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670 History of Three-Color Photography
superpose by projection and obtain a negative in complementary colors and print from this on to lenticular film for projection.
The K.D.B. Societe48 patented the use of black separator papers, the front surface being in contact with the emulsion and provided with a protective coating; thus being somewhat on the lines of the Autochrome system. This was for amateur use of lenticular films. A camera49 was also patented for obtaining stereoscopic pictures with lenticular film. In order to ensure even illumination the use of a telephoto lens50 in reverse position was patented. To reduce the size51 of the elements to the same order of magnitude as the definition of the usual projection lens, or less, the size of the elements should not exceed 0.04 millimeters. Such elements acted as pinholes and might be formed by hollowed engraved dies. Later52 it was proposed to reduce the size of the elements, so that diffraction would have a preponderating effect in the formation of the images of the ocular disk. The elements numbered 500 per mm., and this number might be so selected that the diameter of each corresponded to the diffraction disk. In printing from lenticular films on to like film, the moire or watered pattern might be removed53 by embossing the elements by cylinders at a suitable angle relatively to each other. This angle might be 30, 90, 120, 210, 270 or 330 degrees, but preferably 90 degrees. The interposition of a collective lens54 in close proximity to the film so as to make the rays, proceeding from the camera lens, strike the elements normally and to correct astigmatism and curvature of the field was patented. Minute cone-shaped figures, either arranged in quincux or hexagonal forms55 on which the emulsion should be coated. These might be formed by rolling the film or applying a plastic substance. In lieu of lenticular elements the support56 might be provided on that side opposite the emulsion with a series of minute perforations. The lenticular surface might be turned towards the condenser57 or a negative lens be placed58 between the film and the trichrome element.
The Diffraction Process. — This is an extremely beautiful use of the phenomenon of diffraction by gratings, but may be justly described as belonging rather to the laboratory, for practically the results can only be seen by one person at a time, or to very few, as the scale on which they can be thrown on a screen is limited by the great loss of light, common to the use of all gratings. It was devised by Professor R. W. Wood, of the University of Wisconsin.59
The theoretical basis of the system is comparatively simple. With diffraction gratings it is found that the greater the number of rulings per unit length, the greater the distance between the central white image and the spectra on each side. It is obvious then that one can so choose the number of rulings in three gratings that any three colors will fall on any given vertical, as shown in Fig. 197, in which 5" is the central white image, and I, II, III three spectra under one another. The vertical HH