COLOR PHOTOGRAPHY. 193 



The dark violet which was produced by the violet of the first illumi- 

 nation was made red by the red of the second, and under other colors 

 took on a rather indistinct color, which, however, inclined toward 

 theirs. 



In general, then, each colored substance remained unchanged under 

 similar illumination and was under different illumination altered or 

 destroyed. An exception to this rule occurred in the case of the yellow, 

 or rather orange, as the color produced by pure yellow illumination 

 appeared of a more orange color (see page 177). This color was not 

 changed by the illumination of the neighboring red and green, and 

 was not easily altered by the blue, since in this case the mixed color, 

 green, resulted. 



These facts would contradict the explanation of the color reproduction 

 given if there were not a cause for failure which justifies the explana- 

 tion ; if, namely, the orange colored substance is not completely light- 

 sensitive for red and green it can exist at the same time with red under 

 red and with green under green illumination, without being again 

 decomposed. If, however, this substance is the more stable under the 

 action of light, it will finally gain the ascendancy, and this was in fact 

 observed. 



The originally narrow orange yellow strip broadened out in both 

 directions with increasing duration of exposure. Its breadth was, for 

 example, in a field exposed 24 minutes about 1 millimeter and in one 

 exposed five times as long 3 millimeters. This broadening was in some 

 experiments more considerable toward the red than toward the blue 

 part of the spectrum. In other experiments this appeared not to be 

 the case. This may have been due to slight differences in the method 

 of preparation of the sensitive film. 



The fact that this displacement takes place agrees well with the fol- 

 lowing phenomenon. An exact investigation showed, namely, that in 

 short exposures, for example 4 minutes, a red and not a yellow color 

 results from illumination with sodium light, which gradually takes on 

 the orange coloring. It therefore appears that the yellow substance is 

 a product of the decomposition of the red. This process must be 

 explained chemically, and need be taken in consideration in the present 

 investigation only as explaining the one-sided displacement of the 

 orange yellow strip with increasing time of exposure. For, in accord- 

 ance with what has been said, the red preliminary product would more 

 readily be produced by red than by green illumination. 



It must be observed that the deviations of the characteristics of the 

 photographic substances in use from those of color- receptive sub- 

 stances lead to deviations from a correct reproduction of colors. For 

 that color in Poitevin's process, however, which with prolonged expo- 

 sure is continually correctly reproduced — namely, orange yellow — the 

 conditions are fulfilled. All other colored substances produced are 

 sensitive to orange-yellow light, and are decomposed by it. 

 sm 96 13 



