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 reproduction of the compound colors. The whole secret of the photography 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 extremely inconvenient. The chemical activity of the rays of this color is very slow. They impress the plates so weakly as to permit photographers to use red light without danger while developing 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 impressed the negative plates, however sensitive. While the red shows itself very slowly on the sensitive plate, 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 resume the same attitudes—conditions which would make the faithful 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 diminished for the violet, in such a way that by successive applications it was possible to equalize the sensitiveness of the plate for the different regions of the spectrum, and therefore for the dif-