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24 History of Three-Color Photography
fuse, and that paucity of coloring exists. Three superposed transparent films, such as anilin-dyed gelatin, are usually free from this defect, and, therefore, we obtain a feeling of greater homogeneity, and frequently a crudeness of coloring that is objectionable.
Color Change by Juxtaposition. — The principle employed in the screen-plate processes, is that of juxtaposition of color elements, wherein light is added to light. In three-color typographic processes, less so in collotype and still less so in photogravure, we have to recognize not only juxtaposition but also superposition. The individual half-tone dots or grains do not in all cases superimpose, but lie closely juxtaposed, and the final color effect in the two cases is by no means the same. For instance, vermilion and ultramarin, when juxtaposed in minute dots, appear reddishviolet; but when superposed they form a reddish-brown. In the same way scarlet-red and green will give the impression of yellow, when contiguous ; but an olive-green when superposed. These examples might be multiplied at will, but every worker with screen-plates and superimposed films can find his own examples. In the tri-color half-tone process, we have unfortunately to reckon with both factors, and the result must be different as the one or other method of admixture occurs, and this happens no matter what the spectral composition of the colored inks may be. It applies to broad and narrow banded colors, though naturally the final effect differs. Theoretically one can not have perfect color formation by superposed inks, for with three there must be some black formed. But as von Hiibl38 has pointed out, although we may mix yellow and blue, having chosen two theoretical inks, which lie at 120 degrees from one another on the color circle, there will be formed a green that is not strictly pure, in fact, it may contain as much as 50 per cent black, yet it will give the visual impression of being pure green.
The same result is obtained with an admixture of black and white, and with a mixture of from 10 to 20 per cent black, we are scarcely conscious of the fact, unless we compare it with a standard white. Equal parts of white and black mixed on the color top, appear a light grey and certainly not as lying midway between the two ; a mixture of 80 parts of black and 20 parts white appears as a "mean" grey. The effect is, however, different when black and white are juxtaposed, as may be done by means of closely contiguous lines or dots. Then we obtain the impression of a "mean" grey, when the black spaces are equal to the white interspaces.
Obviously there is here the difference between objective brightness and subjective sensation. In observing the lined or dotted surface, a part of the retina is not excited by the black element, therefore, the sensation corresponds to the quantity of white actually existing; whilst to us the differences in luminosity of the homogeneous ground tones appear proportional to their white content. Fechner39 has clearly laid down the fundamental laws on this point, and the correctness of his law can be proved