214 PROGRESS IN COLOR PHOTOGRAPHY. 



One of the greatest advantages of the indirect methods is that they 

 permit of reproductions by photomechanical processes, and thus lend 

 themselves to illustrative purposes. The colors may be either super- 

 posed or juxtaposed. Superposition, however, requires at least two 

 transparent colored inks. Hence, the mixture of colors is generally 

 produced by juxtaposition of pigments. This is done sometimes in 

 photocollography by producing grained images ; but more frequently by 

 phototypogravure, which has the great advantage of being suited to the 

 production of numerous copies. 



The three-color process of mechanical polychrome illustration was 

 for a long time made use of in England and America. Of late, however, 

 a number of French firms have had excellent success in the form of 

 illustration. 



A great objection to indirect color photography is that it requires 

 three negatives, which consume time for their production. The red, 

 especially, requires a very long exposure. Nevertheless, this process 

 is much employed for landscapes, and not long since M. Montpillard 

 presented to the French Photographic Society a very excellent colored 

 typophotograph representing a landscape photographed by him from 

 nature and engraved by M. Prieur. 



There has been much effort to simplify the process. One of the 

 first methods proposed was to use a camera provided with mirrors, so 

 that the three exposures could go on simultaneously. But the images 

 thus obtained are by no means equal to those produced when the 

 apparatus is used in the ordiuary way with three separate exposures. 

 It has been proposed to use three separate lenses, but this, of course, 

 accentuates the same defect. It is very difficult to have three objec- 

 tives which will give three images precisely identical. 



Another device, proposed by John Joly, of Dublin, about three 

 years ago, but really a re-edition of a procedure published in 1869 by 

 Louis Ducos du Hauron, consists in obtaining the three negatives upon 

 a single sensitive surface. A glass plate is ruled with fine parallel 

 lines colored in rotation with the three fundamental colors in trans- 

 parent pigments. This prepared plate is superposed on the sensitive 

 surface, and thus is produced in close juxtaposition a combination of 

 the three negatives corresponding to the three primary colors. A 

 positive transparency of this negative is projected upon a screen 

 through a color grating exactly like that first used, and the colors of 

 the original are thereby closely reproduced. Louis Ducos du Hauron 

 applied this same process to photomechanical reproduction by print- 

 ing a black image upon a paper ruled like the screen in red, blue, and 

 yellow. 



The colors of the original are fairly rendered in this way. However, 

 it is easy to see that if the original presented a considerable blue 

 space this would be represented in the picture by a space one-third 



