EDWARD ELSWORTH. 173 



that chloride of silver, after a preliminary exposure to 

 white light, was by prolonged exposure to the red end 

 of the spectrum colored a brick red, and by a similar 

 exposure to the blue end colored a metallic blue. 

 Eighty years experimenting with this phenomenon, 

 however, produced no practical results. 



In 1865 a series of experiments along another line were 

 commenced by Henry CoUen, of England, which have, 

 through the scientific investigation of several French 

 and German chemists and physicists, and also by Mr. F. 

 E. Ives of Philadelphia, yielded the only approximately 

 satisfactory solution of the problem under discussion. 

 The result of the last named investigator's experiments 

 I shall present later as Mr. Ives' process. 



In mentioning other processes, it may be well to state 

 that Dr. R. Kopp, a Swiss photographer, claims to have 

 discovered a process whereby all the colors of nature 

 may be impressed upon a single film, which is simple, 

 practicable and permanent. 



As the samples of his work furnished, so far as I can 

 learn, do not sustain his claim, and as he declines to re- 

 veal the process whereby he accomplishes his alleged re- 

 sults, he may be dismissed for all present purposes. 



Various other processes for the purpose of repro- 

 ducing pictures have been published from time to time, 

 but these are all more or less mechanical in their nature, 

 depending upon the application of pigments at some 

 stage of the process and do not properly belong to this 

 subject. 



A little more than a year ago the photographic world 

 was startled by the announcement that Mr. Gabriel 

 Lippman, a distinguished physicist of Paris, had suc- 

 ceeded in photographing the colors of the spectrum. 

 His process, which was based upon well known physical 

 laws, has not so far justified the pretentious claims at 

 first announced and bears about the same practical re- 



129 



