15G PHOTOGRAPHY IN THE COLOPtS OF NATURE. 



isolating successively each ray, or at least tlie rays of the same shade. 

 - - - This analysis is difficult to make with colored glasses; it 

 might be done, as by Newton, by monochromatic lighting and succes- 

 sive exposures to simjile rays of the same shade. - - - Tlie synthe- 

 sis is made by means of black i)ositive images and rays of the same 

 nature as those which produced the corresponding negatives. - - - 

 It will then only be necessary to jdace one above another the colored 

 images so obtained, so as to form one virtually and really. It will be 

 identical with the model, because it will be lormed by the same rays, in 

 the same relation of intensity." This also couhl not then be carried 

 out because no photographic sensitive plates were sufficiently sensitive 

 to yellow, orange, and red spectrum rays. 



In 1873, Br. H. W. Vogel discovered that bromide of silver can be 

 made sensitive to the less refrangible spectrum rays by treatment with 

 certain dyes, and the subseqiuMit discovery of other and better color 

 sensitisers supplied the means for carrying out cither Collen's oiPoiree's 

 idea. 



Duhauron, one of the first to avail himself of these discoveries, made 

 some practical progress, and, in 187G, abandoned Brewster's color theory 

 and ])ateute<l a modified process,* based ni)on the observation that, 

 while there appeared to be seren " principal " spectrum colors, three 

 coloring substances wonld " serve to express them." The coloring sub- 

 stances he named for this purpose are blue, carmine, and yellow, and he 

 decided that, in order to make such a i)rocess rejn'oduce the colors of 

 nature, the negatives should be made by the action of orange, green, 

 and violet spectrum rays, Avhich are complementary to the coloring sub- 

 stances. Some i)ersons have thought that he had the idea of making 

 negatives to represent i)rimary color sensations, but this sui)positionis 

 negatived, not only by the absence of any de(;laration to that effect, 

 but also by the fact that orange does not represent a primary color sen- 

 sation, either in fact or according to any theory recorded in the text- 

 books, and the violet rays are not the ones which most i)Owerfully excite 

 the blue (violet) vsensation. The plan was also utterly indefinite as 

 regards the relative effect of intermediate spectrum rays, and Duhau- 

 ron himself, owing to the fact that he never tried the method upon the 

 spectrum, had no accurate knowledge of its capabilities. In his latest 

 and perfected process (1878) t he employed no plate sensitive to 

 either red or orange light. One negative was made chiefly by yellow 

 light, another by green, and the third chiefly by violet and invisible 

 nltra-violet rays. 



Albert, of Munich, also took advantage of the discovery of color 

 sensitisers to try to carry out Collen's principle according to Duhau- 



* British patent, July 22, 1876, No. 2973. 



t "Traits Pratique <le Photograpbie des Conlenrs," Paris, 1.S78, l'hoto(jraphic News, 

 1878, p. 115. 



