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BOOK ILLUSTRATIONS 277
the yellow pigment absorbs the blue rays, the magenta pigment absorbs the green, and the cyan-blue absorbs the red.
Let us now briefly trace the processes necessary for the production of a three-colour illustration of some subject which can only be viewed for a relatively short time.
Two methods of obtaining a representation of the " colour value " of the subject are open to us. The first is to use orthochromatic plates and take three photographs— one using a red screen and a plate which has been sensitized for red rays ; a second using a green screen with a suitably sensitized plate ; and a third using a violet screen, also with a special plate. This would necessitate three exposures, and although even when using colour filters the time of exposure is very small, still the colour value of the subject depends upon the light which it receives, and this is liable to alter owing to atmospheric changes.
Until the invention of the autochrome plates the above was the method adopted. Now, when it is possible to obtain such excellent representations of nature by the Lumiere autochrome process, it is much better to take a photograph, using one of those plates. Then from this plate, under conditions in the studio which can be perfectly regulated, the three necessary negatives are obtained by the use of the proper plates and colour-screens (filters).
These colour-filters must be of the proper value to ensure the best results, and this is by no means an easy matter to arrange.
It is assumed that the camera has been arranged with a suitable half-tone screen when used in the preparation of the three negatives.
Blocks are prepared from the three half-tone negatives in the manner already described. The negative prepared with the red filter will be employed for the block which will be used to print the colours reflecting all the rays of