of Minerals in the Thin Section. 363 



index /3 presupposes only slight differences between'the refrac- 

 tive indices of the mineral in order that the errors thus caused 

 may not be too large. 



(1) t P 



A, -80° 17°"5 2V AlA3 =l7°-4 



A,... -130 23 2V PlAl = 17 -5 



P" +15 18 -4 



i 



( 2 ) 



A 1 ■ -10° 8° 2V AlA2 = 18°-3 



A 2 _. -17-1 10-6 2V PlAl =21 



Pj.. +42 21 



(3) 



A,.. +44° 28°'3 2V AlA2 =18°-0 



A 2 +85 19 -5 2V PaAa = 17 -6 



P a . —15 '5 18 -4 . 



A,-— -56° ll°-2 



A 2 -140 14-3 2V AlAa =l7°-5 



P x - +29 23 -4 2V PlAl = 16 



P 2 +158 26-0 2V P2A2 =23 



The values for 2Y AlA2 do not differ over 1° from __ the true 

 value, while those for 2V PlAl differ as much as 5° from the true 

 value. 



Muscovite, (b) 



<p p 



A x 0° 35°-6 2V AlAo = 71°'2 



A 2 .. -35 -6 



Double Screw Micrometer Ocular. — The data given below 

 appear also only in corrected form ready for plotting in projec- 

 tion, the actual scale readings having been reduced to equivalent 

 angles in air and these in turn figured to true angles within 

 the crystal, by means of the refractive index /3. The errors 

 observed above in aragonite sections because of strong birefrin- 

 gence apply equally well here. In the following tables H 

 indicates the horizontal and V the vertical micrometer screw of 

 the ocular. 



Topaz. 



(1) H v 



A, 3°-5 0° 2V = 64°-5 



P, 5 -5 17 '5 



(2) 



A 1 5°-3 -3 2V = 65°-5 



P,._. 7-6 17-2 



