184 COLOR PHOTOGRAPHY. 



chloride and pure silver. This is doue in this research for the proce- 

 dure of Becquerel. It must be equally possible to distinguish in the 

 process of Seebeck, where chlorine compounds are also made use of. 



I may here offer a remark concerning a conceivable improvement in 

 color photography by the interference method. Lippmaun's color repro- 

 ductions have, to be sure, the advantage of the possibility of fixing 

 and of greater sensitiveness to light over those of Becquerel. They 

 are, however, inferior, in that the colors are more dependent on the 

 angle of incidence and in the necessity of observing them in a definite 

 beam of light. In BecquerePs method the colors are changed so little 

 with the angle of incidence that it was not for a long time shown that 

 any change at all occurred, and they can be observed in diffuse light. 

 Thus these colors have the characteristics of body colors without being 

 such . They receive this quality by reason of the high index of refraction 

 of the film. 



Lippmann's color reproductions would share in this advantage, and 

 would, indeed, be suitable for copying on paper if it were possible to 

 make some addition to the gelatine such as to give it a higher index of 

 refraction or even to replace it by some other substance with such an 

 index. It cannot, however, be said a priori whether this is possible 

 without the loss of other advantages of the method. 



Till. — Becquerel's Color-Bearing Film viewed prom the 

 Back. — Second Proof for the Interference Nature of the 

 Colors. 



For the purpose of Chapter II it was necessary to separate the color- 

 bearing film of the Becquerel plate from the silver backing. This was 

 accomplished with gelatine according to the directions of Wernicke. 1 



In so doing I observed the remarkable phenomenon that the colors of 

 the reverse side by reflected light were very considerably displaced 

 from what they were originally when viewed from in front. 



The tone of the colors was also somewhat changed. Such a change 

 of color is not conceivable for body colors and can only be explained 

 through interference. This observation furnishes, therefore, a second 

 proof of the interference nature of the colors and at the same time of 

 the correctness of Zenker's explanation of them by the assumption 

 of stationary light waves. 



Such color shifting has also been observed with Lippmann plates 

 when examined from the glass and film sides. I can not, however, 

 recognize the explanation which I found given for this as correct. 



These phenomena are the necessary consequences of facts hitherto 

 overlooked. It would nevertheless lead me too far from the subject 

 of this investigation to discuss this matter here. I must reserve the 

 consideration for another publication. 



1 Wernicke, Analen der Physik und Cheruie, 30, page 462, 1887. 



