Three-colour Processes.—These are dealt with separately (see " Three-colour Photography " and "Screen-plate Colour Photography"). The only other process with which it is necessary to deal is the— Prismatic Dispersion Process, which practically splits up the light reflected from any object into its spectrum, and allows this to act on the sensitive surface, and then reconstitutes the image. In this process no dyes or filters are used, and it may be considered to be nothing more than a method of optical synthesis, in which the whole spectrum and not three colours are used. The first suggestion for the process was made in an English patent, No. 16,548 of 1895, taken out by F. W. Lanchester. The essentials of the process are as follow : A grating of black lines with clear interspaces is arranged between the object and the camera, a prism is arranged behind the lens with its axis parallel to the bars of the grating, the dispersion of the grating being such that when the lens is focused on the grating the images of the clear interspaces form a series of spectra on the plate, which are broken up by the light reflected from the object so that the image consists of lines of shaded intensities. From this negative a transparency is taken on an ordinary lantern plate and placed in the position occupied by the original dry plate, and the coloured picture is then reconstituted by placing a light behind the grating and viewing the picture at the distance of normal vision. In
the British Journal of Photography for Jan., Dr. Rheinberg suggested a similar process, and it has since been taken up by Cheron, Bruignac and Raymond, and the latter seems to have obtained the most practical arrangement. The following description is taken from " Colour Photography," the supplement to the British Journal of Photography for Mar. 19o7 : To an ordinary camera is fitted a diaphragm behind the lens, and this diaphragm should preferably be a narrow rectangle. In front of the plate is fitted an ordinary cross-line screen, with preferably opaque lines wider than the inter spaces ; behind the screen is placed a prism of from 3 to 8 degrees, and then the plate. The exposure is made in the usual way, and a positive from a negative made and placed in the position of the negative. At present the process is but in its infancy, but it promises well, the only disadvantages being the prolonged exposures required, and the difficulty of exhibiting the pictures to more than one person at a time.