172 COLOR PHOTOGRAPHY. 



Seebeck's process, for example. That such compounds are formed 

 most readily under the action of light is not difficult to suppose. 



But why are the compounds formed of the same color as the illumina- 

 tion ? Why, for example, should a red photochloride be formed under 

 the action of red light in preference to some other ? 



It has the physical advantage that it reflects this color better than 

 compounds of other colors. Colored light that is reflected is not 

 absorbed, and can therefore cause no decomposition, for which the 

 absorption of light is requisite. Of all possible compounds which can 

 result from the disturbance of the chemical equilibrium by the action 

 of the light, the red compound possesses the advantage of stability on 

 continuing the exposure. According to the conceptions of the kinetic 

 and newer chemical theories, we must, however, assume that in the dis- 

 turbance of the equilibrium all possible compounds are, temporarily, 

 actually produced by some of the groups of molecules. Among these, 

 only the red continue unchanged, while those of other colors absorb the 

 red light and are by it further decomposed. 



This is an explanation of the phenomenon easily deduced from 

 admitted facts and observations. Its correctness may be readily 

 tested, for it requires that, for example, the red compound shall be 

 decomposed by other than red illumination while stable in red light. 



Such an experiment appears in the researches of Carey Lea. 1 He 

 threw a spectrum upon the rose-colored photochloride. All the colors 

 except red changed it in such a way as to impress their own hue upon 

 it more or less, but " in the red it remained unchanged." 



I myself made experiments to test this explanation, in which the 

 sensitive plates were exposed to the action of two spectra crossing each 

 other at right angles. These experiments, which are described in Chap- 

 ter XIII, confirmed the correctness of the explanation. 



A substance which, corresponding with this explanation, has the 

 characteristic of giving corresponding colors, I call a color receptive 

 substance. This characteristic is discussed in Chapter XII. 



By this connection a new foundation is laid for further methods of 

 color photography, for the phenomenon is not restricted to particu 

 lar substances. It appears that any coloring matter which can, under 

 certain conditions, be decomposed by light is suited to use with new 

 methods. 



It must be remarked that the difficulty to fix these colors appears 

 from the nature of the method of their formation inevitable, for the 

 capacity of the sensitive substances for reproducing colors is due to 

 their decomposition by the action of light. Indeed, the colors in the 

 older processes could only be tixed in a very limited degree. It is inti- 

 mated in Chapter XIV in what manner success may perhaps be reached 

 in this direction. 



In connection with the characteristic of color reproduction the acqui- 

 sition of a capacity for resisting outside actions may be called an 



i Carey Lea. American Journal of Science (3) 33 ; p. 363. 1887. 



