of Minerals in the Thin Section. 367 



For the different positions of IT„, the extinction angles for a 

 given angle of revolution about Y 1 were: 



H, 



H 2 



H 3 



V: 



v 5 



145° 



123° 



333° 



17° 



31 



1 45 -5 



122 



a 



a 



ii 



144-5 



'124 



a 



<t 



a 



144-3 



123 ; 5 



a 



a 



u 



On plotting these values in projection, it was found that 2V 

 was about 64° — 67°, but a more decisive result was not attain- 

 able. The method is not accurate and can only furnish very 

 rough approximations. 



In the second method, which involves revolution about an 

 axis normal to that of the above, the values observed were: 



Hj 



H 2 



H 3 ■ 



v, 



v. 



148°-5 



33° 



332° 



17° 



31° 



148 -5 



34 



a 



u 



a 



149 



32 



a 



a 



a 



and from these angles, 2V was found to lie between 64 and 68°. 

 The determination cannot be termed satisfactory and this 

 method, like the above, can furnish only rough approximations 

 to the true values of 2V. 



Summary. 



(1) The optic axial angle of minerals in the thin section 

 can be determined under the microscope in either convergent 

 or parallel polarized light. 



(a) In convergent polarized light, methods for the meas- 

 urement of the optic axial angle are available for all sections 

 in which at least one optic axis appears within the field of 

 vision. Of these the method requiring the use of the Becke 

 drawing table is of general application and furnishes results of 

 a fair degree of accuracy — the usual probable errors being 

 about ± 1° if both optic axes be visible, and ± 5° if only one 

 optic axis be visible. More accurate and somewhat simpler in 

 manipulation and of the same general application is the method 

 involving the new double screw micrometer ocular, described 

 above. This ocular combined with the method of projection 

 of Professor Wulff, is a general extension of the Mallard 

 method, and, like the Becke method, utilizes the rule of Boit and 

 Fresnel which defines the planes of vibration for any direction 

 of wave propagation. With this ocular the probable errors of 

 determination on sharp interference figures should not exceed 

 1° if both optic axes are visible, nor 3° if only one optic axis 

 appears in the field. 



