of Minerals in the Thin Section. 365 



Direct preliminary determination. 



H, 



180 





H, 



H 2 



H s 



V: 



v 2 





A ] 



180° 



90° 



294° 



35° 



-•5° 2V 



= 66°-6 



A 2 



a 



cc 



a 



36° 



-•5° 





3orrectec 



I by me 



sthod 



of optical curves. 











1ST = 



0° 



N = 30° 



N = 45° 



H, 



H 3 



v 2 



""Ax 





V. 



V, 





A,""" 



Aj A 2 



Aj A 2 



80° 



294° 



1° 



33-5 - 



-33-5 



37 —39-5 



33 —37 



85 



a 



cc 



35 



-36 



36 —38 



33-5 —37-5 



90 



a 



u 



35-5 - 



-36 



34 -37 



34 —36-5 



95 



a 



cc 



34 



-34-5 



33 —36 



34 -37 



100 



a 



cc 



33 



-34-5 



33 —34 



35 —37 



After proper reduction of these angles, the corrected angle, 

 obtained directly from the stereographies plat, is 2V = 66°- 5. 



Topaz. Section nearly normal to an optic axis. — The deter- 

 mination in this case can be most readily accomplished b} 7 first 

 locating A, accurately by optical curves and then fixing the 

 position of A 2 in projection by means of the principal ellipsoi- 

 dal planes. 



Optical curves for A,. 











N = 0° 



N = 30° 



N = 45' 



H, 



H 2 



H 3 



v 2 



V, 



V, 



Vi 



1S0° 



80° 



225-5 



-1 



4-5 



4-5 



5 



cc 



85 



cc 



cc 



5 



5 



5'5 



u 



90 



a 



a 



6-5 



6 



5 



cc 



95 



i< 



cc 



7 



6 



5 



a 



100 . 



cc 



cc 



8 



5 



4 



After reduction to true angles, the position of A 1 in projec* 

 on was found to be : H„ 180°, H 2 , 90°, H 3 , 225°-5, Y T , 6° and'Y 2 , 



1°. The 7/3 ellipsoidal plane was located by : Hj, 180°, H 2 ? 

 0°, H„ 315°, Y 1 -, Y 2 , 26°, while for the ay ellipsoidal plane, 

 the readings were: H„ 180°, H 2 ,90°, H 3 , 225°-5,Y 1 , -, Y„— 1°. 

 The optic axial angle thus determined in projection plat is 

 2Y=64°. In such cases, where the section is nearly normal to 

 an optic axis, the method of extinction curves is not of practical 

 value, owing to the difficulty of determining extinction angles 

 with requisite accuracy. 



Topaz. Section nearly normal to obtuse bisectrix. — Optic 

 axial angle was found by first locating the principal ellipsoidal 

 planes a/3 and ay and then measuring the extinction angle of 

 the section when a coincided with the microscope axis and 



Am. Jour. Sci. — Fourth Series, Vol. XXIV, No. 142.— October, 1907. 

 25 



