of Minerals in the Thin Section. 



351 



In actual work with this method, it happens occasionally 

 that the determination of the location of A 2 is not accurate 

 because of the acute angle between the radius and the small 

 circle A 2 A 2 '. In such cases the writer has been able to apply 

 with favorable results one of the two following new methods, 

 which, like the preceding method, are based on the measure- 

 ment of extinction angles for different ang]es of inclination 

 about one of the horizontal axes of revolution of the univer- 

 sal stage. The new circle V 2 may render hereby valuable 

 assistance. 



In the first of the new methods, the horizontal position of 

 the section is exactly that of the above method (fig. 24) ; A„ 



24 



25 



having been previously located accurately, is brought to coinci- 

 dence with ON, and the extinction angle of the specimen in 

 the horizontal position ascertained ; and then instead of being 

 revolved about the horizonal axis V, (the line OL in projec- 

 tion), it is revolved about V 2 (or ON in the projection) as an 

 axis,* a given angle (apparent angle in air equivalent to true 

 angle in crystal). A 1 travels during the revolution of stage to 

 A/ in the projection, the direction of extinction wanders from 

 OE to OE' and the plane OA 2 containing A 2 , from OA„ to 

 OA 2 ', the angle E'OA/ being by construction = E'OA/. 

 By recording the angle of revolution of the stage about ON 

 (V a ) required to bring the section to its new position, it is not 

 difficult to find in the projection that small circle, parallel to 

 OL, whose arc A 2 A/ intercepted by the lines OA 2 and OA/ 

 is equal to the above angle of revolution and thus to locate A,. 



* The same effect can be produced by revolving the specimen 90° about 

 Hi and about Vi as an axis. 



