The history of three-color photography (1925)

676 History of Three-Color Photography distance apart of these fringes would depend for a given angle of the wedges, on the color of the light used. By semi-silvering the lower surface of the film and the adjacent surface of the glass plate, the fringes would be rendered much more distinct. If the film so mounted were pressed against the surface of a sensitive __jK Celluloid II Glass Glass III Glass Fig. 199 (Page 675). plate, the latter would on development be made into a diffraction grating, of which the ruling would be determined by the color of the light. In Fig. 199, III, B is the Canada balsam, C the celluloid and S the sensitive surface. The plate could be examined by slightly oblique light, and at a certain angle would appear colored with the light used. Lastly if such a film were placed in the camera in the usual way, a combined photograph and grating would be produced. This printed and viewed at a certain angle would appear in colors, similar to those produced by Wood's process. Provided, therefore, that the wedge film can be obtained, the production of colored pictures by its means would be perfectly simple ; moreover, they could be multiplied in the same way. Several methods suggest themselves for the production of the wedged film. For instance, Wood described70 an "echelette," which he had made by ruling a soft metal with a carborundum crystal, which gave a series of parallel wedge-shaped grooves. A cast in celluloid of such a surface could be taken, the grooved surface of the celluloid might be semi-silvered, and then mounted grooved side down in Canada balsam on a sheet of glass previously semi-silvered. The thickness of the celluloid would