OPTIC AXIAL ANc.I.i:. 197 
MI-ASI KI-MI-NTS WITH Till-: INIVKKSAl. STA'.i;. 
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 refractive index of the mineral and n 
( = 1.5239) of the glass hemispheres used. The letters 7/ lf 7/ 2 , 77 3 and V\, V* 
designate the different circles of the universal stage (see Plate 6, Fig. i) on 
which the angles were read. The angle after the letter A" designates the 
angle made by the principal plane of the lower nicol with the plane of sym- 
metry of the microscope. 
Topaz. Section after ooi (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. The direct preliminary determination was as follows: 
77, 77, 77, 1', J', 
.1, 180 90 294 35 -0.5 2F = 66.6 
-I: 180 90 294 -36 -0.5 
The following shows the corrections by method of optical curves: 
A* = o A* = 30 A = 45 
Vi Vi Vi 
77, 77 2 II 3 T 2 Ai J A,. At 
1 80 
80 
294 
33 
5 
-33 
5 
37 
-39-5 
33 
-37 
1 80 
85 
294 
35 
-36 
36 
-38 
33-5 
-37 
5 
1 80 
90 
294 
35 
5 
-36 
34 
-37 
34 
-36 
5 
1 80 
95 
294 
34 
-34 
5 
33 
-36 
34 
-37 
1 80 
IOO 
294 
33 
-34 
5 
33 
-34 
35 
-37 
After proper reduction of these angles, the corrected angle, obtained 
directly from the stereographic projection plat, is 2l' = 66.5. 
Topaz. Section nearly normal to an optic axis. 
The determination in this case can be most readily accomplished by first 
locating Ai accurately by optical curves and then fixing the position of A* 
in projection by means of the principal ellipsoidal planes. The optical 
curves for A i are as follows : 
A = o A = 30 A = 45 
r 2 i', r, r, 
45 4-5 5 
5 5 5 5 
6-5 6 5 
7 6 5 
8 5 4 
//, 
Hi 
IT, 
180 
80 
225-5 
1 80 
85 
225.5 
180 
90 
225-5 
1 80 
95 
225-5 
1 80 
IOO 
225-5 
