OPTIC AXIAL AN<;i.K. l<)<) 
tion angle measured in that position. By trial that position of 77 2 was found, 
for which A\ coincided with the principal plane of the- lower nicol, and the 
optic axial angle thus ascertained by measuring the extinction angles of the 
section in different positions of Vi and comparing the data of observation 
with those obtained by graphical methods from the projection plat on the 
assumption that J t did actually coincide with tin- principal plane of the 
lower nicol. In like manner the section was rotated about V and extinction 
angles measured until theory and observation furnished identical results. 
The principal ellipsoidal planes of the section were determined by the 
readings : 
//, II, II, }\ V, 
07 plane .......... 180 90 333 +16 
0o plane ........... 180 90 241 .5 .. i 
For the different positions of 77^, the extinction angles for a given angle 
of rotation about T'i were: 
Hi 
77 2 
77, 
Vi 
K, 
145 
123 
333 
17 
31 
H5 -5 
122 
333 
17 
31 
144 -5 
124 
333 
17 
31 
1 44 -3 
123 -5 
333 
17 
31 
On plotting these values in projection, it was found that 2 V was about 
64 to 67, but a more decisive result was not attainable. The method is 
not accurate and can only furnish very rough approximations. 
In the second method, which involves rotation about an axis normal to 
that of the above, the values observed w r ere : 
77, 
77, 
7/5 
V 
i 4 8. 5 
33 
332 
17 
148 .5 
34 
332 
J7 
149 
32 
332 
i7 
and from these angles, 2 V was found to be between 64 and 68. 
The determination can not be termed satisfactory, and this method, like 
the above, can furnish only rough approximations to the correct values 
of 2V. 
RECAPITULATION-. 
(i) The optic axial angle of minerals in the thin section can be determined 
under the microscope in either convergent or parallel polarized light. 
(a) In convergent polarized light, methods for the measurement of the 
optic axial angle are available for all sections in which at least one optic 
axis appears within the field of vision. Of these, the method requiring the 
use of the Becke drawing-table is of general application and furnishes results 
of a fair degree of accuracy the usual probable errors being about =*= i if 
both optic axes be visible, and 5 if only one optic axis be visible. More 
accurate and somewhat simpler in manipulation and of the same general 
application is the method involving the new double-screw micrometer ocular 
