136 THE ART OF PROJECTING. 



overlap each other. Turn the prism on the beam as 

 an axis ; the images will turn about each other. 



Place a thin piece of mica between the orifice and 

 the lens. The two disks upon the screen will appear in 

 complementary colors, save where they overlap, which 

 will be white. Turn the prism as before ; the colors of 

 the two disks will change, always being complementary 

 to each other. 



Again, remove the reflector, and place the lens close 

 to the orifice. Fix the prism near the focus so that a 

 large part of the light passes through it ; and then, 

 with lens and Nicol's prism near it, project the images 

 of objects placed close to the double-image prism. In 

 this case the latter acts as a polarizer. 



When large Nicol's prisms can be had, one of them 

 may be substituted for the reflector upon the porte 

 lumiere. The light passing through it will be polarized. 



Fig. 104. 



The object to be examined, o, may be placed near to it 

 in front, then projected with any convenient lens, in 

 the focus of which place the other Nicol's prism. This 

 allows a large, amount of light to be used, and is one 

 method in use with lanterns. The only hinderance to 

 the use of these larger prisms is the costliness of them. 

 All of these experiments may be performed with a 

 lantern, with one of the more powerful lights. The usual 

 method of polarizing the light is to have an elbow in 



