190 COLOR PHOTOGRAPHY. 



It is conceivable that the regularly absorbing light-sensitive sub- 

 stance may be decomposed by the action of light to form colored 

 substances also regularly absorbing and light-sensitive. 



I will designate as a color-receptive substance a black regularly 

 absorbing light-sensitive substance, whose products of decomposition 

 consist only of monochromatic regularly absorbing light-sensitive sub- 

 stances of at least three radically different colors, and, besides these, of 

 a white substance which, however, is the least readily formed. These 

 colors must be radically different in order that by their mixture with 

 one another and with white all compound colors may be possible. In 

 distinction from these compound colors the unmixed colors will be called 

 ground colors. The monochromatic substances reflect only one color 

 well. They must absorb the others the more completely the more they 

 differ from them. With these preliminaries it may be shown that a color- 

 receptive substance reproduces the color of the illumination correctly. 



First, let the color of illumination agree with a ground color. It 

 will be absorbed by the black body and produces a decomposition 

 substance whicb, by hypothesis, is regularly absorbing and light-sen- 

 sitive. In this decomposition different colored substances are formed. 

 Those not agreeing in color with the incident light, absorb it, since, by 

 the hypothesis, they are monochromatic, and must absorb all illu- 

 mination different from their color. Since these are regularly absorb- 

 ing light-sensitive substances, they are also decomposed by the light 

 which they absorb. On the other hand, the substance of the same 

 color as the incident light is not decomposed, since it does not absorb. 

 In the end, therefore, it alone can remain in company with the white 

 substance. The amount of the latter is, by hypothesis, very slight, 

 and its effect upon the color is therefore noticeable only under strong 

 illumination. 



Where the color of the illumination differs from that of a ground 

 color, but is intermediate between two ground colors — as would, for 

 example, be the case with green, were yellow and blue ground colors — 

 the colored substances would suffer least decomposition which reflect 

 green best, that is, the yellow and blue. A green mixture would thus 

 arise besides the small quantity of white. 



In white light all the color substances would be decomposed, leaving 

 white alone. 



In the absence of illumination, the substance would remain black. 



It may thus be seen that all colors would be correctly reproduced. 

 The duration or intensity of the exposure must, however, be properly 

 limited; for, if carried too far, white would begin to predominate, and 

 the colors must gradually be extinguished. 



It is possible that a light-sensitive substance should have only par- 

 tially color-receptive qualities. Such an one would reproduce colors 

 but partially. If the substance is not black, then black could not be 

 reproduced. If not a regularly absorbing light-sensitive substance, it 



