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SCIOPTICON MANUAL. 89 ^ The crystallization of many other substances, such as bichromate of potash, alum, &c., and the precipitation of iodides of silver, mercury, and other salts, all form beautiful objects on the screen. THE DEVELOPMENT OP A PHOTOGRAPH ON THE SCREEN. —For this we require a tank with one of its faces of yellow glass, which side should be next the condenser. Place a small statuette in the rays of the lantern, and having prepared a small plate with collodion and sensi- tized it, expose in the camera for about a minute; then, having filled the trough with developing solution, place in it the slide, and as tbe development proceeds the image will gradually appear on the screen. A trans- parency might then be made from this, and, after drying, shown on the screen, thus illustrating the formation of a photographic lantern slide. CHANGING COLORS. —A glass coated with a mixture of gelatine and chloride of cobalt, when placed in front of a slide, will give a rosy effect to the picture, which, however, from the effect of the warmth of the lantern, will gradually change to purple and then to blue. On becoming damp again it will resume its red color, and can be used over and over again. COMPLEMENTARY COLORS. —A number of beautiful ef- fects, showing complementary colors, may be obtained with the Sciopticon. If we insert a piece of green glass, having any design cut out of black paper and pasted on it, we shall see on the screen a black design on a green ground; but by bringing another light into the room or turning up the gas, the black design will at once appear to the eye as a brilliant pink. By making apertures in a card slide, as circles, squares, 7