338 F. E. Wright — Measurement of the Optic Axial Angle 



axes appear in the microscopic field. By using projection 

 plats, either stereographic or orthographic, results of a fair 

 degree of accuracy can be obtained in a very few minutes. 



On sections in which both optic axes of the interference 

 figure are visible, the exact position of A x and A 2 can be meas- 

 ured directly, and after plotting, the value 2V obtained from 

 the projection by direct reading. Such values should be 

 accurate to ± 1°. 



To form an idea of the relative degree of curvature of the 

 axial bar in the diagonal position for sections of biaxial miner- 

 als cut at different angles with an optic axis (binormal) and for 

 the optic axial angles 2V= 0°, 15°, 30°, 45°, 60°, 75° and 90°, 

 the writer has constructed by graphical methods the following 

 fio'ures : 



Fig. 13. This figure illustrates the positions in the interference figure of 

 the dark curves of no extinction (axial bars) as they would apjsear in the 

 field were observations made in air on a series of biaxial minerals having a 

 mean refractive index of 1'60 and the optic angles indicated, and cut normal 

 to one of the optic axes (binomials). From the figure it is evident that the 

 radius of curvature of the axial bar increases with the optic axial angle, so 

 that for 2V = 90°, the axial bar is practically a straight line. 



Fig. 14. The conditions of construction for this figure were similar to 

 those of fig. 12, except that the section is considered cut at an angle of 

 a = 10°, fi = 10° (small circle coordinates from the center) with one of the 

 optic binormals. 



In these figures, the dark lines represent the curves of 0° 

 extinction (zero isogyres or axial bars) of the interference fig- 

 ure in orthographic projection, the plane of the optic axes 

 making angles of about 45° with the plane of vibration of 

 light waves from the lower nicol. 



