of Minerals in the Thin Section. 



337 



12 



of procedure in measuring the optic axial by means of the 

 double screw micrometer ocular consists in : (a) revolving the 

 microscopic stage until the dark axial bar is parallel to the 

 horizontal cross hair of the ocular ; (h) moving the horizontal 

 cross hair by means of the vertical micrometer screw V until 

 it coincides precisely with the center of the dark axial line 

 (fig. 12, AjCJ ; (c) revolving the nicols (not the stage as may 

 be done in the Becke method) about a suitable angle (usually 

 30 or 45°), the exact position of the optic axis A, then being 

 the intersection of the axial bar with the horizontal cross hair 

 (fig. 12, AjCj with AjPj) ; (d) moving the vertical cross hair by 

 means of the horizontal micro- 

 meter screw until it coincides with 

 this intersection and recordingboth 

 vertical and horizontal micrometer 

 screw readings; (e) the stage is then 

 revolved about an angle of 180°, 

 and similar readings for A 1 taken 

 in its new position, A/. This 

 last step is necessary in order to 

 locate accurately the center of the 

 field O. The position of A 1 is 

 thus fixed accurately and can be 

 plotted directly, after proper re- 

 duction to true angles within the 

 crystal, in stereographic (small circles) or orthographic (co- 

 ordinates) projection. Any point P of the dark curved axial 

 bar can now be determined by two micrometer readings 

 (coordinates from the center) and, after reduction to angles 

 within the crystal, plotted in the projection. From this point 

 on the method does not differ from the foregoing methods. 

 The optic axial angle is determined by construction from the 

 projection plat thus obtained. 



In plotting the angles corresponding to the coordinate read- 

 ings of the double screw micrometer ocular, it should be noted 

 that these angles apply to small circles, the angle for each 

 micrometer screw indicating the position, from the center of 

 the projection, of a certain horizontal or vertical small circle. 

 The intersection of the horizontal and vertical small circles thus 

 obtained from the two micrometer screw readings for a par- 

 ticular point of the interference figure determines the location 

 of that point in the projection. 



Measurements with the double screw micrometer ocular are 

 more delicate, and therefore more accurate, than those with 

 the Becke drawing table, and the values for 2Y correspond- 

 ingly more trustworthy. This method is of general application 

 to all sections cut in such a way that either one or both optic 



