The advance of photography : its history and modern applications (1911)

258 THE ADVANCE OF PHOTOGRAPHY process through a coloured vie wing-screen, but it differed from that plate in that the same screen was not used both for taking and viewing. The tints of the colours used in the two screens were slightly different, since those of the taking-screen were adapted to the plate, while those of the vie wing-screen were suited to the eye, the reason for this being that the plates used were not sensitive to the various colours in the same relative degrees as the eye. The number of lines per inch in the viewing-screen must be exactly the same as in the taking plate. The viewing-screen had of course to be so adjusted that the colours were in perfect register with the corresponding lines on the positive, then the two plates could be bound together. The plate so produced had to be viewed by transmitted light, and unless its image was projected upon a screen, the observer had to stand so that the rays of light which passed through the plate in a direction at right angles to its surface reached his eye, otherwise an incorrect colour rendering was the result. The Dufay Dioptichrome Plate is the result of still another attempt to supply the photographer with a means of obtaining photographs in natural colours. The important difference between this and other screen plates lies in the method adopted to prepare the threecolour screen. W arner-Pomrie Process. — An interesting addition to colour photography has recently 1 been made by the combined efforts of Miss Warner and Mr Powrie of Chicago, and is known as the Warner-Powrie process. The method adopted by these inventors is the same in principle as the JoLy process, but has not the same drawbacks from a commercial standpoint. The coloured screen is obtained by calling in the aid of the bichromated colloid process, and a ruled grating 1 Photographic Jonrn., Jan. 1908.