420 F. E. ^Wright — New Ocular for Use with the 



Fig. 5. 



Fig. 5. Like fig. 4, except that the directions are indicated whose bire- 

 fringence is 10 per cent less than that of the optic normal located at the 

 center of the projection plat. The positions of the curves corresponding to 

 optic axial angles 2 V=0°, 15°, 45°, 60°, 75°, 90°, are indicated in the figure. 



Fig. 6. 



Fig. 6. In this figure the directions whose birefringence is 10 per cent 

 less or greater than that of the acute bisectrix (optic axial angle 2 V=45°) 

 are shown by the dotted curves. In this figure the dotted curve which 

 passes through the center point (acute bisectrix) marks the directions whose 

 birefringence is equal to that of the acute bisectrix (y-(3) or (j3-a) as the case 

 may be. 



is ascertained by means of convergent polarized light. In 

 actual work it is not always easy to find a plate cut precisely 

 normal either to the optic normal or to one of the bisectrices, 

 and it is of interest to know the percentage error caused by 



