OPTIC AXIAL ANGLE. 
179 
Such curves have been called optical curves by Fedorow and are obtained 
most readily by first placing the crossed nicols in any given but fixed posi- 
tion, then turning # 2 through angles of 5 respectively, and for each position 
of //j determining the angle of inclination about V\ for which the section 
is in the darkest position (o extinction) (Fig. 109); the same results can 
also be attained by first turning the preparation about V\, a specified angle, 
FIG. 109. In this figure the method for locating the position of the optic axes by 
means of optical curves is illustrated. The figures o, 20, 30, and 45 opposite the 
curves indicate the angles which the plane of vibration of the polarizer at the time of 
observation made with the plane of symmetry of the microscope. 
and then about H* until darkness ensues. By this method those directions 
in the crystal are obtained (after proper reduction of observed angles to crystal 
directions by means of the refractive index) for which the extinction is zero 
for a given position of the nicols. The curve uniting these directions in 
projection is the optical curve for the particular position of the nicols to 
the axes of rotation. Analogous curves for other and different positions of 
the nicols are to be obtained and plotted in similar manner. All such curves 
pass through the optic axes. Their points of intersection in the projection 
determine, therefore, with considerable accuracy, the exact position of the 
optic axis or of both axes, in case both axes can be brought within the field 
of vision. Since such optical curves are intended solely to increase the accu- 
