424 F. E. Wright — New Ocular for Use with the 



There the different methods for measuring the optic axial 

 angle are treated at length, especially those of Mallard, 

 Eecke and Fedorow, together with a new method which 

 requires for its application a double screw micrometer ocular 

 or a cross grating ocular like plate b (fig. 9). The method of 

 procedure for both double screw micrometer ocular and cross 

 grating ocular is the same, — the observed coordinates being 

 first reduced to equivalent angular values, and these in turn, 



Fig. 10a. View of bi-quartz wedge plate showing relative positions of 

 right and left-handed wedges and underlying left and right-handed quartz 

 plates, all normal to the optic axis, and in combination forming the most 

 sensitive device for the determination of the exact position of total extinc- 

 tion of minerals in the thin section. To be used also in adjusting the nicols 

 in the petrographic microscope. 



Fig. 10b. 



Fig. 10b. Top view of bi-quartz wedge plate. The position of dark line 

 of zero rotation or exact compensation is indicated at C. 



after reduction to values obtaining within the crystal, plotted in 

 stereographic projection in order that the axial angle values may 

 be found graphically. In the article mentioned the different 

 steps requisite for this method are described at length and need 

 not be repeated here. By means of this ocular, with ruled cross 

 section slip, the optic axial ano;le of a favorable section 



on 



which both optic axes are visible can be determined within 



