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



lying plat, and then finding and sketching that great circle 

 whose intersection C with the vertical diameter is at 90° from 

 P (to be counted from P, each of the great circles of the 

 projection being 2° apart). Similarly, the great circle A^,, 

 containing A, and the extremities of the horizontal diameter 

 DE, is located and drawn. The great circle which indicates 

 the plane of vibration for P, can be found by either one of 

 two methods : {a) it is that great circle containing P and 

 tangent at P to the small circle, GPL (fig. 9), which parallels 



Fig. 9. In this figure the points P and Aj are the two points obtained from 

 the original drawing. The plane of vibration for the point P in its position 

 of darkness is indicated by the great circle PC tangent at P to the small cir- 

 cle GPL, which is parallel to FOC, the trace of the plane of vibration of the 

 polarizer when the zero isogyre passes through P. Further details of con- 

 struction are added under fig. 10. 



the trace GOF in projection of the principal plane of the 

 lower nicol ; (b) it is that great circle passing through P and 

 the intersection jC of the polar great circle A/A 2 ' C with the 

 trace of the principal plane FOC of the lower nicol.* In 

 actual work, however, it is not necessary to draw this great 

 circle PC, since the point C is the point sought and deter- 

 mines at once the direction of extinction for the given section. 

 The simplified construction is illustrated in fig. 10, where C is 



*In his paper, Professor Becke determines this great circle as the one 

 which is tangent at P to the straight line parallel to the trace in projection 

 of the principal section of the lower nicol. This method does not facilitate 

 the finding of the great circle in any degree, and introduces an error, as 

 Professor Becke himself recognizes, which decreases the accuracy of the 

 method to just that extent. It seems advisable, therefore, to use one of the 

 new methods described above which are theoretically correct and equally 

 simple. 



