OPTIC AXIAL ANGLE. 
183 
the first method, the great circle CF, indicating the original position of the 
plane OAt, can be constructed and should pass through A t on the line OA\. 
The second new method differs from the first only in the fact that instead 
of placing the optic axis Ai in the plane OE (Fig. 113) and then measuring 
the extinction angle of the section in the horizontal position, the actual 
direction of extinction OE is brought to coincidence with the axis of rotation 
of the universal stage ( V\ or 7 2 ) ; the section is then rotated a given angle 
about this axis and, from the extinction angles, the lines OA* and OAJ are 
determined; the arc A 2 A 2 f is equal to the angle of rotation. The point At 
is then the desired direction of the second optic axis. 
FIG. 112. 
In both new methods the determination can be varied by inclining the 
specimen first about Vi as an axis and then determining a series of extinc- 
tion angles for different angles of inclination about F 2 (F 2 in this case 
being normal to Vi) and thus locating At afresh with each extinction. By 
establishing a set of observations about F 2 for each new position of V\ it 
is possible to extend the number of observations indefinitely and thus to 
locate A 2 with great accuracy. In fact, the position of A\ in the projection 
is immaterial so long as its position be definitely known with respect to the 
axes of rotation (V\ and F 2 ), since with A\ located at any point in the pro- 
jection it is still possible to locate A* by means of extinction angles for 
different angles of inclination about V\ and F. This method, involving 
the use of both V\. and Vt, is therefore a method of general application and 
is capable of furnishing reliable data on all sections so cut that one optic 
axis at least falls within the field of vision. 
Still another method, which furnishes trustworthy results and is of 
general application, consists in determining first the positions of the planes 
of symmetry and the axes of the ellipsoid within the crystal (Fig. 114). 
In this method, practically all the graduated circles of the stage are brought 
