PROGRESS IN COLOR PHOTOGRAPHY. 211 



Two principal difficulties are encountered in this process. The first 

 consists in the discovery of colored substances which not only have 

 the proper colors (corresponding respectively to the three fundamental 

 colors), but which have besides the same laws of sensitiveness. A 

 second difficulty, as yet unsurmounted, is found in fixing the colors 

 when obtained. The Lumieres have, however, made some progress in 

 fixing, by means of metallic salts which form insoluble compounds 

 with the colors employed. Nevertheless it is very hard to avoid modi- 

 fying the tint in this way. Commandant Colson has made tentative 

 experiments in fixing color photographs made by Poitevin's process, 

 and utilizes a property of dry ink to desensitize the color-sensitive 

 substances. 



To sum up the matter, it may be said that these processes appear 

 unlikely to give more than a partial solution of the problem. 



Color photographs were obtained in 1848 by Edmond Becquerel by 

 another species of the direct process, and his researches have been 

 repeated and completed by Niepce of St. Victor. As a sensitive surface 

 Becquerel employed a silver plate superficially chloriuized, either by 

 means of electrolysis or by some strictly chemical process. Becquerel's 

 colored images could not be fixed, and the theory of their formation 

 was, in his time, little understood. 



To Prof. Gabriel Lippmann belongs the honor of showing in 1891 

 the true theory of the phenomenon, and of finding out why the colors 

 could not be fixed, and finally of giving an ingenious solution of the 

 problem of photography in colors, which has come to world-wide fame. 

 Since it is based on the principle of interference, this discovery forms 

 a beautiful confirmation of the undulatory theory of light. It will be 

 recalled that in Lippman's as in Becquerel's process the image is formed 

 by a series of layers of silver separated by distances varying with the 

 colors. If hyposulphite of soda be used to fix colored images of the 

 Becquerel type, it dissolves the intervening chloride of silver which 

 supports this system of layers, so that the whole structure crumbles and 

 the colors are destroyed. 



In Lippmann's process this difficulty is avoided, because the sensitive 

 salt is imprisoned in a transparent mass of albumin, gelatin, or collodion, 

 upon which the hyposulphite has no action. Only the bromide of silver 

 not modified by the light is dissolved, and the substratum forms a solid 

 foundation which maintains invariable the distance separating the 

 layers of silver from one another. 



In reality, as Wiener has shown in the memoir already cited, though 

 these interference colors predominate in the Becquerel images, yet 

 there are also present body colors, due, as in the direct processes 

 already mentioned, to the modification of color-sensitive substances. 



Since the communication by Lippmann to the Academie des Sciences 

 in February, 1891, many photographers have attempted to repeat his 

 experiments. But, despite numerous researches undertaken to explain 



