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



To 



insure accuracy, 



several different angles of 



this measurement should be repeated for 

 revolution and A a determined in 

 each case. As in the first method, the great circle CF, indi- 

 cating the original position of the plane OA/, can be con- 

 structed and should pass through A„ on the line OA„. 



The second new method differs from the first only in the 

 fact that instead of placing the optic axis A t in the plane OE 

 (fig. 25), and then measuring the extinction angle of the section 

 in the horizontal position, the actual direction of extinction 

 OE is brought to coincidence with the axis of revolution of 

 the universal stage (V, or Y 2 ) ; the section is then revolved a 



26 



Fig. 26. In this figure, the great circles a0'y, a0y' and a'/3y of the stereo- 

 graphic projection denote the traces of the principal planes of the optical 

 ellipsoid within the crystal. They are fixed in position by determining the 

 positions of H 3 and V 2 for which the section remains dark for all positions 

 of inclination about the horizontal axis Vi (V" 2 being normal to V"i) ; the 

 lines 00', 0/ and Oa' are thus fixed both in direction and length and also 

 the great circles a(3'y, afiy' and a'py, the planes of symmetry of the ellipsoid, 

 the intersections a, and y of which are in turn the ellipsoidal axes. 



given angle about this axis and from the extinction angles the 

 lines 0A 2 and OA/ determined whose arc is equal to the 

 angle of revolution. The point A 2 is then the desired direction 

 of the second optic axis. 



In both new methods the determination can be varied by 

 inclining the specimen first about V 1 as an axis and then deter- 

 mining a series of extinction angles for different angles of 

 inclination about Y 2 (Y 2 in this case being normal to Yj) and 



