210 PROGRESS IN COLOR PHOTOGRAPHY. 



Iii June, 1895, Otto Wiener published in the Annalen der Chemie und 

 Physik a memoir entitled "Color photography and mechanical color 

 adaptation in nature," which has been discussed by Bernard Brunhes 

 in the Revue Generale des Sciences, 1895. In this memoir Wiener 

 shows that the reproduction of the spectrum as obtained by Seebeck, 

 and Poitevin's process of color photography upon paper coated witli 

 the subchloride of silver, were both based upon the same principle as 

 the direct polychromy of Charles Cros, though Wiener does not cite 

 Cros and seems to have overlooked his work. Wiener gives the name 

 of color-sensitive films to coatings capable of taking on the color of the 

 light incident upon them. 



In the process of Poitevin the color- sensitive substance, according to 

 Wiener, is a mixture of the chloride and subchloride of silver, capable 

 of appearing in various colors, and given by Carey Lea the name of 

 "color salt." 



By the aid of researches described in the memoirs of Wiener, he con- 

 cludes that the ideal color- sensitive substance would be a black absorb- 

 ent mixture containing at least three different-colored substances, each 

 of which should absorb all the colors of the spectrum except one and 

 be destroyed by the colors which it absorbed. A single-colored light 

 falling upon such a substance would leave intact only the color the 

 same as its own. We have already seen, in quoting from Charles Cros 7 

 what would be the effect of a light of mixed colors on such a substance. 



This kind of color adaptation by means of a color-sensitive substance 

 is not infrequently met with among animals. Certain species take on 

 the color of their surroundings. Thus Danais chrysippus, green in 

 nature, becomes white, red, orange, or black when brought up in boxes 

 constructed of white, red, orange, or black paper. 



Immediately after the publication of the interesting research of 

 Wiener, Emile Vallot devoted himself to the task of obtaining a good 

 color-sensitive coating. He recommends a mixture in equal parts of 

 the following three solutions: 



Cubic centimeters. 



Alcohol 50 



Aniline purple 0.20 



Alcohol.... 50 



Tumeric 0.20 



Alcohol 50 



Victoria blue 0.20 



Paper floated for several minutes upon this bath and dried in dark- 

 ness is dyed black. If placed in a colored image cast by full sunlight, 

 it gives a good photographic reproduction in the proper colors. Unfor- 

 tunately two or three days' exposure are necessary, and the yellow is 

 not altogether satisfactory. For the red, the aniline purple may be 

 replaced advantageously with safranine. 



A. and L. Lumiere have obtained a more sensitive paper by employing 

 cyanine for the blue and quinoleine for the red. 



