready for use.
§ 71—24. Indoor Colour- Plate Portraiture.—
Owing to the prolonged exposure necessary it is ex-
tremely difficult to produce effective indoor portraits in
colours. The difficulty may, however, be got over by
using a flashlight powder in conjunction with a filter
specially adapted to the light and the plate. The
Lumiere Co. provide their " Ideal " Autochrome Flash-
powder, and a special " Auto P.O." filter.
The necessary installation is extremely simple, and
may be completed in a few minutes in a studio or
ordinary room. It consists of —
(1) The " Ideal " Flash-lamp (L) (Fig. 20), which by
means of a pneumatic release fires a cap, which ignites
the charge of Flash-powder.
(2) A strip of white semi-transparent fabric (B) such
PHOTOGRAPHY IN COLOURS
as muslin, placed about 18 inches from the lamp, to
diffuse the light.
(3) A white screen (E), to reflect the light on those
parts of the sitter not directly lighted. (A white fabric
stretched on a frame is suitable.)
FIG. 20. — Diagram showing position of Camera, Sitter, Flash-
light, and Screens.
Copied by permission from Messrs. Lumiere's Pamphlet.
(4) The camera (0), to the lens of which is fitted the
Auto P.O. screen.
(5) A background (F) completes the installation. M
indicates the position of the sitter.
§ 72—25. Lantern Projection in Natural Colours.
This can readily be done with any of the colour
positives, made by the Sanger-Shepherd, Carbon, or
Pinatype processes. The Dufay, Omnicolore, and
Thames Colour Plates lend themselves admirably to
projection, and can be used without further alteration,
or if too large can be reduced by copying to the lantern
size (3J inches square). Autochrome positives are also
SINGLE COLOUR-SCREEN PLATES 135
excellent, provided they are correctly exposed and
developed, but as a rule they are too dense for pro-
jection purposes. In this case they should be reduced,
either by one of the reversal solutions diluted with five
or six volumes of water or by Farmer's method (for
which see tables). If, however, the plates are fully
exposed, or were slightly over-exposed and correctly
developed, they should be quite thin enough. But
with these plates lime or arc light should always be
used. Autochromes are often considerably improved
by intensification, even if the colours seem bright
enough when viewed by transmitted light, because the
enormous magnification reduces the brilliancy of the
original very considerably.
In order to prevent the film from melting or cracking
by the heat of the light, it is well to soak the positive
in chrome alum or Von Hiibl's solution. It is
doubtful whether varnishing is any protection at all.
In any case it is as well to place one or two sheets of
mica immediately in front of the condenser, if you
cannot use an alum trough. (See Appendix 17.)
§ 73 — 26. Re -sensitising Colour- screen Plates.
— If a packet of plates happens to be stale, or if it is
desired to increase the sensitiveness of a plate, it may
be revived in the following way.
Make up the following solution : —
Alkaline solution of Pinachrome 1 : 500 ... 2 c.c.
Absolute alcohol (96 per cent.) 50 ,,
Distilled water 200 „
Immerse the plates tor two minutes in the bath in
absolute darkness, or faint green light. Dry rapidly,
136 PHOTOGRAPHY IN COLOURS
preferably on a whirler, or in front of an electric fan.
If neither of these is at hand, shake the plate rapidly
to and fro for five minutes. If this be not done, the
alcohol is apt to soak through the plate and dissolve
the Methyl Green, or else the plate may become
fogged. As the result of this treatment the reds
become rather too pronounced, and the greys become
a little too warm in tone, but in every other respect
the plate will resemble a fresh one. By this means
the plate will be rendered three or four times more
sensitive. The red tone may be got rid of by using a
Dufay screen instead of an ordinary Autochrome one,
since it is a purer yellow than the latter, and gives a
slightly colder tone, thus correcting the excess of red
which is just what is wanted in this case.
Mr. T. H. Grant re-sensitises in the following way.
He makes a drying cupboard out of a light-tight box
of sufficient size to contain four plates resting against
the sides, and to allow a small box containing calcium
chloride, having a perforated lid, to be placed in the
centre, so as to dry rapidly the plates. A 7 x 5
porcelain dish to contain the sensitiser, some absorbent
blotting paper, and a whirler or electric fan complete
the outfit.
Prepare the following solutions. For a half plate
mix 30 c.c. of alcohol (90 strength) with 90 c.c. of
distilled water, thus reducing the strength to 22 J degrees.
To 80 parts of this add 10 parts of ammonia solution
and 10 parts of the dye solution as supplied by Mr.
Charles Simmen (to be obtained from Mr. Grant, of
the Lumiere Co., 89, Great Eussell Street, W.C.). The
plates are now immersed in the bath for four minutes
SINGLE COLOUR-SCREEN PLATES 137
in complete darkness, the time being ascertained by an
alarm clock, or a watch, observed by a screened- off
dark-room lamp. The dish should be rocked, and the
plates placed on end in total darkness to drain. They
are then whirled or fanned, and afterwards placed in
the drying-box with the calcium chloride until the
morning. To hasten matters, one plate can be whirled
while another plate is soaking in the dish. Such plates
will retain their sensitivity for at least a month after
being treated. Mr. Grant says that with these plates
he can get good results with an exposure of ~ second
at F/5, or ^ second at F/4. (See Appendix 18.)
§ 74—27. Preparing Light Filters for Colour
Photography (von Hiibl). — For a normal Autochrome
filter make the following solutions : —
Nelson's gelatine 1 : 10 . . . 40 c.c.
Rapid filter yellow 1 : 10 . . 12 „
Echt Hot 1 : 10 14 „
For use take 7 c.c. of the dyed gelatine for each
square decimetre of the glass to be coated.
To correct the predominant blueness of neutral tones
in the shadows, and especially snow scenes in sunshine,
and in all cases in which insufficient exposure is
unavoidable, or on dull, cloudy days, use a filter made
according to the following formula : —
Gelatine solution 1 : 10 . . . 40 c.o.
Rapid filter yellow 1 : 10 . . 14 „
Echt Rot 1 : 2000 14 „
The starch grains of the Autochrome, or even the
discs and squares of most of the other screen-plates,
138 PHOTOGRAPHY IN COLOURS
are not noticed when the lantern screen is more than
10 or 12 feet from the spectator.
A fine natural-colour slide is a remarkably realistic
and beautiful object when projected, and is admirably
suited for educational purposes, and making medical
records. Butterflies, flowers, portraits, and views with
plenty of colour in them, are charming subjects and
always command the applause of the spectators.
§ 75. Stereoscopic Effect of Colour Pictures.— I
have repeatedly observed that colour slides show a
marked stereoscopic effect when projected on the sheet,
and many of my friends have remarked the same thing.
It is needless to add that it is not a real stereoscopic
projection, although the illusion is often very striking.
§ 76. Colour Screen Filters for Monochromatic
Light. — These may consist of (1) Pieces of coloured
glass ; (2) stained gelatine, either separate or coated on
glass, and protected by a second piece of glass, as used
for the Autochrome plate ; or (3) thin glass troughs
with parallel sides, containing the coloured fluid and
hermetically sealed.
Most of the following substances or solutions have
been recommended by Professor B. W. Wood. They
correspond to the chief colours of the spectrum, and
ultra-violet and infra-red rays, and will be found to
afford very effective monochromatic light filters for
Microphotography and laboratory work when using the
Mercury vapour lamps.
Ultra-violet light (line 316 to 326) is produced by a
chemically deposited film of silver on a quartz lens or
plate. This filter lens was employed by Wood when
photographing the moon and landscapes by ultra-violet
SINGLE COLOUR-SCREEN PLATES 139
light. The silver film should be of such thickness that
a window in front of a brilliantly lighted sky is barely
visible through it.
Ultra-violet light (line 365) is obtained by a very
dilute solution of methyl violet 4 E (Berlin Anilin
Fabrik) and nitroso-dimethyl aniline.
Deep violet (line 405) may be made with Methyl
violet and Chinin sulphate in separate solutions.
Blue violet (corresponding to the primary colour).
Ammoniated saturated solution of Sulphate of Copper ;
a solution of Sulphocyanate of Cobalt.
Blue (line 436). Cobalt glass (thick) and jEsculine
solution.
Green. Solution of bichromate of potash and a
solution of Neodymium chloride. The Bichromate
transmits the Green and the two yellow lines. The
Neodymium absorbs the Yellow, leaving the Green.
Green (another line 492). A mixture of Guinea green
(B extra) and Chinine sulphate.
Yellow (line 579). A thick layer of Bichromate of
Potash ; or (2) a solution of Eosin and Chrysoidin.
Deep red (line 690). Very dense Cobalt glass and a
layer 1-2 cms. thick of saturated solution of Potassium
Bichromate. This was used by Wood for photograph-
ing infra-red landscapes. A clear blue sky is nearly
black through it, and sunlit foliage comes out nearly
white.
Infra-red. Saturated solution of Iodine in Bisulphate
of Carbon. It is quite opaque to the eye and all visible
rays, but freely transmits the infra-red rays.
A good monochromatic light can be obtained by
saturating an asbestos cylinder with a strong solution
140 PHOTOGRAPHY IN COLOURS
of Chloride of Lithium. A good green light may be
obtained by placing a bead of fused metallic thallium
in a loop of platinum wire in the extreme outer edge of
the flame.
For most work, the Mercury Arc will be found very
satisfactory. (For further details see Appendix 19,
p. 285.)
CHAPTEB IX
THREE-PLATE AND TWO-PLATE COLOUR PHOTOGRAPHY
§ 77. The Theory of Three-colour Photography.
— The process of M. Lippmann is of scientific interest
merely; comparatively few workers have attained
much success with it, and it is only to a very limited
degree suitable for exhibition purposes. Those pro^
cesses, however, which depend on the " three-colour "
principle are daily growing in favour, many of the
positives being of great beauty. There are two forms
of this process, the " subtractive " one which is worked
by Sanger-Shepherd & Co., and the " additive " method,
of which the Lumiere Autochrome, the Omnicolore, and
the Thames plates are excellent examples.
The principle of three colours being used to reproduce
all colours was discovered independently by Fredk.
Ives,1 of Philadelphia (the inventor of the " Half-tone "
process, which has revolutionised the art of illustration
of books and newspapers), and Ducos du Hauron, of
France. It is founded on the Young-Helmholtz theory
of colour vision, elaborated by Clerk Maxwell. As this
theory is of fundamental importance, a certain amount
of repetition will be pardoned. Every colour in nature
1 One of my critics in "Knowledge" (Feb. 1911) denies this
statement, but further investigation of the subject only confirms
what I have stated, viz. that he discovered it independently.
142 PHOTOGRAPHY IN COLOURS
can be formed from one or more of the three primary
colours (or, more strictly speaking, coloured lights), red,
green, and blue -violet. Thus, orange-red light and
green light, when combined, produce the sensation of
yellow ; green and blue, that of greenish-blue, orange-
red and blue, purple; while brown may be produced
by the admixture of much red, a little green, and less
blue. The different shades of these coloured lights
may be produced by varying the intensities of the
mixtures.
A mixture of all three coloured lights in the right
proportion will produce the sensation of white. On
the other hand, the superposition of the same colours
in the form of pigments will produce the sensation of
black, since when combined each pigment absorbs the
colour which otherwise would be reflected by the other
pigment ; hence no colour is reflected and the result is
black.
Ives devised a camera having two reflecting mirrors
by which three negatives of the same object were
simultaneously obtained — one behind an orange glass
or filter, one behind a green glass or filter, and one
behind a blue-violet glass or filter. In a later form of
camera the stereoscopic principle was employed, and
the three pairs of positives were viewed in a stereoscope
which he called a Kromskop.
§ 78. Ives' Kromskop. — By using a triple lantern
Ives superimposed the three coloured images on a
sheet. Now, although true stereoscopic pictures in
relief cannot thus be obtained, since one cannot combine
stereoscopic images on a screen as is done in the
Kromskop, an apparent plastic relief not observed in
THREE-PLATE COLOUR PHOTOGRAPHY 143
black and white slides is obtained. I have heard this
remarked by many people. The effect is even more
pronounced when the picture is observed with one eye
only. Possibly the explanation lies in the fact that
the colour increases the sense of reality in the picture
and enables the mind to supply the plasticity which
experience tells us must exist in the actual object.
This is only carrying a step further the well-known
fact that if we look with one eye through a short tube
at an engraving or painting, it will convey a sense
of plasticity which is wanting when the same picture
is regarded by both eyes.
Fig. 21 shows the essential parts of Ives' Kromskop.
A, B, and C are sheets of red, blue, and green glass
FIG. 21. — Ives' Kromskop, showing how the pictures are
combined.
respectively, on which the three pairs of stereoscopic
positives made for these colours are placed. H, D,
144 PHOTOGRAPHY IN COLOURS
and E are mirrors inclined at 45°. F is one of a pair
of stereoscopic lenses, i.e. of two convex prism lenses,
the prisms having their bases directed outwards, and
the lenses are each of slightly longer focus than the
distance OF.
The mirrors and glass plates are so arranged with
respect to F that the distances AE -f EF, BD + DF,
and OF are all equal, so that the images of each colour
enlarged by the lenses FF will exactly coincide, and
give rise to a single coloured aerial image in stereoscopic
relief at the near point of the observer. This coloured
image appears to stand out in the most vivid relief, and
if the three positives are equally illuminated by an
even light, by means of a fine ground glass placed in
contact with the outer side of the three pairs of posi-
tives, and the colours correctly chosen, the result is
exceedingly beautiful.
A difficulty arose in connection with the two mirrors
marked D and E which Ives overcame in a most
ingenious manner. It was necessary that they should
be unsilvered, since they had to transmit light, and
unsilvered mirrors give at least two images — one from
the front surface, and a second, not so bright, from the
back surface. The difficulty was overcome by using
for each mirror a coloured glass which absorbed the
kind of light which entered it and would otherwise
have been reflected. For example, the mirror E
had to reflect red light from the red transparency A ;
so to quench the red light that entered it, it was
made of a blue-green pot- metal glass that absorbed
red light, i.e. one in which the colouring matter is
spread throughout the substance of the glass, this being
THREE-PLATE COLOUR PHOTOGRAPHY 145
more effective for the purpose than a flashed one.
The only light reflected from B was the red light which
was reflected at the first surface. Similarly the mirror
D was made of chromium green glass which absorbed
the blue from the blue positive B. On the other hand,
the glass D was transparent to the green light from G,
while the glass E transmitted the light from both the
green and the blue transparencies.
§ 79. Colour Filters. — In order to make negatives
for reproduction by an additive method such as Ives'
Kromskop or by any of the subtractive ones to be
described, it is necessary to use colour filters in order
to ensure that only the light actually wanted shall be
recorded on the photographic plate. The colour
filter is understood to mean the coloured glass or
gelatine film placed in the path of the rays to exert a
selective action on the light which falls on the
photographic plate. The colour screen is a
plate covered with coloured dots, grains, or lines,
and is placed immediately in front of the panchromatic
film. Neither " pot-metal " nor " flashed " coloured
glasses are suitable as colour filters, because they
absorb too much lig^it and the colours do not admit
of adjustment, nor can coloured glass of the requisite
quality always be obtained. Gelatine films stained
with aniline dyes are now generally employed, and
these are usually sealed in between two pieces of
glass. The great number and variety of such dyes
admits of almost any desired absorption being obtained.
Some few workers prefer to use the unprotected film
only, and to place it in the diaphragm slot of the lens
fixed on a Waterhouse stop. Such an unprotected film
146 PHOTOGRAPHY IN COLOURS
is very liable to damage, and it is more usual to seal it
between glasses with canada balsam and to bake the
filter at a gentle heat for some days in order to make
the balsam set.
The glass that is used for this purpose has to be
plane and optically worked, or the definition of the image
may be impaired. While it is quite true that the use
of such glass plates introduces a slight elongation of
the focal length, it is generally even safer to employ
thick glasses than thin ones, for thin filters are very
liable to distortion, which renders them useless. Care
should always be taken that the filters are not held
too tightly in their rings.
The position of the filter is a matter of some import-
FIG. 22. — Sanger-Shepherd's Three-colour Plate Camera.
ance. Unless a special camera is used, it is generally
most convenient to fix it on the hood of the lens. In
THREE-PLATE COLOUR PHOTOGRAPHY 147
this position the light has some distance to travel after
passing through it, and therefore it must be made of
good quality glass. On the other hand, the filters may
be of the same size as the photographic plates, and
fitted immediately over these as in the camera depicted
in Fig. 22. Here the optical quality of the glass is not
of the same importance, but they must be very even
in colour and have no bubbles or flaws, as shadows of
these would be reproduced in the image.
It is always advisable to focus the image through
one of the filters, owing to the shift in the position of
the image produced by the filter, considered as a plate
of glass and quite apart from its absorption effects.
There are two effects produced by a plate of glass
under these conditions. The most obvious is the
alteration in the focus. If the plate is placed behind
the lens the image of a distant
object is thrown back by an
amount equal to /^ — ' where
t is the thickness of the plate and /x,
its refractive index. This amount
is approximately one-third the
thickness of the plate, since /A
is always about 1*5. The devia-
tion of the light is indicated in
Fig. 23. On the other hand, when
the plate is in front of the lens
there is no shift of the image
or alteration of focus unless the
object is near. The second effect of a plate of glass
placed behind the lens is to produce a slight alteration
FIG. 23.
148 PHOTOGRAPHY IN COLOURS
in the size of the image, and therefore it is im-
portant to see that all the filters of a set are of exactly
the same thickness, as any variation in this respect
will throw the images slightly out of register ; a serious
matter in large-sized pictures and process work.
§ 80. The Testing of Colour-plate Filters.— The
test which is now usually applied to filters is to photo-
graph a spectrum of white light through them. Nega-
tives of a spectrum are made upon the plate to be used
through each of the three filters, and from these it can
be seen exactly what light is recorded. This method is
also valuable, because it at once shows if any ultra-violet
light is being passed by the filters. This cannot be
seen by the eye, but the plate is very sensitive to it.
The best filters now in use show under this test a slight
overlap between the red and green filters in the yellow,
and between the green and blue filters in the blue-
green. There should be no unrecorded gaps.
Much may be learnt from a simple visual examination
of the filters. If the red and green filters are super-
posed and a bright light is viewed through them, then a
person of normal colour vision should see a dark yellow
colour. The blue and green filters superposed should
produce a dark bluish-green. Although the ultra-violet
light cannot be seen, one can ascertain if the red filter
passes any extreme violet by superposing the blue one
upon it, when only a very deep red without any violet
tinge should be seen. Any red filter that fails to pass
this test is valueless.
§ 81. Making Three-colour Negatives.— When
the light filters have been accurately chosen, a plate is
exposed behind each filter in a camera. It might be
THREE-PLATE COLOUR PHOTOGRAPHY 149
thought possible to take all three pictures at once by
using a wide camera with three lenses, but this is impos-
sible, because unless the three negatives are taken from
exactly the same spot, the copies cannot be accurately
superposed, since the difference in the point of view
would give stereoscopic images which cannot be made
to coincide. Each plate must receive such an exposure
that a white object may be represented by a deposit
identical in position and area in each of the three
negatives. This forms the key to successful printing.
Sets of three-colour negatives may be made with an
ordinary camera, provided that some simple holder is
made to hold the niters one at a time in front of the
lens. The operation consists simply in exposing three
panchromatic plates behind the red, green, and blue
niters successively, and then developing them in the
ordinary way. It is most important that the correct
exposure be given to each plate, so that a scale of greys
is rendered in the same manner in all. To judge of
this, trial exposures may be made on a piece of crumpled
white blotting-paper, and the exposures altered until
the images of it in the three negatives are identical.
A very convenient attachment to a camera for this
class of work is a repeating back having a long dark
slide to hold the three plates, or one long plate, and a
frame holding the niters in front of it. The frame and
the dark slide move along together, so that plate and
filter are both changed by the one movement (Fig. 24).
§ 82. Butler's Three-plate Camera.— Mr. E. T.
Butler has designed a useful camera on the principle of
Ives' Kromskop. It will be found very useful for pro-
curing the negatives for the Sanger-Shepherd method.
PHOTOGRAPHY IN COLOURS
The camera is of the box form and fitted with grooves
to hold three double-backs, two above and one behind
(Fig. 24). The first sensitive plate, F, has a red filter
in contact with it, the second, G, a blue filter, both
made of patent plate, but the third sensitive plate, H,
has none at all. In order that the light, after passing
through the lens should reach the plates F and G two
glass plate reflectors placed at 45° are required. Since
FIG. 24.-
-Diagram of path of light in Butler's Three-plate
Camera.
both the front and back surfaces of a glass plate reflect
light, it would give rise to double images, were it not in
some way prevented. Ives got over the difficulty by
employing thin wedge-shaped reflectors and covering
their backs with coloured varnish. Butler has got over
the double images by employing two reflectors set at
an angle of 45° to the axis in the following ingenious
way. The first reflector consists of bluish-green glass
(the complementary to red) which absorbs the red
THREE-PLATE COLOUR PHOTOGRAPHY 151
light and transmits the blue-violet and green rays only.
Thus the white light after passing through the lens
reaches the first plate and is partly reflected directly
upwards, and partly reflected and partly refracted from
the back surface of the plate. On undergoing a second
refraction into air the ray passes up parallel to the
main pencil. On reaching the red sheet placed im-
mediately in front of the sensitive plate, the green and
blue rays are absorbed while the red rays pass through,
thus all the light reflected from the back of the plate is
absorbed and never reaches the sensitive film at all.
In the same way the second reflector consists of
yellow glass the complementary to the blue, so that
the rays reflected from the back of this plate are
absorbed by the blue filter and only the blue rays
reach the second sensitive film. Thus, again, the rays
reflected from the back of the plate become absorbed
and never reach the film. The remainder of the light,
which is green (since it has lost its red and blue con-
stituents), passes directly on to the third panchromatic
plate, and, of course, requires no filter in front.
The order of the three coloured filters must be so
arranged that each will get its proper share of the light.
Hence the red plate which requires a long exposure
must have the greatest volume of light. It is therefore
placed at F, so as to receive the full reflected beam of
white light. The green filter is really in the brightest
position, since it receives all the transmitted light.
Since a red filter can be made which lets all the red
through, but a green filter cannot be made to let all the
green through, it has been found best to keep the green
for the direct light. Of course, with this camera the
152 PHOTOGRAPHY IN COLOURS
exposure is the same for all three negatives and cannot
be varied as is the case with other cameras.
The negatives which receive reflected light, viz. the
red and blue ones, will be reversed, whereas the green
negative which receives direct rays will not be so. This
may be rectified by turning the green negative film-side
out, taking care to allow for the change of focus owing
to the thickness of the glass support.
§ 83. Two-plate Colour Photography. — The
difficulties attendant on three-colour photography, and
especially on making all three exposures at one time,
have led to attempts being made with two colours.
Gurtner has invented and patented a very simple process,
which, while ignoring the red element, still enables one
to produce charming pictures of natural scenery. He
first takes a chlorobromide emulsion plate, very thinly
coated (in other words a lantern plate), stains — in the
dark in a bath of naphthol orange, or aurantia, dissolved
in water — dries it, and then places it in contact with a
panchromatic plate, film to film. The two plates are
then placed in the dark slide, taking care that the glass
side of the lantern plate faces the lens. The ground
glass is reversed, as is done when taking an autochrome
picture in order to compensate for the thickness of the
glass, and an exposure made in the ordinary way.
The orange lantern -plate absorbs the blue rays, which
act on the plate and form the image, and allows the
red, yellow, and green rays to pass through the semi-
transparent film. These act on the panchromatic film,
and form a second image by the action of the red and
orange-green rays. The orange plate, which has a
dark image when the blue rays have acted, serves to
TWO-PLATE COLOUR PHOTOGRAPHY 153
print the yellow image, while the panchromatic plate,
which gives a dark image under the red-green rays,
serves as the negative for the blue image. A little
trouble is necessary to adjust the density of the orange
stain so as to give the relatively correct exposures for
the two plates. In fixing, the yellow stain dissolves
out. A print is then obtained from the panchromatic
plate, either by making a positive and staining it blue,
or by making a blue print on a ferro- cyanide paper
direct. A positive is made from the lantern plate
(which has now lost its colour) either on a second
lantern plate, or on a detachable celloidin paper. These
copies are best fixed in ammonia, without being toned,
and stained lemon or orange-yellow. The blue and
yellow glass positives are now dried and placed in
position, face to face ; and bound together like a lantern
slide with binding adhesive paper. If a paper print
is required, the celloidin print is squeezed down on to
the blue paper print, after careful adjustment.
The results of this process are often very satisfactory,
and it has the advantage of simplicity, since any
ordinary camera will suffice, no filters or dyes are
needed, as is the case with three-colour processes ; and
only two prints need putting into register. The ferro-
cyanide (blue-printing) paper can be obtained in packets
from any dealer. It is very useful to judge the effect
of a negative, as the prints are fixed in ordinary water,
and are made in a minute.
CHAPTEK X
THREE-PLATE PHOTOGRAPHIC COLOUR PRINTING
§ 84. Colour Prints. — The colour processes hitherto
described only furnish single diapositives, i.e. transpar-
encies in which the picture is illuminated from behind,
and seen by one person at a time, or projected on to a
screen. But people naturally call for pictures which
can be hung up on a wall, or placed in an album, and
seen by reflected light. Such pictures also should be
capable of reproduction. These two problems are by no
means so easy of solution as would appear at first sight,
although there are quite a number of ways by which
they may be accomplished. Some of these methods
yield at best only poor results, while others require an
amount of experience and care which is possessed by
very few persons. The following processes, however,
are quite successful in the hands of careful workers.
§ 85. Practical Details for working the Three -
Plate Method with Butler's Camera.— Mr. Butler
prefers Wratten and Wainwright's Panchromatic plates
for all three negatives, or Wratten's Panchromatic plate
for the red exposure, and the Gem " Tricol " for the
green and blue. But any ordinary plate will do for
the blue just as well.
Having inserted the three plates into the camera
behind the three filters, as described in § 81, proceed
THREE-PLATE COLOUR PRINTING 155
to judge the exposure by a Watkin's or Wynne's
actinometer. If the paper changes until it matches
the dark green sector in any time up to 45 seconds or
even a minute, you can expose as the actinometer
indicates ; but if the time exceeds one minute, double
the exposure. If it exceeds two minutes, multiply the
exposure by three. Sunsets require from 5 seconds to
15 seconds, with an aperture of from F/4 to F/6.
Each colour is represented on each negative in vary-
ing densities of the silver images in the same way as
if each plate were exposed separately, and of course
each print must be made in the complementary colours
to the niters through which the pictures were taken.
Method of Procedure. — Three negatives are first
made in the camera :
No. 1 negative (the red), making the blue print.
No. 2 negative (the green) „ red „
No. 3 negative (the blue) „ yellow „
No. 1 negative is taken direct, and a print made from
it by contact will, of course, face the right way. It is
placed in the enlarging apparatus (glass side to the
lens), and the exposure is made in the usual way to
make a positive. With daylight the ordinary acti-
nometer tint must be used, which may be halved or
doubled. Develop with Metol-Hydroquinone for the
black tone lantern plate which is to record the enlarged
positive. Develop until all details are out, but stop
immediately the high lights begin to veil over. The
positive will be thin, but quite strong enough if the
details show up when the positive is placed on a white
ground. Fix in Hypo, wash well in running water for
a quarter of an hour, then place in an 8 per cent.
156 PHOTOGRAPHY IN COLOURS
solution of Ferricyan^de of Potassium (red prussiate)
until bleached. Wash well for 5 minutes, then place
in an 8 per cent, solution of Perchloride of Iron for one
minute. Wash well for 30 seconds, then place in Hypo
for one minute. Wash again in running water for 5
minutes. Place for a moment in a weak solution of
Sulphuric Acid (a few drops to 8 ozs. of water) until
the yellow stain is removed. Finally wash well for
3 minutes to get rid of the acid. This forms the posi-
tive from which the blue picture is obtained.
No. 2 negative (red printer) is placed in the enlarging
apparatus, glass side to the lens, and a copy made on
a black-tone lantern plate, as was done in the case of
No. 1 negative.
No. 3 negative (yellow printer) is placed glass side
to the lens as before, and similarly treated.
No. 2 and No. 3 are developed for 5 minutes to form
the positives, so as to give good printing density. Fix,
wash, and dry, and use for printing the positives.
Printing plates are now made from these three lantern
plates by the Pinatype method (q.v.) by sensitising
gelatine-coated plates in a solution of 1-25 per cent, of
Potassium Bichromate.
Printing the Plates. — These sensitised plates are
first dried in the dark, and are then placed, film to
film, with the No. 2 and No. 3 lantern plate positives,
and exposed to light. The time of exposure is best
judged by means of an "Akuret" actinometer. One
piece of ordinary P.O. P. paper is placed in the " Akuret "
(in No. -»9 for the red, and in No. 12 for the yellow.
When No. 9 is nearly black No. 2 plate will be almost
rightly printed, and when No. 12 is nearly black No. 3
THREE-PLATE COLOUR PRINTING 157
plate will be right. After washing out the sensitiser,
dissolve out the silver with Farmer's Eeducer, wash
well and place No. 2 printer in the pinatype red dye,
and No. 3 printer in the pinatype yellow dye. The
free dye is now to be well washed out of the print
plate with running water, and then the printing paper
is applied (red preferably first). The time of soaking
in the dye-bath depends largely on the depth of print-
ing. For the red plate take about 5 or 15 minutes.
Examine and wash off the free dye until a vigorous
image is made. If necessary, the plate may be re-dyed.
The yellow should be examined after one or two
minutes, so as not to overdo it. It may, however,
take 10 or 15 minutes. (The yellow image will be
barely perceptible.)
If the Pinatype method be adopted for the red and
yellow, place the print plates in contact with the Pina-
type transfer paper, first with the red print plate, and
then with the yellow one, being careful to keep them
in register.
If a P.O.P. print is to form the base, the red print
must be brought in contact with the P.O.P. image — it
can be seen through the back of the print plate. When
in register, squeegee and allow it to stand, usually for
about five minutes, but you may judge by turning up
the corner. The yellow print is transferred in the
same way. This also takes about five minutes. The
enlarged negative from which the P.O.P. print is made
by contact may be formed as follows : — A positive is
made by contact with No. 3 negative, and placed in
the enlarging apparatus (glass side to the lens), and
a black- tone lantern plate exposed in the usual way.
158 PHOTOGRAPHY IN COLOURS
This paper negative should be rather smaller than the
positives, so as to allow for the expansion of the paper
when wet. By always using the same make of paper,
the amount of expansion can be readily judged. With
Gem Dry-plate Co.'s P.O. P. paper, the necessary altera-
tion can be secured by advancing the plate \ inch
nearer the lens than the position in which all the posi-
tives are taken, and at the end of the enlarger place
the smaller positive ^ inch further from the lens.
The red-yellow print, prepared as already described,
must now be brought in contact with the converted
blue plate. The register can readily be seen through
the back. If the balance of colour is correct, squeegee,
and when dry, the picture may be viewed through the
back of the blue glass. The plate may now either
remain in contact with the print, or the print may be
stripped off, leaving the bare glass — provided, of course,
that the converted blue plate has been taken on a
stripping plate, the glass of which has been prepared
before coating with the emulsion. These plates can
be bought at any of the large houses.1
§ 86. Three-colour Half-tone Process. — It is in
photo-mechanical processes that three-colour work has
found its greatest application, and that chiefly in the
half-tone process. Large numbers of colour prints
are now executed in this manner. The colour plates in
this book may be taken as examples, and if they are
1 This description has been almost entirely taken from Mr.
E. T. Butler's paper read before the iSociety of Colour Photo-
graphers on January 26, 1911, and introduced here by his kind
permission. He supplies the apparatus direct from his address,
see Table 26, Appendix.
THREE-PLATE COLOUR PRINTING 159
examined with a magnifying glass, it will be found that
they consist of dots of the same three printing colours
(yellow, magenta pink, and cyan -blue) as are used in
the other " subtractive " process described elsewhere. -
The use of half-tone printing in this connection has
the great advantage, that once the printing blocks are
prepared, a practically unlimited number of prints may
be obtained. The preparation of these blocks, however,
involves the comparatively complicated process of half-
tone block making.
As it was originally worked, the process consisted in
first preparing the usual three negatives of the subject
through the colour filters, exactly as has been described
for other methods; then from these negatives three
transparent positives were made on ordinary photo-
graphic plates. From this point the operations become
those of preparing half-tone blocks, for the positives
are each in turn illuminated from behind, and a half-
tone screen negative is made from each. In the
making of these negatives a transparent screen with
two series of ruled opaque lines crossing each other at
right angles is placed in front of the photographic
plate, when the varying tones of the positives become
translated in the negative into dots of varying sizes.
This screen must not be confounded with the " colour
screens " described in Ch. VIII. and IX. Such negatives
are printed on to copper or zinc plates coated with a thin
film of bichromated fish-glue, and then washed in cold
water. This dissolves away all the still soluble gelatine
unaffected by the light, leaving a positive print in
insoluble glue which, after being heated to harden it,
acts as a resist to the etching liquid with which the plate
160 PHOTOGRAPHY IN COLOURS
is afterwards treated. The etched metal plate when
mounted on wood or metal, " type high," is called a
" block," and has innumerable dots of varying sizes
standing up to a common level. This can be printed
like type on an ordinary printing press.
This form of the process is still in use for much work
that has to be photographed away from the studio — for
copying pictures in galleries, for instance —but by em-
ploying either fine-grained dry-plates or collodion emul-
sion, and having the colour filter and the half-tone
screen both in position at the same time, the colour
negative can also become a " screen " negative, and
thus the number of photographic operations may be
reduced. By whichever method the three blocks are
produced, they are printed in succession on smooth
surfaced paper, superposed in absolute register, the order
being usually the yellow first, then the red, and lastly
the blue. The yellow ink, being the most opaque, is
printed first, and the blue, the most transparent, last, as
any slight opacity in the second or third printings
would tend to obscure the effect of the inks underneath.
All " subtractive " methods are somewhat at a disad-
vantage compared with screen-plate and other additive
methods with regard to their reproduction colours.
While there is no great difficulty in obtaining the
correct red, green, and blue for " screen " plates, it is,
however, still impossible to procure quite the right cyan-
blue, magenta-pink, and yellow for any of the subtractive
methods, and the three-colour half-tone process suffers
most from this drawback. The cyan-blue ink is
usually the most defective ; it rarely absorbs all the
red light which it should do, while its reflection of
THREE-PLATE COLOUR PRINTING l6l
green light is never all that could be desired. The
magenta-pink ink does not reflect sufficient blue and
violet light. The yellow ink is fairly satisfactory.
As a result, many colours are incorrectly rendered,
greens are too dark, purples and greys too red, and
reds often too orange, and the correct hues have to be
recovered by locally etching the blocks ; " fine etching "
as it is called.
In spite of this disadvantage great strides have been
made with the process, and much good work is done
by it. The comparative inexpensiveness of half-tone
printing is also greatly in its favour.
§ 87.— Collotype Colour Process.— Collotype is a
printing process similar in many respects to lithographic
printing. The prints are obtained in printer's ink
from a reticulated gelatine surface. Sheets of thick plate
glass are coated very thinly with a solution of gelatine
and bichromate of potash. These are dried level in an
oven or large heated drying-box, at a temperature too
high to allow the emulsion to set. When cool they are
exposed under the negatives, and are afterwards
washed in cold water until the bichromate is entirely
removed. The sheets of plate glass are then stood up
to dry. When dry the image, looking similar to a
steel engraving before it is inked up, can be seen on the
plate. The plate is then soaked in glycerine and water,
and this is termed in the trade "etching" it. After
about half an hour's soaking the solution is mopped off
with a dry cloth.
It is easy to understand how the glycerine solution
will penetrate freely through the soft high lights of the
image, while the shadows and half tones, hardened by
M
1 62 PHOTOGRAPHY IN COLOURS
the action of light on the bichromate, absorb it less
readily, or not at all, in proportion to the amount of
light received, so that the image consists of varying
degrees of moisture and dryness. When the inked
(lithographic) roller is passed over the surface, the
greasy ink adheres freely to the dry shadows, but is
refused by the moistened high lights. The reticulation
of the gelatine surface gives an extremely fine grain
over the plate, invisible to the naked eye.
This method of printing can be done on almost any
variety of paper suitable for general printing, and has
the great advantage over half-tone printing blocks
etched upon copper, or other metal, in that an
artificially coated paper with a polished surface is not
essential for the best results. The printing is done on
a " scraper " press, not unlike a lithographic press.
Since a print can be taken in this way with a litho-
graphic ink of any colour, all that is required will be
to make three collotype plates from the three negatives
in the way already described ; then to ink each of them
over with one of the three complementary printing
colours, and finally to take a print on a single sheet of
paper from all three plates successively, taking care
that the three impressions lie in exact superposition.
In practice two main and several minor difficulties
arise. In any process of three-colour printing the
balance of the respective printings must be perfectly
maintained in all the prints taken. Collotype plates
are peculiarly liable to variation with changes in
humidity ; so that care has to be exercised to prevent
the weather from affecting the conditions of the
printing room.
THREE-PLATE COLOUR PRINTING 163
The variable climate of England can scarcely be
said to favour the process. Then from its very nature
there is but little opportunity of doing any local work
on the plates to remedy defects, so that it is very
necessary that both niters and inks should be as
perfect as possible, and whatever adjustments appear
to be requisite must be done on the negative before
making the print on the chromated gelatine. The
inks of the primary colours also do not work so
kindly on the rollers as does black ink, and therefore
it is difficult to print evenly with them. The yellow
tends to clog up the shadows. The red is too greasy
and gives harsh contrasts. The blue is fairly easy to
work with.
The other difficulty is fundamental. We refer to the
superposing of the whole image of each colour. Of
course, each of the three negatives must have been
correctly exposed, but even then the yellow if printed
first is apt to be overwhelmed, and the blue on the top
will have become unduly prominent.
Moreover, considerable experience is necessary in
order to obtain perfect uniformity when any consider-
able number of copies are being printed. It is often
desirable to print a fourth impression made from a
collotype plate obtained from a fourth negative, or
sometimes the " yellow " plate is reprinted in a soft grey.
In spite of its difficulty, some very fine work has
been recently done in colour by this process, and it
has an assured future in connection with colour
photography.
§ 88. Sanger-Shepherd's Imbibition Process.—
This is a practical method, fairly easy of application, and
164 PHOTOGRAPHY IN COLOURS
some what resembles his method for making transparency
pictures. Three negatives are first taken through the
colour filters, as has been already mentioned. The
positives from them are printed upon a special
celluloid film coated with gelatine containing bromide
of silver, sensitised by immersion in the sensitising
bath of potassium bichromate for three minutes and
dried in the dark room.
The prints are made upon the film' by printing through
the celluloid — the celluloid side being placed in contact
with the film side of the negative and exposed to day-
light until, on examination in weak light, all the
details are visible on the film as a brownish-yellow
print, very similar in appearance to an undeveloped
platinotype print. The printed film is immersed in
warm water, and in a few minutes the unaltered
gelatine dissolves away, leaving a perfect white image
full of detail attached to the celluloid base. The print
is next fixed in ordinary clean hyposulphite of soda
solution until the white bromide of silver dissolves,
leaving a transparent, low relief in clear gelatine.
After washing in water for ten minutes, the prints are
ready for staining up. The print from the green filter
negative is stained up in the pink bath, and the print
from the blue-violet filter negative is stained up in the
yellow bath— the staining being stopped as soon as the
two prints, when held over the greenish-blue print, give
neutral tints in the grey shadows of the picture.
Should one of the positives be accidentally overstained
it may easily be reduced by merely soaking in clean
water. They are then successively squeegeed on to a
piece of paper coated with a thin layer of gelatine.
THREE-PLATE COLOUR PRINTING 165
This absorbs the dye from the relief surface of the
hardened gelatine. The gelatinised paper is first well
soaked in water and spread over a glass plate, coated
side uppermost. Then the pink-dyed positive (from
the negative taken through the green screen) is
squeegeed on to the gelatine paper until the whole of
the colour has been taken up by it. In the same way
the yellow-dyed positive (from the negative taken
through the blue screen) is carefully adjusted in
register on to the pink impression, and squeegeed down
on to it. Lastly, the blue-dyed positive is squeegeed
on to the pink and yellow image, which is kept wet to
get an even impression. If any one of the colours is
too weak, the printing plate for that colour may be re-
dyed and used again. Thus a paper print is obtained
on which an image built up of three colours is impressed.
This may be squeegeed on to ground glass or polished
glass, according as to whether a matt or glossy surface
is desired. The print is now finished, and if the process
has been correctly carried out, especially the correct
exposures in the first instance, the result will be an
extremely charming effect of colour. The skies are
often very fine, indeed, much superior to autochromes,
as the proper rendering of the sky is the chief defect
in the starch-grain method. According to Sanger-
Shepherd, the following are the chief sources of
failure, with their remedies : —
The printing plates are liable to stick to the paper,
because the paper has not been soaked long enough
before use. It should be soaked in clean, cold water
for at least ten minutes.
The printing plates take up the colour all over when
1 66 PHOTOGRAPHY IN COLOURS
immersed in the colour bath. This is owing to an
over- printed relief ; the relief must be thin. The best
results are obtained by slow development in water at
100° F. to 105° F. It is because of the necessity
for using a very low relief that a thin negative is
recommended.
Dark, muddy prints. — This arises from printing plates
being stained too deeply, or from the relief being over-
printed.
Slurred prints. — This fault is due to the paper being
too wet, or because too long time has been taken in
the transfer, owing to an unsuitable relief. With a
correctly printed relief the whole of the ink should be
transferred to the gelatinised paper within five minutes.
The finished print should be at once pressed, surface
dry, between clean blotters, and pinned up to dry in
a current of air.
Full detailed instructions are sent out by Sanger-
Shepherd & Co., along with all the materials necessary
for carrying out their process.
§ 89. The Pinatype Process. — This method, in-
vented by Dr. E. Konig, has considerably grown in
public favour of late, and is well able to hold its own
among competitors. The process, like Sanger-Shep-
herd's, depends on the selective action of certain dyes
on the gelatine. Thus, supposing the three gelatine
bichromate printing plates have been prepared as in
the last process, then the parts exposed to light will
be hardened, the rest remaining soft. Now, it has
been found that dyes may be classified thus —
1. Those (and they are the majority) which stain
the whole surface, either uniformly or partly, by a
THREE-PLATE COLOUR PRINTING \6j
selective action ; the dye being in some cases removed
by the water, in other cases remaining fixed.
2. Those dyes which stain the hardened parts of the
gelatine more than the unhardened parts, since they
enter into composition with the hard (light-impressed)
parts which contain oxides of chromium.
3. A few dyes exist which do not touch the hard
gelatine, but stain the (unacted on) soft gelatine. Such
stains are called pinatype colours, and they constitute
the dyes used in this process.
It will thus be seen that the pinatype method is the
reverse of the Sanger-Shepherd, since the latter depends
on dyes which adhere to the raised hard gelatine and
come off on to the paper.
Pinatype colours should possess the following
properties : —
1. They must be fairly soluble in cold water.
2. They must stain the soft gelatine strongly, and
hardly touch the hard parts.
3. They must be fixed dyes, and incapable of being
washed out.
4. They must readily stain the paper brought in
contact.
5. The picture must retain its detail and sharpness
after drying, and must not suffer from prolonged
washing.
6. Lastly, the colours must not be liable to fade.
Fortunately all these properties can be found among
the red, yellow, and blue dyes.
It may also be noticed that since these dyes do not
stain the light-impressed gelatine, but only the parts
unacted on, the pinatype print will be a facsimile of
1 68 PHOTOGRAPHY IN COLOURS
the original negative. In other words, the original
negative taken through the colour-screen is reproduced
by the printing plate exactly as in the Sanger- Shepherd
process, but with this difference : By the latter
method the print is made from the light-hardened
gelatine which receives the dye, whereas by the
pinatype process it is the unchanged gelatine which
takes up and transfers the colour. In order, therefore,
to make a positive print, our bichromated printing
plate must be made from a transparency (diapositive),
and not from a negative, as in the other method.
To sum up, the pinatype process consists of five
stages.
1. Making the Negatives. — Three negatives of the
subject are taken through their respective screens.
2. Copying the Negatives. — Three transparencies
(diapositives) are made from the negatives on a fine-grain
emulsion, such as is used for lantern slides. These
can be made to any size, so that if an enlargement or
reduced print be wanted, the diapositives can be made
to the size required in the print. The qualities of a
lantern slide are transparency, brilliancy, and contrast,
but the pinatype diapositives should be soft, without
any great amount of density anywhere. This quality
can be readily obtained by giving a full exposure, and
using a diluted developer.
3. Transferring the Image to the Printing Plate. —
From the diapositives three printing plates are made.
These are glass plates thinly coated with gelatine and
sensitised in a 2J per cent, solution of bichromate of
potash (15 grains to 2 oz. of water), dried in the dark
(6 to 8 hours), and then successively exposed behind the
THREE-PLATE COLOUR PRINTING 169
diapositives. Each plate should be marked B, R, or Y
in the corner, to indicate the colour to be used. The
sensitising solution must be kept cool (60° to 65° R),
and the time for each printing regulated by a
photometer or actinometer (Warnerke's or Sanger-
Shepherd's). It is about the same as for collodion
P.O.P. The image appears faintly drawn on a yellow
ground. The plates should be well washed until all
the yellow has disappeared from the water. They are
then dried, and are ready for use at any time.
4. Dyeing the plates. — Three baths are to be made.
A blue bath of 10 tablets pinatype blue to 9 oz. of water
for the plate from the red screen negative (immerse for
15 to 20 min.) ; a red bath of 10 tablets pinatype red to
1 drachm of ammonia '880 and 9 oz. of water for
the plate from the green negative (immerse for 10
to 15 min.) ; and a yellow bath 10 tablets of pina-
type yellow to 7 oz. hot water (immerse for half
an hour).
5. Printing ike picture on paper. — A sheet of transfer
paper is soaked in water until it expands no longer.
It is then gently and evenly squeegeed down on to the
blue-dyed plate, which is taken wet from the bath. A
piece of oiled paper is laid over the print to enable the
roller squeegee to run smoothly. The progress of the
transfer of the colour to the print must be watched
by turning up one of the corners from time to time.
On an average about ten to fifteen minutes will suffice.
The blue print is then removed and transferred to the
red-dyed plate. In this case it is well to place a thin
transparent sheet of celluloid between the two, and as
soon as the two are in register, to hold the top of the
1 70 PHOTOGRAPHY IN COLOURS
print firmly and slip the celluloid from underneath,
and then to squeegee as before. This precaution is
necessary to prevent the transfer of colour before
register is secured. In the case of the yellow dye this
is not necessary, as it acts more slowly.
Lastly, the print now dyed with blue and red is
squeegeed down upon the yellow-dyed plate. The
order is therefore Blue-red-yellow, but you may make
it Bed-blue-yellow, or even Blue -yellow-red, but only
experience will teach you which is best for each case.
If any of the prints have been dyed too deeply, the
colour may be thinned down by squeegeeing them on
to a piece of paper coated with gelatine until sufficient
colour has been abstracted. In the same way an
unfixed print which is too weak may be reinforced
by squeegeeing it on to its printing plate. Eetouching
may be done on any of the wet prints with a brush
soaked in the dye.
One great advantage of this process lies in the fact
that the three impressions of colour are superposed
on the single support, and not on separate gelatine
layers which require to be accurately placed in register.
The weakest part of the picture lies in the blues,
which are apt to become too red owing to the varying
effect of the green filter.
§ 90. The Colour Carbon and other Processes.—
Many other beautiful and useful processes exist, such
as the Rotary Co.'s Stripping Pigment Films, in
which the printing is done through thin sheets of
celluloid and each developed pigment image is in turn
transferred to a piece of single transfer paper. The
Hesekiel-Selle carbon process and the Perscheid screen
THREE-PLATE COLOUR PRINTING 171
process may also be mentioned in this connection, but
it is beyond the scope of this work to enter into details
respecting them, since they are quite unsuitable for the
amateur by reason of the apparatus and skill required.
§ 90A. Raydex Colour Process. — This is a new
colour printing process which seems to be rapidly
gaining in favour, as it is easy to work, reliable, and
permanent. In brief, the process is as follows : Three
separate negatives are made behind red, blue and green
filters, respectively. From these negatives bromide
prints are made which, when washed and fixed, are
soaked in water and laid in contact with Raydex colour
sheets of the complementary colours for twenty minutes.
The colour sheets are now stripped off the bromide prints,
and each is squeegeed on to a celluloid plate and im-
mersed in hot water until the unacted- on colour is
dissolved off. The three-colour prints are now dried,
and then one is placed on a paper support under water
and again dried when the colour picture on the support
is stripped off. Then two of the prints are carefully
placed in register under water, taken out and dried,
and, finally, the third colour print is adjusted in register
on the other two. The three prints now adherent are
again dried and the celluloid stripped off. The finished
picture is then trimmed and mounted, and can be
framed and hung up on the wall. The initial outlay
is comparatively small, since any ordinary camera and
lens will suffice, although (as previously pointed out) an
astigmat lens is preferable. The only provisos being
that the camera should not be of the roll film type, and
further, that the plate holders should be of the hinge
type, and provided with springs if the filters are to be
172 PHOTOGRAPHY IN COLOURS .
placed in contact with the plate. A long slide carrying
the three plates side by side is to be preferred, but
double backs may be used, the slide being turned round
for the second exposure, and a second slide employed
for the third exposure. This process has the further
advantage that a small camera may be used, since
enlargements may be made from the original negatives
and colour prints obtained from two to three times the
size of the negatives. In a good light, all three exposures
will only occupy about 12 to 15 seconds with a long
slide, and a few seconds more with separate slides.
Details of the Process. — The Negative. — First,
three negatives are made on panchromatic plates
through three colour filters exactly as in the Sanger-
Shepherd process (see p. 163). The filters may
either be cut the size of the plates, and placed in
the slide in contact with them, or they may be
placed in holders and fitted to the lens in front or
behind. Some photographers prefer coloured glass
filters ; others, thin filters made of dyed gelatine.
Any of the panchromatic plates on the market will do.
They should preferably be backed to avoid halation.
These should be marked E, G, and B to correspond
with the filters. As regards the time of exposure
behind the " blue " plate, give with F/8 a quarter of
the time that the paper of Watkin's or Wynne's meter
takes to match the tint. Thus, if the Actinometer takes
10 sees, to turn dark, give about two and a half sees.
Each box of Wratten's plates gives the ratio of the
exposures for the three colours. All three plates
should be developed in the same dish, and at the
same time in deep Virida light.
THREE-PLATE COLOUR PRINTING 173
The Bromide Prints. — From each of these negatives
when fixed, washed and dried, a print is made on
Eaydex Bromide paper. Each should be marked E, G,
B, in the corner, to correspond to the negatives taken
under the red, green, and blue filters, and care should
be taken to cut them all from the paper the same way
of the grain, so as to secure equal expansion, and
consequently correct register when soaked in water.
As soon as the papers are soaked, place them side
by side in a clean porcelain dish, and develop with any
good developer. The Eaydex Company recommend their
single solution developer, diluted 1 in 20. A weaker
solution, 1 in 30 or 40, is slower, but allows of greater
control. This weak solution is advisable in the case of
beginners, as if development is too rapid a weak image
will result. The prints must then be fixed in Hypo
solution — an acid fixing bath is to be preferred — in
which they should be left for at least ten minutes.
They are then removed and thoroughly washed free of
Hypo, and then dried for future use ; or when half dry
they may be placed on a chemically clean glass plate,
which acts as a support — and a colour print made
directly from them.
The Colour Prints. — While the bromi,de prints are
half dry on their glass supports, cut three pieces off
the Eaydex colour sheets a quarter of an inch larger
than the Bromide paper, prepare three Eaydex trans-
parent supports by rubbing over each support with a
few drops of the wax solution with a soft rag, and
then polish it off with • a fresh clean rag, avoiding
streaks. Take two porcelain dishes. Fill the larger
one with clean water and place the smaller one
174 PHOTOGRAPHY IN COLOURS
in front of you to sensitise the colour sheets in.
Next place the three colour sheets in the water, and
allow them to soak until they uncurl and flatten out,
which they will do in a few moments, and then hang
them up to drain. Or, if you prefer it, you may
merely sponge over the backs of the colour sheets with
a wet sponge, taking care not to touch the colour side.
They will at once curl up, but will soon flatten down
again.
Now proceed to develop the colour. Measure out
one dram of each of No. 1 and No. 2 solutions and add
to them an ounce of water. This will be sufficient to
sensitise three quarter-plate sheets — or even three
half-plate sheets with care. Immerse the colour
sheets in the solution for about two minutes, moving
them about all the time. Now take the bromide print
marked E (made from the negative taken through the
red screen) in the one hand, and in the other hand
lift up the blue sheet from the bath, drain off the
excess of solution and slide it over the bromide print
still adherent to the glass support under the surface of
the water in the larger dish, taking care that there is
a margin of colour sheet all round the print. Withdraw
the two sheets in contact, and rapidly squeegee the two
sheets together so as to get rid of all bubbles, being
careful not to shift the papers, as chemical action is
going on all the time, and any movement will give
rise to a double image. Next, dry the back of the
top sheet with blotting paper, and strip both papers
off together with a flat knife off the glass, and then dab
the moisture off the other side. Then hang up the
two sheets stuck together to dry. Leave the prints in
THREE-PLATE COLOUR PRINTING 175
contact for twenty minutes so as to allow the bromide
paper to become thoroughly bleached. Treat the other
bromide prints in the same way, i.e. place the " green "
Bromide print in contact with the red colour sheet, and
the " blue " print in contact with the yellow colour
sheet. Eenew the water in the big dish the moment
it becomes coloured. Hang 'up the three combined
sheets which have been roughly dried for twenty
minutes, when the action will be complete.
It is worth while pointing out that the bromide
prints, after they have fulfilled their purpose, may be
washed and redeveloped, and again used for a fresh
set of prints. Now, take a celluloid sheet previously
cleaned with benzole and coated with waxing solution.
Strip one of the colour sheets off the bromide paper
(this should require a slight pull to detach, and not
come off too readily, or it will show that it had not
been dried enough), and, after dipping it for a second
in water, lower the centre of the colour sheet, face
downwards, on to the waxed celluloid surface, and
then smooth down the two ends. Squeegee it firmly
in contact with the celluloid, using first a flat squeegee,
so as to exclude all air bubbles. Then squeegee again,
under blotting paper, with the roller squeegee, using
several pieces of blotting paper, so as to dry the colour
sheet as much as possible. Treat the two other sheets
in the same way. Then proceed to develop.
This is done by placing the three sheets in a large
dish of hot water at about 110° P. or 115° P. until the
colours begin to ooze around the edge of the paper.
Now strip off the paper support, and rock the plate in
the water until all the superfluous, unacted colour is
176 PHOTOGRAPHY IN COLOURS
dissolved off, and the high lights are quite clear and
transparent. As soon as you are quite sure that all
the unaffected colour is dissolved off, rinse in a fresh
dish of cold water. If any colour remains and blocks
out the high lights, pass a smooth, broad camel's hair
brush over the surface, taking care that the brush is
free from all traces of grit. Then stand the print up
to dry.
Combining the Colour Positives. — Single Transfer. —
We have now three prints — a blue, a red, and a green
one — attached to three separate sheets of celluloid.
These prints have now to be brought into register.
Soak a piece of Eaydex single transfer paper in water
until it becomes soft. Dip the celluloid plate carrying
the yellow-coloured image into water, and slide the
image over the paper support. Eemove it from the
water, drain, and then squeegee into contact, using a
roller squeegee with blotting paper between. Brush
over the blue and red positives with a little Eaydex
combining solution with a broad camel's hair brush.
Leave them to dry on a flat surface out of the dust.
As soon as the yellow positive is quite dry, strip it off
the support, which can be readily done by bending it
slightly. Then remove all traces of wax by gently
rubbing the surface with a soft rag moistened with
benzole. Soak in water along with the blue positive,
and roughly register them under the water. Then
remove and lightly squeegee, and slide the print until
the two are in perfect register, avoiding all pressure,
so as to prevent the two surfaces from sticking and
tearing until' they are precisely in apposition. Allow
them to dry thoroughly, and detach the two prints,
THREE-PLATE COLOUR PRINTING 177
now firmly adherent, from the celluloid. Eub with
benzole, and apply the red print in the same way.
When the final celluloid support is detached, the picture
will have a fine polished surface. If a mat surface is
desired, clean the surface with a soft handkerchief
dipped in benzole, and then dip the print in water for
a moment and allow the picture to dry quite flat. The
picture is now ready to be trimmed and mounted.
The picture will be found to be reversed as regards
right and left. This is immaterial for many subjects,
such as flowers and similar objects, but whenever it is
imperative that the picture should be the right way
round as seen by the eye, the original negatives must
either be taken with the glass side of the plate facing
the lens, or the double transfer method must be used.
Double Transfer Method. — Soak a piece of Eaydex
temporary paper support in water for a few minutes,
and apply it to the blue or red positive as previously
described. In this method the yellow image must be
laid on last, and the combining solution should be
brushed over the last two images to be superimposed.
As soon as all three colour positives are on the tem-
porary support, clean with benzole, and soak in water
at about 62° F. for a minute along with a piece of
Eaydex final support a trifle larger than the picture.
Now remove both in contact and squeegee, and place
between sheets of blotting paper under a flat weight
for ten minutes.
Tn order to remove the temporary support, float on
the surface of a dish of water at about 120° F., avoid-
ing air bubbles, taking care that the support is on the
top and kept dry. After two or three minutes, turn
N
178 PHOTOGRAPHY IN COLOURS
over so that the temporary support is on the top, and
then submerge the whole under the water. Gently
remove the temporary support, and wash off all the
soluble gelatine. The picture is now ready to be
trimmed and mounted.
If the beginner fails to get satisfactory results the
Eaydex Company state that they will be only too
pleased to put him on the right road, and it will greatly
facilitate matters if the prints are sent for inspection.
OHAPTEE XI
COLOUR PRINTING FROM SINGLE-PLATE TRANSPARENCIES
§ 91. Uto-color Printing from Single-Plate Trans-
parencies.— The reproduction of colour transparencies
on to paper so that they can be framed on a wall, or
pasted in an album, has until recently only been effected
by making three separate glass negatives, or printing
blocks, and then either superposing transfer films stained
with the complementary colours, or by direct printing of
these colours, as is done in the half-tone colour process.
Such methods will be found described in Chapter X.
Although copies of remarkable delicacy and beauty can
be produced by these methods, they all require three
separate negatives to be made, besides entailing a cer-
tain amount of apparatus coupled with a degree of
delicate manipulation which can only be obtained by
constant practice. Hence any method by which a
colour transparency can be directly printed on a sheet
of prepared paper would be a great desideratum. In
fact, the difficulty of reproducing colour transparencies
has been the main cause of the want of popularity of
colour photography among amateurs.
This difficulty has at length been more or less
overcome by the Bleach-but Process of colour print-
ing, which, although very far from perfect, is daily
being improved upon. It has at length reached such
l8o PHOTOGRAPHY IN COLOURS
a stage, that, with a suitable colour transparency, very
effective and faithful copies on paper are reputed to
have been obtained in Europe.
We will now proceed to describe the methods and
the principles on which it is founded.
§ 92. The Theory of the Bleach = out Process.— It
is well known to everybody that the colours of wood
and textiles change under the prolonged action of light.
Wall-papers, carpets, watercolour paintings, etc., all
tend to lose the brightness of their colours, and partially
bleach-out in sunlight. Most colouring matters (dyes)
when added to fabrics are not permanent. They tend
to fade, and are for the most part removable by wash-
ing. A few dyes like Indigo and Safflower immediately
impart a permanent colour to any fabric when it is
dipped or boiled in the dyeing vat. Hence they are
called substantive colours. Most woollen goods, and to
a certain extent silks, form a permanent compound
with dyes, and require no further treatment ; but the
vast majority of stuffs require a special re-agent called
a mordant, which will form an insoluble compound both
with the fabric on the one hand and with the dye on
the other. The colours which require such treatment
are termed adjective colours.
Alum, Cream of Tartar, Tannin, and the oxides of
Iron and Tin are the principal mordants employed.
The power of resisting the action of light varies
enormously with different colouring substances, and
their resistance depends very largely on the colour of
the light which attacks them. It is by making use of
these facts that we are able to place certain colours or
pigments on to the paper which are affected by different
UTO-COLOR PRINTING l8l
coloured lights, and at the same time are quite under
control. Of course, it is easy enough to make a direct
copy in almost any single colour, but it is a much more
complicated problem to get the various colours to re-
produce themselves on the same sheet of paper by
selective action of the light. The direction in which
the solution of the problem may be sought for appears
to lie in the bleaching-out process.
In order to understand the theory, we may as well
begin by explaining what is meant by a pigment. A
pigment is a colouring matter or fluid which permits
a portion of the rays of white light to pass through it
and absorb the rest. If a pigment in a fluid state such
as water or oil paint, or aniline dye, be spread over a
sheet of white paper, a certain portion of the light will
pass through the layer to the white paper, and after
being reflected will again pass through the coloured
layer, and give rise to the sensation of colour in the
layer in question. If only a small amount of light can
pass through to the white paper, it is called an opaque
colour. Such, for example, are Chinese White and
Naples Yellow. Such a colour is said to have body, and
is called a body colour, or opaque colour, because it hides
the colours beneath it. If a large quantity of light
passes through, it is called a transparent colour, of which
most of the aniline dyes, the Umbers and Siennas, are
examples. They allow the subjacent colours to shine
through, or merely modify them. Colours made
luminous by transmitted light, such as coloured glasses,
and the stained glasses- used for windows, as well as
the colours of the spectrum, are all good examples of
transparent colours.
1 82 PHOTOGRAPHY IN COLOURS
§ 93. Permanent and Fugitive Colours. -When
white or coloured light-rays are absorbed, they may
either be changed into heat, without altering the com-
position of the pigment, in which case they are called
permanent or fast colours • or the light-rays may alter or
split up the molecules of the colouring matter, in which
case the colour will be altered, or may even entirely
disappear. These pigments are called sensitive or
fugitive colouring matters.
If the rays of light which fall on the surface of a
body are scattered diffusely, such a surface will appear
white, and we call the body a white one. Snow, milk,
lime, white lead, all reflect light diffusely, and therefore
convey the sensation of white ; but if a surface reflect
light regularly, so as to form an image, it no longer
appears white, but constitutes a mirror, and will have
a metallic lustre. The surface of still water, glass,
quicksilver, polished metals are familiar examples of
what we mean.
If, on the other hand, the rays of light are nearly all
absorbed, little or no light is reflected, and the surface
of the body appears black. As we have already ex-
plained, no substance is absolutely white or black, since
the whitest body known, viz. fresh fallen snow, only
reflects about 85 per cent, or 90 per cent, of the light,
and the blackest material, such as black velvet, powdered
charcoal or soot, reflects about 1 per cent, of the light.
Grothus, writing in 1819, first explained the theory
of the action of light on coloured bodies. He says,
" Coloured light seeks to destroy in bodies upon which
it acts those colours which are opposed to its own
(i.e. complementary colours), while it endeavours to
UTO-COLOR PRINTING 183
retain its own or another analogous colour." Thus,
red light would seek to discharge its complementary
colour blue-green, while retaining the orange-yellow ;
and green light would discharge red, while retaining
the green. In other words, a coloured body, whether
it be a fast colour or one sensitive to light, is not
affected by light of its own colour, since it either
allows all rays of such light to pass through it, or
else reflects them away. It is only the absorbed rays
which decompose or destroy the colour. Now, a red
coloured body does not absorb red rays, but it will
absorb rays of other colours. If this coloured body
is a fast colour, the only change observed, when other
than red rays reach it, will be a rise of temperature,
but if it is a body sensitive to light these rays will
destroy the colour. And the same thing will happen
in the case of any other colour. Hence we may
formulate it in the following sentence, which Dr. J. H.
Smith calls the Bleach-out Law : —
"A coloured body sensitive to light
will only be affected by the light it absorbs,
but not by the light of its own colour."
The rays which are absorbed cause a chemical
change in the molecules of the sensitive colouring
matter whereby the colour is destroyed. In other
words, the colour bleaches out. Now, if we coat a
sheet of white paper with a suitable gelatine emulsion
containing a purple-red dye, and then superpose a
yellow dye, and lastly a blue dye (taking care that
these are the proper colours and in the right propor-
tions), we shall obtain a black mixture. As a matter
of fact, the result is rarely a pure black. It generally
184 PHOTOGRAPHY IN COLOURS
has a greenish or greyish tinge, but a good black is
what is aimed at, and with the increased experience
which the manufacturers of printing-out colour papers
are gradually obtaining, the colour is approaching nearer
and nearer to an ideal black. Such a three-coloured
emulsion layer on a white backing forms the basis of
a printing-out colour paper.
The Effect of Coloured Lights on the Paper. — Let us
suppose that a copy on paper is to be made of an
Autochrome Transparency. Instead of a complicated
landscape, let us, for the sake of simplicity, suppose
that the transparency consists of six stripes or squares,
A, B, C, D, E, F (Fig. 25), of the following colours :
Black, Blue, Bed, Green, Yellow, and White. A sheet
of the sensitive pigment paper is placed in a printing
frame with its colour (Black) surface in contact with
the film side of a colour transparency (or with a piece
of thin celluloid between the two in order to prevent
the surfaces sticking), and then put in the sunlight.
What happens is this. At A the Black stripe blocks
out all the light, so that no action takes place, and the
black surface of the paper remains unaltered.
At B the Blue stripe allows mainly the blue rays to
pass through. According to the Bleach-out law, the
Blue layer will be unaffected, since the Blue rays are
not absorbed, but pass through it ; but the light will
be absorbed by the yellow and red pigments, and they
will be destroyed and bleached out, so that ultimately
when the action is completed, all the light which passes
through the blue layer will reach the White surface of
the paper, and be reflected through the Blue layer.
At C the Bed stripe allows the Bed light to penetrate
UTO-COLOR PRINTING 185
the Blue, Yellow, and Bed pigments. The green light
will only be absorbed to a small extent by the Blue
pigment, since the blue and green waves overlap to
a considerable extent, forming a greenish- blue or
bluish-green mixture. For the same reason the green
light will only act in a feeble manner on the Yellow
pigment, because the Yellow and Green overlap, form-
ing a greenish - yellow or yellowish - green mixture,
according to the nearness of the waves to the red
or the blue end of the spectrum respectively. But
the green rays will bleach out the Eed pigment almost
entirely, so that we have a Yellow and a Blue layer left
practically intact, and these two together make a green
colour.
At E the Yellow stripe will permit Yellow rays to
pass, and these will bleach out the Blue and the Red
so that only the Yellow layer is left.
Jle/bre JZicparure
FIG. 25.— Diagram showing the action of light on the emulsion
colour layer. Three separate colour layers are shown, as
occurs in the Szczepanik paper, and not a single layer of
mixed colours, as is the case in the Uto paper. This is
done for the sake of clearness, but the action is identical in
both papers.
At F we have a White or clear strip of glass left.
In this case nearly all the rays which compose the
whole spectrum pass through, so that all the three
layers are acted upon. If they are all acted upon
equally, they will all be bleached out together, and the
1 86 PHOTOGRAPHY IN COLOURS
White of the paper will shine through the gelatine film
unopposed, and the stripe will appear White. This
White is formed by a subtractive process, i.e. by removal
of all the superimposed colours, and its appearance is
due to exactly the opposite process to the White seen
in a diapositive (transparency), for in this latter case
the White is formed by the combination of the three
colours (additive process).
If, therefore, the pigments have been perfectly
chosen, and have been acted on by the light so as to
harmonise with the exact colours of the stripes, a perfect
facsimile of the original ought to be produced. But,
for obvious reasons, an exact copy can only be approxi-
mated, even with the most perfectly chosen pigments.
For even if the pigments were right, it would always
be impossible to regulate the light, so that the bleaching
action can be stopped exactly at the right moment.
Up to the present time the writer has never once
succeeded in getting a satisfactory print from any
negative by this method. Perhaps the climate of S.
Africa is unsuitable by reason of the extreme dryness.
§ 94. Details concerning1 the Bleach = out Pro=
cess. — Having explained the principle of the Bleach-
out process, we will now consider it in detail.
Difficulties to be Overcome in Procuring /Suitable Dyes.
— It is extremely difficult to find dyes which possess
all the requisite properties for producing a suitable
bleaching-out paper. For example, (1) they must have
no chemical effect on one another, and (2) they must
bleach out equally. If one of them bleach out quicker
than the other two, it is necessary to find some agent
or compound which has to be added in order to retard
UTO-COLOR PRINTING 187
its action to the requisite degree. (3) After the colours
have been bleached out by the light in the parts which
represent the picture, those that remain must be capable
of fixation so that the light will have no further action.
(4) Since highly sensitive dyes cannot be permanently
fixed, it has been found necessary to employ less
sensitive dyes, in order to secure the essential property
of stability on fixation. (5) These moderately sensitive
dyes must be capable of being made temporarily highly
sensitive, and then of being rendered less sensitive
afterwards. (6) Lastly, the dyes must not only possess
all these properties, but they must have the right shades
or tones of Red, Yellow, and Blue, so that together
they will produce Black, Greys, Browns, Greens, and
Purples of the proper tones, so as to give correctly
balanced mixtures. It will, therefore, be readily seen
how extremely complicated the problem is, and what
enormous difficulties have to be overcome, in order to
produce ideal light-sensitive dyes.
Supposing that we have the right dyes, we have
then to mix them and dissolve them equally and
thoroughly in an emulsion of either gelatine or collodion.
Such an emulsion must have no chemical or injurious
action on the dyes, and should permanently retain a
certain amount of moisture, because moisture favours
oxidation, and therefore a bleaching-out action.
Now we may adopt one of two methods. We may
either mix all three colours, red, yellow, and blue dyes
with the gelatine or collodion, and then spread the
mixture over the paper, as is done in the U to-colour
process ; or we may coat the paper with three layers,
each holding its respective dye, as is employed by
1 88 PHOTOGRAPHY IN COLOURS
Szczepanik in his three-layer bleach-out paper. Each
process has some advantages and some drawbacks.
Both answer the purpose fairly satisfactorily, in fact,
the single layer (Uto-color) process differs more in
theory than in practice, because, so long as the emulsion
is wet the colours gravitate, and thus arrange them-
selves into layers according to their solubility. Thus
the red particles which are less soluble than the other
two colours will be deposited first, then the yellow,
and, lastly, the highly soluble blue. So that ultimately
we get three layers just as in Szczepanik's process,
the only difference being that the single layer resolves
itself ultimately into three very thin layers, whereas
the triple layer remains as a single thick layer. The
Uto-color process has therefore the advantage of the
three layers being closer together, and hence there is
less parallax. Moreover, the Uto-color method can
be produced much more cheaply, since there is only
one layer to coat instead of three.' On the other
hand, with Szczepanik's method one may isolate
each layer in turn and so prevent the three dyes
from acting chemically on each other, this isolation
allowing of a larger range of colours. Szczepanik has
now overcome the difficulty of isolating and fixing the
intermediate layer, which was at first a serious objection.
He does this by spreading on first a layer of red stained
gelatine, then a coating of varnish which has been
stained yellow; and, lastly, a blue stained layer of
gelatine.
In practice both methods will produce equally good
prints. The Uto-color process is, however, the one
most in favour at the present time.
UTO-COLOR PRINTING 189
Methods of Increasing the Sensitivity of the Dyes. —
The dyes alone are not sufficient. It is necessary
to increase their sensitivity to light. This is effected
by means of either an ethereal oil such as Anethol, or
Thiosiamine, which is a compound of Urea, discovered
by Dr. Smith, the inventor of the Uto-color process.
It is interesting to note that Urea itself, or one of its
products, has been used by platemakers to increase the
sensitivity of plates for many years past. Peroxide of
Hydrogen has also been recommended, but as it rapidly
decomposes into oxygen and water it is of no value.
The special qualifications of a good sensitiser are that
it should be permanent as far as its composition
goes, it should be non-volatile, and easily removable.
There is still a great demand for a good sensitiser,
as all those which we know of possess certain draw-
backs.
In selecting a support for the gelatine containing the
three colours, the paper should be perfectly neutral
and indifferent, i.e. it should possess no chemical
properties which can act on the gelatine, and it must
be protected by means of an insulating material from
becoming stained by the dyes. When the Uto paper
was first issued it was found to be stained pink by the
acid red dye (Erythrosine) which leaked out of the
gelatine. At the present time a basic red dye is used
instead, being better adapted for several reasons.
§ 95. The Nature of Dyes. — Dyes are of three
kinds, basic, acid, and neutral. Basic dyes are salts of
the colour bases, which latter form the real colouring
principle. It may be just as well to explain here the
meaning of a few terms which are constantly used in
190 PHOTOGRAPHY IN COLOURS
connection with this subject, viz. Bases, salts, acids,
and colloids.
A Base is a substance which combines with an acid,
and neutralises it to form a salt. It is usually an
oxide, and sometimes a derivative of Ammonia, and
when it combines with an acid, water is formed.
Bases are nearly always alkaline, or if not, neutral, and
have the opposite character to acids. Thus a Base
usually turns red litmus paper blue, whereas an acid
turns blue litmus paper red. When it unites with an
acid the acid is neutralised, and a salt is formed. For
example, Sodium Carbonate (Na2C03) is a base. If
Hydrochloric acid (HC1) be added to the base, a neutral
salt (common salt, or NaCl) is formed, which is com-
posed of the metal sodium and chlorine gas, while water
and carbonic acid are liberated. A neutral body such
as common salt is neither acid nor alkaline, and there-
fore has no effect on litmus paper.
A Salt, then, is a compound formed when the
Hydrogen in an acid is partly, or entirely, exchanged
for a metal.
An Acid is a substance containing Hydrogen which
reddens litmus paper ; and when it unites chemically
with a base, the Hydrogen is replaced by the metal of
the base (if one is present) and a salt is formed. Thus
KHO and K2O are bases. Sulphuric acid (H2S04) is
an acid. If one of the two comes in contact with the
acid, either one or two atoms of Potash are capable of
exchanging places with the Hydrogen to produce one
or other of the salts, Potassium Sulphate (K2SO4) or
Hydro-potassium Sulphate (KHSO4).
A Colloid is a viscous or jelly-like substance, which
UTO-COLOR PRINTING
in solution will not pass through an animal membrane.
Indiarubber, Gelatine, and Collodion are examples of
colloids. They are very intractible and difficult to
analyse. A colloid is necessary to hold the colour in
solution in a printing-out paper.
Acid and Basic Colours. — These are nearly all
Analines (Coal-tar products), and comprise the vast
majority of dyes used in commerce to-day. The real
colouring matter of such a dye lies in the colour base.
If such base is combined with an acid it forms a basic
dye which is merely a salt of a colour base. The most
important dyes used in colour photography are the
following : —
Reds. — ^Ehodamine, Saffranine.
Blues. — Methyl violet, Diamine blue, Methylene
blue.
Yellows. — Chrysophenine, Auromine, Chrysoidine.
Acid Dyes. — These form by far the largest group
of dyes. They are all salts of the colour acids. Just
as in the former class a colour base or alkali forms the
real colouring principle, so in this case an acid colour
body does the same. When this acid is combined with
a colourless base, e.g. Soda, Lime, or Ammonia com-
pounds, we get an acid salt. And as in the former
case the acid has no effect on the colour, so in this
case the addition of the base neutralises the acid, and
consequently does not change the colour.
Acid dyes are of course acid, and they dye substances
best in an acid bath. The chief acid dyes used in
colour photography are as follows : —
Reds. — Echt Eot, Glycinred, Krapplak, Biebrich
scarlet, Erythrosine, Irisine, Isocyanine, Eose
PHOTOGRAPHY IN COLOURS
Bengal, Eosine, Methyl red, Nitrate of JEthyl-
red, JEthylred, Chinolin red.
Yellows. — Phenosaffranine, Bapid filter yellow
(Hoechst), Naphthol yellow, Normal gelb, Pina-
chrome, .ZEsculme, Methyl orange, Aurantia,
Picric acid, H. Naphthol orange, Eosine yellow.
Blues. — Cyanine, Fast blue, Methylene blue,
Normal blue, Milori, blue, Peacock blue.
Greens. — Fast Green, Acid green.
Mineral and Vegetable Colours. — These form a
class by themselves, and are largely used for printing-
inks in colour photography. Such are, for example : —
Chrome yellow, Cadmium yellow, Zinc yellow, Prussian
blue, Madder lake, and its artificial derivative Ali-
zarin. They are all mordants (that is, self-fixing), and
are very permanent.
All the dyes required in photography may be obtained
from one or other of the following firms :—
1. Actien Ges. fur Analinfabrikation,
Hoechst factory, Frankfort-a-M.
2. Bayer Elberfeld, Germany.
3. Berger u. Wirth, Benthstrasse, Berlin, for Normal
Blue and Normal Bed.
4. Bohringer Mannheim (for .ZEthylred).
Griibler und Holborn, Chemiker, 63, Baierische
Strasse, Leipsig, or their agent, Charles Baker,
244, High Holborn, London, W.C.
East in Ehinger (for inks), A. and E. Lumiere, Lyons.
Dr. G. Miindie, Mik. Chem. Institut, Gottingen.
Hanover.
§ 96. Uto-color Paper.— In the year 1906, Dr.
J. H. Smith, of Zurich, produced a paper covered with
UTO-COLOR PRINTING 1 93
a gelatine layer impregnated with the three primary
colours. He called it Uto paper, the name being
taken from a range of mountains near Zurich. This
paper was at first of a greyish-green colour, but later,
efforts produced a black paper, which, as we have
already pointed out, is the correct and in fact the only
possible colour which the paper can have to give the
effect of dark shadows. Obviously Black cannot be
formed by any conceivable bleaching action, and if any
black is to appear at all in the finished picture, it must
be supplied by the unaltered paper. This paper proved
very disappointing and unsatisfactory owing to the
irregular way in which the colours printed-out, and to
the great want of sensitiveness of the colours to light.
Besides, in order to sensitise it before printing it was
necessary to add Peroxide of Hydrogen. Subsequently,
Dr. Smith discovered that Anethol was not only a better
sensitiser, but could be incorporated in the colour layer,
thus rendering the process of after-sensitising un-
necessary.
Uto Rapid Paper. This paper is now manfactured
and sold by the Societe Anonyme Uto-colour, Eue de
la Pointe, La Garenne-Colombes, Paris. It was intro-
duced in October, 1911, and perfected a year later, and
is a great improvement over the old paper. It consists
of a paper covered with the following layers : —
1. The white paper support.
2. A white Baryta layer, which gives the pure
whites in the finished picture after the colour layers
are entirely bleached-out. The reader must re-
member that in the print we are dealing with colour
formation by subtraction (subtractive process, see
o
194 PHOTOGRAPHY IN COLOURS
p. 68), whereas in the formation of the positive the
colours are produced by addition (additive process or
colour synthesis). In the paper print the whites are
produced by more or less complete removal of all
three colours in the gelatine layer. The object of the
Baryta layer is to afford a purer white than the paper
alone would give.
3. The colourless insulating layer. This layer is
necessary to prevent the colours from soaking through
and staining the Baryta layer and the paper beneath.
4. The Gelatine Emulsion Layer. This holds the
mixed red, yellow, and blue dyes, which are adjusted
to form as dense a black as possible, and give rise to
the blacks and dark shadows in the finished picture.
The black has a slightly greenish tinge, but this is
hardly noticeable. The gelatine emulsion is coated by
machinery, and holds the three dyes and the sensitisers
Thiosinamine and Anethol, and other compounds. A
little glycerine is added to prevent the layer from
becoming too dry.
Under a suitable positive, the colours print out
fairly evenly, and give quite a good effect.
According to the makers, the paper keeps well in its
original wrapper for about a year, if it is kept in a
cool dry place. Each sheet is packed with the sensitive
surface protected by a piece of brown tissue paper.
This, as the author has pointed out, is a mistake, as in
hot climates the tissue paper will stick so firmly as to
render it impossible to remove it. He has suggested to
the company that the paper should either be packed in
air-tight and light-tight grooved boxes, so that the
black surface should not touch anything, or else the
UTO-COLOR PRINTING 195
sheets should be in contact with celluloid films which
would not stick. If the sheets become too dry, the
emulsion films become brittle and crack; if they are
kept in a damp place, they stick to the positive, and
print out too green.
§ 97. Practical Details of Printing on Rapid
Uto Paper.
1. Subjects jor Printing. — Any colour screen-plate
will do, and any subjects may be copied, but to get the
best results, the following points must be attended to : —
(a) The colour transparency or positive should
possess strong and brilliant colours since the pictures
always lose a good deal of the original brilliancy and
appear flat. This is due to the fact that the paper
pictures are seen by reflected light, and not by trans-
mitted light as the originals are.
(#) The positives should not possess great contrasts
of light and shade, because to obtain these the colours
must be entirely bleached in some parts and remain
unaffected in others, and that is a result which is very
difficult to obtain, since the light parts print out too
quickly compared with the dark parts, which we know
is the case in ordinary black and white photography.
(c) The illumination must be evenly distributed over
the entire plate. The best subjects for printing are,
therefore, positives which have rich saturated colours,
without possessing great contrasts of light and shade,
i.e. subjects which are not too hard.
2. Preparing the Subjects for Printing. — The positive
must first be coated with a- special varnish supplied by
the Uto-color Company, since the ordinary varnishes
have too low a melting point. If the Author's alum
196 PHOTOGRAPHY IN COLOURS
trough dish be used no varnish is required, since the
alum trough absorbs all the heat. The plate must first
be gently and evenly warmed (but not heated, or the
film will be injured), and the plate is then held either
between the fingers at the sides, or on a pneumatic
rubber glass holder, and the varnish is then liberally
poured over the middle of the plate, and by gently
rocking the plate, is made to distribute itself up to the
four corners alternately, and the remainder is then
poured back into the bottle by tilting up the opposite
end. A little practice will enable the beginner to pour
it on evenly. The great point is not to have the
varnish too concentrated, otherwise it becomes very
difficult to apply evenly. The beginner should practise
it on a spoilt negative a few times first, as he is apt to
spoil the picture when he tries to dissolve it off in order
to get rid of the lines and ridges. The plate should be
gently warmed again after varnishing until it is no
longer sticky, but has become quite dry.
3. Printing the Picture. — This is conducted exactly
in the same way as with an ordinary printing-out
paper. The paper is taken out of the packet in one's
shadow at the end of the room away from the window,
the black side of the paper being in contact with the
film of the positive in the printing frame. The Author
invariably fits a plate of clear glass inside the frame
against which the glass side of the negative rests.
This is advisable for many reasons, and is indispensable
if the negative (or positive) is smaller than the frame.
When all is adjusted, the frame is placed in the open
air and, if possible, in direct sunshine, so as to reduce
the time of printing.
UTO-COLOR PRINTING 197
The Author has already recommended the use
of an alum trough. This consists of either an all-
glass trough, or, what is better, a sheet of ordinary
clear glass fixed with putty into the bottom of a wooden
frame about \\ ins. high. This is then filled half-
way up with a 3 per cent, solution of common
alum in tap water (that is, about 15 grains to the
ounce of water). The trough should be a little larger
than the frame so as to prevent any shadows being
on the picture. The object of the alum trough is to
absorb all the heat rays, which latter would otherwise
tend to injure the positive and soften the gelatine layer
of the paper Any scratches or marks or unevenness
in the plate at the bottom of the trough will not affect
the print, since the glass is too far away from it.
The time of exposure varies from an hour to three
hours in the full sunlight in summer, according to the
nature and density of the positive. In diffused day-
light the time will, of course, be considerably increased.
From time to time a corner of the print should be
examined to see how the printing is going on.
Sometimes the prints will be found to stick to the
positives. This always occurs if either the varnish or
the black colour layer is not perfectly dry, but it may
occur even if they are dry. This will happen if the
gelatine layer of the paper or the varnish of the positive
gets too hot during the printing in the sun. We can
-avoid this in several ways. We may largely prevent it
by using the alum trough above mentioned. Or we may
put one or two drops of olive oil on the centre of the
black layer of the paper, and gently rub it evenly over
the surface with the finger, or a smooth soft pad, being
198 PHOTOGRAPHY IN COLOURS
careful not to put too much on so as to leave rings or
lines of oil behind. These must be mopped up, or
they will leave marks in the printing. Lastly, we may
place a very thin layer of celluloid between the two
surfaces, a piece of a Kodak roll-film carefully cleaned
will do perfectly.
The Uto people claim that more than a hundred
prints can be made from a single diapositive, but they
do not say what effect such prolonged exposure to the
sun's rays will have on the originals. For my part I
would not risk more than two or three prints off it.
Should a greater number be required, it would be
much safer to take a copy by direct contact, or through
the camera, and then print off the copy.
§ 98. Methods of Improving the Print. — It is found
to be very difficult to keep the blacks pure, while at the
same time bleaching out the high lights so as to get
good pure whites. Worel, Miethe, and others suggest
overcoming the difficulty by making a negative copy
on an ordinary gelatine plate, and then taking a posi-
tive from that. This black and white positive is then
substituted for the colour positive and a preliminary
print is made on the Uto paper so as to bleach out the
high lights. The colour positive can then be substi-
tuted for the black and white positive (being careful to
secure exact register), and the colours printed out in
the usual way.
Printing-filters of yellowish-green of different shades
(marked G and MG, MG being twice as dense as G),
can be had from the Uto-color firm. Either one of
these may be placed over the positive so that all the
light will filter through it. These filters are useful to
UTO-COLOR PRINTING 199
check unequal printing. Thus, if the blue and yellow
dyes bleach out quicker than the red (which is usually
the case) a green filter will allow most of the blue and
yellow rays to pass through, and very few red rays.
According to the bleach-out law, since the blue and
yellow rays pass through the filter, they will to a great
extent bleach out the opposing colour red, but will
only slightly affect the other two colours. Hence by
using the filter for a certain time, we can regulate the
effect on the three colours, so as to get an even balance
between them. In the same way, if the green colour
tends to predominate we can adjust matters by using
a light red filter. According to the Uto-color manu-
facturers, if the paper is slightly damp, greens will
predominate, or in other words the blue and yellow
colours will bleach out faster than the red.
§ 99. Fixing the Uto-color Prints.— The prints
must now be fixed. This is effected, 1st, by removing
the sensitive products resulting from the bleaching out
of the dyes, and 2nd, by making the dyes stable to light.
Unfortunately up to the present neither of these
essentials can be perfectly fulfilled. Still the prints
can be fixed quite sufficiently to render the colours
permanent if kept in an album. If mounted and hung
on the walls they must be placed where no direct light
from the window can reach them. If the print has
been oiled, the oil must first be removed with a soft
pad soaked in benzine. Then the picture is placed for
half an hour in a bath containing 35 grams of Tannin
to 100 c.c. of methylated- spirits (15 grains to the ounce).
About 3J ounces will suffice for 4 quarter-plate prints.
The tannin solution dissolves out the sensitisers and
200 PHOTOGRAPHY IN COLOURS
bleaching products, and in addition hardens the film.
Next wash the print for three minutes under the tap,
and then place it in the fixing bath. This bath con-
sists of a liquid sold by the Uto-color Co. in a concen-
trated form. It is a green strongly smelling liquid.
One ounce of this is to be diluted with 3 ozs. of water.
The prints should be left in for about three minutes.
They are then rinsed for about a minute under the tap,
and dried as quickly as possible. This is best done by
squeegeeing the print film downwards on to a sheet of
glass, or japanned iron if a glossy surface be desired,
or ground glass for a matt one. The back of the print
is then dried with blotting paper, and the print left to
dry. The glass on which the print is squeegeed must
first be thoroughly cleansed, and then wiped over with
French chalk, or a little wax dissolved in benzole in
order to prevent the film sticking.
§ 100. Uto-color Stripping-paper.— When a
photograph is taken on an ordinary plate the picture
is reversed, i.e. the right side of the object appears on
the left, and the left on the right side of the negative,
but when the print is made the position is reversed
again, so that the print corresponds to the object. If
a colour positive is taken, the position of the object is
correct, because the plate is put into the camera with
the glass side, and not the film side towards the lens ;
and so when a print is made, whether coloured or
plain, the position is reversed. The picture will only
correspond to the original when it is held up to the
light and looked at from behind.
One may obtain a correctly placed picture in three
ways.
UTO-COLOR PRINTING 2OI
1st. We may use a reversing prism when taking the
positive, and the print made from this will be correct.
But reversing prisms are very expensive, and few
amateurs possess one.
2nd. We may employ a colour-film fixed on a very
thin celluloid sheet, and print through the back. Such
films can be had from the Neue Photographische
Gesellschaft (New Photographic Company), Steglitz,
Berlin.
3rd. We may use a bleach-out paper which can be
stripped like carbon tissue. This is now supplied by
the Uto-color Co., and is likely to supersede the
ordinary rapid Uto-color paper.
The stripping-paper is used exactly in the same way
as the Uto paper just described. It may be oiled just
before printing. Then the film is lifted up at one
corner with a penknife, and stripped off with the
fingers. It is then carefully laid on the film side of
the positive, and a piece of black paper is laid on the
top of it. The cover of the frame is placed on it and
fastened down, and the positive exposed to the light.
In order to watch the progress of the printing, one
half of the frame-cover is lifted up, and a sheet of
white paper is inserted a little way between the posi-
tive and the film, since it is impossible to see the
picture on a black support.
After printing, the film is placed on a glass plate
and the oil removed with a pad soaked in benzine. It
is then laid face upwards on a piece of stout Baryta
paper which should be slightly larger than the film. A
sheet of paper or cardboard is now selected on which
to mount the film, and the latter with its Baryta
202 PHOTOGRAPHY IN COLOURS
support is squeegeed face downwards on to the mount.
As soon as the gelatine has set, the Baryta support is
pulled off, leaving the film (now right side up) on the
paper or cardboard mount. It is then left to dry in a
dark place.
§ 101. Uto Lantern Slides.— The stripped Uto-color
film may be printed and mounted on a glass plate in-
stead of on paper, and thus can be used as a grainless
lantern slide. The glass plate must be dipped into a
3 per cent, solution of gelatine, just as was done in the
case of the mount, and the film squeegeed on, while the
gelatine is hot. It is recommended to cover the film
with a thin sheet of rubber to prevent the film being
damaged while it is being squeegeed. The glass trans-
parency is then allowed to dry in a dark place, and
afterwards fixed in the usual way, except that a 3 per
cent, solution of Chrome Alum is to be preferred to
the tannin bath. A cover glass is finally added and
bound round with binding strips, or, what is much
better, with a long strip of adhesive surgical plaster
ygths of an inch (11 mm.) in diameter. Uto lantern
slides possess much more brilliancy than Uto papers
since they are seen by transmitted light, and moreover
they do not require to be printed quite so deeply.
Whether their colours will stand the prolonged action
of the luminant remains to be proved.
CHAPTER XII
KINEMATOGEAPHY BY MEANS OF COLOUEED LIGHTS
§ 102. Projection of [Cinematograph Pictures in
Colour. — The extraordinary popularity of screen
pictures of moving objects has led to innumerable
attempts to still further the illusion by representing
the scenes in colour. This has been frequently done
by staining the film, and in this way effects in which
browns or greens are predominant, or moonlight
scenes, can be obtained; but although such effects
are pleasing at first, the eye soon gets tired of the
unreality. Colouring the films by hand is possible,
but tedious beyond measure. When one considers
that many films measure 1000 feet and contain over
16,000 separate pictures, the time and expense required
to colour such a film are very great. In spite of
the difficulties, hand-coloured films are still produced
to a considerable extent. The colours are never brilliant
and as far as I can ascertain are limited to pink,
bluish-green, yellow and brown. Notwithstanding
this the films are quite a success, and in the writer's
opinion are far more pleasing and agreeable to the eye
than the kinemacolor films.
§103. The Urban-Smith "Kinemacolor" Pro-
cess.— Mr. Charles Urban and his collaborator, Mr. G.
Albert Smith, after a prolonged series of experiments,
204 PHOTOGRAPHY IN COLOURS
have at length succeeded in exhibiting kinematograph
pictures in colour, which for brilliance of colour effect are
unequalled by any other method. Although only two
colour-filters are used in projection, a red and a green,
these brilliant colours and their orange and yellow
combinations, as well as browns, greys, eau-de-nil, and
even blues and indigoes, are in evidence. The green
filter used is one which transmits a considerable
amount of blue light, and therefore the resultant
picture gives not only the effect of blue sky and water,
but a very considerable range of combination of all the
colours, as well as white and black.
The principle of the Kinematograph depends on
what is called " persistence of vision " and the con-
tinued perception of the changing object. When light
is reflected from a moving object it forms an image at
the back of the eye, and produces a nerve current which
passes along every one of the fibres which receive the
image and collectively carry the impression along the
optic nerve to the brain. This sensation is not instan-
taneous, but is divided up into four periods : 1st, a
latent period which is almost instantaneous, and
during which nothing appears to happen ; 2nd, a
very short period — probably less than ^ of a second
— during which the sensation reaches the maximum ;
3rd, a much longer period, ^ to ± of a second
(the time varying directly with the intensity of
the illumination), during which the sensation slowly
diminishes ; and 4th, a short period of decline,
during which the effect dies away. In the case of
a moving object on which attention is directed, the
fourth period remains unnoticed, because a new image
KINEMATOGRAPHY IN COLOUR 205
takes the place of the old one before that period
arrives. The whole of kinematography depends on
this third period, by which the first impression, A
(Fig. 26), lingers until replaced by the second one, B,
and the second one is again replaced by a third one, C,
and so on.
We have suggested this in the diagram, in which the
height of the curve represents the intensity of the
light stimulus, and the width or base the time.
FIG. 26. — Curve representing the four periods 'of a visual sensa-
tion. The first sensation is represented by a thick line, the
two succeeding ones by dotted lines. 1. Short latent period ;
2. Period of increase to maximum ; 3. Long period of per-
sistence of sensation with gradual decline ; 4. Period of
rapid fall and obliteration of the sensation. In reality the
sensations overlap very much more than here shown, but
they are separated in the diagram for the sake of clearness.
This explains why, when a lighted stick is whirled
round, it forms an unbroken circle of fire, and why a
stream of water allowed to drop from a pipe appears
to form a continual stream, and not a series of droplets,
as is really the case ; and' just as the first impression of
a moving object melts into the next one, so a series of
colours pass before the eye, as in the familiar colour-top
206 PHOTOGRAPHY IN COLOURS
which carries a card divided into sections painted
blue, green, and red. If the top be spun rapidly, each
colour fuses into the next, and a combined sensation
of white appears to the eye. It is this last prin-
ciple that Mr. Urban and Mr. Smith have so cleverly
made use of in their " Kinemacolor " apparatus.
In working out his method of kinematograph pictures
in colours, Mr. Smith based his first experiments on an
instrument somewhat similar to the Ives' Kromskop,
and also on the same inventor's triple projecting
lantern. The principle of the analysis of the colours
in the object photographed, and the subsequent build-
ing up of the colour-records to produce a coloured
result, are similar in both cases. In his earliest experi-
ments, he made use of strip-film negatives taken
alternately through red, green, and blue filters. When
he had made a positive film from this negative film,
and proceeded to project his pictures on to the screen
by red, green, and blue light respectively, the results
were almost colourless, on account of the excessive
actinic action of the blue light which had produced the
blue negative record; and the correspondingly over-
powering effect of the blue light which reached the
screen through the blue filter. This obliterated both
the other two images. In other words, the exposure
necessary to get satisfactory red and green records
was utterly out of scale with that required for the
blue record.
Another serious objection to the use of red, green,
and blue was that the normal speed of the kinemato-
graph film, which is one foot per second (each foot
carrying sixteen exposures), required to be increased
KINEMATOGRAPHY IN COLOUR 207
to three feet per second (forty-eight exposures in the
same time). Such an increase of speed would, of
course, involve prohibitive expense and complicated
and expensive mechanical devices for the manipulation
of the films at this high speed.
Further experiments with the Ives' Kromskop and
the comparison of the appearance of the coloured
image when viewed in daylight (illuminated with
white sky) as compared with artificial illumination,
led Mr. Smith to the following discovery. If the
blue light in the former case were cut off, the appear-
ance of the coloured image was utterly spoilt. In the
latter case, however, the blue light could be dispensed
with altogether without seriously altering the effect of
the coloured image. He attributes these phenomena
to the fact that most artificial lights are very deficient
in blue — a fact well known to every photographer.
Our eyes are to some extent accustomed to this excess
of red and green rays (i.e. yellow rays) and deficiency
of blue ones.
As a result of the above, Mr. Smith has perfected the
" Kinemacolor " apparatus to use red and green filters
only, the want of blue being met by using a green filter
which passes a considerable amount of blue light.
Herr Kasimir Proszynski in a Paper read before
the Eoyal Photographic Society (see Journal R.P.
Society for March, 1913) considers that the continuity
of kinematograph vision is a purely psychological
illusion, and is not dependent on the persistence of
vision hitherto believed to be the cause. He employs
a shutter consisting of three similar wings separated
by three intervals of precisely the same shape, for
208 PHOTOGRAPHY IN COLOURS
according to him three wings are necessary. By
revolving this shutter 15 times a second, we obtain
45 alternations of light and dark, and 15 separate
pictures, which is just sufficient to eliminate all flicker-
, ing. He states that intervals of ^th to ^jth of a second
comprise the limit of our perception, and that this
interval is necessary to avoid flickering. Although the
writer has studied Herr Proszynski's paper carefully,
he cannot see anything in it which refutes the usually
accepted theory, nor does he see wherein his modifica-
tion of the shutter is superior to the ordinary form
in use.
§ 104. The " Kinemacolor " Camera is similar to
that used in ordinary black and white kinematography,
except that it is built to run at twice the speed, viz. two
feet, or thirty-two exposures, per second. The shutter
used is, of course, a rotary one, and so geared to the
handle by which the film is moved that the light only
reaches the film whilst it is at rest. The essential
difference between the ordinary kinematograph camera
arid the " Kinemacolor " camera is that the latter has
a rotating colour-filter which is placed between the
lens and the shutter. This filter consists of an
aluminium skeleton wheel having one segment filled
in with red-dyed gelatine, and a similar one filled in
with green -dyed gelatine, and is so geared that the
exposures are made through the red gelatine and the
green gelatine filters alternately. The film, when thus
exposed, is developed and from it a positive film is
made by contact in the ordinary way.
If the negative " Kinemacolor " film be examined it
will be found to consist of images in pairs, which
PLATE XI.
Through
green filter
Through
red filter.
Through
green filter.
Through
red filter.
JUtiliL
Kinemacolor negative film.
Kinemacolor positive film.
To fact p. 208.
KINEMATOGRAPHY IN COLOUR 209
differ from each other inasmuch as they are records
of the red and the green in the object photographed.
We have selected for our illustration (Plate XI.) four
successive pictures of a plant, with bright red flowers
and green leaves, standing in a brown flower-pot. The
first exposure is made through the green filter. As
nearly the whole of the red and much of the blue are
absorbed by the filter, the red flowers in the negative
will show hardly any silver deposit, while the green
leaves will be quite dark, and the brown flower-pot
will have an intermediate hue. On making a positive,
everything will be reversed. The red flowers will show
a dense deposit, the green leaves hardly any, and the
brown flower-pot an intermediate one.
The next exposure, taken through the red filter,
will exhibit exactly the reverse. If you look at the
positive (Plate XI.) you will notice the red flowers show
hardly any deposit, and the leaves are quite dark, while
the brown pot, since it is merely a mixture of red,
green and a trace of blue (all degraded with black),
has the same density in both positives.
§ 105. In the " Kinemacolor " Projector the rotat-
ing colour-filter is placed between the condenser and the
gate or fire-proof shield. When the two pictures are
rapidly projected one after the other on to the screen
the combined effect gives rise to red flowers and green
leaves in a brown flower-pot — as it should be. Had
the positive film been placed in the wrong order, i.e.
the " green " picture opposite the red light, the colours
would have been the complementary ones, all through
the film. So in order to prevent mistakes, a white
dot is placed on the film opposite the " green " picture
p
210 PHOTOGRAPHY IN COLOURS
in the black and white image which has to be projected
by green light. If on rotating the handle the colours are
seen to be complementary, it may be instantly remedied
by shifting the mask one whole picture width either up
or down, and at the same time recentring the arc or
limelight. Furthermore, whenever a title appears on
the film, it should be so threaded as to show red
letters on the screen. If this is done all the colours
will come in their right order. It will thus be seen
that with a little care both the taking and exhibiting of
" Kinemacolor " pictures are very little more difficult or
troublesome than ordinary black and white pictures,
while there can be no doubt that with films correctly
exposed and developed and in perfect register, the
pictures in colour are often more pleasing to the eye
than the ordinary black and white.
The chief defects of kinemacolor pictures are : 1st.
If the objects move rapidly, fringes of red and green
are seen bordering the objects. Thus, if a person
suddenly raises his bared arm, it will appear at one
moment red, at another moment pink, and again
greenish ; or else the arm will have a fringe of red or
green. 2nd. The range of tints and hues is very
limited. This is inevitable, seeing that all the effects
are produced by means of the rotation of two colours —
an intense red, and an intense green disc. 3rd. The
colours are painfully intense and vivid. There is an
entire absence of greys and neutral tints, which are
always present in nature, and which soften and tone
down the harsh and saturated colours. But consider-
ing the limitations, the results reflect great credit on
the inventors.
PLATE XII.
The Kinemacolor Projector, showing Colour Filter in position.
To face p. '210.
KINEMATOGRAPHY IN COLOUR 211
In preparing the rotary colour-filter it is necessary
to bear in mind that red is more vivid to the eye than
green, so that the balance of colour in the two dyed
gelatines must be correctly maintained. This is done
by making the red the standard, and adjusting the
green to equalise it.
A single film of red gelatine and one of green
being fitted to the shutter, a second film of green
gelatine is added to the first, trimmed to the requisite
width, by trial, and then affixed to it. As a rule it
is made to occupy the middle third of the gelatine.
If it is too narrow the green will preponderate and
yellows will have a greenish hue; if it is top wide
the green will be too dense and the red will be in
excess, so that yellows will have an orange hue. If
of correct width the filter on being rotated will show
a disc on the screen of a neutral white hue.
The perfection of the picture on the screen is largely
due to the correctness of the exposure and the skill of
the photographer. If more than two feet of film were
exposed per second the objects would appear to move
slowly ; if less, too fast. If the exposure is too short
the colours will be unnaturally vivid, if too long the
colours will be dull. On the other hand, if the sub-
ject is too brightly lighted the picture will be white
or colourless. If too dark the colours will be poor,
clogged, and without detail.
It might rightly be asked, if only red and green
filters are used, how can blue effects be produced?
One may either use a yellow-green filter which would
give the correct hue to grass and foliage, or a deep
blue-green which would give an imperfect colour
212 PHOTOGRAPHY IN COLOURS
to grass, but which in artificial light would give an
effect indistinguishable from a pure blue itself. Hence,
Mr. Smith has made a compromise by sacrificing a
little of the purity of each. If the green used for
projection (which, by the way, is a different shade from
that used for exposing the film) be examined by the
spectroscope, it will be noticed that quite a considerable
amount of blue passes through in addition to the green.
In the next place advantage has been taken of the
effect of yellow light on greens. Now, if blue be
mixed with a very large proportion of white light
such as we find in our Northern blue skies, or re-
flected from large sheets of water, and it be seen
through a green glass — although it appear greenish
in daylight — yet in artificial light it will appear indis-
tinguishable from blue, and that is what happens
when projected on the screen. The blues of the sky
and sea or rivers appear to the eye on the screen as
greenish-blues, although taken through a green filter, and
the effect is sometimes natural and pleasing. Further-
more the combination of the red and green lights gives
rise to the sensation of greenish-yellow, pure yellow, or
orange, according to the preponderance of green or red
light in this mixture. In any case there is always a
trace of residual yellow and residual blue in the high
lights. Hence, the addition of this residual blue to the
yellow will give rise to white. This is the explanation
of how the whites and the blues are reproduced by
" Kinemacolor."
§ 105A. Gaumont's Method of Colour Cinemato-
graphy.— Although this method is the oldest of all, it
is in the writer's opinion superior to the Kinemacolor
PLATE XIII.
To face p. 212.
CINEMATOGRAPHY IN COLOUR 213
process, and is briefly as follows. Three small cinemato-
graph pictures are taken with an exposure of the l/30th
second through three lenses simultaneously, each being
furnished with its respective colour filter, and after
each exposure the film is moved on over the space
occupied by the three pictures, i.e. about 56 mm., or
roughly two and a quarter times as much as for the
ordinary cinematograph picture. Projection is accom-
plished by three lenses, each carrying its own colour
filter, which is slightly different from the one used in
taking the negative, and registration is effected by
moving the top and bottom lenses in three directions
by a very ingenious mechanism. This method only
reached a practical stage by the production of exceed-
ingly rapid and highly colour-sensitive emulsions.
CHAPTEE XIII
COLOUR PHOTOMICROGRAPHY
§ 106. Permanent records of microscopic objects are so
essential that no worker can afford to be without a
photographic camera, since the old methods of drawing
objects through a camera lucida can never be absolutely
reliable, and are besides tedious to execute. Objects
which require a high magnification to be seen, are
nearly always transparent, and can be made out just
as well in monochrome, provided that a panchromatic
(or "Wratten M plate) with a suitable colour filter be
used. Still, since many anatomical and botanical
structures take on a selective action with certain stains,
it is advantageous to reproduce them in the colours
seen. Moreover, when slides are projected on to a
screen, the effect is greatly enhanced by colour, and
often details can be shown which are lost in mono-
chrome.
The five methods at present in use are the Auto-
chrome, the Dufay, the Paget screen plates, and the
three and two separate plate methods.
As regards the choice of methods, the three and two
colour separate printing methods are undoubtedly the
most satisfactory, but at the same time the most
tedious, and present far more technical difficulties to
COLOUR PHOTOMICROGRAPHY 215
the amateur except in practised hands. They are
entirely free from all traces of grain, and thus show up
the finest detail, and by reason of their great trans-
parency form ideal slides for the lantern. Neverthe-
less, the single-plate methods are so simple that most
workers will use nothing else. The Dufay and Paget
plates are much the best for lantern projection, but the
Autochrome plate is preferred by many, because the
colours are truer to nature, and the losses through
failures are fewer than by any other method. Since
the working details of all these processes are given
very fully in other parts of the book, they need not be
referred to here.
It is in opaque objects that colour is best shown, and
such objects rarely require high magnification, i.e.
above 30 or 50 diameters. Low power microphoto-
graphy presents far fewer difficulties to the tyro than
high power work, since none of the adjustments
require anything like the same precision in regulating,
in fact, below 50 diameters a substage condenser will
rarely be needed.
We may divide the subject into two heads : low
power, and high power photomicrography.
§ 107. Low power Photomicrography. — A few
points may be useful to the reader regarding the
apparatus.
The two most essential things are : first, to secure
rigidity and freedom from all vibration in all the parts ;
and secondly, correct centering of the axial rays.
The table. — A strong kitchen table answers every
purpose. A couple of rails or guiding rods, made of
mahogany, and extending the whole length of the table
2l6 PHOTOGRAPHY IN COLOURS
on either side of the middle line, and about five inches
apart, should be screwed down.
The, camera. — Bach end of the camera should consist
of a square frame of mahogany. The front part should
rest below on a broad wooden or metal support, which
fits exactly between the guides so as to allow of its
sliding freely backwards and forwards without any side
shake. The sides of the guides facing each other
should have a rectangular groove (or rabbet) running
the whole length, and each wooden support should be
provided with a corresponding tongue which should fit
the rabbet. This allows the supports to be clamped
in any position along the table by means of a good
broad screw, which screws through the support and can
be made to press with its flat end against the table,
the counter pressure being made by the tongue against
the rabbet. Both the front and back of the camera,
as well as the microscope, water-trough, screen, con-
denser, and radiant, should each be provided with a
similar support, but as the camera front and back are
the only parts liable to shift (owing to the spring of
the bellows), the other accessories do not require a
clamp. The camera should have at least 1^ inches of
rising front which can likewise be clamped in any posi-
tion, while the back which holds the focussing screen
and double slide should be fitted so as to allow its being
moved laterally across the support through a distance
of about 2^ inches on either side of the middle line,
and clamped in any position. The use of this will be
explained presently. The ocular end of the microscope
should be connected with the central aperture in the
front of the camera (just inside the flange of the lens)
COLOUR PHOTOMICROGRAPHY 21 J
by means of a large metal collar which while excluding
all extraneous light allows of free movement between
the microscope and camera without the slightest con-
tact between them.
The back of the camera must be square so as to
allow of vertical or horizontal pictures being taken at
pleasure. My camera is a whole-plate one, with
adapters for half-plate, 5x4 and quarter plate sizes.
This is the apparatus which I have used continually
for many years past, and I have never been able to
improve upon it. It was originally made from the
description given in Dr. E. C. Bousfield's book,1 which
teems with original and valuable information.
The sliding support for the microscope should have
a recess to hold each leg in position, or if the micro-
scope has the German tuning-fork shaped foot, each
prong can be clamped by a movable wooden button.
My camera has an extension of 4 ft. 4 ins., which
just permits of 50 magnifications with a 25 mm.
(1 in.) Protar Anastigmat screwed on to the front.
Any ordinary anastigmat of one, two, or three inches
focal length will give just as good an image as the
best microscope objective. Zeiss introduced a series
of Planar objectives of 25 mm. and 50 mm. focal
lengths, and F/4, 5 aperture, which are identical in
construction with ordinary photographic objectives,
and can be employed equally well for the microscope,
bioscope, or snapshot camera. They are fitted with
an iris diaphragm, and have the standard microscope
screw. When screwed on to the front of the camera
1 " Photo-micrography," by Dr. E. C. Bousfield. J. & A.
Churchill, London.
2l8 PHOTOGRAPHY IN COLOURS
enlargements may be photographed without a micro-
scope or any other accessory apparatus.
§ 108. Illumination. — A bright, even illumination
over the entire object to be photographed is of the
highest importance. As a rule daylight can be em-
ployed with advantage, but in many cases, especially
where high relief or shadows are necessary, artificial
light is imperative. The simplest method is to place
an incandescent bulb mounted on a stand on each
side of the lens. It is well to fix a mirror or tin
reflector behind the luminant, so that the light is con-
centrated on the object, and at the same time screened
from the lens. By using only one light, or by placing
the pair at different angles and distances from the
object, the latter can be thrown into any required
degree of relief or shadow. If two 60 c.p. Osmium
incandescent bulbs be used, the exposure will be very
short for unit magnification. I find 3 or 4 minutes
ample when using a Planar lens, working at F/8. In
order to ascertain whether the condenser is correctly
centred, move the back of the camera with the screen
fitted in its place, until the margin of the circle of
light projected by the lens is brought into view. Then
adjust the condenser or the radiant, until the margin
of the circle is as sharp and free from colour as possible.
If a projecting eyepiece is used, this same method will
inform you at once which is the best distance of
separation between the front and back elements of
the ocular. Having made the necessary corrections,
the back of the camera is swung round to its original
axial position.
§ 109. Methods of ascertaining the Magnifica-
COLOUR PHOTOMICROGRAPHY 219
tion.— It is well to remember that whatever the focal
length of the lens, the camera must be extended as
many focal lengths + 1 from the lens as the number of
magnifications (m) required, while the distance of the
object from the lens will always be the 1/mth of that
distance + 1. For example, suppose a magnification
of 8 times (diameters) be required, and the lens
chosen is of 3-in. focus. Then the camera must be
extended (8 + 1) X 3, or 27 inches from the optical
centre of the lens, while the object must be 27/8 inches
from the centre of the lens = 3f inches. Again,
suppose the object is to be taken natural size with
the same lens. Then as the magnification = 1, the
camera must be extended 1 + 1 times F, or 6 inches,
while the object will be 1/mth of that distance, or
1 + 1/1 times = 6 in., i.e. the same distance as before.
Lastly, suppose the object is to be reduced to | the
size, then the distances will be the same as in the
first example, only image and object will have to
change places, so that the object will now have to be
27 inches away, and the image 3| inches.
If we require the total magnification of the camera
extension, plus the magnification produced by the
microscope, we have to consider three factors.
First, we have the initial magnification produced by
the objective. This is given in all the catalogues issued
by the makers. It may be easily reckoned by adding
a nought to the denominator of the focal length
expressed in inches or fractions of an inch, and divid-
ing this by the numerator. Thus, in the case of a
2-inch objective, the denominator being 1, we have for
the initial magnification 10 divided by 2 = 5 times.
220 PHOTOGRAPHY IN COLOURS
In the same way a 1-inch objective magnifies 10 times,
a J-inch objective magnifies 40 times, a ^-inch
objective 120 times, and so on. This initial magnifica-
tion is again magnified by the eyepiece, which as we
have shown is modified by the tube length as measured
from the collar where the objective screws in, to the
top of the tube, or a little below it. In most of the
Continental makes this is 160 mm., while most of the
English models measure 10 inches, or 250 mm. As a
rule, the magnification is marked on the ocular for the
tube for which it is to be used, but if it is to be used
for any other tube length the factor is
Actual tube length A
Standard tube length Focal length of eyepiece
A being the standard tube-length. Thus, if the focal
length of the eyepiece be 1 inch, or 25 mm., we shall
have a magnification of 160/25 or 6| times for an
average Continental microscope, or 250/25 or 10 times
when used with an English standard microscope.
Lastly, in order to get the total magnification of a
compound microscope attached to a camera, it is
necessary to multiply all three factors together. Thus,
suppose we are using a ~ inch objective with an
inch ocular on a 10-inch tube, and the camera is ex-
tended 20 inches. The factor for the camera extension
extension in inches extension in mm.
= ~ — ^TFT- — iT~ ~or ' r.gn — , which in
10 inches 250 mm.
the above case = f§ = 2 times. Hence the total magni-
fication = 120 X 10 X 2 = 2400 times. In order to
check this it is quite easy to measure the magnification
COLOUR PHOTOMICROGRAPHY 221
direct. We place a microscope slide ruled in l/10ths
and l/100ths of a mm. (which can be obtained of any
dealer) on the microscope stage, and focus for one of
the l/10th divisions if for a low power, or a l/100th mm.
if a high power. Suppose in the latter case the
interval between two lines measures 6*5 mm. on the
focussing screen of the camera, then the magnification
is clearly 100 X 6-5 or 650 times.
§ 110. Exposure. — The correct exposure is, as we
have more than once pointed out, of supreme import-
ance. Many elaborate calculations have been given in
the text-books for arriving at the correct exposure by
multiplying various factors together, but the simplest
and, as a rule, the best and most accurate way, is to
make a series of trial exposures on a portion of the
plates to be used. It is well worth while to have a
special carrier made to fit into the slide. A quarter-
plate can readily be cut up lengthways in a subdued
Virida or Wratten " Safe-light " into three or four
equal strips, and one of them placed in the carrier
after the object has been focussed on the ground-glass.
The shutter of the slide should have previously been
ruled on the inside with white vertical lines, say 2 cm.
apart, and numbered consecutively. When the slide
is in position draw out the entire shutter, and expose
for what you would consider to be about a quarter the
correct exposure (say two seconds). At the end of the
two seconds cap the lens, close the slide 2 cm., and
expose the rest of the plate for another 2 seconds.
Repeat the process and expose for 4 seconds, again for
4 seconds, and finally for 8 seconds. In this way the
first two centimetres will have had 2 seconds exposure,
222
PHOTOGRAPHY IN COLOURS
the next two centimetres 4 seconds, the third 8 seconds,
the fourth 12 seconds, and the last piece 20 seconds
respectively. When the strip of plate is developed,
one of the portions will almost certainly be correctly
exposed. This will be shown by giving a clear plucky
negative, with full details in the half-tones and shadows,
which latter should not be too dense to print easily,
and a little experience will at once decide this when it
is held up to the light. Once the correct exposure is
known, any modification in the factors which influence
it can at once be arrived at by calculation.
§ 111. Factors which Influence Exposure. — The
following are the chief factors which influence exposure :
1. Character of the light.
2. Degree of magnification.
3. Numerical Aperture (N. A.).
4. Speed and colour sensitivity of plate.
5. Colour and density of screen.
1. Character of the light. — The following table gives
the approximate factors for the most appropriate
luminants : —
Factor for
Approximate
Light source.
candle-
power.
Panchromatic
"M" plate.
Ortho-
chromatic
plate.
Ordinary
rapid
plate.
Oil flat-wick lamp .
15
1
4
8
Incandescent gas
35-60
1
t
i
Nernst lamp (1 amp.)
200
&
i
i
Acetylene ....
40
A
i
i
Direct arc (4 amp.) .
300
&
B1*
&
Direct arc (30 amp.)
3000
TOGO
7&J
Too
COLOUR PHOTOMICROGRAPHY
223
Theoretically the exposure is directly proportionate
to the candlepower. As, however, different kinds of
luminants vary enormously in their richness in blue-
violet rays, as well as in the amount of yellow light,
it is impossible to calculate off-hand what the exposure
should be with any other kind of luminant. The above
table based on practical experience will be found very
useful. They are for the most part taken from the
" Kodak " pamphlet, " Photomicrography," issued by
the Kodak Co., Ltd.
2. Effect of magnification. — The exposure varies as
the square of the magnification. The following table
is based on this law : —
Magnification.
Exposure.
10
Too
25
TV
50
1
100
1
250
6
500
25
1000
100
3. Effect of N. A. — The exposure varies inversely as
the numerical aperture.
224
PHOTOGRAPHY IN COLOURS
Objective.
2" or 50 mm.
Average N.A.
0-15
Exposure.
10
1" or 25 mm. . . 0-25 . . 4
§" or 16 mm. . . 0-35 . . 2
£" or 12 mm. . . 0-45 . . 1J
J" or 8 mm. . . 0-50 . . 1
i" or 6 mm. . . 0-8 . . f
£" or 4 mm. . . 0-85 . . £
i"or 3mm. . . 0-9 . . i
JM ' - iM°ri
apoch. oil imrn.
JL" or 2 mm. , . 1 to 1 imm. . . do. do.
12 o 4-
4. Speed and colour sensitivity of plate. — These factors
are generally given on the boxes and indicated by
H. and D., Watkins, or Wynne. (See Appendix.)
5. Effect of colour and density of screen. — This varies
considerably with different kinds of luminants.
EXPOSURE FACTOR OP WRATTEN'S " M " PLATES WITH
VARIOUS SCREENS AND LUMINANTS.
Screen.
Transmission.
Oil.
Nernst.
Arc.
Incand.
Gas.
Acety-
lene.
A Scarlet . . .
("red end to line |
3
6
6
6
5
B Green ....
600-460^/i
12
12
12
12
12
G Blue-violet . .
510-400
25
16
12
12
12
E Orange . . .
red end to 560
2
3
6
6
4
F pure Bed . . .
„ „ 610
6
6
8
12
8
G strong Yellow .
„ „ 510
1£
2
4
4
3
HBlue . . . .
540-420
24
16
12
16
16
K3 for orthochro.
production . .
_r_
1£
1J
1|
3
2
A and D deep Red
red end to 640
60
90
240
240
120
B and E Yellow-
green ....
560-600
120
60
250
120
90
G and H pure
Green ....
510-540
1000
1600
1600
1600
1600
B and C Blue-
green ....
460-510
1000
600
600
1000
800
D and H Violet .
420-460
200
150
64
160
90
B and G Green .
510-600
25
25
64
20
30
COLOUR PHOTOMICROGRAPHY 225
According to Messrs. Hind and Bandies, liquid
screens of the same light transmission have, as a rule,
lower factors.
The way to make use of these tables is as follows :
If, after having made a trial experiment in the way
previously indicated by giving a series of different
exposures, you merely require to alter one of the factors,
say, the magnification, it resolves itself into a simple
rule-of-three sum to obtain the correct exposure.
Thus, suppose your first magnification was twenty-five
times, and you require a second negative with a mag-
nification of 100 diameters, then, if the correct expo-
sure was 3 seconds, with an oil lamp having a factor
of 1, with the new magnification and employing incan-
descent gas having a factor of \, the correct exposure
will be 3 x ( -OF) X J, or 16 seconds. Of course, all
the other factors must be dealt with in the same way.
§ 112. High-power Photomicrography. — It is
assumed that the reader is acquainted with the general
principles of high-power objectives and substage illumi-
nation, and they will, therefore, not be further discussed.
The microscope, camera, and condenser system may be
arranged either in the horizontal position or vertically,
in which case the light is thrown vertically upwards
along the optic axis by the ordinary substage mirror.
The advantages of the horizontal form are that one
can sit comfortably while arranging the specimen
under the microscope, and when focussing on the
screen. Further, that a very great extension of
camera may be employed.
The objections are that fresh liquid preparations
Q
226 PHOTOGRAPHY IN COLOURS
are inadmissible, and that the viscosity of the cedar
oil often interferes with the final focussing when using
immersion lenses, by rendering it necessary for the
slide to be held down by clips, which are absent in
most mechanical stages.
In most horizontally placed apparatus there is some
difficulty in observing through the microscope. This
has been eliminated in the large apparatus of Zeiss by
the employment of two separate tables, one for the
microscope and illuminating systems, and one for the
camera.
The vertical form of apparatus has the advantage
when photographing liquid preparations, and it obvi-
ates the necessity of swinging the instrument into the
horizontal position every time it is used for photo-
graphy. Also the slide will remain in focus by its own
weight and the ordinary clamps of the stage. I find
that most of the fine adjustments retain their foci
better when the instrument is vertical. Since the
vertical camera is usually short, the fine and coarse
adjustments can be used without any extending rods
or gearing for the purpose of focussing. The inevit-
able drawbacks to the vertical apparatus are first that
a camera of limited length is almost compulsory. If
one wishes to obtain a magnification of, say, one thou-
sand diameters, using a 2 -mm. objective and la No. 4
eyepiece, the camera screen must be about 50 cms.
from the plane of the projection ocular, a distance
which is only just within reach of the fine adjustment
screw when the operator is observing the image on the
screen. Again, it is much less fatiguing to focus when
the image is vertical than when it is in a horizontal
COLOUR PHOTOMICROGRAPHY 227
position, especially when the head is enveloped in a
focussing cloth.
Bearing these things in mind, I felt convinced that
the ideal apparatus for high-power work lay in the
combination of a vertically placed microscope with a
horizontal camera. It is true that such an arrangement
can be contrived with the most recent form of the
large Zeiss apparatus, in which the camera can be
racked up to a sufficient height to meet the eyepiece of
the vertically placed microscope ; but the whole appa-
ratus is not only very expensive and cumbersome, but
is of such a length that a fairly large room is required
to use it with comfort. This apparatus also suffers in
common with all other horizontal arrangements by the
necessity of a gearing for the transmission of the fine
adjustment motion to the operator's hand when focus-
sing the image. However perfect the gearing may be,
it cannot be trusted to keep in focus while the dark
slide is being introduced, or until the exposure is com-
plete. After having constructed and experimented
with the two forms of reflex apparatus, I have adopted
the following arrangement, which I have found most
satisfactory. Fig. 27 shows the apparatus diagram-
matically. A A' is a strong table 25 mm. (10 inches)
wide (shaded in the diagram). C is an asbestos-lined
cover with changeable circular diaphragms in front.
D is the usual condenser which collects the light to a
point close to the mirror of the microscope. F is
another stand to carry any- further fittings, such as an
auxiliary lens, or a trough with colour filter solution,
etc. B B' is a strong, rigid table, carrying G the
microscope in a vertical position, upon a strong stool,
228
PHOTOGRAPHY IN COLOURS
having at least 3 inches clear space below it. This
clearance, together with the fact that the stool top is
B B'
m, Mitled, ktad for reusing or lou'eriruj stage.
V ,/7t«.7rtinx'u7rt/ red wi-fK, delineate. cbi,tcfi> TV
-nvilte£ not- of JSerger fine, motion..
S,$ 8c$S.
Jv.lt! Tath* o-J 'retys
<£ fta.ctia.nt-
t Blackened tuLe nvaJci
joint: itritfv hcocl of rewrslng prism,
The- dotted circle ske-ws area, covered.
<Z*t:
FIG. 27.
quite open between the feet of the microscope, will be
explained later. H is a mount holding a small revers-
ing prism with silvered hypotenuse at 45 degrees to
the two optic axes, and provided with . a blackened
PLATE XIV.
o
•a
[To face p. 228.
COLOUR PHOTOMICROGRAPHY 229
hood which makes a non-contact, light-tight junction
with the camera front. J J' is the camera made to
slide on two round bars, so that it may be bodily with-
drawn to a short distance from the microscope.
Another view of the central part of the apparatus is
shown in Plate XIV. The camera is drawn back from
the prism hood so as to show the rod leading to the
slow motion behind the camera. In the foreground,
on the optical bench, is an extra iris diaphragm as
well as a tank for light -filtering solutions. The ordi-
nary mirror of the microscope is in position.
This arrangement presents at least two great advan-
tages. Firstly, the operator is enabled to sit com-
fortably at the point B, and perform any focussing or
other adjustments with the microscope, thereby dis-
pensing with a complicated revolving table, etc.
Secondly, a straight, light rod of aluminium or other
metal can be instantly set in action with the small
milled head of the Berger or other fine adjustment.
It requires for this purpose only a very simple fitting.
All gearing for transmitting the fine motion is there-
fore done away with, and, if only a small, cloth-lined
V support be fitted near the observer's hand, the rod
need not be lifted or lowered at all. Owing to the
vertical position of the microscope, and the consequent
elevated position of the camera, the aluminium rod
passes beneath the camera to the right of its supports.
It will be found advantageous to have a cloth-lined
gutter arranged alongside .the apparatus to hold the
rod when not in use, so that it may not sag or bend
when at rest. A further advantage of this contrivance
is that the microscope can be protected in a moment
230 PHOTOGRAPHY IN COLOURS
when work is finished by simply covering it up with a
bell-jar as it rests on the stool. The 3-inch clearance
below the latter is to enable the first-surface mirror to
reflect the light upwards. This enables a more perfect
reproduction of the original beam of light to be formed,
and also permits of longer patterns of aplanatic con-
densers to be inserted below the stage. Further, it
allows of room for the insertion of an arrangement for
the quick changing of colour filters. It is sometimes
useful to arrange the three-colour screen of a Sanger-
Shepherd repeating-back over the foot of the micro-
scope, especially if it has a horseshoe stand. If a
mirror be adapted below the stool it may be fixed
exactly at 45, so that the light will remain per-
manently centred. Of course, in this case the optical
bench must be correspondingly lowered. The whole
arrangement can easily be constructed by any one
with a little mechanical skill, the only precaution to
be observed being that the camera supports at either
end must be narrow enough to enable the slow-motion
rod to pass straight on to the milled head. The rods
which slide in supports with the camera are easily
made from brass-plated curtain rods, but a solid bar
of square or prism- shaped section would be preferable
to the two round rods. The only objection to the form
of apparatus above described is that there is no way
of varying the height of either the microscope as a
whole, or of the camera, to allow for the difference of
focus of different lenses. This is not a serious objection
at all, and can be easily surmounted by having a front
to the camera which can be raised or lowered within
a small distance, or else by slightly withdrawing or
COLOUR PHOTOMICROGRAPHY 231
closing the drawfcube, and at the same time correcting
the magnification by lengthening or shortening the
camera. A still better way is to employ a microscope
like the Zeiss pattern I S, in which the stage itself can
be racked up and down independently of the rest of
the microscope. This instrument is very solidly built,
and for this reason is eminently suited for photo-
micrography, and it is to be hoped that British
manufacturers will furnish some analogous form of
stand.
Details relating to high-poiver colour photography. —
The majority of high-power colour work consists of
pathological and histological, and especially bacterio-
logical preparations. Passing on to the actual subject
of making photomicrographs in colour, I take it that
the object of the great majority of operators in this
field is not to produce transcendental ingenuities in
colour for their own sake, but to represent as accurately
as possible any microscopical preparations which they
wish to place on record, or to demonstrate with the
lantern. I shall write, therefore, from the point of
view of the bacteriologist and pathologist. The
majority of the preparations which a bacteriologist
will wish to accurately represent are slide or cover-
glass preparations of cultures or secretions stained
in one colour. The colours will be either blue
(methylene blue), violet (gentian or methyl violet),
or red (fuchsine).
(1) The preparation is stained with methylene blue.
Let us suppose a magnification of 1000 diameters is
required. We take a rapid isochromatic plate (Wrat-
ten's isochromatic is strongly recommended for this
232 PHOTOGRAPHY IN COLOURS
kind of work). Use a deep orange screen or a scarlet-red
screen (Wratten's " G " screens superimposed answer
well). Select a 2-mm. oil immersion apochromat or
semi-apochromat of 1,4 or 1,3 N. A., and a No. 2
projection ocular. (By No. 2 we mean an ocular
giving two magnifications.) The camera must be
racked out to a distance of 100 cm. from the hypo-
teneuse of the inverting prism to the focussing screen.
Of course, if a No. 4 ocular be used, the camera must
be racked out 50 cm. It is a good plan to have a
scale fixed along the whole length of the table marked
out in inches and centimetres.
Exposure. — The tables for finding the correct ex-
posure have already been given under the heading
of low-power photography, and they apply equally
well in this case. I may add, however, that with a
radiant of about 750 cp., which I obtain with a star
pattern triple filament (thick type) Nernst lamp, the
exposure would be about 90 seconds for a bacterial
culture. In all photographs of this nature the object
is to get as great a contrast as possible, and in the
negative the bacteria should appear as nearly clear
glass, and the background as black as possible. We
must therefore employ a contrast or hard developer.
For this purpose I have found the following developer
very efficient : —
Solution A. Solution B.
Hydroquinone . . 9'5 grms.
Sod. sulphite . . 50 grms.
Citric acid ... 3-5 grms.
Pot. bromide . . 3 grms.
Water . . 500 c.c.
Sod. hydrate (pure) 90 grms.
Water . . 500 c.c.
COLOUR PHOTOMICROGRAPHY 233
To develop take equal parts of A and B and add from
half to an equal part of water.
Push development to the full, and fix thoroughly
with acid hypo. It is always easy to clear up with
hypo and ferricyanide of potash, by which the contrast
will at the same time be increased. The negative
having been obtained, make a clean positive on a
lantern plate, clear if necessary, so that the background
is perfectly clear glass. Wash thoroughly, and tone
with the following toning solution : —
Potassium ferrocyanide . . 28 grams
Water 280 c.c.
Bleach the plate in this, and wash for ten minutes.
Then place in Sanger-Shepherd's " Minus Red stain-
ing solution," 1 part to 2 parts water for 1J minutes.
Transfer to hypo (1 to 5). Keep on applying fresh
hypo until a clear blue image is obtained. Clear if
necessary in sulphuric 'acid (1 to 300 water). Any
other good blue formula may be used. Then wash
well.
Now, if the original preparation had been stained
with carbol-methylene blue, it will tend to be of a
greenish-blue colour, and the lantern plate in its
present 'stage will probably be a very accurate re-
production of it. If, however, the stain was an
alkaline methylene blue (Loeffler), it will be more of
a true spectrum blue. To obtain this the lantern
plate should be dried, then soaked in distilled water,
and flooded with very dilute nitric acid, and then
again thoroughly washed. If there is any blue in
the background, a very rapid treatment with dilute
234 PHOTOGRAPHY IN COLOURS
potassium oxalate solution, followed by thorough
washing, will remove it.
(2) In the case of a violet-stained film, proceed
exactly as before, excepting that the light-filter may
be of a lighter orange colour, without any green.
One may use either two or three superimposed
Wratten " G " filters, or a trough containing bi-
chromate of potash solution of corresponding depth
of colour, according to the intensity of the violet
stain. Take a flat celluloid gelatine-coated film,
sensitise in ammonium bichromate, or in the S anger-
Shepherd film-sensitising salt, and dry in the usual
way, out of reach of dust and light. Then print this
out with the uncoated slide against the film of the
negative. Expose until details are visible as a pale
silver image, or, better still, against a Chapman-Jones
" Fraction tint actinometer." Expose simultaneously
to the extent shown to be correct by previous trials.
Then develop the film with warm water and dry.
Make a solution of methyl or gentian violet (methyl
violet 6B), called crystal violet, answers well. A 0*5
per cent, solution of crystal violet added to 100 c.c. of
distilled water. Stain the film in this, and then wash
until the bacteria appear deeply stained on a colourless
ground. When dry, varnish with Sanger- Shepherd
film varnish. The film will look better and clearer if
mounted quite dry and warm between lantern cover-
glasses in melted hard neutral Canada balsam. Wait
until the balsam is quite dry, and then paint round
the edge with hard asphaltum varnish. Bind up as
usual. If it is desirable to add a circular mask in
imitation of the microscopic field, which certainly adds
COLOUR PHOTOMICROGRAPHY 235
to the reality of the image, then omit the balsam on
the celluloid side of the print, and insert the mask
between the cover-glass and this side, after the balsam
of the film-side is dry.
(3) In the case of a red- stained preparation such as
a film of B. Typhi, or Vibrio Cholerse stained with
fuchsine, a green screen must be used for making the
negative, and the positive film must be stained in a
solution of fuchsine. A suitable solution is made by
adding 4 c.c. of a 0*5 per cent, solution to 100 c.c. of
distilled water. Sometimes in examining a fuchsine-
stained slide by daylight, the red colour may not
appear so brilliant as that of the original preparation.
This is because the dye fuchsine, or magenta, transmits
a varying amount of blue in addition to the red rays,
according to the makers' formulae, or the manner of
making the solutions. The yellow rays of the lantern
will largely correct the slight proportion of violet in
the red image. If further correction be desired, a
little eosine (yellow shade) may be added to the
fuchsine solution used for staining up. Lantern slides
made as described above, when looked at through a
blackened tube by transmitted light, give a remarkably
exact reproduction of the picture seen, when the
original preparation is examined under the micro-
scope. Hitherto, only single-stained preparations
have been considered, and it is very desirable to
master the making of these before passing on to
double and triple-stained objects.
A good simple example of double-stained high-power
work is a film of tubercular sputum stained by the
Ziehl-Neelsen method. In this process the tubercle
236 PHOTOGRAPHY IN COLOURS
bacilli appear as brilliantly red minute rod-shaped
objects, while the pus cells, debris, and other bacteria
are stained blue. Put two Wratten " M " plates into
a dark slide, and expose one of them under a green
filter, and the other using a red filter. The first nega-
tive will show the tubercle bacilli very clearly marked,
and must be printed on a gelatine film and stained up
with fuchsine as directed for single-stained red pre-
parations. The other plate will show only a faint
image of the bacilli, but very distinct pus cells. From
this negative a black lantern plate should be made,
and toned as described for blue-stained preparations.
As the gelatine film was printed through the back, if
the two are placed face to face in register, a very good
representation will be the result.
The following are the details of a successful two-
colour slide of tubercular sputum stained by the
Ziehl-Neelsen method on a slide 1 mm. thick.
Magnification. — 1000-2 mm. Apochromatic oil im-
mersion.
Condenser. — Centering achromatic N. A. 1-0 when
used dry — used here oiled to slide.
Radiant. — Nernst lamp. Large projector 3 filament,
star pattern, 220 volt — continuous.
1st " M " plate, Wratten B screen, 90 seconds ex-
posure.
2nd " M " plate, Wratten A screen, 80 seconds ex-
posure.
Developed with hydroquinone in total darkness for
4 minutes. There was a good deal of development fog
which was cleared up by Farmer's reducer, resulting
in very good negatives, except that there was a faint
COLOUR PHOTOMICROGRAPHY 237
image of the tubercle bacilli on the second plate. This,
it was found, could be eliminated by using a liquid
screen, in addition to the Wratten filter.
This liquid filter was made by adding 0*7 c.c. of a
0-5 per cent, solution of diamant fuchsine to 230 c.c. of
water, and the thickness of the layer of fluid was 3 cm.
The theoretical photographer may criticise the above
results, in that a perfect negative was not at once
obtained, and that accurate spectroscopic observations
would have eliminated the necessity for a repetition of
the second plate. Actual work with high powers will
soon convince any one that practice often renders
theory nugatory in colour microphotography, at any
rate in its present stage of development. This is
especially true in the matter of exposure under very
high magnifications. An exposure of, say, a simple
violet preparation under such conditions as I have
indicated above, would work out under the theoretical
formulae at something like 600 seconds, whereas in
practice an exposure of 80 or 90 seconds will generally
yield a good negative, providing development is pro-
perly conducted. The study of absorption spectra is
not only desirable for any one wishing to excel in
colour photography, but, after working out all the con-
ditions for any given preparation, the practical worker
will often retreat to the primitive refuge of placing
certain colour filters before his microscope condenser
in succession, being guided by the visual results thus
obtained. One must, of course, do this under the
same light to be employed in taking the photograph.
In photographing sections stained by the old-
fashioned methods of haematoxylin and eosin, great
238 PHOTOGRAPHY IN COLOURS
difficulties will arise when one wishes to reproduce the
colours with any exactitude. There is a table given in
Messrs. Wratten and Wainwright's pamphlet on
"Photomicrography," which gives the absorption
spectra for three different hsematoxylin formulae ; but
all hsematoxylin and hsematein solutions change as
they are kept in vitro, and sections stain all kinds of
different shades, according to the alkali used to blue
the preparation, and vary from other causes. It is not
of vast importance that a lantern slide of a haema-
toxylin-eosine preparation should show pure blue and
pink, and one may therefore adopt an artifice as fol-
lows : take the negative of a blue and pink hsema-
toxylin section through a red or red plus orange screen,
then develop, and print a black lantern plate. Tone
the lantern plate as pure blue as possible, and stain
up afterwards in weak eosin or erythrosin.
The tout ensemble is generally quite sufficiently
realistic and educative, eosin being a diffuse counter-
stain at the best, and only fitted for blood studies. It
is far otherwise when a section is stained by some
method which entails delicate selective qualities.
Suppose a section be stained first with Unna's
polychromic methylene blue, then treated with differ-
entiating agents, fixed and counterstained with orange-
tannin mixture, and finally counterstained with acid
fuchsin (Kubin S.). In the epithelial cells one may
have a very red-violet tint, which becomes a peacock
blue in certain secreting cells, while red blood-cor-
puscles become a bright orange, and the connective
tissues a whole gamut of brilliant tints, to say nothing
of accidental greens caused by the yellow elements in
COLOUR PHOTOMICROGRAPHY 239
the tissue combining with the blue of the stain. In
such cases it is obviously necessary to resort to one of
the three-colour processes; but, as has been pointed
out, high-power work is seldom required for anatomical
or pathological sections— at least, not over five hundred
magnifications ; but even with these magnifications, no
one plate method will do. The grain of the plate is
too coarse. Hence, for all such work, one of the three
separate-plate methods is far superior. For the
beginner it is much simpler to adopt the Sanger-
Shepherd process as detailed in the booklet issued by
the firm.1 All the necessary apparatus can also be
obtained from them for their processes, as well as for
most of the other three-colour processes. It will be
found that a plate 9 inches X 3 inches is quite large
enough to make lantern slides from, and very little
carpentering ability is required to adapt the whole
arrangement to the back of the micrographic camera.
Instead of finding the exposure ratios through the
three screens by means of crumpled wool or a plaster
cast, one must employ a black and white microscopic
preparation, such as one of the old-fashioned micro-
photographs of engravings, or part of a lantern slide
diagram, or (for high powers) a stage micrometer.
When the ratios have been found for any particular
light and plate, it is only necessary to carefully follow
the directions given in the booklet to obtain the finest
slides possible. But it cannot be too much insisted on
that the instructions issued by the company should be
i « Working Instructions for the Sanger-Shepherd Process of
Natural Colour Photography." Sanger, Shepherd and Co.,
5 and 6, Gray's Inn Passage, Red Lion Street, London, W.C.
240 PHOTOGRAPHY IN COLOURS
rigidly adhered to, for, although many of the instruc-
tions may appear frivolous to the ordinary worker, the
infringement of any one of them is apt to entail much
disappointment and loss of time. In my opinion, the
final cementing of the glass positive, the two celluloid
positives, and the cover-glass is the most troublesome
and tedious of all the stages, and the amateur must
not be discouraged if he should fail at first in doing it
to his satisfaction.
In conclusion, we would strongly recommend the
following works on this subject, which we constantly
refer to ourselves : —
(1) Spitta's " Photomicrography."
(2) Article by Dr. Duncan Reid, British Jour. Phot.
Almanac for 1915.
(3) " Photomicrography," issued in pamphlet form
by Messrs. Wratten and Wainwright, Ltd., Croydon,
England.
(4) " Photomicrography " (Pamphlet). Kodak, Ltd.,
Kingsway, London, W.G.
(5) "Photomicrography," by Dr. E. C. Bousfield.
J. & A. Churchill, London.
(6) "The Photography of Coloured Objects," by
C. E. Kenneth Mees, D.Sc. Lond., published by The
Eastman Kodak Co., Eochester, New York.
(7) " Handbook of Photomicrography," by Messrs.
Hind and Randies. Routledge and Co., London, 1915.
CHAPTER XIV
AET IN COLOUR PHOTOGRAPHY
§ 113. What constitutes Art ?— Unlike science, art
is not governed by well-ascertained laws, but is largely
a matter of education and individual taste. Of course
it has its rules, and is governed by well-established
principles ; but when we come to special cases there
is room for much argument and difference of opinion.
In fact, the only branch of pictorial art which is based
on rigid and undisputed rules is that of perspective.
The more, therefore, we go into detail the more we
must expect hostile criticism. Before dealing with art
itself, it is necessary to have a clear understanding as
to what we understand by colour, with its various
hues and shades as applied to art.
§ 114. Primary, Secondary, and Tertiary
Colours. — A long time ago Sir David Brewster pointed
out that instead of Newton's seven colours, white
light could be resolved into three, viz. red, blue, and
yellow, and for this reason they were termed the three
primary colours. Each was said to be the complement
of the other two. Thus, red was said to be the com-
plement of blue and yellow, yellow the complement of
red and blue, and blue the complement of red and
yellow. A mixture of any two was called a secondary
242 PHOTOGRAPHY IN COLOURS
colour. Thus, green was a secondary of yellow and
blue, and violet a mixture of red and blue. Conse-
quently a mixture of all three in various proportions
formed a tertiary colour. Scientifically this theory is
entirely wrong, and this becomes very obvious when
dealing with spectral coloured light, although true in
the case of pigments. Hence, it is very convenient,
and even necessary, when dealing with pigments.
For instance, pigments mixed in certain proportions
will form various shades of grey, which coloured lights
never do. This fact is a most important point to
remember in water-colour painting, and in retouching
prints made from photographs by the three-plate
method, or process blocks, and it will be found
extremely useful in forming shadows over coloured
parts. It was formerly believed that if pigments
could only be obtained absolutely pure, that a pure
white could be formed by mixing the three primaries
in certain proportions. As a matter of fact, this can
never be done, since in the sense formerly attributed
to the word, every colour is a primary, i.e. every
spectral coloured light can be mixed with some other
coloured light which will produce white light. The
following are the fundamental complementary pairs of
coloured lights : —
Ked and Blue-green (sea-green)
Yellow-orange and Blue (cyan blue)
Yellow and, Violet-blue (indigo-blue)
Greenish-yellow and Violet
Green and Purple (red and violet).
Besides these fundamental complements, there are
subsidiary complements, which are formed by pairing
ART IN COLOUR PHOTOGRAPHY 243
intermediate shades of colour. We can illustrate this
in a very convenient way by means of a chromatic
circle. In this the colours of the spectrum are sub-
divided, so that twenty-four hues are shown in all
FIG. 28. — Chromatic circle showing the complementary pairs of
colours, which are indicated by the same numbers.
which may be arranged in a circle corresponding to
their actual position in the spectrum. In this circle it
will be found that each colour is exactly opposite to its
complementary, i.e. 180° away from it.
Although any two complementaries will produce the
244
PHOTOGRAPHY IN COLOURS
sensation of white, it is not a true white, for Helmholtz
pointed out long ago that the only two colours which
would produce an absolutely pure white were yellow
(or yellow-orange) and blue in certain proportions. If
you examine the colour diagram you will observe that
Violet.
Indigo-
blue.
Cyan-
blue.
Blue-
green.
Green.
Greenish-
yellow.
Yellow.
Red
Purple
Dark
Rose
Light
Rose
White
Whitish-
yellow
Gold-
yellow
Orange
Orange
Deep
Rose
Light
Rose
White
Light
Yellow
Yellow
Yellow
Yellow
Light
Hose
White
Light
Green
Light
Green
Greenish-
yellow
Greenish-
yellow
White
Light
Green
Light
Green
Green
Green
Light
Blue
Sea-
blue
Blue-
Green
.&
Blue-
green
Sea-
Blue
Sea-
blue
Cyan-blue
Indigo
Blue
FIG. 29. — Diagram showing the effect of Spectral Colour Fusion
after v. Helmholtz.
green is the only colour which cannot find a partner to
form white, or even an imperfect white, since the com-
plementary colour to green is purple, which is a mix-
ture of red and blue, and not a primary at all.
§ 115. As regards colour in art, we have to dis-
tinguish between hue, tint, and shade.
ART IN COLOUR PHOTOGRAPHY 245
Hue. — This may be defined as an extremely narrow
portion of the spectrum which corresponds to a definite
wave-length, in other words, it corresponds to a
certain definite colour. It corresponds to the pitch of
a musical note. In a wider sense, it comprises a mix-
ture of any pair of primaries in any proportion.
Painters employ it in this sense.
Tint. — This is a hue diluted with white. The
amount of admixture of white defines the tint. It
corresponds to quality in the case of musical sounds.
The addition of white will alter the tint without affect-
ing the hue.
Shade. — This differs from a tint in that the hue is
altered by the addition of black, or, indirectly, by vary-
ing the illumination. It corresponds to loudness when
referring to musical sounds. For example; the hue
" red " gives every variation of tint from red to white,
and every variation of shade from red to black.
Hence, a tint is any colour to which white has been
added, a shade any colour from which white has been
subtracted.
§ 116. How one can shade or produce Shadows
in a Coloured Drawing or Photographic Print in
Colours. — It is a common error with a beginner to
imagine, if he wishes to make a coloured painting or
print look darker or in shadow, he must add more
colour and make it thicker. Making the colour thicker
only makes it deeper in tint and more saturated ; it
never makes it darker or more in shadow. If he wants
to make a colour darker, i.e. to place it in shadow,
he must make it greyer. This can be done, not by
adding Hack paint, however diluted with water, but by
246 PHOTOGRAPHY IN COLOURS
painting over the surface with a mixture of the three
primaries, in other words, a mixture of red, yellow, and
blue paints in certain proportions, the proportions
depending on the degree of grey or shade required.
As a rule, much yellow, a little red, and more blue is
required, and if one does this to any water-colour
drawing, it will have the effect of being in shade. The
best way to throw a light-yellow surface into shadow
is to add a small quantity of red while the paper is
still wet. This will give it an orange colour. Then
add somewhat more blue paint. In the same way,
in order to darken a blue surface, add some red, and
then a good deal more yellow. For a red surface,
wash over with a little blue, and then considerably
more yellow. By varying the proportions of these
three colours, you can obtain any shade you please.
In each case the colours must be mixed on the paper.
Instead of spreading the two-shade colours on with a
brush, it often greatly improves the effect if the colours
be finely stippled over. The dots are quite invisible at
a very short distance, and the colours blend together,
and, if well done, the effect is often most charming,
giving rise to a very soft and delicate surface. Useful
pigments for this purpose are * —
Yellows. Reds. Blues.
Chrome. Burnt sienna. Cobalt blue.
Gamboge. Brown madder. French blue.
Ochre. Crimson lake. Prussian blue.
Eaw sienna. Vermilion. Ultramarine.
Light red.
1 These colours are taken from Mr. H. A. Eankin's admirable
little book entitled " The Teaching of Colour," published by Sir
Isaac Pitman and Sons.
ART IN COLOUR PHOTOGRAPHY 247
§ 117. General Hints as to Colour.— A few of
the following hints may prove useful to the beginner
in relation to this subject.
Always avoid very large areas of the same colour
when taking a coloured photograph, as they weary the
eye and detract the attention from the general compo-
sition. The more brilliant and prominent the hue, the
smaller it should be in the case of a coloured print.
But in the case of a transparency, very much larger
areas may be employed with effect, owing to the light
shining through the positive, and in this case the
colours cannot be too brilliant.
Of all the colours, red, and especially a bright
scarlet or vermilion, is the one which requires the
greatest judgment in photographing. It- is surprising
how little red or orange there is in Nature, and especi-
ally in landscapes, if we except the red skies at sunset ;
and when it does occur, it is almost invariably toned
down by a liberal admixture of browns, greys, and
other sombre colours. I tested this by taking a large
number of photographs through a spectrum-blue glass,
which cut off all the red and orange rays, leaving the
yellow, for the most part, unaffected. And I found, to
my astonishment, that many of the prints showed very
little difference from those taken on a panchromatic plate
through a yellow screen, which, of course, is sensitive
to all colours, including red and orange. Still, in almost
every colour print, and certainly in every transparency,
one or two large patches of bright red, together with
others of a more sombre shade, greatly improved the
effect.
It is well to remember that both bright red and
248 PHOTOGRAPHY IN COLOURS
orange have a stimulating effect on the senses and
exhilarate; bright sky-blues, nearly saturated, give a
pleasing effect. On the other hand, very pale blues
and light greys are anything but pleasing over large
areas, while greens have a soothing effect and are
especially restful to the eyes.
Large areas of white fatigue the senses and dazzle
the eyes, whereas small areas have the opposite effect.
As we have just stated, the amount of unmixed red in
landscapes is so small that it is often advisable to
shift the point of view so as to include if possible some
considerable amount of red or other bright colour in
the foreground or middle distance, in order to brighten
up the picture. This is especially the case if the view
is very largely made up of green foliage.
In order to make a pleasing picture, it is most
important to see that whatever colour is dominant —
by which we mean that it occupies the greater part of
the picture— the colour should not be of the same
uniform hue or shade, but that it should be repeated,
or echoed, as it were, in various shades and tints
throughout the picture, so as to give a sense of repose,
while at the same time the main subject of the picture
should have the most pronounced hue of all, so as to
fix the interest on that spot. Every picture should
exhibit unity of purpose throughout, as well as one
and only one idea and episode, all the other parts
being contributory and accessory to it. The com-
position of a picture is every whit as important in
colour photography as in an oil or watercolour paint-
ing. No one will dispute the fact that a photograph of
a house or cottage will have a more pleasing effect if
ART IN COLOUR PHOTOGRAPHY 249
it occupies only a portion of the picture instead of the
whole width, and that the house should be balanced
by setting it off with a certain amount of foreground.
Again, it will be far more artistic if the building be
photographed from one side, so as to represent one of
the side walls in addition to the front, instead of being
taken horizontally so as to show merely the front of
the house without any depth or sense of perspective
whatever. No one with any artistic feeling will take
a photograph of a road which stretches vertically up
through the centre of the picture, nor will he set his
camera right in the middle of the road, with the latter
reaching nearly up to the lens, so that the print will
exhibit a vast isosceles triangle of bare road cutting
out the greater part of the view. He would certainly
improve his picture if he raised his camera as high
above the road as possible. By this means the nearest
distance which forms the immediate foreground of the
road can be photographed a very considerable distance
away from the camera, so that the sides of the road
would appear more nearly parallel, and thus produce
a less violent perspective.
In order to contribute to depth, it is well to arrange
the main lines of the picture obliquely, or more or less
in diagonals to the sides. This will allow of a con-
vergence of the perspective lines towards the vanishing
point, as well as gradations in size of similar objects
as they appear to recede, and this will largely add to
the sense of depth. This is always an important
element in a picture, because, in Nature, every object
possesses three dimensions which in consequence
give rise to a solid stereoscopic effect; whereas a
250 PHOTOGRAPHY IN COLOURS
picture has of necessity only two dimensions, viz.
height and breadth, and to give the sense of three
dimensions, the artist has to create a number of
illusions and contrivances. To this end he employs
shadows, aerial effects, vanishing lines, contributory
curves, and various other devices for increasing the
sense of perspective and depth. Moreover, in colour
photography, any colour can be intensified or reduced
by local intensification or reduction, or the whole can
be modified so as largely to contribute to these qualities.
§ 118. Shadows. — Shadows are especially useful in
order to give depth and plasticity (stereoscopic effect) to
the picture, and so remove the appearance of flatness
which is so conspicuous a fault in most of the photo-
graphs made by beginners. This fault may be avoided
— if there is no choice in taking the view— in a very
simple manner by merely altering the position of the
source of illumination, or if — as in landscape work —
that is impossible, one must alter the direction of the
view, so as to get the source of light more on one side.
If that is of no use, one must wait until the sun occupies
another position, or else defer taking the picture until
the evening, when the shadows will be longer, and the
high lights have diminished. The position of the sun
is of paramount importance, and its effect on the
picture should be a matter of careful study if you
really wish to excel in your work.
Never photograph any one in a white dress, or
indeed, a large, white object of any kind if it be
illuminated by a light directly in front of it, i.e. behind
the camera. Always secure a side light, or even one
well above and in front of you, if the sun be sufficiently
ART IN COLOUR PHOTOGRAPHY 251
high up to allow of the lens being cast into shadow.
By this means you will be able to get as many half-
tones as possible. White, unless in comparatively
small quantities, is very unsatisfactory. In ordinary
monochrome photography, it is easy enough to re-
produce pure white over a surface of any size, but in
colour photography it is almost impossible to secure
a large, unbroken surface of pure white by combining
the three primary colours. If, however, the surface is
small, or largely broken up by half-tones and shadows,
very pure whites may be reproduced and remarkably
pleasing effects obtained. Beware of going to the other
extreme, and allow black shadows to fall on the figure.
This is especially important when photographing the
face and other exposed parts.
Never take snow scenes with the sun directly behind,
or you will get a flared picture with no half-tones,
whereas, if the sun is nearly or even quite in front of
the camera and fairly high up, the results are often
magnificent, the snow and ice being full of lovely
purple half-tones. But to secure this effect you must
screen the lens from direct sunlight by means of a
flap-shutter, or your hand or hat, or otherwise you will
inevitably spoil the picture by the light striking the
lens obliquely and causing flare and fog.
§ 119. Choose Simple Subjects. — The beginner
is generally impressed by the beauty of a distant
panoramic view, and naturally concludes that a colour
photograph of what is spread out before him will
make a most impressive picture. When he has taken
and finished the picture, he is invariably disappointed
with the effect produced. The reason for this is two-
252 PHOTOGRAPHY IN COLOURS
fold. In the first place, the view which so enchanted
him embraced an angle of about 150 degrees or more,
whereas the actual angle of the picture taken only
includes about 40 or at most 45 degrees; in fact, a
mere slice of the panorama. No wonder that it proved
disappointing. Again, unless a very large plate be
used, everything in the far distance appears dwarfed,
and all detail is lost, so that the eye wanders aimlessly
over the picture without anything large enough for the
eye to dwell upon with any satisfaction, since, unlike
an ordinary photograph, it can only be enlarged with
difficulty, and not at all if taken on a single colour-
plate. The experienced photographer, on the other
hand, will select a far more modest subject, often one
which to the casual observer would appear entirely
destitute of interest ; such, for example, as a dripping
well, set off with a group of ferns, or even an old
rustic porch, or perhaps a bank covered with primroses.
Such subjects would seem very commonplace, and he
would certainly never dream of photographing them ;
and yet they are the subjects which are most frequently
selected for a medal. Wherever possible, introduce
figures, and see that they wear "bright colours, and never
black. Life of some kind, whether animals or human
beings, always make a picture more interesting ; they
balance the surroundings, throw back the distance,
and give strength and plasticity to the whole. But
when you introduce figures into the landscape, don't
on the one hand commit the fault of making the
figures so small as to be lost in the landscape, nor, on
the other hand, of placing the figures right in the fore-
ground so as to reduce the landscape to a mere back-
ART IN COLOUR PHOTOGRAPHY 253
ground of the figure. Of the two, this is by far the
worst fault. In a landscape the figures should always
be selected to harmonize with, and balance the picture.
An inspection of Mr. H. P. Eobinson's picture entitled,
" Wayside Gossip," or of Gale's " Sleepy Hollow," re-
produced in the former's little handbook of " Art
Photography," x will show what is meant.
It is also important to see that the colour of the
dress harmonises with its immediate surroundings.
Thus, a red parasol or cloak will often work wonders
in brightening up a landscape. You must be sure and
see that there is sufficient contrast to bring the figure
into relief, so as to catch the eye at once. If, there-
fore, the person is wearing dark clothes, do not place
him in the shadow or in front of dark foliage, or the
broad trunk of a tree, but, rather, select a position in
which the surroundings are as light as possible. A
bright object placed next to a dark object or a deep
shadow will make the bright one appear still brighter
and the dark object darker. The more contrast the
greater the prominence and the depth. Try and imi-
tate Turner, Claude, or Cuyp, and employ every artifice
to give the effect of depth and plasticity to your pic-
ture. A visit to the Turner room in the National
Gallery will well repay a visit. Turner was in the
habit of selecting some point such as the sun, which
he placed above the horizon as the point of fixation, so
as to give the impression of infinite distance, and all
the objects in the foreground and middle distance he
artfully arranged so as to direct the observer's eye
towards this point.
1 See " The Amateur Photographer's Library," No. 4. Pub-
lished by Hazel, Watson, & Viney, Ltd., London.
254 PHOTOGRAPHY IN COLOURS
One of the great secrets of pictorial photography is
to endeavour to compensate and balance all leading
lines and masses. If the lines incline in one direction,
try and arrange the point of view so that other lines
compensate by their inclination in a nearly opposite
direction. By this means a sense of stability is secured,
and the observer becomes unconsciously satisfied.
§ 120. Portrait Photography. —When taking a
group, endeavour to arrange the figures so that they
stand or sit down naturally, just as they would do if you
happened to come upon the group unawares. If some
of the persons are standing with their backs to the
camera, or are sitting down, or happen to have their faces
turned round in another direction, so much the better,
as it will appear more natural. Very often the effect
will be greatly enhanced by arranging the figures in
an irregular, pyramidal group, with the heads at dif-
ferent heights, and the faces turned towards each other
in a natural manner, as if in conversation. This idea
is carried out to perfection in H. P. Eobin son's cele-
brated photograph, " A Merry Tale," which will be
found reproduced in the book referred to on a previous
page. His work on " Picture Making by Photography"
contains many illustrations which bear out in a
graphic way the maxims I have laid down in this
chapter.
Most amateurs, and even many professional photo-
graphers, are in the habit of arranging a group as if
the members were drawn up on parade, with the
result that there is no picture at all. One sees merely
a row of individuals, all staring straight in front of
them. This is a fatal mistake, and the further it is
ART IN COLOUR PHOTOGRAPHY 255
departed from the nearer it will be to an artistic
picture. Often a portrait taken with the back turned
round so that the face is invisible in the camera, will
actually be a better portrait, and more characteristic of
the person, than a full-faced one. One of Whistler's
greatest triumphs — a portrait of his mother — was taken
in this way.
Let us take for example a photograph of a couple of
children. The photographer who has no idea of art
will place them side by side right in front of the
camera, and staring into it like a couple of dolls, or
else leaning their heads together in an idiotic fashion,
while he sets the figures off by a hideously painted
background, or, worse still, by a wall-paper decorated
with endless baskets of impossible flowers. On the
other hand, the artist will go to work very differently.
He will think out some natural scene out-of-doors.
Perhaps he will arrange the children playing at hide-
and-seek on each side of a moss-covered trunk of a big
tree, and just catch the expression of the one looking
round the corner with the face in full view and laugh-
ing merrily, while the other has its face turned partly
away, and is peering round the opposite side of the
tree. Or he may photograph them unawares, sitting
down on a bank or under a hedge gathering black-
berries or buttercups, which they are busy placing in
a basket. What could be more charming than the
picture of two children by Millais, entitled, " Cuckoo " ?
This is art, because it illustrates a scene in their real
lives. They are doing something which they are
accustomed to do, in a natural manner and without
any pose. All that is required in either case is to
256 PHOTOGRAPHY IN COLOURS
arrange the dress surroundings .and illumination so as
to harmonise perfectly, and quietly await the oppor-
tunity when the children are unconscious that their
portraits are being taken, and as a result you will have
a real picture. I recollect seeing in one of the photo-
graphic exhibitions in London a photograph of a
number of fishermen and boys leaning over a sea-wall
with some shipping in the background. It was called
"A Stern View," and hardly a single face could be
seen. But this picture was wonderfully true to life,
and was so excellent that I believe it was awarded a
medal. It cannot be too often repeated that unless
you mix brains with your colours, whether in a paint-
ing or a photograph, it will fail to have much value.
Although 'a photograph is incapable of producing the
full individuality of the artist, nevertheless a colour
photograph will always contain evidence of the photo-
grapher's soul and inspiration in every part of the
picture, if it is to rank as a really artistic production.
§ 121. Backgrounds. — A suitable background greatly
adds to the beauty of any coloured object which may be
selected for reproduction. Pottery, flowers, fruit, fish,
butterflies, and other natural objects will often appear
tame and commonplace without a background, or with
an unsuitable one, but will be made beautiful and
striking when shown up by an artistic background.
If the object requires a strong relief, a very dark twill,
or, even better, a piece of black velvet or velveteen,
will do admirably, provided the object be light in
colour, and especially if it is a bright yellow or orange.
On the other hand, a dark object, or one showing a
fully saturated colour, such as crimson, emerald green,
ART IN COLOUR PHOTOGRAPHY 257
or Prussian blue, will show up best against a creamy
or bluish-grey background. It is rarely advisable to
select the exact opposite or complementary colour, but,
as we have already mentioned, a hue some 20 or 30
degrees away from the opposite side of the chromatic
circle of colours (see Fig. 28) may with advantage be
chosen. The complementary colour generally affords
too severe a contrast to blend harmoniously with the
subject.
Sometimes the same hue in a much darker or lighter
shade is pleasing. Often greys, browns, or some other
neutral or composite hue will serve the purpose best.
As a rule, a simple colour without any pattern should
be chosen. The reason for this is that any pattern,
however inconspicuous, will, to some degree, draw the
observer's attention away from the subject of the pic-
ture. It is, however, quite permissible to vignette the
background, or vary the tint or shade over certain
parts. In many cases, should strong relief be desired,
the light should be so arranged that the object will
cast a deep shadow over a portion of the background.
This, of course, can be readily effected by arranging
the lighting so that it emanates from one side only,
the rest of the room being more or less in subdued
light.
The backgrounds which I can recommend from
experience are brown, light-brown, pale-green, dark-
green, and dark-blue twill, or stiff linen cloth, which
should unroll quite flat, without any creases. Also
red, purple, and cream-coloured velvet or velveteen
are most useful for exhibiting many brilliant objects
such as jewels, coins, butterflies, and beetles, etc.
s
258 PHOTOGRAPHY IN COLOURS
These show up splendidly by contrast on cream or
coloured velvet, and they can be made to appear in
high relief by shutting out all superfluous light and
confining the illumination to one side of the object, so
as to cast deep shadows.
APPENDIX
THEORIES OF COLOUR VISION.
SEVERAL theories have been made to explain the
phenomena connected with colour vision. But none
of them will explain all the facts, although each of
them in turn has its special advantages. The two
theories most in favour are those known as the Young-
Helmholtz and the Hering theories.
1. Young -Helmholtz theory. — This theory, which was
originally suggested by Thomas Young about the year
1807, and slightly modified by Helmholtz, assumes
that there are three types of nerves in the retina, each
tuned to respond to one of the three primary colour
sensations, viz. — red, green, and blue-violet. By
decomposition of the three photo -chemical substances
stored up in the retina, the nerve fibres are stimulated
to respond to the frequencies of vibration corresponding
to these colours. These vibrations generate impulses
in the nerve ends which are conveyed to the visual
centres in the grey matter of the brain, and the mind
perceives the colours developed.
By suitable mixing of these three colours, every
shade and hue can be produced. Thus White is the
result of the fusion of all three colour sensations, or of
any two complementary coloured lights, while Black
260 PHOTOGRAPHY IN COLOURS
on the other hand results from the absence of all
stimulation of those parts which are capable of
responding to colour stimuli.
Helmholtz constructed a scheme to illustrate the
effect of stimulating the photo-chemical substances
which produce the three colours in different degrees
according to the different colour observed. Thus
yellow will be produced by the fusion of much red
and green, together with a trace of blue ; while blue is
caused by the full stimulus of blue substance with a
FIG. 30. — Scheme illustrating the Young-Helmholtz Theory of
Colour Vision.
The curves represent the intensity of stimulation of the
three colour substances.
little green and a mere trace of red. It will be noticed
from the annexed figure (Fig. 30) that it is impossible
to stimulate anyone of the primaries without at the
same time affecting to some extent the other two.
Unfortunately, if we try to imitate in practice any
of the colour sensations in the same proportions as the
curves in Helmholtz's diagram, an almost colourless
or dirty white will result. Hence some physiologists
have suggested that the curves should take a different
form.
There are many objections to the Young-Helmholtz
APPENDIX 261
theory in addition to those mentioned in § 22. Thus
we are not conscious that the sensation of white is a
blend of two or more colours, as we invariably are in
such mixtures as peacock green or purple. Again,
towards the periphery of the retina we can perceive
whites and greys notwithstanding that part is colour
blind. Moreover, we perceive black as a real impres-
sion, although Helmholtz explained it as being due to
a state of quiescence or rest of the visual cells.
2. Hering's theory. — This theory also assumes three
photometric substances which give rise to six different
qualities of sensation, arranged in three pairs, one
sensation in each pair undergoing assimilation, while
its fellow undergoes disassimilation. Thus we have a
white-black substance, which when acted upon by
light undergoes disassimilation, and gives rise to the
sensation of white, while the same substance becoming
assimilated gives rise to a black sensation. In the
same way a red-green substance and a yellow-blue
substance exist in the retina, each of which by assimi-
lation or disassimilation results in the sensation of one
of its components. These six sensations can be
tabulated as follows : —
Photochemical substance. Retinal process. Sensation.
-D , /Disassimilation = Red.
Red-green , . (Assimilation = Green.
. XT- ,, v, /Disassimilation = Yellow.
Yellow-blue . . {Assimilation = Blufi>
TH7U-4. vi i (Disassimilation = White.
White-black . . {A-gsimilation = Black<
This theory gives a definite objective cause for the
sensation of white, black, and yellow, and in this
respect is superior to the Young-Helmholtz theory. It
262
PHOTOGRAPHY IN COLOURS
also accounts for yellow as a distinct sensation which
the physiologists demand. Moreover, it is in harmony
with the fact that in certain birds and reptiles we find
yellow as well as red oil globules in the bacillary
layer, and also in the majority of tapeta we find not
only red and green but also intense yellow colours over
large areas. Instead of complementary colours red and
green should be termed antagonistic colours. When
they act simultaneously on a retinal cone, the effects
RG w
,J£xis
Muf -yellow subjtance
FIG. 81. — Scheme to illustrate the Hering Theory of
Colour Vision (after Foster).
The curves above the axis, xx, illustrate catabolic changes
(disassimilation), those below the axis anabolic changes
(assimilation).
neutralise one another and the result is white, or, as
Heron would say, the disassimilation effect remains
over which produces white. It will be seen from the
above description that the Young- Helmholtz theory
agrees best with physical, while Bering's theory agrees
best with physiological, phenomena.
APPENDIX
263
1. TABLE OF EXPOSURES FOB SEPARATE AND
COMBINED COLOUR PLATES
The following table giving approximately the correct
exposures for the Autochrome, Omnicolore, and Paget
plates, has been revised by Messrs. Lumiere, Messrs.
Jougla, and Mr. Dawson respectively, to which the
Dufay has been added. The ratio of the five plates
with their proper filters is as follows in seconds (") or
in minutes (').
Autochrome 1", Dufay f",1 Omnicolore f", Paget
separate plate \ to £, or taking the Paget separate
plate as unity, we get —
Paget separate I", Omnicolore 5", Dufay 4", Auto-
chrome 3 to 4". The following table gives the ex-
posures between the middle of May and middle of
August, with lens working at F/8 and time of day 10.30
to 2.30. Bright sky, white clouds or sun. In cloudy
weather increase exposure 3 to 6 times.
For the benefit of those who do not understand
ratio-apertures, the following table will be found useful.
If we assume the exposure with a stop of F/8 = 1 sec.,
then —
F/4; requires £ sec.
F/5, 6 4
F/6, 3 §
F/6, 8 f
F/8 1
F/ll 2
F/16 4
F/22 ' 8
F/32 16
1 According to the Author's experience Dufay plates should
have 1J" and Omnicolore 1J", i.e. slightly longer exposure than
the Autochrome,
264
PHOTOGRAPHY IN COLOURS
Subject.
Bright sunshine 10.30-2.30, May 15-Aug. 15, F/8.
Omni-
colore.1
Dufay.1
Auto-
chrome.
Paget.
Portrait or flower
study, well lighted
room near window
with white reflector
30"-60"
25"-50"
20"-40"
7"-15"
Portraits, flowers,
fruit studies, studio
well lighted .
15"-25"
12"-20"
10"-16"
3"-5"
Ditto, ordinary room
not near window .
4'-6'
3J'-5'
3i'_5'
l'-2'
Ditto, ditto, in open
air, bright sunshine
4"-6"
sr-5-
3J"-5"
l'-2'
Stained-glass window
north aspect with
45"-80"
35"-60"
27"-48"
9"— 16"
much ruby glass.
With addition of Kl
filter to the lens
3'-5'
2'30"-4'30"
2'-3' 30"
40"-80"
Open landscape, no
heavy obj ects in fore-
ground, well lighted
1"— 2"
§"— li"
iP'-li"
1."-!"
Ditto, strong fore-
ground ....
3i"-7"
3"-6"
2i"-5"
1"— 2"
Ditto, very heavy
foreground .
10"-20"
9"-18"
6"-12"
2"-4"
Eiver view, water in
foreground, no dark
objects, well lighted
r-i"
r-r
i"-§"
JL"_I"
Open lake or sea view,
sun or bright light
on water
j"~~i"
r-f
i"-^"
JL." i "
No large objects near,
with addition of Kl
filter
2A"-4i"
2"-4"
12" 31"
i"-l"
Snow and ice scene .
r-r
l"-¥'
tw-9
1 "__!_"
No dark rocks, well
5 o
lighted, with Kl
filter added . . .
2"-4"
1A"_31"
1V-22"
l"-f"
Ditto, ditto, in win-
0
ter, much snow, and
Kl filter added . .
4"-8"
8J"-7"
2§"-5J"
l"-2"
1 According to the Author's experience, the exposure of the
Dufay and Omnicolore plates should be, if anything, longer than
that of the Autochrome. If the exposure of the Autochrome
be taken as 1", that of the Dufay should be about 1£", and
the Omnicolore \\". The exposure of a Paget plate is about
one -third that of an Autochrome.
APPENDIX 265
Stained glass windows with much ruby glass require
a very full exposure, or the reds will appear brick-red
and weak. All ice and snow and seascapes require a
second filter to suppress the excess of violet light;
Wratten's Kl will do. It should be fixed in front of
the lens during exposure. A K2 filter1 or a second
Lumiere filter held in front of the lens during one-
half of the exposure is recommended by some workers.
2. TABLE OF EXPOSUEES OF SUNSETS FOB AUTO-
CHROMES (V. Cremier).
60 minutes before scheduled time of sunset (F/8) 1| sees.
45 „ ,, ,, ,, 3 „
30 „ „ „ „ 6 „
15 ,, ,, ,, ,, 12 ,,
5 „ „ „ „ 22 „
At sunset 30 „
5 mins. past 2 mins.
1 These niters can be obtained from Wratten and Wainwright,
Photographic Plate Manufacturers, Croydon, England.
266
PHOTOGRAPHY IN COLOURS
3. TABLE OP ADDITIVE COLOUR EFFECTS, OR COLOUR
SYNTHESIS (Helmholtz).
Colour.
Violet.
Indigo.
Cyan-
blue.
Blue-
greeu.
Green.
Greenish-
yellow.
Yellow.
Eed
Purple
Dark
Eose
Light
Eose
White
Whitish-
yellow
Golden-
yellow
Orange
Orange
Dark
Eose
Light
Eose
White
Light
Yellow
Yellow
Yellow
Yellow
Light
Eose
White
Light
Green
Light
Green
Greenish-
yellow
Greenish-
yellow
White
Light
Green
Light
Green
Green
Green
Light
Blue
Sea-
blue
Blue-
green
Blue-
green
Deep
Blue
Sea-
blue
Cyan-blue
Indigo
4. TABLE OF RELATIVE BRIGHTNESS OF A STRONGLY
ILLUMINATED SPECTRUM (Vierordt),
Yellow-green being considered as 100.
Eed 2
Orange 12
Yellow 78
Yellow-green ........ 100
Green 37
Blue 12-8
Dark blue 0-8
Violet . 0-07
APPENDIX
267
5. SLOWEST EXPOSURES' NECESSAEY TO SECUEE
SHARPNESS.
Conditions.— Focal plane or other highly effective shutter.
Lens, 5 to 6J-in. focus. Nearest object, 50 feet.
sec. sec.
Ordinary street scenes with traffic. No rapid motion 1 to ^
Trees, moving with light breeze 2*5 *° s'o
,, strong wind 2tbto3oo
Yachts, motor boats, 10 knots per hour, viewed end on J^ to ^
„ „ ,, ,, broadside on T^
Trains, 30 miles an hour, nearly end on, beyond 50 ft. J^ to ^QQ
For trains nearly broadside on, motor-cars, horses galloping,
divers, birds on wing, etc., all calculations are useless. You
must use quickest shutter, and largest diaphragm compatible
with density of negative and sharpness of image.
With F/4 aperture and bright sunlight in June and July be-
tween 11 and 3, ordinary street scenes beyond 50 ft. can be
taken with J^" exposure on Paget (separate) plates. If the plates
be resensitised by Grant's method, an exposure of ^ sec. can be
given with F/4 stop.
6. FACTOR NUMBERS (WatkM).
Developer.
Temperature 60° F. to 65° F.
Soft.
Normal.
Hard.
Adurol
4
5
6
Amidol l <.
7
10
12
Azol (Johnson) .
20
30
35
Diogen
8
12
15
Dionol (Diamidophenol) ....
Edinol
44
14
60
20
75
25
Eikonogen
8
12
15
Glycin (soda)
6
8
10
(potash)
9
12
16
Factor.
1 Amidol (2 grains to the ounce) has according to some writers
a factor of 18 for normal contrast, and Pyro Metol 14.
The "Agfa" Co. give the following factors: Amidol 18,
Eikonogen 9, Glycin 10, Hydroquinone 5, Pyro-soda 5, Imogen-
sulphite 5, Metol 30, Metol-hydroquinone 14, Ortol 10, Bodinal 30.
268
PHOTOGRAPHY IN COLOURS
Developer.
Factor.
Temperature 60° F. to 65° F.
Soft.
Normal.
Hard.
Hydranine
5
7
9
Hydroquinone
3
4'5
5
Imogen ...
4
6
8
Kachin
7
10
12
Kodak powders . . .
13
18
23
Metol (Hauff)
20
30
35
Metol-hydroquinone
Metaquin
10
9
12
12
15
14
Ortol
7
10
12
Paramidophenol
12
16
18
Paraphenylene
20
25
30
Pyro-catechin
7
10
12
„ (Crystals) ....
Pyro-metol
22
6
30
9
35
11
Pyro-soda without bromide 1 gr.
» » » 2 ,,
>» i> » " »
»> » a 4 ,,
» »> » 5 1>
Pyro-soda with bromide (half the
above factors) . . .
13
9
7
6
5
18
12
10
8
6J
22
14
12
10
8
Pyro-soda (Imperial)
Quinomet
4
9
4f
12
5£
14
Kytol (Burroughs Wellcome & Co.)
Eodinal
10
30
12
40
15
50
Synthol ...
22
30
35
NOTE. — The factor (at least in the case of Pyro and Amidol)
varies inversely with the percentage amount of the active
ingredient, and inversely with the amount of restrainer (Bromide,
etc.).
The factor governs the contrast thus: For more
contrast, use a higher factor ; for flat negative, or soft
contrast, use a lower one.
Eoughly speaking, for soft contrasts use three-fourths
of the normal factor ; for strong contrasts, add one-fifth
to the normal factor.
APPENDIX 269
Example. — Metol-hydroquinone is used as the de-
veloper. The image first appears after 20 sec. Since
the factor is 12, the plate must be left in the developer
for 20 x 12 sec., i.e. 4 min. If soft contrast be desired,
the plate must be left in for 20 x 9 sec. = 3 min., and
for hard contrasts, for 20 x 15 sec. = 5 min.
Rule for factor developing. — Multiply the number of
seconds that have elapsed between pouring on the
developer and the first appearance of the image by the
factor number. The product gives the time that the
plate should remain in the developer.
Double emulsions, such as Cristoid films and
Thomas's plates, require at least double the time of
the factor. Other plates, whether slow, fast, or iso-
chromatic, do not appear to affect the result.
Rule for combination developers. — If equal quantities
of each be used, half the sum of the two factors will
be the factor of the mixture. If the mixture contains
unequal parts, proceed as follows : —
Let/ = factor — number of solution A ;
f = factor — number of solution B ;
x - number of ounces of A ;
y = number of ounces of B.
Then the combined factor number
TP fo+fit
x + y
Example. — A mixture is made of 4 oz. of hydro-
quinone and 1J oz. of metol. What is the combined
factor F? The factor of hydroquinone is 5, that of
metol is 30, therefore
s + (1-5 x so) = 12
5-5
270 PHOTOGRAPHY IN COLOURS
This does not hold strictly true with pyro developers,
which affect the speed of other developers in a different
way.
As regards the ultimate image, all developers appear
to give the same, or nearly the same, result, but the
rate at which the image first appears, as well as the
time necessary to acquire a standard density and
gradation, differ enormously.
Thus, in the case of rodinal, metol, and dianol
(diamidophenol) the image flashes out quickly, but it
requires to be developed for a long time in order to
acquire sufficient density, while, in the case of strong
pyro-soda, adurol, and hydroquinone, the image takes
a long time before appearing, but requires a short
development to secure the necessary density.
INSTKUCTIONS FOE DEVELOPING
AUTOCHBOME PLATES.
7. PYROGALLOL DEVELOPER FOR AUTOCHROME
PLATES.
Some operators still prefer Lumiere's " Pyro " de-
veloper, which is as follows —
1st Development (Stock Solutions).
A A. Sod. Bisulphite (commercial solution) . 2 drops
Pyrogallol (dry) 3 grms.
Bromide of Potassium 3 „
Distilled water 100 c.c.
BB. Anhydrous Sodium Sulphite ... 10 grms.
Ammonia (0-923 or 22 B) 15 c.c.
Distilled water 85 c.c.
For a half-plate take equal parts of AA. and BB.
(10 c.c. of each) and add 100 c.c. (3J ozs.) of distilled
water.
APPENDIX
271
Develop (if correctly exposed) for 2J minutes at a
temperature of about 60° F. This bath cannot be used
a second time.
2nd Development.
Diamidophenol 0-5 grms.
Anhydrous Sodium Sulphite . . 1-5 „
Distilled water 100 c.c.
Leave positive in this bath in full daylight from
3 to 4 minutes or more. Wash and allow to dry.
8. LUMIEBE'S FOBMULA (1908).
Quantities sufficient for
whole plate or 7£ X 5 plate.
Time of action.
Remarks.
A.—
Quinomet, 1*5 gms.
2| min. for
Image should appear in
(23 grns.) l
correct ex-
10 to 14 seconds.
posure
Water (distilled) 100
Factor = 12.
c.c. (3J oz.)
Sodium Sulphite, 10
Temperature
Hence time of develop-
grms. (154 grns.)
of baths 15°
ment = No. of sees.
Potassium Bromide,
C. (60° F.)
before image first
0-6 gm. (9 grns.)
Ammonia (density
appears multiplied by
Factor. Thus, if image
0-923) 3-2 c.e. (55m.)
appears after 13",
then 13" x 12"= 156"
or 2J min. (approx.).
Wash 1 min. in dark.
Dissolve the Quinomet in the water, add the Sulphite and
Bromide, then the Ammonia.1
AA.— For use.
distiUed).
To one part of A add 4 parts of water (preferably
1 Quinomet is another name for Metoquinone, and is a chemical
compound and not a mechanical mixture. It consists of Metol
in powder 6*2 gms., Hydroquinone 8*8 gms. It is sold in a solid
form by the Lumiere Co.
272
PHOTOGRAPHY IN COLOURS
Quantities sufficient for whole
plate or Ik X 5.
Time of action.
Remarks.
B. — Reversing bath —
Potassium Perman-
ganate, 0'2 grm. (3
grains)
Sulphuric Acid 1 c.c.
(17 m.)
Water, 100 c.c. (3£
oz.)
2-4 min. ac-
cording to
appearance
of image as
observed
from time
to time in
daylight
After plate has been
in B for \ min., the
dish may be carried
into full daylight and
the transparency ex-
amined. Wash for 1
minute in several
changes of water. '
C. — Redevelopment —
Plate is returned to
AA bath
4 min.
Must be carried out in
full daylight or nega-
tive must first be ex-
posed six inches in
front of 1 ft. of Mag-
nesium ribbon. Wash
2 mins.
D. — Hardening and
clearing bath — •
Powdered Chrome
Alum, 6 grms. (90
grns.), or alum 6
grms., Citric Acid
in powder 0-6 grm.
(9 grns.)
Tap water, 4 ounces.
Quantities need
not be measured.
This bath is optional,
but brightens up and
toughens the film.
Wash 2 minutes.
Then leave to dry.
Fixing in Hypo op-
tional. (Wash 3
minutes.)
Or instead use a 1/500 bath of Permanganate of Potassium
This is preferred by Lumiere to the alu
hot weather Chrome Alum must be used.
'O CU .L^VITX/ UCVUJLL VJ- J- W iJ_LiOULJ.Q«IJ.i«(Uf WJ. J. V/ UGUOO Jt Ct-J-Lt
(without acid). This is preferred by Lumiere to the alum and
citric bath. In
9. LUMIERE'S IMPROVED GRADUATED DEVELOPER FOR
UNCERTAIN EXPOSURES (LATEST FORMULA, 1910).
This developer allows of greater latitude in exposure
than the last mentioned. The other baths remain the
same.
APPENDIX 273
Solution 1. — Place in glass measure (for a quarter-
plate or 5x4) 40 c.c. (1 fl. oz. 2 drms.) of water.
Add Quinomet (concentrated developer) 2-5 c.c. (42 m.).
Call this A.
Temperature about 15° C. (60° F.).
Solution 2. — Also in a second (small) measure put
7-5 c.c. (2 drms. 8 m.) of concentrated developer. Gall
this B.
Place your watch close to green (Virida paper) safelight.
Put plate in dish in nearly total darkness, and pour
developer A over it the moment the second hand reaches
60 seconds. Bock the dish (screened from the light) for
14 seconds, then bring the dish containing plate close
to the light for a moment to see if a trace of the image
has begun to appear (ignoring sky). The moment this
occurs note number of seconds which have elapsed.
Then add immediately contents of small measure (B)
while rocking dish. Put cover over dish and consult
the following table, which you should have previously
written in Indian ink on the outermost Virida paper of
the lamp.
If time which has elapsed Development should be continued
since first appearance (from the commencement) for
of image is minutes, seconds.
12" to 14" 1 15
15" to 17" 1 45
18" to 21" 2 15
22" to 27" 3 0
28" to 33" 3 30
34" to 39" 4 30
If image fails to appear after 40", add 22 c.c. of
Quinomet solution (B).
minutes, seconds.
40" to 47" 3 0
48" to 60" 4 0
T
274 PHOTOGRAPHY IN COLOURS
If no image appears at the end of a minute,
the plate is hopelessly under-exposed. Then wash
in the dark, place in reversal bath and redevelop accord-
ing to instructions given in Table 8. If after reversal the
image looks heavy and dull, i.e. wanting transparency,
it shows under-exposure. This can sometimes be partly
remedied by placing for a moment in a fresh hypo
bath and looked at frequently, until the colours are
more transparent. Then wash well, redevelop, and
intensify.
In hot weather, or if temperature of the solutions
exceeds 68° R, use Chrome Alum bath immediately
after reversal to prevent frilling.
.Reduction or intensification is preferably carried out
immediately after the redevelopment (before positive has
begun to dry). After intensification it must be immersed
in the fixing bath, otherwise the latter bath may be dis-
pensed with. If the second development has not been
very thorough, and no intensification performed, fixa-
tion had better be omitted, as there is risk of producing
a flat, dull image.
As Professor Namias's modification of Lumiere's
formula is very highly spoken of by many amateurs, I
give it here.
9A. Professor Namias's Method of Developing
Autochromes. — Make the following stock solution : —
Soda sulphite (crystals) .... 100 grms. (or 2 oz.)
Ammonia 22 per cent. (Beaume) . 32 c.c. (4J drams)
Pot. bromide 6 grms. (54 grs.)
Metol 4 grms. (35 grs.)
Hydroquinone 12 grms. (105 grs.)
Water , 1000 c.c. (20 oz.)
APPENDIX 275
After reversal, clear in the following solution :—
Potassium ferricyanide 3 per cent, solution . 50 c.c. (3 oz.)
Ammonia 4 ,, (2 drms.)
Hypo 10 per cent, solution 50 ,, (3 oz.)
Dilute with twice its bulk (200 c.c.) of water.
Intensify by the mercury method : —
Bichloride of mercury 0'5 grm.
Common salt 1 ,,
Hydrochloric acid ....... a few drops
Water 100 c.c.
Then rinse well and re-develop in the original
developer. If less effect be desired, use a 5 per cent,
solution of sodium sulphite. In commencing develop-
ment Professor Namias uses two baths (1st) 5 c.c. of
the above-mentioned stock solution to 50 c.c. of water,
and (2nd) 20 c.c. of stock solution to 80 c.c. of water.
He immerses in first solution to find the time which
has expired before the image begins to appear (neglect-
ing the sky), and then he immediately puts the plate
in the second solution and develops by Lumiere's table
(vide previous section, p. 273, Table 9).
10. OTHER DEVELOPERS.
Instead of the Pyro or Quinomet solutions, Eodinal
may be used, diluted 1 to 5 of water for under-exposed
plates, or 1 to 6 for normal exposures, and 1 to 15 or
1 to 20 for over-exposed plates. Develop for about 2
minutes, or, with diluted developer, for 4 to 8 minutes.
Rodinal is a very powerful developer, and is eminently
suited for travelling, as it occupies so little space.
Ordinary tap water may be used. Of course, any
276 PHOTOGRAPHY IN COLOURS
one of the developers mentioned in the Table of
Factor Numbers (No. 6) may be used, and the time of
development regulated by the factor number. Among
those specially recommended are Metol-hydroquinone
(Meto-quinol), Glycin, Eytol, and Kodinal (1-20 or 1-30
of water). This last developer is highly recommended
and very useful for travelling, as it takes up no room
and requires no acceleration.
The Author has discarded Lumiere's graduated de-
velopment method and prefers Metp-quinol, using 12
as the factor number. He covers the dish with a card
the moment the image begins to appear, and does not
uncover it until at least two-thirds of the time has
elapsed.
11. INTENSIFICATION FORMULAE.
Lumiere recommends the following : —
(1) Pyrogallic Acid .... 3 grms. j
Citric Acid 3 grms. / Label F.
Distilled Water . . . 100 c.c.
(2) Nitrate of Silver . . . 5 gr. ) T
Distilled Water . . . 100 c.c. \ Label G'
For use, take 5 c.c. of G and add 5 c.c. of F ; pour into
50 c.c. of distilled water. Pour this over positive, and
rock until colours are sufficiently bright, or until the
solution becomes turbid. Be sure you use the solution
the moment you have added the silver to solution F,
for precipitation of the silver commences in about half
a minute, and the solution becomes black and useless.
Another excellent method is : —
(1) Perchloride of Mercury ...» \ grm.
Common Salt 1 grm.
Water . 100 c.c.
APPENDIX 277
(2) Sulphite of Soda (Cryst.) .--.;.; 5 grms.
Water 100 c.c.
These baths may be used repeatedly.
Leave plate in (1) until the image is completely
whitened. Wash for a minute and place in sulphite
solution. The process may be repeated if more inten-
sification be required, and one may either redevelop
with amidol or metol-hydroquinone developer, or 1 %
of strong ammonia solution may be used instead of the
sulphite bath. This gives a very black image. Then
fix in hypo.
Either of the above methods can be used to intensify
the image or any of the plates mentioned.
As soon as the image has been intensified it must be
fixed in hypo, since during intensification in either
method a silver compound is formed which will be
acted upon by the light.
12. EBDUCTION FORMULA.
(1) Either use the acid permanganate or the acid
bichromate reversal bath diluted 1 to 10, or use —
(2) Farmer's solution, viz. : —
Hyposulphite of Soda 15 grms.
Ferricyanide of Potassium ... 1 grm.
Water 100 c.c.
This gives an even reduction all over the plate. Or —
(3) Persulphate reducer , viz. : —
Persulphate of Ammonia ... 2 grms.
Water . , 100 c.c.
This latter reducer acts first on the denser parts and
leaves the half-tones unaffected, and is for this reason
278
PHOTOGRAPHY IN COLOURS
to be preferred to the others when selective action is
desired. It softens a hard negative. Eock the dish
and examine the plate every half-minute. Wash
immediately the reduction is sufficient.
N.B. — Use the moment the bath is made Up, as it
does not keep.
Stop the action as soon as the image viewed by
transmitted daylight is clear enough. Then, if neces-
sary, re-intensify a little to bring up the colours.
After intensification, clean in neutral permanganate
bath (same as reversal bath for Lumiere plate, only no
acid is added). Then fix in hypo.
13. INSTRUCTIONS FOR DEVELOPING OMNICOLORE
PLATES. (MAKERS' FORMULA.)
Quantities suitable for 7J x 5 to whole plate size.
Solutions.
Duration.
Remarks.
A.— 1st Developer, 13° C.
to 18° C.—
Water distilled, 100
c.c. (3J oz.)
Metol, 0-4 gm. (6 grs.)
Hydroquinone, 0'2
gm. (3 grs.)
Sodium Sulphite
(anhyd.) 5 gms. (77
grs.)
Carbonate of Potas-
sium (dried) 3 gms.
(46 grs.)
Bromide of Potas-
sium, 0-1 gm. (1£
grs.)
Sodium Hyposul-
phite (1 % sol.), 1-5
c.c. (25 m.)
2 to 5 minutes
Wash for 20 seconds in
the dark.
APPENDIX
279
Solutions.
Duration.
Eemarka.
B.— Reversal Bath—
Distilled Water, 100
2 minutes
Wash for 2 minutes in
c.o. (8£ oz.)
the light.
Bichromate of Potash
or Soda, 0'8 gm.
(12* grs.)
Sulphuric Acid, 1-2
c.c. (20 m.)
C. — Bedeveloper — Use
Leave plate in solution
1st developer (A)
in bright daylight
or make up fresh
until it is quite black.
developer
Usually 4 minutes
D. — Fixing Bath —
Water (tap) 100 c.c.
Not to exceed
will suffice.
This bath is optional,
Hyposulphite of soda
(crystal) 20 gins.
2 minutes
but many workers
consider the colours
Bisulphite of soda (or
metabisulphite) 3
are more permanent
and the positive is
gms. (46 grs.)
less liable to change
if it is used.
NOTE. — Any one of Lumiere's formulae (Tables 7, 8 or 9) may
be used for developing Omnicolore or Dufay plates, and the Omni-
colore formulae may be used for Autochrome plates. After the
plate has been in the reversal bath for half a minute, it should
be repeatedly examined in artificial light so as to stop the
moment all details are visible, otherwise the high lights may be
eaten away.
The plate must be thoroughly washed for 2 minutes
after reversal to get all the bichromate out. It is
advisable to put the plate for one minute in a 1 %
solution of sodium bisulphite after washing. Then
place in the old developing-bath and redevelop in bright
daylight. If the plate is veiled or grey, place in a little
of the old reversing solution, diluted 1 to 100 with water
280 PHOTOGRAPHY IN COLOURS
for 20 seconds or so. Then wash and leave to dry. If
the colours are not bright enough, intensify and, after
washing, fix in hypo and again wash thoroughly. If the
image is not intensified, the hypo bath may be omitted.
14. INSTRUCTIONS FOB DEVELOPING THE DUFAY
PLATE.
The instructions given for the Omnicolore plate will
answer perfectly for the Dufay. Or Lumiere's im-
proved Quinomet developer will answer equally, but
it is recommended to use the acid bichromate reverser
instead of the permanganate, as it has greater pene-
trating power — although the latter may be used quite
successfully.
The instructions given with the Dufay plate are as
follows : —
A. Developer
Metol 6 grms.
Sulphite of Soda crystallised . . 75 ,,
Hydroquinone ....'... 2 ,,
Bromide of Potassium .... 2 ,,
Ammonia, -880 12 c.c.
Water 1000 „
For use dilute with equal parts of water.
For a correctly exposed plate, develop for 4 to 5
minutes.
As soon as all the details have shown up, wash for
20 seconds and place in —
B. Reversal Bath
Bichromate of Soda or Potash . . 5 grms.
Sulphuric Acid 10 c.c.
Water 1000 „
APPENDIX 28l
The moment the plate is covered by this solution, take
the dish and negative into daylight, and hold up the
plate for a moment from time to time and examine by
transmitted light, until the image appears quite distinct
in its natural colours. Then wash under the tap imme-
diately. If this is delayed too long the highlights will
be eaten away. The usual time for this bath is 2
minutes.
. Then wash for a minute or two and place in a 10 %
sulphite of soda bath. If the air or water is too
warm (i.e. over 68°), place the plate in a solution of
chrome alum for a minute or two. Then redevelop in
bright daylight in the first bath (A) until the plate is
quite black.
For intensification, soak first in a saturated solution
of perchloride of mercury for 5 minutes, or until quite
bleached through.
Perchloride of Mercury (Sublimate) 40 grms.
Alcohol (Meth. Spirit) 200 „
Water 800 c.c.
Then into a 10 % solution of sulphite of soda in
water.
These two baths can be repeated if desired, and may
be used over and over again.
For under-exposure the density may be reduced by
Farmer's solution (see Table 12), or the Persulphite
reducer, or the reversal bath diluted 1 : 10 or 1 : 15
may be used, if the Hypo, bath be objected to.
282 PHOTOGRAPHY IN COLOURS
15. INSTRUCTIONS FOE DEVELOPING PAGET PLATES.
After exposure the Panchromatic Plate should be
taken from the dark slide and developed in the ordinary
way. The Taking Screen will, of course, be kept for
future exposures.
Most developers may be used, provided the resulting
negative be clean and soft. The best results are
obtained with Eodinal, 1 in 30, and development
should be complete in two minutes.
Unless a Green Safelight is used development must
take place in total darkness. On no account should a
Bed Light or one of any colour other than the Safe
Green be used. Development in total darkness pre-
sents no difficulty, as if the exposure given is about
right, the time of development with Kodinal as given
above will be correct.
Einse the plate and fix in the following bath : —
Hypo 6 ozs.
Potass. Metabisulphite \ oz.
Water 20 ozs.
Wash again for about 15 minutes, and put to dry.
Making the Transparency.
From the negative made in accordance with the
foregoing instructions a contact transparency is made,
and to obtain the best results the following conditions
must be observed.
The Transparency should be of black tone, perfectly
APPENDIX 283
clear, and free from fog, brilliant and full of detail.
These conditions can be secured by using the special
transparency plates issued in connection with this
process, and adhering to the instructions therewith.
Registration.
When the Transparency is dry it is ready to be
registered with the Viewing Screen. The method of
doing this is to place the Transparency upon the
Viewing Screen, film to film, and holding them up
together in this position so as to look through. Keep-
ing one of the plates stationary, move the other about
slowly, maintaining contact all the time, and altering
the position by minute steps, until all signs of any
pattern have disappeared, when a slight movement
either to the right or left will show the picture in its
correct colours. Clip the plates together with strong
letter clips and bind in the same way as with ordinary
lantern plates.
16. ELIMINATION OF GREEN SPOTS.
Mr. J. Mclntosh recommends that the green spots
should be cut out with a sharp knife, and then a
lantern plate exposed in contact with it. The ex-
posure must be very brief. On development, a grey
spot will be seen corresponding in position and size to
the hole. After fixing and drying it the spot can be
retouched and worked up with Aniline colours to
harmonise with the picture. The negative must then
be bound up with the positive film to film.
284 PHOTOGRAPHY IN COLOURS
17. RECENT DEVICES FOB PROTECTING THE COLOUR
SLIDE FROM THE HEAT KAYS OF THE LANTERN.
M. Massiot protects the slide by separating the two
halves of the condenser by means of a freely ventilated
wooden box, which is placed outside the lantern. It
measures about 8" or 10" long. In order to further
diminish the heat, the to and fro carrier is provided
with two converging lenses of a focus selected to suit
the objective. These lenses being thrown outside the
path of the rays with each change of slide, have time
to cool, and thus become only moderately heated. It
must be explained that each lens attached to the
carrier really forms part of the condensing system,
and its addition is necessary to completely fill the slide
with light. Zeiss in his Epidiascope projecting lantern
has quite overcome the difficulty by employing a
mirror which reflects the light through the trans-
parency at such a distance from the source as to
render it perfectly safe from injury. It is now sold in
Paris under the name of the " Frigida " projecting
lantern. It is, moreover, a very much cheaper form of
lantern than the Zeiss model. A somewhat similar
device is fitted to the projecting lantern sold by the
firm of Bouch and Lomb, which may be obtained from
Staley and Co., 24, Thavies Inn, London.
18. SENSITISING AND EESENSITISING COLOUR
PLATES.
Dr. Konig, in a recent number of the "Photogr.
Rundschau," has strongly recommended the following
sensitising bath : —
APPENDIX 285
Alcohol 100 c.c.
Pina chrome-violet (1 : 1000) 3 c.c.
Orthochrome, Pinaverdol, or Pinachrome (1 : 1000) . 3 c.c.
Water (distilled) 200 c.c.
Bathe for three minutes ; do not wash.
The doctor points out the immense superiority of
these dyes over pinachrome behind a red filter, and
over a mixture of pinacyanol and orthochrome when
exposing behind a green filter.
19. COLOUR-SCREEN FILTERS AND MONOCHROMATIC
LIGHT.
The Mercury vapour spectrum yields the following
lines : —
Yellow . . . Wave-length 579 ^ and 576 /u/i
Green . . . „ 546^
Blue .... „ „ 436 ^
Violet. ... „ ,, 407 /iju and 405 /u/i
Bed is entirely absent.
The three following filters will be found useful with
this lamp : —
To transmit yellow light only of \ = 579 ^ and 576 pp.
Potas. bichrom 15 grms.
Copper sulphate . . . r ^ . 3-5 grms.
Sulphuric acid 1 c.c.
Distilled water 300 c.c.
To transmit green light, of A. = 546 /cp
Picric acid . . " . . . . 0'4 grin.
Copper sulphate 3*5 grm.
Didymium nitrate .... 15 grms.
Water. 300 c.c.
286 PHOTOGRAPHY IN COLOURS
To transmit blue light only of \ = 407 w and 405 /*/i
Copper sulphate 1 grm.
Distilled water 225 c.c.
Ammonia (0-880) .... 75 c.c.
20. A. B. HITCHINS' DEVELOPEE.
Owing to his vast experience in colour portraiture
this formula can be recommended with every con-
fidence : —
Metol 6-5 grms.
Sod. sulphite 40 ,,
Hydroquinone . . . . 2-1 „
Pot. bromide 2-5 „
Sod. hyposulphite ... 0-1 gram.
Ammonia (0-880) ... 20 c.c.
Water 1000 „
Carry on development until the high lights and
flesh tones just begin to show reversal and trans-
parency when viewed against the green Virida safe-
light, i.e. in about 3 to 4 minutes. Einse in water
and then place in reverser. This is best made up as
follows : —
Potas. bichrom 4 grms.
Sulphuric acid 15 c.c.
Water 1000 „
Then redevelop with —
Sod. sulphite (anhydrous) ... 21 grms.
Diamidophenol 6 grms.
Pot. bromide (10% solution) . . 100 minims.
Water 1000 c.c.
Continue development for 4 minutes. Temperature
of water 65 F.
N.B. — If a very clear transparent positive be desired,
APPENDIX
287
or for lantern exhibition, add to first developer
4-7 grms. of Ferrocyanide of Potassium (not Ferri-
cyanide), and omit the Hypo.
21. METRIC EQUIVALENT TABLES.
Solid measures (Metric).
Solid measures (British).
1 Milligram =£$ grain
1 Centigram =g7g= 0-154 grain
1 grain =65 milligrams
2 „ =13 centigrams
3 , =19-5
1 Decigram =1'543 grains
0-1 Gramme = 1*5
4
5
= 26
= 32-4
i
0-2 ,, =3
6
, =39
,
0-3 , =4£
7
= 45
f
0-4 „ =6
8
, =52
,
0-5 „ =7*
9
> =58
»
0-6 „ =9
10
,- =65
.
0-7 „ =11
11
= 72
9
0*8 =12£
12
» =78
t
0-9 =14
13
, =84-5
,.
1 „ =15-43
14
=91
,
2 =31
15
, =97-4
f
3 =46
16
, . = 1-04 gra
names
4 „ =62
17
= 1-1
,
5 =77
18
= 1-17
t
'-' J)
6 —92-5
19
„ =1-23
t
,, — <ja v
7 „ =108
20
= 1-3
8 =123
30
, =1-95
i
9 =139
40
=2-6
i
10 „ =154
50
=3-24
t
14 „ =216 = £oz
avoir.
60
= 3-9
a
20 „ =308 grains
28 „ = 437 grains = 1 oz. av.
£ oz. avoir. = 7
£ „ „ =14-17
t
1 ,, ,, =28*35
9-
llb. = 16oz.=454
1
NOTE. — Gramme is generally written " gr.," but English
writers usually indicate it by.gm. or grm., to distinguish it
from gr. (grain), but the occurrence of c.c. in the one case, or
ounces and minims in the other case, will enable the reader to
know at once which measure " gr. " stands for. On the Conti-
nent all liquids as well as solids are sold by weight in grammes.
288
PHOTOGRAPHY IN COLOURS
Fluid measures (Metric).
Fluid measures (British).
1 c.c. = 17 minims (m.)
2 =34 „
1 minim (m.)
5 ,,
= J-r c.c. =0-06 c.c.
= 0-29,
3
= 51 „
10
= 0-59,
3-5
= 60=3i (1 drachm)
20
= 1-18,
4
= 68 minims
30
= 1-77,
5
= 85 „
40
= 2-36,
6
= 1 dr. 41 m.
50
= 2-95,
7
= 2
Idra
chm (31)
= 3-5 ,
8
= 2
15,,
2
= 7 ,
9
= 2J
2£
= 9 „
10
= 2
49,,
3
= 10-65 c.c.
11
= 3
6,,
4
(J fl.oz)
= 14 ,
12
= 3
23,,
5
= 17-5
13
= 3
40,,
6
= 21-3
14
= 4
7
= 24-7
15
A
14 tt
In.
3Z. (Si)
= 28
20
= 5
8,,
2 ,
(B")
= 57
25
= 7
3 ,
(3iii)
= 85
28
= 3i(lfl.oz.) = 480m.
81 ,
(Smss) = 100
30
= 8J dr.
4 ,
(?iv)
= 113
40
= 1 fl. oz. 2 drms.)
1 pint (Oi) '
= 568
50
75
= 5i3vi(lfl.oz.6dr.)
35-2 fl. oz.
1 quart
= 1000 c.c. = 1 litre
= 1-136 litres
100
= §iiis (3i fl. oz.)
1 gallon
=4-546 litres
1000
=1 litre =35-2 fl. oz.
Conversion of grammes per litre into grains per
ounce : multiply the grammes by 0-44, product is
grains per ounce. For c.c. per litre into minims per
ounce, multiply by 0-48. Conversion of grains per ounce
into grammes per litre : multiply grains by 2-3, product
is grammes per litre. Thus 40 grs. in 16 ozs. = 2J grs.
per oz., and 2-5 x 2-3 = 5-75 grammes per litre. For
minims per ounce into c.c. per litre, multiply the num-
ber of minims by 2-3. Thus 20 mm. in 4 ozs. = 5 m.
per ounce, and 5x2-3 = 11-5 grammes per litre.
APPENDIX
289
MEASURES OF LENGTH (METRIC).
1 Kilometre = 1000 M. = 1094 yards = g mile.
1 Metre (M.) = 10 decimetres = 100 cm. = 39-37 in.
1 Decimetre (dm.) = 10 cm. = 3-937 in.
1 Centimetre (cm.) = 10 mm. = 0*3937 in.
1 Millimetre (mm.) = 1000 microns = ^ in. = 0*03937 in.
1 Micron j/t) = 1000 micromillimetres = ^Jooo m-
1 Micromillimetre (ju/t) = 10Ang-\_ , j
strom units (often written A.U.)/ ~ iffirtroiy mm- - 2 sznroooTy in
MEASURES OF LENGTH (BRITISH),
1 mile = 1609 M.
1 foot =1 30-47 cm.
1 furlong = 201 M.
1 inch = 25-4 mm.
1 yard = 91-41 cm.
1 line = 2 mm.
Inches to millimetres.
Centimetres to inches.
Inches mm. cm.
cm. inches.
,V = 1-58 = 0-16
1 = §
£ = 3-17 = 0-32
2 = «
i = 6-35 = 0-63
4 = ]#
| = 9-5 = 0-95
A = 12-7 = 1-27
5 _ j^i
f = 15-9 = 1-59
6 = 2|
f = 19 1-9
7 = 2|
| = 22-2 = 2-2
8O_5
~ ?2
1 = 25-4 = 2-54
9 _ gla_
2 = 50-8 = 5-08
10 = 3^f
3 = 76-2 = 7-6
H -_ 4_5_
4 = 101-6 = 10-1
12 = 4-1
5 = 127 = 12-7
13 = 5J
6 = 152 = 15-2
14 = 5£
7 = 177 = 17-7
15 = 5ff
8 = 203 = 20-3
16 = 6^
9 = 229 = 22-9
17 — 6|£
10 = 254 = 25-4
18 = 7TV
11 = 280 = 28
19 = 7|°
12 = 304 = 30-4
20 = 7|
13 = 330 = 33
21 = 8*
14 = 355 = 35-5
22 = 8|
15 = 381 = 38-1
23 = 9
16 = 406 = 40-6
24 = 9§
17 = 431 = 43-1
25 = 9|
18 = 458 = 45-8
26 = 10*
19 = 483 = 48-3
27 = 10|
20 = 508 = 50-8
28 = 11
The above values are correct to
The above values are correct to
-i mm.
aV in-
u
2 QO
PHOTOGRAPHY IN COLOURS
22. ENGLISH AND FOREIGN SIZES OF PLATES.
Centimetres.
4-5 X 6-0
9 X 12
12 X 16
13 X 18
Continental S
Inches.
If X2§
3-54 X 4-72
4-72 X 6-30
5-12 X 7-08
izes of Plates.
Centimetres.
13 X21
18 X 24
24 X 30
30x40
.7.0*
T Inches.
\ 5-12 X 8-25
-&43-X 8-25
9-44 X 11-80
11-80 X 15-75
English Sizes of Plates.
Inches. Centimetres.
Inches.
Centimetres.
3£ X 2£ 8-9 X 6-4
7X5
17-8 X 12-7
3J X 3£ 8-25 X 8-25
21-5 X 16-5
4J X 3|
10-8 X 8-25
10 X 8*
25-4 X 20-3
5X4
12-6 X 10-1
12 X 10
30-4 X 25-4
6J X 4|
16-5 X 12-0
15 X 12
38-1 X 30-4
23. COMPARATIVE PLATE SPEEDS.
H. and D.
Watkins.
Wynne.
1
H
8
2
3
11
3
4,5
14
4
6
16
5
7J
18
8
12
22
10
15
24
N.B. — Hurter and Driffield's 2, Watkin's 3, and Wynne's Meter
11 correspond to Wellcome's Plate Speed No. 12, which is the
correct number for Autochrome and Dufay plates out of doors.
The makers give for Autochrome Plates Watkin's 3 and
Wynne 11 and for Paget Plates (separate), Watkin's 7£ and
Wynne 18. In both cases with filter in position.
APPENDIX
291
24. WAVE LENGTHS OP VISIBLE SPECTRUM.
768/j.fj. Visible limit of spectrum
Oxygen line
Water vapour
Oxygen „
Lithium „
Hydrogen vapour
Sodium
Iron
Magnesium
Hydrogen
Lithium
Iron ,,
Visible limit of spectrum
Calcium vapour
Ultra-violet begins
rave length of dark red
A line =
759
,
„ deep red
a
»
=
733
,
red
B
>>
=
687
,
„ light red
—
,,
=
670
,
,, orange red
C
it
=
656
,
,, yellow
D
»
=
589
,
„ green
E
,,
=
527
,
,, bluish- green
b
»
=
518
,
„ greenish-blue
F
»
=
486
}
blue
—
|f
—
460
(
„ blue- violet
G
J}
=
434
,
,, violet
H
)>
=
397
K ,. =
Roughly speaking, blue extends from 400 to 500, green and
yellow from 500 to 600, red from 600 to 700 w.
PHOTOGRAPHY IN COLOURS
25. THERMOMETRIC SCALES
Centigrade,
(Celsius) C.
Fahrenheit
F.
Reaumur
R.
Centigrade
C.
Fahrenheit
F.
Reaumur
R.
0°
32°
0°
26
78,8
20,8
1
33,8
0,8
27
80,6
21,6
2
35,6
1,6
28
82,4
22,4
3
37,4
2,4
29
84,2
23,2
4
39,2
3,2
30
86,0
24,0
5
41,0
4,0
31
87,8
24,8
6
42,8
4,8
32
89,6
25,6
7
44,6
5,6
33
91,4
26,4
8
46,4
6,4
34
93,2
27,2
9
48,2
7,2
35
95,0
28,0
10
50,0
8,0
36
96,8
28,8
11
51,8
8,8
37
98,6
29,6
12
53,6
9,6
38
100,4
30,4
13
55,4
10,4
39
102,2
31,2
14
57,2
11,2
40
104
32
15
59,0
12,0
45
113
36
16
60,8
12,8
50
122
40
17
62,6
13,6
55
131
44
18
64,4
14,4
60
140
48
19
66,2
15,2
65
149
52
20
68,0
16,0
70
158
56
21
69,8
16,8
75
167
60
22
71,6
17,6
80
176
64
23
73,4
18,4
85
185
68
24
75,2
19,2
90
194
72
25
77,0
20,0
95
203
76
100°
212°
80°
RULE. — To convert —
0° into F°. Multiply C° by 9, divide by 5, and add 32.
R° into P°. Multiply R° by 9, divide by 4, and add 32.
C° into R°. Multiply C° by 4, and divide by 5.
R° into C°. Multiply R° by 5, and divide by 4.
F° into C°. Subtract 32 from F°, multiply remainder by 5, and
divide by 9.
F° into R°. Subtract 32 from F°, multiply remainder by 4, and
divide by 9.
NOTE. — Fahrenheit's scale is only used in English-speaking
countries. Reaumur's scale is used by the general public in most
countries on the Continent. The Centigrade scale is now used in
all countries by physicists and chemists, and this scale is therefore
implied in scientific works unless otherwise specially mentioned.
APPENDIX
293
26.
LlST OP ALL THE FlBMS MENTIONED IN THIS
WORK, together with their Postal Addresses and
Telephone Numbers. (Lens and Camera Makers
are omitted.)
Telegraphic Address and
Name of Firm.
Postal Address.
Telephone No.
Actien Ges. fur
Berlin.
.
Analin fabrika-
tion.
Autotype Co. . .
74, New Oxford St., W.C.
Central, 873
Baker, Charles .
244, High Holborn.
Bayer & Co. . .
Elberfeld, Germany.
Berger v. Wirth .
Benthstrasse, Berlin.
Bohringer . .
Mannheim, Germany.
Burroughs Well-
Snow-hill Buildings,
Central, 13300.
oome & Co.
Holborn Viaduct, E.G.
Butler, H. T. .
26, Craven Park, Willes-
den, London, N.W.
Dufay . . .
22,RueChateaudun,Paris.
London Agents, Auto-
type Co. (^.v.)
Fuerst Bros. . .
17, Philpot Lane, E.G.
Fuerst, London.
LondonWall,4350.
T. K. Grant (suc-
89, Great Russell St.,
Diamido, London.
cessor to Lu-
London, W.C.
Gerrard, 3419.
miere Co.
Grubler, Chemi-
63, Baierische Strasse,
ker.
Leipzig.
Ives Inventions,
939, Eighth Avenue, New
Ltd.
York.
Johnson & Sons .
Mfg. Chemists, Ltd., 23,
London WaU, 677.
Cross St., E.G.
Jougla, J. & Cie. 45, Rue de Rivoli, Paris T. N., 105-75.
Koenig, Dr. E. See Fuerst Bros.
Lumiere,A.&Sons Monplaisir, Lyon, France Lumiere, Lyon.
T. N., 11-19.
Lumiere: A.& Sons 89,Great Russell St., W.C. Diamido, London.
Gerrard, 3419.
294
PHOTOGRAPHY IN COLOURS
Name of Firm.
Dr. G. Mundie .
Natural Colour
Kinematograph
Co., Ltd.
Paget Prize Plate
Co., Ltd.
Kaydex Co., Ltd.
Kotary Photo Co.,
Ltd.
Koyal Photogra-
phic Society
Sanger-Shepherd
&Co.
Smith, Dr. J. H.
&Co.
Postal Address.
Mik. Chem. Institut,
Gottingen, Hanover.
Wardour St., W.
Watford, England, and
244, High Holborn,
London, W.
71, Lavender Hill, S.E.
12, New Union St., .E.G.
35, Russell Square, W.C.
Telegraphic Address and
Telephone No.
Kinmacolor, Lon-
don.
T. N., City, 3976.
Rotatoria,London.
Wall, 1109.
Central, 4124.
5, Gray's Inn Passage, Sentido, London.
W.C. Central, 8722.
Wollishofen, Zurich Dryplate, Zurich.
T. N.,484.
Urban. See Natural Colour Kinematograph Company, Ltd.
Utocolor. Societe La Garenne - Colombes,
Anonyme Uto- Paris,
color.
Watkins Meter Co. Imperial Mills, Hereford Watkins.Hereford.
Wratten & Wain- Croydon, Surrey Wratten, Croydon.
wright Croydon, 572.
INDEX
Abney, Sir W., on rendering
plate sensitive to red rays, 30
Absorption of light, coefficient
of, 15
Acid colours, 191
„ definition of, 190
Anastigmat lenses, 96
Aniline dyes, 191
„ ,, firms which sup-
ply, 192
Apertures between leaves form-
ing circles on negative, 23
Aplanat lenses, 97
Appearance of white on Auto-
chrome positive, how pro-
duced, 90, 91
Appendix A. Theories of colour
vision, 259
Table 1. Exposure times for
colour plates, 263
„ 2. Exposure times for
sunsets, 265
„ 3. Colour Synthesis,
266
„ 4. Relative brightness
of parts of spec-
trum, 266
,, 5. Slowest exposures
necessaryto secure
sharpness, 267
,, 6. Factor numbers for
developments, 267
„ 7. Developers for Au-
tochromes, 270
„ 8. Lumiere's formula,
1908, 271
Table 9. Lumiere's gradu-
ated formula,1910,
272
,, 9A. Prof. Namias' for-
mula for develop-
ment of plates,
274
,, 10. Other developers,
275
„ 11. Intensification for-
mulae, 276
„ 12. Reductionformulae,
277
,, 13. Instructions for de-
veloping Omnico-
lore plates, 278
„ 14. Instructions for de-
veloping Dufay
plates, 280
„ 15. Instructions for de-
veloping Paget
plates, 282
,, 16. Elimination of
green spots, 283
,, 17. Recent devices for
protecting colour
slide in the lan-
tern, 284
,, 18. Sensitising and re-
sensitising colour
plates, 284
,, 19. Colour-screen fil-
ters and mono-
chromatic light,
285
,, 20. Hitchins' deve-
loper for Auto-
chromes, 286
296
INDEX
Table 21. Metric equivalent
tables, 287
,, 22. English and foreign
sizes of plates, 290
„ 23. Comparative speed
of plates, 290
„ 24. Wave lengths of
visible spectrum,
291
,, 25. Thermometric
scales, 292
,, 26. List of firms men-
tioned in this
work, 293
Art in colour photography, 241
Autochrome plates, description
of, 84
,, ,, instructions
for deve-
lop ing,
Table 7,
Appendix,
270 - 275 ;
also 286
„ screen, remarkable
resemblance to oil-globule
colour screen in the eyes of
certain birds and reptiles, 45
B
dirty
Background being a
colour, cause of, 127
Backgrounds, choice of, 256
Base, definition of, 190
Basic colours, 191
Becquerel rays, 8
Binding the plates (separate
methods), 122
Black conditions of McDo-
nough, 89
„ true meaning of sensa-
tion, 54, 55
„ spots in positive, 124
Bleach-out process, theory of,
180
,, ,, details of,
186
n ,, theory of
Grothus,
182
,, „ law, Smith's,
183
Blind spot, description of, 55
Blisters in film, 127
Boll, discovery of visual purple,
53
Butler's three-plate camera, 149
,, details for working,
155
Camera, selection of, for colour
work, 96
Carbon colour process, 174
Carrara method of printing
autochromes, 132
Choice of lens, 97
„ of plate, 93
„ of subject, 99
Chromatic circle, 243
Cinemacolor. See Kinemacolor,
203
Cinematograph. See Kinema-
tograph, 203
Clearing the image, 109, 112
Clouds, colours of, 15
Colloid, definition of, 190
Collotype colour process, 161
Colour blindness, 47-54
„ „ total, 47
Colour carbon process, 170
„ filter for colour plates,
101, 137 .
,, filters for monochroma-
tic light, 137,
138, 285
INDEX
297
Colour filters for Autochrome
and other colour
plates, 145
„ „ efiect of thick-
ness, 103, 147
„ „ testing of, 148
, , cause of sensation of , 1 1 ,
12
„ formation, theory of,
67
,, how produced, 12, 13
, , photography, history of,
Chap. II., 29
,, plates, comparison be-
tween, 73
,, vision, 61
,, theories of, 52, and Ap-
pendix, 259
„ permanent and fugi-
tive, 182
„ pictures, stereoscopic
effect, 138
,, plates, defects in, 123
„ positive, processes con-
cerned in, 108
Coloured lights, effect on dyes,
184
Colours, acid and basic, 191
,, additive and sub trac-
tive, 48, 68
,, complementary, 69,
243
,, of screen plate, why
insufficient, 46
„ manufacturers of, 192
,, primary, secondary,
and tertiary, 241
,, pure, where found, 50
„ surface, 18
Combined and separate plates
compared, 78, 79
Condition, black, first and-
second, 89, 90
Cones convey colour sense, 34-
47
Corpuscular theory of light, 2
Crime de Menthe, cause of
colour in, 16
Curves of sensitivity of plates
and of the eye, 56-59
Dark room lamp, 106
Defects in colour plates, 123
Development,
instructions for,
,, Autochrome plate,
Tables 7, 8, 9, 10,
Appendix, 263-
292
,, Omnicolore plate,
Table 14, Appen-
dix, 278
„ Dufay plate, Table
14, Appendix, 280
,, Paget plate, Table
15, Appendix, 282
Development, first, 108
,, second, 111
„ of screen plate,
general in-
structions, 81,
82
,, for uncertain ex-
posures, Table 9, Appendix,
273
Dichroic colours, 17
„ fog, 123
Dioptichrome plate, 76
Dufay's plate described, 76
Dyes, how affected by coloured
light, 184
,, method of increasing
sensitivity of, 189
„ "nature of, 189
„ manufacturers, list of,
192
„ tabulated list of, 191
„ list of firms which supply
them, 192
298
INDEX
E
Edridge-Green on colour, 50
„ on colour blind-
ness, 51, 52
Ether, 3
„ waves, 4
Evolution of colour photo-
graphy, Chap. II., 29-33
Exposure of plate, rules for, 105,
also Appen-
dix, 263-
264
„ „ uncertain, de-
veloper for, Table 9, Appen-
dix, 272
Eye, analogy of, 34
„ compared with a camera,
Chap. III., 34
,, .description of natural
colour filter in, 44
Face of portrait appears thin
and eaten away, 128
Faraday, Michael, 7
Fatigue of retina, 48, 49
Film broken, what to do, 127
,, scratches in, 127
Filters for Autochrome and
other colour
plates, 145
„ „ monochromatic
light, 138
Focal length of eye, 40
Forster, Prof., on the produc-
tion of white in a colour
plate, 90, 91
Fovea (see Macula), 37, 44,
45
Fresnel, 2, 10
Frilling of film, 127
Fugitive colours, 182
Gaumont cinematography in
colours, 212
Goethe's Farbenlehre, 29
Goodall, T. E., on colour effects,
13,60
Grant on protecting plate in
slide, 96
„ on resensitising colour
plates, 136
Green (Edridge - Green) on
colour sense, 50-52
Green spots, removal of, 283
Grothus' bleach-out law, 182
Half-tone processes in colour,
158
Hardening the film, 112
Hauron, Ducos du, 31
Helmholtz, 31
„ colour theory, 259
Hering's theory of colour, Ap-
pendix, 261
Hitchins' developer, 286
Hood, use of, 104
Hiibl, Baron Von, on theory of
intensification of light, 86
Huyghens on wave theory of
light, 2
Hypo fixing bath, cause of re-
ducing image, 127
Image, final improvement of,
121
„ reversal of, 111
Indoor portraiture, 132
Insertion of plate in slide, 100
Intensification by mercury, 116,
117, and Ap-
pendix, 276
INDEX
299
Intensification by pyro and sil-
ver, 116, and
Appendix, 276
of image, 113
,, theory of, 113
Interference of light explained,
63
Irregular plates, character of, 73
Isocyanine, effect of, 31
Johnson, Lindsay, Dr. —
Explanation of yellow colour
of macula, 44
Existence of yellow filter in
the eye, 44
Similarity between auto-
chrome coloured starch
layer, and layer of coloured
oil globules in birds and
reptiles, 45, 46
Explanation of use of visual
purple, 53
Explanation of how white is
primed in an autochrome
picture, 90, 91
Joly's ruled-line screen process,
71,72
Jougla's Omnicolore plate, 76
K
Kerr phenomenon, 8
Kinemacolor projection lan-
tern, 209
„ camera, 208
„ projection of
pictures, 209
„ principle of, 204
Ko'nig, Dr,, on panchromatic
plates, 32
„ Pinatype process,
166
Kromskop, Ives', 142
Lambert's law, 15
Lamp, dark room, for colour
plates, 105
Lantern projection of colour
positives, 134, 284
Lens, choice of, 97
„ best focal length to use, 98
„ human, rapidity of, 38
Light, corpuscular theory of, 2
,, electromagnetic theory
of, 7
„ filters, preparation of,
137
,, interference of, 63-67
„ nature of, 1
,, sources of, 1
„ white, nature of, 9
Lippmann, Prof., photography
by interference colours, 30,
43, 63-67
Lucas, H., 46
Lumiere's Autochrome plate, 84
„ screen anticipated by
birds and reptiles, 46
M
Macula lutea, description of,
37, 44, 45
„ „ why yellow, 44
Manufacturers of dyes and
stains, list of, 192
Mariotte's blind spot, 54, 55
Massiot's heat - protecting
lantern, 284
Maxwell, Clerk, theory of
light, 7
McDonough's two black con-
ditions, 89, 90
Monochromatic light, filters
for, 138
Mercury intensifier, 116, and
Appendix, 276
300
INDEX
Miethe, Prof., on sethyl red, 31
„ on choice of lens,
98
Mother - of - pearl, interference
colours of, 66
Muscles of the eye, what is
their purpose in animals, 38
N
Newton, Sir Isaac, 2
,, and Lucas on the use
of grey, 46
Newton's rings, 66
Nicol prism, 61
Obernetter on dyeing the film,
30
Omnicolore plates (Jougla's)
described, 76
Over-exposure, remedy for, 123
Paget plate (separate) de-
scribed, 79, 80
„ „ (combined) de-
scribed, 84
Paper, Utocolor, 192
Parallax, explanation of, 75
Permanent colours, 182
Persistence of vision, 204
Photomicrography in colours —
Low power, 215
High power, 225
Pigments, colours of, how
caused, 18
Pinacyanol, 31, 57
Pinatype process, 166
Plate, choice of, 93, 94
,, combined and separate,
78,79
Plate, insertion of, in slide, 99
Plates, resensitising, 135, 284
Portraiture indoors, 131
Positive, disappearance of co-
lour in, 129
„ drying, 118
,, dull and opaque ap-
pearance of, 128
„ protecting by glass,
119
,, red and orange spots
in, 129
„ thin appearance of,
128
Powders, colour of, 14, 15
Printing by Utocolor paper, 195
Process, carbon colour, 170
„ Pinatype, 166
„ Sanger-Shepherd's im-
bibition, 163
,, three-colour half-tone,
158
Projection of transparencies in
colour on screen, 134
Purkinje phenomenon, 59, 60
„ „ proof of,
60,61
R
Raydex process, 171
Rayleigh, Lord, on colour of
sky, 27
Red tone in positive, 126
Reduction, methods of, 117,
and Appendix, 277
Reflection, theory of, 21
Regular plates, character of, 73
Resensitising plates, 135, 284
Retina, description of, 34
Reversal of image, how ob-
tained, 111
Rod vision and cone vision,
34-47
Rods act as dampers, 39
,, double function of, 39
INDEX
301
S
Salt, definition of a, 190
Sanger-Shepherd, 32
,, „ imbibition pro-
cess of colour photography,
163
Screen plates, comparison be-
tween, 73
Second development in colour
photography, 110
Sensitising plates, 284
Shadows, 22, 250
„ coloured, 24, 25
„ production of, 245
„ why black, 24
Silver intensifies, 116, and
Appendix
Single-plate processes, theory
of, 86, 87
Sky, colour of, 26, 27
Smith, Dr. J. H., inventor of
„ Utopaper, 33
„ bleach-out law, 183
Smith and Urban, 33
Smith's Kinemacolor projec-
tion, 203
Soap-bubbles, colour of, due to
interference, 66
Speeds of plates compared,
85
Spots (black) in positive, 124
,, green) 125
white)
red)
orange)
125
125
129
Stains "(brown) 124
(yellow) 123
manufacturers of, 182
Stereoscopic effect produced by
colour, 135
Subject, choice of, 98
Szczepanik, 32, 33
Table showing characteristic
features of colour plates,
74
Tapetuin Lucidum, 43
Tar colours due to interference,
66
Testing colour filters, 148
Thames plates described, 77
Thin positive, 123
Three-colour half-tone process,
158
„ „ photography,
theory of, 141
„ „ negatives,making
of, 155
printing, 67, 68
Three-plate camera (Butler's),
154
Translucency, 20
Transparency, only relative, 20
Two-plate colour photography,
152
Tyndall on clouds, 27
U
Underexposure, remedy for,
123
Urban - Smith's Kinemacolor
method, 203
, , principle of Kine -
macolor explained, 204
Utocolor, fixing the print, 199
lantern slides, 202
methods of improving
the print, 198
paper, 192
paper printing, 196
rapid printing colour
paper, 193
stripping paper, 200
302
INDEX
Varnishing the plate, 119
Veiled fog, 126
Violet-blue tone in image, cause
of, 126
Virida paper for dark-room
lamp, 107
Vision, persistence of, 205
Visual purple, use of, 51, 52
Vogel, rendering plates sensi-
tive to orange rays, 30
W
Wave theory of light, 5
White, how produced in an
Autochrorne, 90, 91
„ light, nature of, 9
Wiener, theory of bleaching, 30
Wrattens Kl filter, 103
Yellow, a true sensation, 20, 44
„ not a primary colour, 19
„ spot, reason for its
colour, 44
„ stains in plate, 123, 124
Young, Thomas, 2, 31
Young-Helmholtz' theory of
colour, Appendix A, 259
Zeeman effect, 8
Zenker, stationary waves, 30, 31
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