The advance of photography : its history and modern applications (1911)

276 THE ADVANCE OF PHOTOGRAPHY Now suppose a pull from a yellow block has been made on pure white paper, then, instead of the three primary light colours being reflected in equal proportions, only a mixture of green and red results. When on the top of this a magenta colour is printed, since this can reflect only red and violet, the green from the layers beneath will be suppressed. Finally, should a blue be printed on the top of the other two colours, the result would be simply a deep black, for this blue can only reflect the green and violet rays, and the substance on which it is placed has absorbed these rays, so no reflection of light takes place. This is what would happen if the pigment colours used were perfect, and each was printed to its full strength. Printers' inks, however, leave much to be desired, although with the best obtainable some excellent results are now produced with only three workings, in fact, so much so, that it is very seldom that a large number of workings are now resorted to, although in days gone by this was the method adopted to produce coloured illustrations. A good ink for this purpose must not only be of the correct colour and body, but the pigments which produce that colour must be of a sufficiently transparent nature to allow the effects of former workings to be seen. In order to ensure the building up of effects, the reflecting power of the colours should be as high as possible. If white light is spectroscopically examined after passing through a very thin layer of an ideal coloured printing ink, only two-thirds of the spectrum should be visible, the light rays producing the colours making up the remaining third having been absorbed by the ink. The three regions absorbed by the three ideal inks should of course just make up the complete spectrum without any overlapping. We may thus consider that