Page:Proceedings of the Royal Society of London Vol 69.djvu/45

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Assuming now that the absorptions should be abrupt, the next question is, what limits should they have to give the best effects ? If one is at A and the other at B in the accompanying diagram in which equal heights of each colour would give white the yellow and

blue inks will be complementary to the colour sensations, but they will be very pale, for they reflect large quantities of all three colours. The pink ink will be very dark, it will reflect only a very small amount of the red sensation, and as the violet reflected is not equal to the red, it will not be complementary to the green sensation. It could be made so by shifting A further to the violet, so as to reduce the violet reflected by the pink ink in fact, until the areas of the red and violet are equal. It is obvious that this would give absurd inks ; the yellow and blue would be so pale as to be mere tints, and the pink so dark as to appear purple.

If the absorptions are supposed to end at A' and B', the spectrum will be much more evenly divided between the three colours, each will be slightly pale, but now neither will be the complementary of the colour sensation. For instance, if the yellow ink is to be conplement- ary to the violet sensation, it should absorb either violet or violet and equal amounts of red and green, measured by equal areas on the above curves. But at A' (or at any other line between A and B) the green sensation is evidently in excess. In the same way if the absorption of the blue ink extends from the red end of the spectrum beyond B, green is absorbed as well as red, and the ink cannot be complementary to the red sensation. To make this ink comple- mentary, it must have another absorption band somewhere in the violet, and the difficulties we have been considering above will be introduced, for there will then be a part of the spectrum absorbed by more than one ink, namely, by the yellow and the blue, and in places where these inks are printed side by side the absorption of this colour will be twice as great as in those where they are on top of one another.