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674 History of Three-Color Photography
pictures were prepared in the same way, but the subsequent gratings were imposed at an angle, preferably 10 degrees, to the ruling of the first. The three grating pictures were then superimposed and viewed in an apparatus, in which the source of light was incandescent gas, for instance, and mirrors placed on the surface of an ellipsoid of revolution, which reflected the light from the light-source through slits parallel to the rulings of the gratings.
F. E. Ives63 said that owing to the use of dichromated gelatin or albumin, variations in the depth of printing could not be relied upon to produce" corresponding variations in the apparent luminosity of the different tones of the image. He proposed to improve the process by breaking up the diffraction lines by making the exposure through a black and white line screen with the lines at right angles to the rulings of the grating. The distance between the line screen and the grating was so adjusted that the shadows thrown by the opaque lines were deepest in the center, and had a penumbra shading off regularly to the commencement of the next penumbral shadow. It was also claimed that a screen with a granular surface might be used; further that the interruptions might be secured by the incorporation of semi-transparent granules, such as silver bromide in sufficient coarse grain, either in a film to be placed in contact with the diffraction grating or incorporated within or upon the sensitive film itself, and dissolving the granules afterwards.
H. E. Ives64 said that one of the fundamental defects of this process was that the three gratings were superposed on the assumption that their separate effects were added. But disturbing effects were produced, partly due to the inability of the gelatin surface to take several grating impressions without mutual blotting out and chiefly to the formation of a new compound grating. For if two gratings of different spacings were superposed, the spacings periodically got in and out of step, and this new periodic structure formed a new diffraction grating, which then formed its own spectra, which subtracted light from the original ones. Therefore, when the two gratings were superimposed, the eye, instead of receiving double the quantity of light, received much less. A still more serious defect was that the new spectra, due to the composite grating, fell in such positions as to produce false colors. That this is possible is proved by placing two gratings with different rulings, at right angles to one another, when two sets of spectra will be formed, one by each grating and parallel to its ruling and in addition a series of spectra diagonally disposed. If the gratings be now turned into the same straight line all the spectra turn, and the additional diagonally-spaced spectra take up positions between the spectra formed by the original gratings. So that while the eye may receive red from one grating and blue from another, one of the spectra formed by the combination of the two gratings may produce another color, such as green.