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Historical and Theoretical Data 31
dyed transparencies and tri-color typographic prints, and proved that none is theoretically correct. Taking as his starting point the negatives with the color records as laid down hy him and Bull (see p. 16), he came to the conclusion that the ideal inks should have abrupt absorptions limited by the complementaries of the same ; that is to say, the yellow should be the sum of all wave-lengths between 5000 and 7000; the red should be the sum of all colors between 4600 and 4000 plus 6000 and 7000, and the blue the sum of all between 5800 and 4000.
R. J. Wallace455 accepted the absorptions of the correct filters as red from 7000 to 5900 ; the green from 6000 to 4900, and the blue from 4000 to 5000. He concluded that the inks must be complementaries to these and advanced proofs in the shape of three superposed films, stained with the correct dyes. L. P. Clerc49 dealt with this subject at some length, and with the ideal and commercial inks. He set the theoretical colors as a yellow, which can be matched by wave-lengths 5695 to 5700; a red, that does not exist in the spectrum, but which can be matched by rhodamin S or erythrosin solutions, which give a color that is variously termed pink, crimson or magenta; and a blue simulated by wave-lengths 4880 to 4885.
Von Hiibl50 suggested the following apparatus, Fig. 3, for the measurement of the colors of pigments. This may be considered as a modified
Fig. 3. Von Hiibl's Colorimeter.
Maxwell color box, or chromoscope. The ink or pigment is placed at O O, and illuminated by daylight or artificial light, a standard light. Its image is projected by the lens m on to the right-angled prism e, thence to the half of the eyepiece 1. The comparison color is formed by synthesis of the three beams W , W , W , passing through the three apertures B, G, R, where are placed the three color filters b, g, r, which are pure blue, green and vermilion. The light from these filters is reflected from the transparent mirrors S1, S2, S3 and projected by the lens L to the upper half of the