EDWAED ELSWORTH. 177 



cessful solution of the question of color photography, up 

 to date, and which differs entirely from all the processes 

 which I have mentioned. 



The first practical suggestion of this process was made 

 in 1865 by Mr. Henry Collen, a painting teacher, em- 

 ployed in the household of Queen Victoria. His idea 

 was to make three photographs of an object, one by the 

 action of red, one by yellow and one by blue, and to 

 print from each pair of these negatives superposed as 

 one a transparent positive having the color represented 

 by the third negative, and to superpose on a white sur- 

 face the three prints thus obtained. 



The obstacles in the way of Collen' s theory were that 

 there was no known process at that time by which 

 plates could be prepared, which were sensitive to single 

 colors only, and no photographic plates were sensitive 

 enough to red and yellow to admit the production of 

 such negatives by exposure through color screens. The 

 result too, says Mr. Ives, would have been very imper- 

 fect in any event. 



In 1868, Ducos Duhauron, of Paris, attempted to 

 carry out Collen' s theory in a slightly different manner, 

 with precisely the result which Mr. Ives suggests. 

 Charles Cros, of Paris, followed Duhauron with some 

 further modifications of the same process, but with no 

 more successful result. 



These two men continued their experiments until 1880 

 witHout solving the problem. 



I will not take up time with examination of the 

 theories of Drs. Stolze and Vogel, of Berlin, except to 

 say that the efforts of these distinguished physicists and 

 chemists, while they did not solve the problem of color 

 photography, undoubtedly incited the invention of what 

 are known as orthochromatic or Isochromatic plates. 

 These are now made by all of the principal manufac- 

 turers of dry plates, and are simply plates which render 



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