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Historical and Theoretical Data 21
overlap. These latter being each recorded in two negatives and reproduced by a single printing color, that is by the yellow ink where the red and green negatives overlap and the blue-green ink where the blue and green negatives overlap. The spectrum is, therefore, reproduced by five colors only, but in ordinary color work we meet the problem of reproducing common absorption colors, each comprising more or less of the whole of the spectrum. Theory at once defines the reflection spectrum of each printing color; but one is limited by the question of permanence.
Specially selected color patches were used and their spectral reflections measured spectrophotometrically and the reproduction by the normal halftone process measured in the same way. His conclusions are : that given the ideal red and blue printing colors, that is, pigments which shall sharply reflect only one of the primary colors and absorb the other two, perfect reproduction would be possible ; the yellow pigments being fairly satisfactory. Certain definite changes of color occur in the course of reproduction, namely, blues become darker and greyer ; blue-greens lose their greenish hue ; greens become darker and greyer ; pinks acquire a yellow hue ; mauves become brown ; reds lose any bluish tint that they may possess ; yellows are lightened without change of hue, while oranges and browns are well reproduced. No change in the character of the reproduction takes place with variation of tone with the colors yellow, blue and green. In the scale of tones between black and white there is a tendency for the middle tones to become reddish. The lightening of the yellows is due to a very light yellow ink being used. The inks are sufficiently transparent for the visual effect of superposed printings to be capable of approximate calculation, and hence the actual reproductions accord fairly well with the calculated result of superposing the component printings. The major errors of the reproductions of the colors would be removed if each ink absorbed one of the three primary regions only. In view of the general characteristics of colors produced by selective absorption, there is little immediate hope of securing printing inks of this theoretical character, therefore, it will be necessary to devise suitable methods of correction as the method of threecolor printing is extended into new fields.
Eder34 made a special study of the absorptions of the filters and reflections of the inks; but it is not necessary to more than refer to this, as the specific nature of the inks he used rather limits the usefulness of his data, and the filter effect is referred to elsewhere.
A. Payne15 basing his arguments on the correctness of the ft Tax well color curves, came to the conclusion that the minus red or blue ink should possess complete and abrupt absorption of the less refrangible rays up to E, wave-length 5270; the minus green or red ink should show abrupt absorption from wave-length 6563 to ¥l/2G at 5137, and the minus blue or yellow ink should have complete absorption to E at 5270. But he came to the conclusion that the height of Maxwell's curves, whilst of great