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



bisectrix (a or y, as the case may be) by the method of principal 

 ellipsoidal planes (page 353), and bring it to coincidence with 

 the axis of the microscope, the plane of the optic axes being then 

 parallel to the vertical cross hair. (Fig. 29.) Revolve section 

 some convenient angle about axis V„ and then about V l (as shown 

 in fig. 29), also through any suitable angle. Measure accurately 



Fig. 29. To use the method indicated by this figure, turn the section so that 

 its obtuse bisectrix coincides with the axis of the microscope (center of the pro- 

 jection plat), and the optic axial plane is parallel to the vertical cross hair ; 

 turn preparation about axis V 2 a convenient angle (reduce to true crystal 

 angle equivalent to apparent angle observed in air or glass), and then about 

 axis Vi (normal to V 2 ) any suitable angle, and measure extinction angle of 

 section in its new position. Plat these data in stereographic projection and 

 find those two points A"i and A" 2 equidistant from the obtuse bisectrix 

 and contained in the plane of optic axes in its new position, for which 

 the observed line of extinction OE bisects the angle included between OA"i 

 and 0A" 2 . 



the extinction angle of the section in its new position. Plot data 

 in stereographic projection after proper reduction of observed 

 angles to true crystal angles ; and find those two points A", and 

 A"„ contained in the optic axial plane and equidistant from the 

 obtuse bisectrix a", which are so located that the observed extinc- 

 tion angle OE, bisects the angle A'^OA", (fig. 29). The angle 

 A'^A'^ is then the desired optic axial angle, 2 V. 



With the universal stage in its present form it is not always 

 possible to execute the movements indicated in the above method, 

 since when the obtuse bisectrix is brought to coincide with the 

 axis of the microscope, the axis of V 2 is in general no longer 

 horizontal and the revolution about V„ is therefore along an in- 

 clined axis. In plotting the observed data, this fact should be 

 carefully noted, otherwise errors may occur and nullify the results. 



