OPTIC AXIAL ANGLE. 185 
be scrutinized very sharply, since the correct position is not always that of 
absolute darkness, but rather that for which the same degree of darkness 
or intensity of uniform illumination obtains throughout. 
From the complete determination (by this method) of the positions of 
a, /3 and y, which should be mutually 90 apart, Fedorow has shown that 
FIG. 114. In this figure the great circles afl'y, ya'0 and 0y'a of the sterepgraphic 
projection denote the traces of the principal planes of the optical ellipsoid within the 
crystal. They are fixed in position by determining the positions of H t and V t for which 
the section remains dark for all positions of inclination about the horizontal axis I', 
(V, being normal to V,); the lines 0/3', Oy'and Oa'are thus fixed both in direction and 
length and also the great circles o0'-y, ya'0, and ffy'a, the planes of symmetry of the 
ellipsoid, the intersections, a, 0, and 7, of which are in turn the ellipsoidal axes. 
the average refractive index of the mineral can be derived approximately 
by use of this fact, although the determination is not of sufficient accuracy 
to be of great practical value. 
