144 THE ART OF PROJECTING. the sensitiveness of the eyes for various colors. Gen¬ erally, after looking steadily at a given color, and the disk is made suddenly white, the outline of the colored part will be seen in a color complementary to the one looked at first. Thus, if a square red glass should be projected the residual image would be a square green one. If a blue one was projected its complementary image would be orange, and so on. A great variety of such effects are obtainable with various colored pieces of glass, or of films of gelatine, by projecting them singly, in juxtaposition, or superposed. Let disks of white cardboard a foot or two in diame¬ ter have partial sectors painted black, with india ink, so that the white and black parts alternate four or five times in the circumference. This is to be rotated while a powerful beam of light falls upon it. The persist¬ ence of some of the elements of white light being greater than of others, the disk will appear of various colors ; purple, green, and yellow being generally well developed. HEAT. — AIR THERMOMETER. A bulb blown upon one end of a small glass tube, five or six inches long, answers for this experiment. A drop of colored water can be made to enter the tube by first heating the bulb a little by holding it in the fingers with the open end of the tube a little below the surface of the water. A bubble or two of air will be expelled, and the fingers may be removed from the bulb. As it cools a drop will be driven into the tube, and with a little painstaking it can be brought to any required place by cooling or heating the bulb. These movements can be shown with the porte lumière and a single lens, as shown in Fig. 17, or it can be put in