168 COLOR PHOTOGRAPHY. 



published in Goethe's "Farbenlehre," 1 in the year 1810. Seebeck used 

 moist chloride of silver, which had become gray by the action of light, 

 spreading this upon paper. 



Schultz-Sellack's objection applies with the greatest force to pro- 

 cesses where paper is coated with the substance sensitive to light by 

 soaking it in different solutions, as, for example, in Poitevin's process. 

 It does not, on the other hand, apply in those which make use of a 

 uniform transparent layer of a substance sensitive to light with a good 

 reflecting background, as the processes of Becquerel, in which bright 

 silver plates are chlorinized to a determined depth by electrolysis. 



A second objection of Schultz-Sellack is raised .against the possibility 

 of the satisfactory production of colors by a mechanical division of the 

 layer brought about by the exposure, the degree of which would be 

 determined not by the colors but by the intensity of the light. The 

 coloring would, under these circumstances, be only accidental. This 

 explanation is in reality shown to be erroneous in chapter five. 



Kew doubt concerning the general validity of Zenker's theory is 

 raised by the investigations of Oary Lea 2 on the haloid salts of 

 silver. He showed that the colored substances produced by the action 

 of colored light on chloride of silver already exposed may be produced 

 by purely chemical methods in the dark. 



H. Krone 3 also has recently given a series of reactions for the Poite- 

 vin process which are carried through by the exposure, and by which 

 different colored bodies may be produced in the sensitive substances 

 by purely chemical means. He announces, therefore, 4 " The method 

 of Poitevin rests upon purely chemical processes," and is "totally dif- 

 ferent from that of Lippman." But he also makes the following 

 remark: 5 "This causal connection" — namely, between the color of the 

 light and the above-mentioned bodies which may be produced by 

 chemical means — "is of a purely physical nature, and is only to be 

 explained with reference to the processes and by the progress of inves- 

 tigation upon wave motion and the nature of light." 



If, now, the Zenker theory has no application in this case, it is not 

 expressly stated wherein the verification fails. On the contrary, Krone 

 asserts 6 "that our present knowledge of the method of photographic 

 production of colors, so far as this has until now been chiefly deduced, 



1 Goetlie Farbenlehre 2: p. 716. The there communicated memoir of Seebeck I 

 found in none of the editions of the complete works of Goethe -which I could com- 

 mand. 



2 Carey Lea: "On red and purple chloride, bromide, and iodide of siver; on helio- 

 chromy and on the latent photographic image." American Journal of Science, 

 series 3, 33; p. 349, 1887. 



:! In an address published in the Deutsche Photographen-Zeitung, p. 327, ff. 1891, 

 and in his book "Darstellung der natiirlichen Farben dnrch Photographie," pub- 

 lished by the Deutsche Photographen-Zeitung, p. 43, 1894. 



4 In the beginning of the same address. 



'■ Loc. cit., p. 49. 



'In his book, p. 38. 



