ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



511 



Fig. 64 is like fig. 63, except that the directions are indicated whose 

 birefringence is 10 p.c. less than that of the optic normal located at the 



Fig. 63. 



centre of the projection plate. The positions of the curves corresponding 

 to optic axial angles 2 v = 0°, 15°, 45°, 60°, 75°, 90° are indicated in the 



Fig. 64. 



figure. In fig. 65 the directions, whose birefringence is 10 p.c. less or 

 greater than that of tne acute bisectrix (optic axial angle 2v = 45 D ), are 



