Visual Education (Jan-Nov 1920)

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Miscellaneous Notes 43 evitably reproduce the exact color of every point in the subject. Chemical and Mechanical Difficulties This is all quite simple but here, for instance, was one obstacle to overcome. The sensitive emulsion ordinarily used in photography is scarcely affected by the red rays. Witness the red light used in photographic laboratories. Inversely, this emulsion is much more sensitive to blue and violet rays than to any other, so much so that if it were employed for this new device the photograph of the green rays would hardly have time to be impressed on the emulsion before the photograph of the blue rays would be over-exposed, and while the red rays would still have produced no effect at all. The panchromatization of the films — that is, the process of sensitizing them to all colored rays — is an industrial process well known for 15 years past. The difficulty in the present instance is this ; hitherto the emulsion has not been equally sensitive for all these rays. It has always been necessary to use screens to retard the action of the blue and violet rays in the landscape, and therefore panchromatization has always diminished the speed of the emulsion. This defect becomes particularly serious when one attempts to use it in motion pictures at high rapidity with the subjects in movement. M. Gaumont, therefore, could not accomplish his work until he had perfected this vital element, namely, an emulsion which was sensitive at the same speed to these three primary colors. He found the emulsion. After this chemical difficulty arose a mechanical one. It was necessary in the new machine to project in a single flash three images and to repeat this action sixteen times a second, while in the standard machines this speed is needed for one image only. In other words, the new film must be moved three times as rapidly, with decided danger of tearing the celluloid. He therefore reduced about one-third the height of each frame in the film (to 14 mm. instead of 19 mm.) and thus needed only double speed to achieve his purpose. He found, Inci dentally, that the oblong form thus given to the little pictures lent itself very happily to landscape and panorama, which will be one of the principal fields, of course, of the new color art. The Apparatus The camera for taking the new pictures is formed of three superposed lens chambers, placed as close as possible to each other so that the three lenses will take the view from almost identical angles. Across the rear of these chambers at each partial revolution of the star wheel passes, in a downward direction, a length of fresh film long enough to receive the three images side by side at a single exposure. It is evident now that this process would give us no new effect whatever upon the film. We simply would have three images instead of one, each produced by the total multicolored rays given off by the subject and nothing more. The function, however, of each of these chambers is to receive rays of different nature and only these rays. If, therefore, we place behind the lens of the upper chamber a disc colored in the primary green, the emulsion at the back of the chamber will receive only the green rays emanating from the subject. Similarly a red disc is placed behind the central lens and a blue disc behind the lowest lens of the three. These discs are called selective screens because they make a veritable selection of the rays from the subject which they will record; but the impression on the emulsion, I repeat again, whatever the color of the ray, is translated onto the film in black and white. The projection apparatus is as follows: The source of light sends its rays through the three negatives of the film; selective screens, respectively green, red, and blue, interposed between the film and the lenses, allow to pass only the same rays which came through said lens when the picture was taken. These three images then are thrown at the same time upon the screen in such a way that they are superposed with abso(Continued on page 64)