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CHAPTER XVII
SCREEN-PLATES— HISTORICAL AND THEORETICAL DATA
In the ordinary tri-color processes it is necessary to use three separate plates for the constituent negatives. L. Ducos du Hauron1 (p. 473) conceived the idea of applying the principle of a particular school of French painters, known as the "pointillistes," and this was the laying-down of juxtaposed minute dots of color, instead of superposing three films. It is obvious that this is essentially an additive process and not subtractive, as to make violet, minute dots of red and blue were placed side by side, and to make yellow minute dots of green and red.
Du Hauron, in his French patent said : "Enfin, il existe une derniere methode par laquelle la triple operation se fait sur une seule surface. Le tamisage des trois couleurs simples s'accomplit non plus moyens de verres colores, mais au moyen d'une feuille translucide recouverte mecaniquement d'un grain de trois couleurs'1 Which may be thus translated: Finally there is another method by means of which the triple operation may be effected on one surface. The separation of the three elementary colors may be effected no longer by three colored glasses, but by means of one translucid sheet covered mechanically by a grain of the three colors.
In a series of articles published in the following year2 the process is more fully described and speaking of his dot method he says : "Assume that we have the surface of a paper entirely covered with the alternating lines of red, yellow and blue, as fine as possible, of equal size and without interspaces. If this paper be viewed at a short distance, one can distinguish the three colored lines ; but at a distance they will fuse into a uniform tint, which will be white if viewed by transmitted light, but grey if examined by reflected light (assuming at least the relative luminosities of the three kinds of rays are so combined that not one of them dominates).3 And if the camera image is received on this paper, the image, viewed at a distance, will appear the same as though the paper was actually white. Such a paper gives the possibility, either by artistic hand work with a black crayon, or by means of light with the aid of the direct or indirect processes of ordinary photography, of obtaining a picture, in which the colors of nature will be represented with a certain degree of truth. Supposing, for instance, that it is desired to produce on this paper, which when examined at a distance, is a uniform grey tint, a red color ; it will be sufficient to cover with hatchings with a dark pencil the yellow and blue lines. If it is desired to obtain violet, it will be sufficient to cover in the same way the yellow lines, and if it is considered necessary to lower the luminosity of the
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