100 



A HALF-HOUR WITH 



double refraction is shown in fig. 17 ; a is a rhom- 

 boidal crystal of Iceland spar. These crystals have 





Fig. 17. 



a, rhomb of Iceland spar ; b t ray of common light ; c, or- 

 dinary ray of polarized light; d, extraordinary ray of 

 polarized light. 



the property of splitting the impinging ray into two ; 

 thus, if a small hole is made in a card, and viewed 

 through a rhomb of Iceland spar, two discs of light 

 will be seen ; or, if a black line is drawn on a 

 piece of paper, two images of it will appear. 



b is the ray of common light which becomes 

 divided as it passes through the crystal. These 

 rays are both polarized. 



Certain varieties of tourmaline, when cut into 

 plates parallel to the axis of the crystal, possess 

 the property of polarizing common light. Fig. 18 

 represents such a plate. 



Having seen how a polarized ray can be ob- 

 tained, the reader will ask, How am I to recognize 

 this condition of light ; for you have already told 



