OPTIC AXIAL ANGLE. 
By projecting these angles and performing the requisite mechanical opera- 
tions, the optic axial angle thus determined on this section was 2V = 62. 5. 
For a second section the values were : 
A,. 
Pi. 
-8 
-72 
For a third section : 
P 
3-i 
P 
8.8 
23 
Pi -58 
The average of these three values is 65. 2. 
Aragonite. 
In aragonite the optic axial angle is so small that both axial bars AI and 
A 2 are visible and the direct determination of 2V AlAt should in all cases 
be accurate within i. The birefringence is so strong, however, that the 
measurements involving a point PI or P 2 on the dark axial bar and conse- 
quent introduction of the refractive index /3 for that point, may be decidedly 
incorrect. The use of the refractive index /3 presupposes only slight differ- 
ences between the refractive indices of the mineral in order that the errors 
thus caused may not be too large. 
(0 
(2) 
(3) 
(4) 
The values for 2V AtAt do not differ over i from the correct value, while 
those for 2V P , Al differ as much as 5 from the correct value. 
Muscovite J(b). 
<* 
Al 
- 80 
A, 
-130 
k ft 
+15 
'Ai 
- 10 
A, 
- 17-1 
Ift 
+42 
\A, 
+44 
At 
+85 
ft 
- 15-5 
[Ai 
- 56 
At 
140 
Pi 
+29 
P* 
+158 
A,. 
A,. 
