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Double-Coated Stock
647
W. V. D. Kelley and J. MasonM patented a printer in which registration of a negative with double-coated positive film was effected by sprocket pins, the one ensuring vertical and the other horizontal registration. Standard positive film was used and certain perforations of each positive picture area were exactly registered with corresponding ones of the negative picture area by pins tightly fitting the perforations from top to bottom in the one case, and from end to end in the other, so that any shrinkage, warping or distortion was taken up here. With standard perforated film this might be satisfactory, but to ensure perfect accuracy a pin on the opposite side was also used, this pin, which fitted opposite perforations of negative and positive stock tightly, had a loose fit from side to side, so that there was allowance for the shrunken negative to slide laterally under the control of the full-fitting pin on the other side. A subsequent patent37 was for the positive film strip transparency thus produced.
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Fig. 182. Brewster's U.S.P. 1,253,137.
J. G. CapstafF8 would solve the difficulty of printing on both sides of the stock by so selecting the light rays that the absorbing action of the emulsion comes into play. He pointed out that the emulsion "as used on ordinary cinematograph film is somewhat yellow in color, is sensitive mainly to ultra-violet, violet, blue and green light, or in other words to light materially absorbed thereby, which comprises the shorter wavelengths. On the other hand, the opacity of the emulsion to light increases with the decrease in the wave-length of the light, so that light of short wave-length penetrates the emulsion to a less depth than light of a longer one. Accordingly, ultra-violet and violet lights do not pass through the sensitized layer on the ordinary positive film as readily as blue and green