500 FRED, EUGENE WRIGHT 
Ramsden ocular) is parallel with the horizontal cross-hair of the 
ocular and its distance in this position from the center of the field 
recorded. The crossed nicols are then turned through a suitable 
angle (30° or 45°). The axial bars rotate in the opposite direction 
but the optic axial points, A; and A,, remain stationary and dark; 
they are situated at the intersection of the axial bars with the 
horizontal line which marked the plane of the optic axes. The 
angular distance between the two optic axial points A, A,, thus 
determined, is directly the optic axial angle 2# in air. 
In case only one optic axis A; appears in the field of vision 
the method is more complicated and less accurate but in view of 
its usefulness it may be briefly outlined. It is based primarily . 
on the rule of Biot-Fresnel which states that for light waves pro- 
pagated in any given direction in a crystal the two lines of vibration 
bisect the angles between the projections of the two optic axes 
on the plane normal to the given direction of propagation. In an 
interference figure the line of vibration for any point which appears 
dark between crossed nicols is evidently contained in the extin- 
guishing plane of the analyzer—otherwise it would not be dark. 
The plane of vibration for all points on the achromatic brushes 
(zero-isogyres) of an interference figure is therefore known. By 
locating the optic axis A, and any point P on the achromatic 
brush it is accordingly possible by applying the Biot-Fresnel rule 
to determine graphically the position of the second optic axis." 
After having made the measurements in the interference figure, 
the observed co-ordinate values are first reduced to equivalent 
angular values in air and these in turn to corresponding values 
within the crystal by means of its average refractive index. These 
angles are then plotted in suitable projection (angle or stereo- 
graphic)—the plane of the optic axes as a great circle parallel with 
the horizontal diameter, the principal plane of the polarizer in its 
two azimuths as diameters of the projection, the optic axis A, 
and the point P on the achromatic brush at the intersection of the 
recorded small circle (almucantar) co-ordinates. In this pro- 
tSee F. Becke, 7.M.P.M., XXIV, 35-44, 1905; XXVIII, 290, 1909; also 
F. E. Wright, Amer. Jour. Sci. (4), XXIV, 316-69, 1907; (4), XXXI, 157-210, 1911; 
Carnegie Inst. Wash. Pub. 155, 147-200, IgII. 
