546 THE POPULAR SCIENCE MONTHLY. 



then, constitute, in pairs, a thin film the thickness of which is 

 precisely that indicated by Newton's theory of the rings ; and 

 thus, according to that law, of which we cite the text, the rays 

 reflected upon these two films give, by interference with one 

 another, the sensation of the corresponding color. Furthermore, 

 each color produces in the plate a similar system of parallel 

 planes, the coexistence of which explains the photographic repro- 

 duction of the compound colors. The whole secret of the pho- 

 tography of colors lies in the enunciation of this principle. 



On observing the reflection of the plate fixed and dried by the 

 process which we have indicated, we shall discover upon it the 

 direct reproduction of all the colors which have been presented 

 before it. The time of exposure plays an important part in the 

 practical execution of the experiment. 



The beginnings of the experiments were very laborious. The 

 first effort was to photograph a spectrum, in which the red was ex- 

 tremely inconvenient. The chemical activity of the rays of this 

 color is very slow. They impress the plates so weakly as to per- 

 mit photographers to use red light without danger while develop- 

 ing their gelatinized bromide-of-silver glasses. Even those least 

 familiar with photography know that red objects are reproduced 

 in black on the positives, and that means that they have not im- 

 pressed the negative plates, however sensitive. While the red 

 shows itself very slowly on the sensitive j)late, the blue and the 

 violet act upon it with great energy, and completely polarize it if 

 the exposure is allowed to continue during the time required to 

 secure the impression of the red. Means, therefore, had to be 

 found to let the exposure to the red be continued for a long time, 

 to the green for a little less long, and to the blue and the violet 

 for a very short time. It is not hard to conceive the trouble 

 which these difficulties, all material, caused at the beginning of 

 the experiments. In fact, they were susceptible of barring the way 

 to every new tentative in the art of practically photographing 

 colors. 



How should one proceed in photographing a human being or 

 a landscape ? A posing before the objective as many times as 

 there were colors could not be thought of. It would, besides, be 

 necessary to fix the person in the same place, to make him re- 

 sume the same attitudes conditions which would make the faith- 

 ful reproduction of his image impossible. The assistance of a 

 practical photographer became necessary in this emergency. 



M. Attout-Tailfer discovered that on plunging an ordinary 

 plate into cyanine, its sensitiveness increased for the red and di- 

 minished for the violet, in such a way that by successive applica- 

 tions it was possible to equalize the sensitiveness of the plate for 

 the different regions of the spectrum, and therefore for the dif- 



