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transition from the hue of the red filter, 625, to the hue of the green filter, 547, the transition being continuous and the admixed white varying continuously in the same way between the amount present in the red and in the green filters.
Now, with the additive process it is essential that our filters should be approximately complementary, because otherwise they will not give white on the screen, and this condition that the two colors should be complementary to one another makes it very difficult to get a satisfactory range of intermediate hues. When working the sub tractive process, on the other hand, the whites are got by the absence of any color at all, and blacks by both colors in full strength, so that we can use colors which are not exactly complementary to each other, and the range of possible colors which can be obtained is greatly extended. Since only two colors are used, it is obvious that all colors cannot be correctly rendered, and in the twocolor subtractive process, the colors for which the process fails are the blues, violets, magentas, and purples; light blues appear bluegreen, and true violets appear black; magentas appear pink; and purples, brownish-red. Fig. 14. There is little differentiation between the yellow-greens and the true greens. A buff or yellow will appear light red, but the psychological effect of such a light red under the conditions is so close to that of yellow that, as a matter of fact, yellows appear to be satisfactorily rendered by the process. Flesh tints of all kinds and all shades of red, orange, and most greens, grays, and blacks are well rendered; as these are predominant in portraits, the results are very satisfying for this class of work. Many of the pictures appear to show blues fairly well, but this is because, by contrast with green, blue-greens look blue, and especially by artificial light the eye is accustomed not to expect very much of blues.
Before passing on to a consideration of the application of color photography in connection with motion picture work some mention should be made on the importance of simplified processes such as the two-color subtractive process in connection with clinical photography and photomicrography. Since blues are almost absent in clinical photography, the two-color process is quite satisfactory for the photography of skin diseases or of pathological and anatomical sections, and lantern slides made in this way are far more effective for teaching purposes than the same subjects reproduced in black and white only. In the case of microscopic specimens, most sections to be dealt with are stained in not more than two colors, and very good results can be obtained by making two-color lantern slides. The method of work is to choose special filters according to the specimen, each of which absorbs one of the colors as much as possible and transmits the other, the two filters being necessarily complementary to the stains used, and from the two negatives made through the filters, prints are made either by the bleaching and dyeing process, already described, or in bichromated gelatine.
Thus, suppose a section is stained red and green. Make two negatives on panchromatic plates, one with a red filter, which will cause the green to appear clear in the negative and will not record
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