Petrograjphic Micr 

 Fig. 3. 



419 



Fig. 3. In this stereographic projection plat (circles 10° apart) the posi- 

 tions of the directions in a biaxial crystal whose birefringence (/-a') is 2 per 

 cent less than that of the optic normal (y-a) are indicated for the optic 

 axial angles 2 V=0°, 45° and 90°. The optic normal coincides with the 

 central point of the figure. 



Fig. 4. 



Fig. 4. Stereographic projection plat showing positions of the directions 

 for which the birefringence (y'-a 1 ) is 5 per cent less than that of the optic 

 normal (y-a) which coincides with the center of the concentric 10° circles. 

 These curves are drawn corresponding to the optic axial angles 2 V=0°, 45°, 

 and 90°. 



