OPTIC AXIAL ANGLE. 
169 
optic axis the zero isogyres become wider and less distinct and also some- 
what more symmetrical because of the factors noted above, whereby the 
ffects of the two refracted waves W\ and W t are superimposed in different 
intensities. Although the figures are not strictly true to nature, they show 
approximately the course of the zero isogyres; and experience has shown 
that the general conclusions deduced from them are valid. 
FIG. ioo. 
FIG. 101. 
FIG. ioo. The conditions of construction for this figure were similar to those of Fig. 99, 
except that the section is considered cut at an angle of X = 10, M = 10 (small circle coor- 
dinates from the center) with one of the optic binomials. 
FIG. 101. This figure differs in construction from Figs. 99 and ioo only in the fact 
that the section is considered cut at an angle X = 20, n = 20 with one of the optic 
binormals. 
The curves of Figs. 99 to 101 are constructed for minerals with a refractive 
index /3= i .60, while those of Figs. 102 to 105 are orthographic projections 
of the actual positions of the directions of o extinction within the crystal 
as obtained by the graphical construction when the refractive index of the 
mineral and the surrounding medium are equal. It is evident from the fig- 
ures that the differences in curvature of the axial bars for the different values 
of 2 V are sufficient to warrant their use in measuring optic axial angles ap- 
proximately. The accuracy of the method depends on the accuracy with 
which the points A\ and H (Fig. 86) can be determined. The positions most 
favorable for these points are located one-half to two-thirds the distance 
from the center of field to its margin. Near the center of the field the errors 
of construction increase rapidly, while near the margin errors due to the 
decrease in distance between equal angular intervals, to imperfections in the 
lenses, and to elliptical polarization tend to modify the interference figures 
and decrease the accuracy to be attained. 
The actual diameter of the field covered by the micrometer-screw move- 
ments of the writer's ocular measures about 600 micrometer-screw divisions. 
The distance covered by the extremes of the curves for o and 90 is less 
than 200 divisions, or about 2 divisions for i on an average. Taking into 
