180 



L. A. COTTON AND M. M. PEART. 



In this case it was possible to take the plane of the 

 stereographic projection so that it was perpendicular to 

 the crystallographic axis C. The direction of transmission 

 C P, (Fig. 1) therefore lies in the plane of the projection 

 and is perpendicular to the trace of the mineral section 

 SCT. 



ot 



-©- 



010 



/ 



— /— o 



/ 



\ 



5 



\ 



\ 



-X 



P 

 010 



100 



Fig. 1. — Stereographic Projection showing the method of finding the direction* 

 of vibration corresponding to the required refractive indices. 



