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36 History of Three-Color Photography
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 colors, each will be slightly pale, but now neither will be complementary to the color sensation. For instance, if the yellow ink is to be complementary 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 can not be complementary to the red sensation. To make this ink complementary, 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 color will be twice as great as in those where they are on top of one another.
"Thus the blue ink should not be the complementary of the red sensation. And in fact the ink will be far from the complementary to the red. I find that by taking the areas of Abney's curves up to B' with a planimeter, that it should very nearly match the green at about 50.3. The yellow ink would be made complementary to the violet by ending the absorptions at A, but it would be very pale, for it would transmit a large amount of white, as there is a large amount of violet beyond A. The result will be an unnecessary amount of white in the final picture. The result will be better if the absorption reaches to A'. Thus this ink also should not be complementary to the violet sensation. The pink at the red end of the spectrum transmits red and green, and at the other end chiefly violet and green. If the violet is equal to the red it will be complementary to the green sensation. This can be achieved by moving A towards the violet end, but only at the expense of the yellow, which would be rendered very pale. So this ink also should not be complementary/'
Clay suggests that the eye is a fairly good judge of the three primary colors — at least, of their hue — so that if the inks be printed in pairs a fair estimate can be made of their correctness. The violet produced by the pink and blue should be almost pure, containing only a very small percentage of white. The green should very closely match the green sensation. But the red will contain some green and should about match wave-length 6740. The only true test is with the spectroscope and then: 1. The inks should each absorb one band of the spectrum in the red, green and violet respectively. 2. Must transmit the remainder unaffected. 3. Must absorb the whole between them. 4. For half-tone work the