1 88 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
coincidence with the axis of the microscope, the plane of the optic axes 
being then parallel to the vertical cross-hair (Fig. 117). Rotate the sec- 
tion some convenient angle about axis V and then about V\ (as shown in 
Fig. 1 1 7), also through any suitable angle. Measure accurately the extinc- 
tion angle of the section in its new position. Plot the data of observation in 
angle or stereographic projection (after proper reduction of observed angles 
to true crystal angles) ; and find those two points A\" and A z " contained in 
the optic axial plane and equidistant from the obtuse bisectrix a", which are 
so located that the observed extinction angle OEi bisects the angle A^'OAi" 
(Fig. 117). The angle A\' A*' is] then the desired optic axial angle, 2V. 
FIG. 117. To use the method indicated by this figure, turn the section so that its 
obtuse bisectrix coincides with the axis of the microscope (center of the projection plat) 
and the optic axial plane is parallel to the vertical cross-hair; turn the preparation 
about axis V t a convenient angle (reduce to true crystal angle equivalent to apparent 
angle observed in air or glass), and then about axis V t (normal to ri) any suitable angle 
and measure the extinction angle of section in its new position. Plot these data in 
stereographic or angle projection and find those two points.!/' and A t " equidistant from 
the obtuse bisectrix and contained in the plane of optic axes in its new position, for which 
the observed line of extinction OE bisects the angle included between OA\" and OA t ". 
O is the center of the sphere of projection. 
With the universal stage in its present form it is not always possible to 
execute the movements indicated in the above method, since, when the 
obtuse bisectrix is brought to coincide with the axis of the microscope, the 
axis of K 2 is in general no longer horizontal and the rotation about \\ is 
therefore along an inclined axis. In plotting the observed data, this fact 
should be carefully noted, othenvise errors may nullify the results. 
With the universal stage it is thus possible to measure the optic axial 
angle of any grain of any transparent birefracting substance in the thin 
section and to distinguish the biaxial and uniaxial minerals. The degree 
of accuracy of this measurement, however, is not of the same order of 
magnitude for all sections, but differs very materially with different sec- 
tions. As a matter of experience, it has been found that the most accurate 
results can be obtained on sections in which both optic axes appear within 
