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546 History of Three-Color Photography
As shown by the equation a variety of results may be obtained with a light-source of given size by merely varying A and a, but on paying regard to the structure of the screen-plate, it will be seen that the actual size of L is very material to the process of copying because of parallax errors. As shown in Fig. 143, the screen-plate image consists of the mosaic multicolor screen R, the gelatin film G, containing the developed image s, and a transparent film separating the two and indicated in the diagram by i. Assuming that the blue elements are covered by the silver deposit s this area will appear yellow, the light-source being chosen of such size that an almost homogeneous yellow is formed on the surface F. But the rays of light are not entirely obstructed by the silver deposit on the blue areas, but pass obliquely to the surface F, so that a portion of them arrive on certain areas m in the diagram, forming white with the red and green which have passed through the screen; thus there is produced, not a degraded, but a whitish yellow on the surface F. The same parallax occurs with all colors, and the result, therefore, is less saturated or whitish colors. This defect is greater the more obliquely the light rays fall on the original plate, that is to say, it depends on the ratio L : A, and it increases towards the edges of the picture. It is necessary, therefore, to choose the lightsource L as small as possible, and its distance A as large as possible. The distance between the two films is then given by the equation :
a=^—c(v-l)
Von Hiibl recommended that in practice a lens should be used in a camera with a circular aperture covered with ground glass and the picture to be copied and the sensitive plate placed in the dark slide, the glass side of the original being laid against the sensitive film, so that the distance between the two a is the thickness of the glass of the original, about 2 mm., the extension of the camera should be about 50 cm., and the size of the circular disk, admitting the light, can be adjusted to comply with the above rules. The camera should be pointed to an even white sky, or the ground glass may be illuminated by artificial light, and magnesium is the best. The results are much improved in the Lumiere arrangement if the circular aperture be covered with ground glass and the usual yellow filter used. Instructions for making filters for artificial light work with screen-plates has been given under Filters.
E. Stenger and F. Leiber6 suggested that it might be possible to print from screen-plates with leuco-bases of dyes, as they considered that no satisfactory method of reproduction was available. They suggested that it would be more convenient to use Autochrome negatives, in the complementary colors ; the first advantage being that the black silver deposit would not produce degradation of the color, but a brightening of the pic