126 THE ART OF PROJECTING, 



DOUBLE REFRACTION. 



A piece of calc spar will be needed to show this. 

 Its size is not very material, though the thicker it is 

 the farther apart will the refracted figures be. It 

 should have smooth faces, but the natural faces are 

 often good enough to permit this phenomenon to be 

 projected. 



Fig> 94, 



Make a hole a quarter of an inch in diameter through 

 a bit of cardboard (unless you chance to have a dia- 

 phragm with holes of various sizes) and place it at the 

 aperture ; the small beam of light which comes 

 through it should be directed horizontally upon the 

 screen. Next place the piece of spar in front of it, 

 and then project the hole with an object lens with a 

 foot or more focus. The two spots will appear upon 

 the screen, and if the spar be rotated the one spot 

 will revolve about the other. Instead of the hole in a 

 diaphragm, it will do as well to make a black spot upon 

 a piece of glass and project it in the same way. 



Either side of the spar may be used for showing 

 this phenomenon. 



A double-image prism may also be used with still 

 better results as the images will be still further separated. 



