PROGRESS IN COLOR PHOTOGRAPHY. 213 



of these three pictures is reduced to a separate photograph and repro- 

 duced in the special color, it only requires to blend again into one the 

 three colored reproductions thus made in order to obtain an exact repre- 

 sentation of the original as regards both color and form." 



When the three cliches representing in analysis the colors of the 

 original have been obtained, their synthesis can be effected by one of 

 two different methods. 



1. By addition of lights. — Ordinary positives of the three cliches are 

 produced and illuminated, respectively, with blue, yellow, and red light, 

 each positive being illuminated by the same light which passed the 

 screen in producing the corresponding negative. When these three 

 separately illuminated positives are superposed, either by means of 

 a projection lantern (thus forming a real image) or by means of chromo- 

 scopes (virtual images), there is obtained a very faithful reproduction 

 of the colors of the original. Prof. Gr. Lippmann has proposed a very 

 ingenious device to make both the analysis and synthesis of the colors 

 with the same apparatus. He employs the principle of reversibility of 

 path of the rays traversing a lens. Three small objective lenses are 

 mounted on a support in a camera and provided with three colored 

 screens. In the foci of the lenses are placed plates sensitive to the 

 colors transmitted, respectively, by the corresponding screens. From 

 the three negatives thus obtained are reproduced positives on glass, 

 which are placed in the positions first occupied by the negatives, and 

 illuminated by the beam of a projection lantern. In this manner a 

 colored image of the original is projected upon a white screen. This 

 image can be magnified or diminished at pleasure by placing a fourth 

 lens of suitable curvature in the path of the beam. 



2. By absorption of light. — Procedures for synthesis of this kind are 

 very numerous. They admit of multiplication of copies. The carbon 

 process was employed first by Louis Ducos du Hauron. Charles Cros 

 communicated in 1881 to M. Oarpentier, of the Academie des Sciences, 

 a process of reproduction analogous to hydrotypy, based upon the 

 employment of coatings of albuminated collodion containing from 2 

 to 3 per cent of bromide of cadmium sensitized with ammonium bichro- 

 mate. This process is analogous to that employed by the brothers 

 Lumiere in producing their beautiful color photographs. A skillful 

 photographer, George Eichard, has also proposed a very ingenious 

 method. Positives from the three cliches are first produced on glass. 

 Then by means of a series of chemical reactions the reduced silver which 

 forms the image is transformed either into a mordant capable of fixing 

 the aualine colors or into a salt capable of reacting upon analine to 

 produce colors upon the plate. The three positives are then super- 

 posed to give color photographs. 



These polychrome proofs have to be viewed as transparencies. It is, 

 to be sure, possible to transfer the films to paper, and thus to have 

 photographs upon paper, but the results thus obtained are far less 

 satisfactory. 



