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436 OPTICAL PROJECTION slides. Professor Joly's process lends itself well to lantern illustra- tion, though the network of lines show rather prominently on the screen. Samples of this method, however, I believe are not now obtainable. Messrs. Lumiere's method consists in taking three negatives through carefully selected colour screens. A positive from one of these is obtained on a glass plate and then dyed a corresponding colour. The plate is then varnished and again sensitised, and the print from the second negative imposed; this again is dyed, varnished, and sensitised, and the print from the third negative superimposed again and treated likewise. Mr. Sanger Shepheard's process is somewhat similar, but in his method the variously dyed positives are taken on gelatine films, and the whole then balsamed together. In Lippman's process the photograph is taken upon a gelatine plate containing finely divided particles of silver bromide. The plate is ' backed' by a thin film of mercury in absolute contact with the sensitive surface. When the exposure is made the mercury film reflects the image into the gelatine plate again, so to speak, and the result is to set up a sort of wave formation in the gelatine itself. This, when viewed at the right angle by reflected light, gives the image in its natural colours, the wave formation by a diffractive effect reproducing the correct tints. Specimens of the process can, of course, only be projected by reflected light, and then care must be taken to ensure the beam being reflected at the correct angle ; but the result, when enough care and pains are taken, is extremely pleasing, and the colours really very correct. The Photochromoscope. —This instrument, designed by Mr. Frederick Ives, attracted great attention a few years back. Perhaps the method can hardly be called colour photography, as the image can only be projected or viewed by means of an instru- ment designed for the purpose. Three negatives are taken through suitable colour screens, as in the Lumiere and Sanger Shepheard processes. Three separate positives are then printed on glass from these, and the images of these positives are then projected on the screen, each being pro- vided with an optical system and colour screen to itself. By means of an ingenious arrangement of mirrors, one single lime-light jet or arc lamp suffices for the illumination of all three systems,