LIGHT. 135 



number of circular crystals will be formed with radiat- 

 ing forms between them. These circular crystals may 

 be made larger and regularly disposed by touching the 

 mass with a fine needle point when crystallization 

 begins. Such ones will form about each point touched. 

 Magnify such objects so much that these circular 

 crystals will appear a foot in diameter upon the screen. 

 The Nicol's prism will show each one with four arms 

 that will turn about the centre of the crystal when the 

 prism is rotated, while the radiating crystals will show 

 as red, yellow, or purple brushes sweeping over the 

 screen. 



By inserting a sheet of transparent mica or thin 

 selenite between the reflector and the object, a colored 

 field will appear as a kind of background, upon which 

 the minute crystals, such as chlorate of potash and 

 oxalic acid, will appear more highly colored. The effect 

 is usually to heighten the color. 



THE DOUBLE IMAGE PRISM. 



Fig. 103. 



With a large lens project the image of the aperture 

 upon the screen, the light being polarized by the black- 

 ened reflector. At the focus of the lens place the 

 prism. Two images of the aperture will appear and 



