COLOUR PHOTOGRAPHY
Photography has introduced many of the general public to a branch of
practical science which otherwise they would never have cared much
about. The action of light upon certain chemicals, the subsequent action
upon the same of other chemicals, such as developers, toning solutions
and so on, form a very well-known region of the domain of science. And
this is, too, a branch of chemistry in which the practical inventor has
been very busy. The efforts, therefore, which have been made to invent
ways of producing photographic pictures which shall give to the objects
their natural colours, will probably be of special interest in a book
like this.
Of these there are two very well-known systems, and to them we will
mainly confine our attention.
It should first be pointed out, however, that what we are discussing is
quite different from the simple "orthochromatic" plates which are used
by many photographers. These latter are coated somewhat differently from
other plates, with a view to their giving a more realistic picture, but
the result is still in one colour. They are, in fact, a little more
sensitive to differences in colour than ordinary plates, so that colours
which appear, when the latter are used, very much the same, appear, when
orthochromatic plates are employed, a little different. But the
difference in colour in the object photographed is only, even then,
represented by a difference in shade in the picture. The object is, it
may be, in many colours, in all the colours, very likely, but the
picture is only in one.
And the step from that to a coloured picture is a very long one. True,
the solution of the problem is very simple in principle, yet the
practical difficulties are so great that even now they have not been
entirely overcome.
Let us first of all examine the principle. Sunlight, by which
photographs are usually taken, appears to the eye white and colourless.
It is not really so, however, as can be proved by analysing it with the
spectroscope. In this instrument a flat beam of light, having passed
through a narrow slit, falls upon a prism of glass, from which it
emerges as a broad band, known as the "spectrum." This band can be seen
upon a screen, or can be examined through a telescope. So far from being
white and colourless, it consists of the most lovely colours. At one end
of the spectrum is a beautiful red, which, as the eye travels along,
imperceptibly merges into orange, which in turn merges into yellow,
after which we find green, blue, indigo and violet, in the order named.
These seven are known as the "primary colours," but it is quite a
mistake to suppose that there are seven clearly defined and distinct
colours. The colours so change, one into another, that their number is
really infinite. The seven names indicate seven points in the spectrum,
whereat the colours are sufficiently distinct from others to warrant a
separate name being given to them. We call the starting colour red, for
example, and as we pass our eyes along we perceive a constant change,
and when that change has become sufficiently pronounced to justify our
doing so, we call the new colour "orange." Continuing, we find the
orange changing into something else, and when it has gone far enough, we
bring in a third name, yellow, and so on to the violet. Thus we see the
division into seven colours is arbitrary, and only for our own
convenience, since the whole number of colours is innumerable.
Passing through a prism is not, however, the only means by which white
light can be split up. When the sun shines upon a blue flower, for
instance, the blue petals perform a partial separation; they reflect the
blue part of the sunlight, and absorb all the rest. A red flower
likewise reflects the red part of the sunlight and absorbs the rest. It
is because things can thus discriminate, reflecting some kinds of light
and absorbing the remainder, that we perceive things in different
colours.
It follows, therefore, that when we look upon a landscape, or a field of
flowers, we receive into our eyes an enormous variety of coloured
lights. The white sunlight furnishes each thing we see with a flood of
white light, and each thing according to its nature, reflects more or
less. A white flower reflects the whole, a pure black object reflects
none, but the great majority of things reflect some part or other of
that infinite variety of which white light really consists.
So a view at all varied sends to our eyes a variety of colours, almost
as manifold as the colours of the spectrum, which, as has been said, are
infinite. And the task of reproducing them, or even of producing a
similar general effect, upon a piece of paper seems at first sight
beyond the bounds of possibility.
But fortunately there is a way by which we can produce, approximately at
all events, the intermediate colours by mixtures of the others. The
second colour of the spectrum, for example, orange, can be obtained by
mixing its neighbours on either hand--namely, red and yellow. We can,
indeed, imitate very closely the imperceptible change from red to yellow
through orange, by skilful mixture of red and yellow pigments. First
there is the pure red, then just a suggestion of yellow is added; more
and more yellow brings us to orange; after which by gradually
diminishing the amount of red we reach the pure yellow. Next, by
introducing blue pigment, we can gradually change the yellow into green,
and further manipulation of the same two colours will lead us on to pure
blue. Indeed by mixtures of red, yellow and blue we can obtain almost
all the perceptible varieties of colour.
And it must be remembered that when, by mixing blue and yellow pigments,
we get the effect of green, that is only the result of an optical
illusion. The particles of which the yellow pigment is made remain
yellow, and the particles of blue remain blue. The one sort reflect
yellow light to our eyes, the other sort reflect blue light, and owing
to what in one sense may be called a defect in our vision, these two
mingling together look as if the whole were green. In the spectrum we
see real green light; from green paint made by mixing yellow and blue,
we only see an imitation or artificial green. If the particles were
large enough, we should see the yellow and the blue ones quite separate,
but since they are too small for us to see at all, except in the mass,
our eyes blend the whole together into the intermediate colour.
Thus we see that, although the variety of colours is infinite, we can
for practical purposes reproduce as much difference as our eyes can
perceive by the judicious blending of three--namely, red, yellow and
blue.
And there is a further fortunate fact--we can filter light. The red
glass with which the photographer covers his dark-room lamp looks red,
and throws a red light into the room, because it is acting as a filter
to the light proceeding from the lamp behind it. The lamp is sending out
light of many colours, but the glass is only transparent to the red. It
holds up all the others but lets the red pass freely. So if we were to
take a photograph through a red screen, we should get on the plate only
those parts which were more or less red in colour. For example, if we
thus photographed a group of three flowers, one red, one orange and one
yellow, the red one would come out prominently, the orange one would
come out faintly, and the yellow one not at all.
Then suppose we took the same picture again through a yellow screen. In
that case the yellow flower would be prominent, the orange would again
be faint, but the red would be absent.
Having got, in imagination, two such negatives, let us make two carbon
prints, one off each. And let the print off the first negative be red,
while that off the second is yellow. Let each be, in fact, of the same
colour as the screen through which the picture was taken. Finally, let
the two films be placed in contact one upon the other. On holding the
two up to the light, what should we see?
We should see a red flower, for there would be a red flower clearly
defined upon one film coinciding with a blank transparent space upon the
other film. We should see, too, a yellow flower, for a clearly defined
yellow flower on the second film would coincide with a clear space upon
the first. We should see also an orange-coloured flower, for there would
be a faint red image of it, and a faint yellow image of it, one on each
film, lying one over the other, producing the same effect as a mixture
of yellow and red pigments. Thus by taking two negatives through two
coloured screens, and then colouring the prints to correspond, we can
obtain three colours in the finished picture.
By taking a third negative, through a blue screen, we could add
immensely to the range of colours obtainable. Indeed, with three films,
red, yellow and blue respectively, made through three screens of the
same colour, a variety of colours practically infinite can be obtained.
So the principle is quite simple; the difficulty is in carrying it out.
For the three kinds of light have not the same photographic power, and
so to avoid upsetting the "balance" of the colours different exposures
would be required for each. Then there is the difficulty of so
manipulating the films as to get them one over another exactly. Anyone
who has tried the handling of carbon prints will readily realise how
difficult this would be. It is possible and has been done, but the
process is too uncertain and too laborious to be of general use.
But the same result can be attained more or less automatically, as the
following descriptions will show.
Let us turn to the Lumiere autochrome process, by which the results
desired can be in a large measure attained by methods of manipulation
comparatively simple.
[Illustration: _By permission of The Mining Engineering Co., Ltd.,
Sheffield_
PNEUMATIC HAMMER DRILL
This tool is used by miners for making holes in hard rock, preliminary
to blasting. Note the spray of water, which prevents the stone dust
rising and getting into the miner's lungs.--_See_ p. 220]
The plates used for this are of a very special nature. In the first
place, there is the basis of glass, but upon that there is laid what we
might term the selective screen. This is a layer of starch grains, of
exceeding smallness. The size of them is as little as a half a
thousandth of an inch and there are about four millions of them on every
square inch of plate. Next, upon the screen of starch grains is a layer
of waterproof varnish, while over that is the ordinary sensitive
emulsion such as forms the essential part of the usual non-colour plate.
Now the starch grains which form the screen are, before they are laid
on, stained in three colours. Some are blue, some red, and some a
yellowish-green, which experience shows is preferable to pure yellow.
The differently coloured grains are well mixed, and when the screen is
held to the light and looked through the effect is almost that of clear
glass. That is because red rays from the red grains, and green and blue
rays from the grains of those colours, all proceed to the eye mingled
together.
This plate is placed in the camera differently from the usual way, since
the glass side is turned towards the lens. The light, therefore, after
entering the camera, passes through the glass, then through the screen,
and finally falls upon the sensitive film.
Suppose, then, that the camera were pointed to a red wall; red light
would fall upon the plate and, passing through the red grains, would act
upon the sensitive film behind them. The blue and green grains, on the
other hand, would stop those rays which fell upon them, and so those
parts of the sensitive film which they cover would remain unaffected by
light. Then, if that plate were to be developed, a dark, opaque spot
would be produced upon the film under each red grain, the film under the
other grains remaining transparent. Hence, when held up to the light and
looked through, the plate would appear a greenish-blue, for all the red
grains would be covered up.
In like manner, if the wall were blue instead of red, a greenish-red
plate would result, while if it were green, the plate would be a purple,
the result of the combination of red and blue.
But this, it will be seen, is a topsy-turvy effect, the exact opposite
of what we want, so that it is fortunate that by a simple chemical
method we can set it right. After a first development in the ordinary
way the plate is placed in another bath and exposed to strong daylight,
with the result that those parts which were darkened by the first
development become clear and the parts which were clear become opaque.
Thus, after this twofold development of the photograph of the red wall,
we find ourselves in possession of a red plate, in which only the red
grains are visible, since all the others are covered up by opaque parts
of the sensitive film. The photograph of the blue wall will also, after
it has been subjected to the double development, show blue only, and the
same with the green.
But suppose that instead of a red wall or a blue wall we focus our
camera upon one which is half red and half blue. Then it is easy to
perceive that we shall get a plate which is half one colour and half the
other. Moreover, it follows that a wall covered with a mosaic of red,
blue and green would give us a plate duly coloured in the same way.
But when we go a step further and photograph, say, a landscape, which
may contain a vast range of colours, we find a difficulty in believing
that they can all be rendered by the simple process of covering or
leaving uncovered grains either blue, red or green. It can be done,
however, since the other colours may be made up of two or more of these
three in varying proportions. For example, should there be something in
the landscape of a darker, more blue, shade of green than the green
grains, then the light proceeding from that object, while passing freely
through the green grains upon which it falls, will slightly penetrate
the neighbouring blue ones as well, and so at that point on the plate
there will be not only green grains visible, but some of the blue grains
partly visible also. The light from the blue grains will enter the eye
along with that from the green grains, and by so doing will add just
that amount of blue to the green as to give it the right shade.
After this manner is the whole picture built up. It is, of course,
really a mosaic, consisting entirely of little coloured patches, but
since they are so small none can be seen individually, all merging
together in the eye so as to form a picture in which colours change
imperceptibly from one into another.
To sum up, then, what happens is this. We start with a layer of coloured
grains; the action of taking and developing the photograph covers up
some of these grains and leaves others exposed, and the action of the
light is such that those which are left visible produce a picture
closely resembling the original, not only in form but in colour.
But there is one other interesting point about this process which
deserves mention. The differently coloured lights are not of the same
power photographically. Red light, as we know well, is very weak in this
respect, wherefore, we use it in the dark-room. A faint red light will
have no perceptible effect upon a plate unless it be exposed to it for
some time. Blue light, on the other hand, is very active, and were the
blue and red lights to be allowed to act equally on the autochrome
plate, the result would be much too blue. It is therefore necessary to
handicap the blue light, as it were, by placing a "reddish-yellowish"
screen either just in front of, or just behind, the lens to cut off a
proportion of the blue rays.
The other very successful process is known as the Dufay dioptichrome
process. It differs very little from the Lumiere except in detail, the
selective screen being formed of small coloured squares instead of by a
mass of little grains.
In both, it will be noticed, the result is a single positive. It is not,
as in ordinary photography, a negative off which any desired number of
positive prints can be made. And, moreover, it is a transparency: it
cannot be viewed except by light shining through it. The results are,
however, extremely beautiful, when well done, and anyone who cares to
try either of these methods of working will be well repaid for the
trouble involved.