OPTIC AXIAL ANGLE. 
I6 7 
On sections in which both optic axes of the interference figure are visible, 
the exact position of A\ and A* can be measured directly and, after plotting, 
the value 2 V can be obtained from the projection by direct reading. Such 
values should be accurate to i. 
To form an idea of the relative degree of curvature of the axial bar in the 
diagonal position for sections of biaxial minerals cut at different angles with 
an optic axis (binormal) and for the optic axial angles 27 = o, 15, 30, 
45, 60, 75, and 90, the writer has constructed by graphical methods 
Figures 98-105. 
FIG. 98. 
The graphical methods of construction adopted for ascertaining these 
curves are based on the rule of Biot-Fresnel and require the use of the stereo- 
graphic projection plat. For the sake of completeness, and especially since 
the figures themselves have been called in question by Professor Becke,* 
they may be accorded brief description. In Fig. 98 the order of procedure 
for finding the course of the zero isogyre for a definite optic axial angle is 
presented. In this figure, which is a stereographic projection plat, the 
graphical solution of the general problem is indicated, which, briefly, is to 
find that point H on the great circle THM which is located on the zero 
*T. M. P. M.. 28, 393. 1910. 
