Petrographic Microscope. 



421 



Fig. 7. 



Fig. 7. Similar to fig. 6, except that the center of the projection plat is 

 the obtuse bisectrix (2 V=45°). As in fig. 6, the directions whose birefring- 

 ence is 10 per cent greater or less than that of the obtuse bisectrix are 

 indicated. 



Fig. 8. 



Fig. 8. Similar to fig. 6, except that the optic axial angle is 2 V=90°. 

 The dotted curves again represent the directions for which the birefringence 

 is 10 per cent greater or less than that of the bisectrix at the center of the 

 projection plat. In this stereographic plat, as in all preceding, the concen- 

 tric circles are 10° apart. 



