COLOR PHOTOGRAPHY. 169 



rests upon Zenker's theory." He implies that the knowledge is not as 

 yet firmly established, and this may be inferred also from the following- 

 sentence, which finishes the treatise above cited: "We may assume, 

 in consideration of the color processes of which we have been speak- 

 ing" — those which I have designated as old — ''that the resulting colors 

 appear the same to us as the colored lights used in the exposure, 

 because the molecules of the layer exposed continue to vibrate with the 

 same wave lengths which they encountered in the light to which they 

 were subjected." 



The assumption thus made in the last sentence leads however to no 

 explanation of the color results ; for since the place exposed does not 

 become self-luminous, the further vibrations of the molecules must 

 result in the absorption of the colors before illuminating and the place 

 would appear of the colors complementary to these. 



In this state of affairs the fundamental question must first of all be 

 proposed : Are the colors appearing in the older processes apparent or 

 body colors — that is to say, produced by interference or absorption ? 



In the first case, which is the one required by Zenker's theory, it 

 must be farther asked: How is it then possible that the same colors 

 may be produced by chemical means"? Is it possible that by chemical 

 action a body may be produced with a stratified structure which is 

 capable of producing interference? Krone 1 indeed alleges this extra- 

 ordinary possibility; and it becomes clear that in this case one may 

 distinguish a process as chemical without contradicting Zenker's theory. 



In the second case however Zenker's theory is not applicable, and 

 one is confronted by the remarkable, and for my science new, issue, 

 the fundamental possibility that colored illumination can create cor- 

 responding body colors. Yet in these circumstances one might perhaps 

 fall back on the consideration that absorption and interference may not 

 be fundamentally different. Thus absorption follows from the inter- 

 ference theory developed by Wrede. 2 Such an assumption is not how- 

 ever compatible with the fact that the metals show the characteristics 

 of absorption at a thickness of about T ^ 7 the wave length of light. 

 It is fundamentally contradicted moreover, as was shown long ago by 

 Stokes and Eudberg, by the fact that absorption is connected with a 

 loss of light whose energy is changed into other forms, as for example, 

 into heat or chemical energy, while with interference alone no light is 

 lost, the reflected and transmitted together being always equal to that 

 incident, and in the case of white light complementary to each other. 



But why is it that the question as to the source of the colors in the 

 older methods of color photography has not already been easily-decided ? 

 Zenker :! gives the answer to this question. The fundamental substance 

 in these is chloride of silver or a loAver chlorine compound of this salt. 



1 In the address above cited. 



3 Wrede's theory and refutation, see Wiillner, Le.hrbuch der Experirnental-Physik 

 2: p. 456, 4th edition, 1883. 



3 Zenker. Photochromie, p. 85. 



