DOUBLE REFRACTION OF LIGHT 



49 



Now every facet at the front of a cut transparent gem forms with any facet at the 

 back (provided they are not parallel) a prism ; and through every such pair of facets can be 

 seen, when viewed in the proper direction, an image of 

 a small flame. As a matter of fact, a large number of 

 such images will be seen, since for any one facet at the 

 front of the stone there will be several at the back, each 

 of which may form with the front facet a prism and 

 give rise to an image. The images given by singly 

 refracting stones are single, as in Fig. 19, while doubly 

 refracting stones give two images very close together, 

 as shown in Fig. 25. This difference enables us to 

 distinguish a singly refracting from a doubly refracting 

 stone. 



For this purpose the stone should be held with 

 the largest front facet, namely, the table, close to the 

 eye, and a small flame viewed through it. On turning 

 the stone about, a position will be arrived at when 

 numerous coloured images of the flame become visible, 

 each being single if the stone is singly refracting, or 

 double if it is doubly refracting. Each image, whether 

 double or single, has originated by refraction through 



a prism formed by one of the facets at the back of the stone and the table at the front. 

 The images seen through a doubly refracting stone are shown in Fig. 26a, while those 

 seen tlirough a singly refracting stone are shown in Fig. 266. 



Fig. 25. Path of light through a doubly 

 refracting prism. (Perspective view.) 



Fig. 26a. Images of a flame observed through a 

 doubly refracting stone. 



Fig. 26J. Images of a flame observed through a 

 singly refracting stone. 



This experiment is best performed in a dark room so that no light other than that from 

 the small flame passes through the stone. 



Instead of using a flame, however, any other convenient object may be observed through 

 the stone, and for this purpose a needle may be used. When the needle is placed in the 

 proper position relative to the stone, there will be seen several coloured single images of it 

 in the case of singly refracting stones, while doubly refracting stones will give coloured 



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