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672 History of Three-Color Photography
Three constituent positives are required from negatives taken through the usual trichromatic filters. A dichromated gelatin plate is placed in contact with the 2,000 line grating and above the grating the positive taken through the red filter. The exposure should now be made to parallel sunlight. Register marks must be affixed to the dichromated film or its support. Wood used an ink dot, this corresponding to some prominent object equally defined in each positive. After exposure, the grating and positive were removed, and the 2,500 grating and the positive from the green-filter negative accurately superimposed on the exposed plate. Another exposure was then made, and the operation repeated for the blueviolet grating and positive. The plate was then developed in warm water and dried.
Wood first imprinted all three gratings on one plate, but the difficulties of accurate superposition led him subsequently to print only the red and green positives on one plate and utilize a second plate for the blue-violet, reversing the positive, that is to say, placing the film side out, thus this last plate could be used as a cover glass for the two-grating picture.
The viewing apparatus consisted of a section of a large reading glass, with an eyehole, or preferably a narrow slit. As the spectra are formed on each side of the central white image, if the distance between the two spectra were so arranged as to coincide with the interpupillar distance of the eyes, it is obvious that one could obtain stereoscopic pictures by this process.
It was further pointed out that it was possible to produce these diffraction pictures directly in the camera on a single plate. For if a plate of fine grain were to be exposed in succession under red, green and blue filters, on the surface of which diffraction gratings had been ruled or photographed, the plate on development should appear as a colored positive, when examined in the viewing apparatus. The ordinary commercial plate has too coarse a grain, but it seems possible that a Lippmann emulsion might be satisfactory.
Fig. 198 shows Wood's illustration of his patent, and 1 is a diffraction grating of uniform grating space, in combination with a lens, the projection of the spectra being diagrammatically shown. In 2 is a similar view with the grating provided with varying and overlapping grating space ; 3 shows the front view of the grating 1, 4 a. front view of 2. A diagrammatic view of a multi-colored object to be viewed is shown in 5, and the distribution of the grating lines in the finished photograph of the same is shown in 6, the viewing apparatus is in 7.
T. Thorp62 patented what he considered an improvement on Wood's process, in that he impressed the grating rulings on dichromated gelatin, or mounted a celluloid replica grating direct on the gelatin, then printed under the positive, removed the grating and developed the plate. All the