OPTIC AXIAL ANGLE. 
I8 7 
cisely with the normal to axis V\\ Ay is then the second optic axis and 
angle A j/3a, equal to angle Azfia and A i0a, is the extinction angle. On rotat- 
ing the section now, about V\, the optic axial point A\ is brought to B and 
the extinction angle B&E for the new position of the section should bisect 
exactly the angle B$A-i '. If this be not the case and the extinction angle 
be too large or too small, the section should be rotated about H-t either 
counter-clockwise (Ai" to A\} or clockwise, A \" to A\, through a small angle 
and a new set of measurements should be made, until after repeated trials 
the corrected position is to be found for which observation and construction 
agree precisely. The angle A i/3a is then half the desired optic axial angle. 
In certain cases this method of placing the one optic axis A i in the plane 
normal to the axis of rotation V\ has been found unsatisfactory, and a new 
method* has been used, which consists in first bringing by trial the one optic 
axis to coincidence with the axis of rotation and then measuring the extinc- 
tion angles for different angles of inclination about V\ (or Vz) and testing 
the results of observation and construction until they coincide. The method 
is shown in Fig. 116 and is so similar to the foregoing method (Fig. 115) 
that further description is unnecessary. 
FIG. 116. 
SECTION NEARLY NORMAL TO THE OBTUSE BISECTRIX. 
For a section nearly normal to the obtuse bisectrix of a mineral both optic 
axes lie again outside the field of vision and the optic normal /3 can not be 
brought to coincidence with the axis of the microscope. The above methods 
do not apply, therefore, and new ones are required to meet the new condi- 
tions, and of these the following has been found practicable by the writer :f 
Place the universal stage in the primary position, the axis of Vi normal 
to that of Vi and the circles HI, H z , and H 3 all in the horizontal position; 
determine the exact position of the obtuse bisectrix (a or 7, as the case may 
be) by the method of principal ellipsoidal planes (page 183), and bring it to 
*Amer. Jour. Sci. (4), 14, 355. 1907. 
tAmer. Jour. Sci. (4). 24, 355. 356. 1907- 
