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 lumiere and a 

 single lens, as shown in Fig. 17, or it can be put in 



