COLOR PHOTOGRAPHY. 171 



able angle with, the normal to the surface. The difference of path of 

 the interfering light waves will, in comparison with vertical incidence, 

 be greatly changed, and according as the colors are thus altered or not 

 are they interference or body colors. A description and theory of this 

 prism experiment follow in Chapter XI, below. 



If, now, the spectrum were produced according to the method of 

 Becquerel, the surprising result appeared that a portion of the spec- 

 trum observed J . arough the prism appeared when compared with that 

 observed directly through the air to be considerably displaced toward 

 the extreme red. This is exactly the color shifting which is to be 

 expected if the theory of Zenker is correct. 



Zenker, therefore, deserves the credit of having, in the year 1868, 

 rightly recognized as the cause of the coloring in the Becquerel proc- 

 ess the action of fixed light waves. 



The photographic substance is in Seebeck's method the same. The 

 only difference lies in the form. Seebeck used powder, Becquerel a 

 homogeneous film of silver chloride mixed with subchlorides. The 

 color displacement in the prism experiment is to be expected to occur 

 with the same prominence as with the Becquerel plates. The displace- 

 ment is in fact, however, not to be observed, and just as little in the 

 Poitevin process. 



The objection of Schultz-Sellack is shown to be valid exactly in those 

 cases where it would appear plausible from previous considerations; 

 for in fine powder and in paper one could expect no regularly arranged 

 stationary light waves. 



But the question may properly be asked, Why are there not created 

 in the Becquerel plates body colors as well, since these plates contain 

 the same substance as those of Seebeck? It does not appear impossi- 

 ble that, besides the interference colors demonstrated to be present, 

 there may also be body colors. I was, indeed, able to show that in the 

 method of Becquerel body colors also cooperate. The proof of this 

 may be found in Chapter XI, below. The colors on plates of Seebeck 

 and Poitevin are, on the contrary, exclusively body colors. 



There are, therefore, methods of color photography which can not be 

 explained through application of the theory of Zenker ; and there are 

 substances in which colored illumination gives rise to corresponding 

 body colors, in which the coloring is not due to interference, but to a 

 characteristic absorption determined by the chemical constitution. 



But how is such a phenomenon conceivable! The idea of a possible 

 answer came to me in reading the above-mentioned memoir of Carey 

 Lea. The various colored compounds of chlorine and silver which he 

 mentions are, according to him, molecular compounds of silver chloride 

 and protochloride, but not to be expressed by definite number relations. 

 He groups them under the name of u photochlorides." These colored 

 compounds are also formed by the action of light upon a ground con- 

 sisting of silver chloride and protochloride, such as is at hand in 



