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 Lumière autochrome process, by which the results desired can be in a large measure attained by methods of manipulation comparatively simple.
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 Lumière 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.