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20 History of Three-Color Photography
it means, when printing, the degradation of the green by the addition of some crimson ink because more crimson ink will print on the green than on the white. Needless to say this is not attained in practice, and a possible compromise in the case of subjects having complementary colors, widely antagonistic in contrast, may be found in narrow banded filters, correct in the principal position. The greens are nearly always the trouble, chiefly because they are of low luminosity and do not get recorded on the plate before the complementary colors (minus-green) are recorded.
"To discover the suitability of the filters, it is not necessary to go to the length of reproducing the spectrum in half-tone in the way we have done, as a simple examination of the spectrum negatives, made in the same ratio of exposure as camera exposures, after a little experience will at once enable one to determine if the records are correct or not, and from them to prophesy with practical certainty what color will reproduce well and what badly. Xor need the spectra be reproduced in order to determine whether inks are suitable or not, for after some experience the mere printing of a solid patch, singly, in pairs, and all superposed, will indicate whether inks are suitable or not. We conclude that: — 1. It is not possible nor is it desirable for any filter or plate to follow either the color sensation, color mixture, or certain calculated curves. 2. The effect of using plates, having maxima with broad banded weak filters, is to cause a degradation of any pure color occurring in the band of insensibility, therefore, plates showing gaps in the spectrum record should not be used for the green record. 3. Ultra-violet should not be recorded, as it will exercise a disturbing effect where it is reflected by colors other than blues and violets, as is the case with some browns, scarlets and yellows, these reproducing with a distinct bluish tint. 4. As much red should be recorded as possible ; ethyl violet in collodion emulsion, or pinachrombathed dry plates, at present give the best record of red. 5. There should be no unrecorded gaps in the visible spectrum, for while these may not be important, for certain mixed colors of pale tints, they are fatal to correct rendering of colors whose spectra do not extend beyond the gap. 6. We think that we have proved that the filter records should be even, end abruptly, and overlap each other as follows, the blue-violet and the green from 4600 as far as 5000, and the red and green should overlap from 5800 to 6000.
Bull also published33 an important paper on the relation of the selective absorption of printing colors to the errors occurring in three-color photography, which should be considered in the original as it is fully illustrated by spectral curves, which space prevents from inclusion. He pointed out that three-color negatives record five color regions ; three where the light is recorded through only one filter, which are reproduced by uniform hues of red, green and blue ; and two regions where the records