Polarizing Prisms* 353 



Let P Q be the trace of the balsam. In an ordinary Nicoi's 

 prism A B would be inclined at about 74° to A D, and P Q 



Fig. 1. 



A 



y k Q 



D 



,/ ^ 





^ > 



T^Z^— — — 



TJ V 





/ ^< 





B 



p 



C 



would be at right angles to AB, AD and B C being 

 parallel to edges of the rhomb of spar. In the case now 

 being considered, A B is at right angles to BC, BC being 

 still parallel to a rhombic edge. 



Consider a ray R $ incident normally on A B. The ordinary 

 ray S T enters the spar without deviation, but is reflected by 

 the balsam at T in direction T K ; the extraordinary ray is 

 refracted at the face A B in direction T U, and turned from 

 its original path in virtue of the extraordinary refraction. It 

 emerges along U V parallel to its original direction, but dis- 

 placed to one side, so that the extraordinary image of the 

 object seen is displaced to one side by the passage of the 

 light through the spar. 



In the prism considered in fig. 1, the optic axis lies in the 

 plane of the paper, making an angle of 57° 30 with B C. 



Suppose now we cut a rectangular parallelepipedon from a 

 piece of spar, in such a way that two of its faces are at right 

 angles to the optic axis while the other four are parallel to it. 



Let A B C D (fig. 2) be a section of the solid by a plane also 

 at right angles to the optic axis, and therefore parallel to two 

 faces and at right angles to the other four ; and suppose that 



Kir. 2. 



B C is about three times A B. Let P Q be inclined at 

 about 20° to B C, and suppose the prism cut in two by a 

 plane at right angles to the paper and passing through to 

 P Q. Then let the faces of section be polished, and cemented 

 together with Canada basam. The optic axis will be at right 



