MICROSCOPICAL PETROGRAPHY 501 
jection the Biot-Fresnel rule is applied by first projecting the 
optic axis A, on the polar circle of the point P; the intersection, 
C, of this polar circle with the diameter which defines the position 
of the principal plane of the polarizer at the time of the observa- 
tion determines in projection the line of vibration of P. The 
angle between the point C and the projection point A’; of the 
optic axis A; is half the angle between the lines of projection of the 
two optic axes. The point A’,, the projection of the second optic 
axis A., is accordingly found by laying off on the polar circle from 
the point C an angle equal to A’:;C. The intersection of the great 
circle through P and the point A’, with the plane of the optic axes 
is obviously the second optic axis A, and the angle between the 
two axial points, A; and A,, is the optic axial angle 2V. The best 
results are obtained by this method on sections in which the optic 
axis A, is located about midway between the center and margin 
of the field and the point P on the achromatic brush in a similar 
position. As noted above, a probable error of +3° is possible with 
this method even on the most favorable sections. The error is 
proportionately larger on poor sections. 
‘ 
GEOPHYSICAL LABORATORY 
WASHINGTON, D.C. 
March 16, 1912 
