Page:Proceedings of the Royal Society of London Vol 69.djvu/41

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As we shall obtain all our colours diluted with white, one is tempted to ask whether printing with a black ink will not enable us to get them pure ; that is, as we can copy any colour, except for the addition of white, by the three printings in yellow, blue, and pink inks, could we not, by the addition of black, match the colours exactly 1

Suppose it is the red we are matching. We shall get a red with a certain percentage of white. For instance, for 100 parts of red light reflected there may be 10 green and 10 violet. If we now print 10 per cent, black what will be the result 1 If the black is a good one, z.<?., if it absorbs uniformly all along the spectrum, we shall now have reflected 90 parts red, 9 green, and 9 violet. In other words, we merely darken the whole, without in the least altering the proportion of white. The addition of black will then not improve this purity of the colours ; it will only make them " dirty."

There is a very fundamental difference between this and the produc- tion of colour by successive addition, as in Ives's triple projection. In the latter case, if any spectrum colour is thrown on the screen by one light say, the green and the same spectrum colour is transmitted also by a second light red, for instance the amount of that particular colour is doubled, and the result is an arithmetical addition of luminosity.

But in printing, if a spectrum colour is completely absorbed by one ink say, the pink and also absorbed by another perhaps the blue the total light of that colour absorbed by the superposition of the inks is not twice that removed by a single one. The absorptions do not successively subtract light. If two inks each would separately transmit one-tenth of the light of a given wave-length, the two inks together would transmit not one-twentieth, but only one-hundredth, of the light of that colour. The law is a geometrical and not an arithmetical one.

In " process " three-colour printing this is of the utmost importance, for there the inks are always printed full strength, and the tint is regulated by the size of the dots, that is, by the percentage area of the paper which is covered by the ink. The dots are produced by placing a ruled screen in front of the negative while it is being exposed, and the dots at any part, when developed and etched, have areas very nearly proportional to the intensity of the light which fell upon that part. Each colour is produced by similar dots, and when the three inks are printed these dots partly overlap one another. As the ruling is very fine and the dots are very closely spaced, it is impossible in