OPTIC AXIAL, ANGLE. 
175 
Although the measurements accomplished by the universal stage methods 
of Fedorow are made in parallel polarized light and with low-power objec- 
tives, the same objectives can be used for weakly convergent polarized 
light with the Bertrand lens and the position of the optic axes may be thus 
determined if it be possible to bring them within the field of vision and 
if they are sufficiently distinct for accurate location under these conditions. 
For general work, however, with thin sections in convergent polarized light, 
the methods requiring the double-screw micrometer ocular or the coordi- 
nate scale ocular are the most accurate and easy of application. 
FIG. 108. 
PARALLEL POLARIZED LIGHT. 
The introduction of the universal-stage methods by Fedorow, in 1893* 
and succeeding years, placed a powerful instrument of attack in the hands 
of petrologists. With his methods it is now possible to obtain the optic 
properties of mineral sections which before were considered practically 
useless. The universal stage (Plate 6, Fig. i) can be attached securely to 
any suitable petrographic microscope; parallel-polarized light only is used. 
By means of horizontal and vertical axes of rotation, a crystal section can be 
brought to any given position and rotated about any axis for optical 
examination. 
In parallel-polarized light an optic axis is recognized by the fact that 
when placed parallel to the axis of the microscope it remains uniformly dark 
during a complete rotation about that axis. By plotting these directions 
graphically in projection, and by determining extinction angles in given 
zones, it is possible not only to measure the optic axial angle, but also to 
determine the position of the optic axes with reference to the crystal plate, 
even though it may happen that neither optic axis appears within the field 
*E. v. Fedorow. Zeitschr. Kryst., 21,574-678. 1893; 12, 320-268. 1804; 25, 225-261,1895; 27,337-398. 
1897: 29, 604-568, 1898; see also F. WalKrant Bull. Soc. Min. Fr.. 19, 356-363. 1806; J. W. Evans. 
Miner. Mag.. 14, 157-159. 9o6. 
