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 

 su:h 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 



