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.
[Illustration: _By permission of The Mining Engineering Co., Ltd., Sheffield_
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.