361 F. E. Wright — Measurement of the Optic Axial Angle 



Aragonite. 



(1) H V 



A 5 7° 9°-8 2V AlA2 =18°-0 



A„.-. —10 -8 9 -8 2Vj, lAl = 18 -5 



Pj- 17-8 6-8 2V,, oA „ = 14 



P„ —17 -5 6 -8 



(2) 



A, 4°-6 20° 



A 2 —13-20 20 2V AlA „ = 17 0, 9 



P x 5 3 2V PoA ;= 19 



P 2 -- -23 -5 3 2V P ; A ;^ 23 



(3) 



A 1 7°-3 l°-8 



A„ -11 '5 1 -8 2V AlA2 = 18°-5 



P,--. 20-6 18'8 2V PlAl = 18 



Muscovite, (a) 



(1) a H V 



A,... 35°-5 0° 2E = 71° 



A 2 —35 -5 



(1)5 



A, 0° 36° 2E = 72° 



A 2 —36 



Muscovite, (b) 



H V 



A 1 30°'l 0° 2E = 60 o> 2 



A„ —30 -1 



A, 0° 30° 2E = 60° 



A„ -30 



Measurements with the Fedorow-Fuess universal stage. — 

 The angles given below were read directly on the different 

 circles of the universal stage and before plotting in projection 

 require reduction to true crystal angles by means of the refrac- 

 tive index /3 of the mineral and /a (= 1*5239) of the glass 

 hemispheres used. The letters H„ H 2 ,' H 3 and Y 15 Y 2 desig- 

 nate the different circles of the universal stage (see fig. 11) on 

 which the angles were read. The angles after the letter N 

 designate the angle made by the principal plane of the lower 

 nicol with that of the microscope. 



Topaz. Section after 001 (acute bisectrix). — A direct 

 preliminary determination of the position of the optic axes in 

 parallel polarized light was first made and the approximate 

 location of each axis determined. These values were later 

 corrected by means of optical curves. 



