The history of three-color photography (1925)

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626 History of Three-Color Photography the green projecting light, and this would cause the bronzing, which was due to excess red. Bennett's plan of admitting a blue band would probably result in better exposure, hence more density in the green negative and consequent greater transmission through the green positive. Possibly the excess of red was contributed to by the characteristic chlorophyll reflection band from B to CJ/2D. The results obtained being comparable to those seen with Simler's erythroscope, though, of course, not so brilliant. If we consider the matter from the point of view of projection we can possibly arrive at some sort of conclusion as to the best filters to use. In the first place one must recognize that some of the colors, such as the deep blues and violets, have to be sacrificed. These are not recorded in the negative and will, therefore, not be reproduced in the positives. But one must not assume that they will all be blacks, for as nearly all natural objects reflect some white light, this will act to some extent on both films. And, moreover, the blues reflect more or less of the green rays, and the violets more or less of the red. The consequence will be that these colors are not always shown as black. One factor that is of some importance in the taking of the negatives is the question of exposure, and to clear this up one must take into preliminary consideration the actual method of obtaining the exposures. If these are in succession, as was the case in the original Kinemacolor process, it is clear that as the revolving shutter was composed of two colored sectors, variation of exposure could be obtained by varying the angles of the sectors, so that if the negative stock was not equally sensitive to both red and green, it was possible to compensate for this. If, on the other hand, and it is assumed that only one lens is used, the image is split up into two beams by mirrors or prisms, then the only method of equalization is that of altering the reflective power of the reflecting surface. Of course, if the reflective surface is silver, that is a half-silvered surface, it is extremely easy to control the ratio of the reflected to the transmitted light by the use of a more or less dense deposit. If the reflective surface is composed of small elements, then varying the areas of these also gives the same effect. Equalization of exposure by varying the density of the filters is out of the question, as it is extremely difficult to do this without altering the absorptions, unless one adds black to one of the dyes, and this would be excluded assuming that the sensitive ratios of different batches of emulsion also varied. On the other hand, there is comparatively wide latitude in the variation of the relative color-sensitivity, because of the overlap of the filters, and the fact that we are not using pure spectral colors but natural objects, which usually reflect a considerable amount of white light. Considering that the additive processes have been relegated to the dead and gone methods we may dismiss these very briefly by stating that the best projection filters are Nos. 25 and 44, issued by Wratten &