512 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



shown by the dotted carves. In this figure the dotted curve which 

 passes through the centre point (acute bisectrix) marks the directions 



Fig. 65. 



whose birefringence is equal to that of the acute bisectrix (y - /3), or 

 (fS — a), as the case may be. Fig. 66 is similar to fig. 65, except that the 



Fig. 6G. 



centre of the projection plate is the obtuse bisectrix (2v = 45°). As in 

 fig. 65 the directions whose birefringence is 10 p.c. greater or less than 



