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468 History of Three-Color Photography
slow emulsions were generally used, and the normal sensitiveness of the emulsion does not extend into this region, and consequently when sensitized there is this gap between the added and the original sensitiveness. E. Stenger44 found three maxima, one at 5300, another from 5600 to 5700 and the third from 6100 to 6200. The curve of sensitiveness was very similar to that of isocol. With short exposures the action extended to 6400, and with longer to 6900. Under given conditions plates were under-exposed in from 5 to 200 seconds, correctly exposed in from 400 to 800, and over-exposed from 1000 to 1400 seconds. With these exposures the orange-red appeared first from 6000 to 6250, and then with increase of exposure this region was prolonged to 6500 with a whitishred; the green showed first at 5500, and blue at 4400 to 5750, after a long exposure the yellow turned to a whitish-yellow. The maximum sensitiveness of the emulsion was at 4250.
Von Hubl45 placed the maxima at 4500, 5500 and the sensitiveness extended to 6500. Mees46 said that the sensitiveness was very similar to a collodion emulsion, and that the sensitizer might be orthochrom T. R J. Wallace made47 a careful examination of the Autochrome plate. He found the diameter of the grains to accord with the author's estimate, and the number of grains found to be in the ratio stated by others. On treatment with hot water, the blue dye was found to be the most soluble, then the red and lastly the green; but on treatment with cold water the green was the most soluble. Ether dissolved the varnish of the starch layer, so that isolated grains could be observed, but it had no influence on the colors. In alcohol the green dye was discharged slowly, and the color film frilled away from the support in one continuous film. Dilute ammonia discharged the colors slowly in the order of red, green and blue and in acid permanganate the colors were equally but slowly discharged. The thickness of the starch layer was 0.01524 mm., and that of the emulsion only 0.004 mm. ; for comparison the thickness of that of the ordinary plate may be put at 0.031 mm. The sensitiveness of the emulsion was found to correspond practically with the results of Stenger. On exposing to the spectrum through the screen elements, without the compensating filter, it was found that with under-exposures the first visible action was at 4100, which gave a bright blue; with increased exposure, the green at 5270 was the next color to show, followed almost immediately afterwards by 5850, which appeared of a deep red, instead of bright yellow. With increased exposure, the blue-violet, from 4300 to 3900, became diluted with white ; while the green and red regions extended and whitened. The ultra-violet was shown throughout as a bright blue.
Mees and Pledge dealt with the examination of the screen as a whole, and stated that the Autochrome, when examined by daylight showed a distinct pink tint, as did also the Omnicolore, whilst the Dioptichrome was greenish, and the Krayn film fulfilled the first black condition best.