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552 History of Three-Color Photography
and the temperature raised slowly to 46° C, which produced a relief. They were then immersed in 2 per cent glycerol and dried. At this stage the prints might be temporarily superposed to check the color rendering, and if satisfactory transferred to a weak bath of cupric sulfate to fix the colors. To superpose them a 10 per cent solution of glue was used at the lowest possible temperature that would keep it liquid. The first print should be immersed in the glue and transferred to well-glazed paper, lightly squeegeed, placed in a press and allowed to thoroughly dry. This print was then immersed in acetone, which dissolved the celluloid. The surface was then well washed with acetone and the other prints transferred in the same way, though the celluloid need not be removed from the top print, as it served as a protection.
Screen-Plate Printing Patents. — Naturally inventors turned their attention to this subject, and the following patents are those which refer directly to this work.
C. L. A. Brasseur and S. P. Sampolo23 patented the use of a line screen in front of a sensitive plate in the camera, with a black and white line screen in front, thus blocking out all colors but one line, and monochromatic filters were used. The International Color-Photo Company24 would print white celluloid, paper or the like, with straight, zig-zag or other lines or dots, coat with emulsion, and negatives taken with similar screens in the camera were placed in register with the patterned support, and a positive made. Printing surfaces for photomechanical work could also be made and impressions from these printed in black ink on paper with corresponding patterns. C. L. A. Brasseur-5 proposed to obliterate the ruling of the screen negative by first making a positive, then copying in the camera in contact with a black and white screen, as shown in Fig 145; 1 represents the original polychrome screen enlarged ; 2 shows the normal spectrum photographed through it. The black and white screen is shown in 3 as covering all but the red line of the positive. Each line was thus exposed, the black and white screen being shifted for each exposure. Later Brasseur26 proposed to partially rotate the black and white screen and the plate as a whole, first to one side and then to the other, around an axis parallel to the lines of the screen, so that the rays would impinge at corresponding angles. Those parts of the plate covered by the black lines would thus be exposed and the closing up of the elements effected. He also proposed27 to use sharp-cutting filters for printing transparencies from screen-plates.
The Vereinigte Kunstseidefabriken28 patented a process, and if line screens were used primarily, it was only possible to obtain prints by using a similar screen at a given angle, otherwise the well-known moire made its appearance. In screens in which only one line was continuous and the others broken up into rectangles, there is also moire, though less pronounced, unless the angle was about 45 degrees. This trouble was to be gotten over by making, in the latter case, another colored line unbroken.