i8 4 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
into play, since not only must extinction angles be observed, but also the 
section rotated about the ellipsoidal axes and the exact position of each 
axis noted. The method of procedure consists in first placing the stage in 
the zero (primary) position, H 3 , H\ t H 2 , and V\ in zero position, and F 2 normal 
to V\\ the section having any orientation and position. The section is then 
inclined about Fj until darkness between crossed nicols ensues; if this be 
not the case, it is turned about Hz a small angle, and the attempt made a 
second time, and so on until darkness is observed at a definite angle of 
inclination about Vt. The preparation is then rotated about V\, and, if 
FIG. 113. 
by chance the correct position be obtained, darkness will continue for every 
angle of inclination about V\. This is usually not the case; by repeated 
trial that position of 7/ 3 , //a is to be found for which the preparation 
remains dark for every angle of rotation about V\. The angle of inclina- 
tion V* and the directive angle HZ determine then the position of one of the 
planes of symmetry of the ellipsoid within the crystal, e. g., the plane afl'y 
of Fig. 114, this being fixed by the line 0/3' ; in similar fashion the planes ay'0 
and 7a'/3 are located and plotted in the suitable projection. This method of 
locating the planes of symmetry of the ellipsoid within the crystal is com- 
paratively rapid and sensitive, and a fair degree of accuracy can be attained 
by its use. The new circles Fj (Plate 6, Fig. i), attached to the large 
Fedorow-Fuess universal stage, have proved extremely serviceable and time- 
savers in this method. 
Having once determined the position of either a or 7 by this method, and 
that of one optic axis A i by optical curves, the position of the second optic 
axis At is readily obtained, since the angle A t a or A\y is by definition equal 
to Aia or A t y respectively. 
After some practice, the exact relative positions of //, H 3 can be found 
without difficulty, for which darkness remains for all angles of inclination 
about V\. To insure accuracy, however, the fact of remaining dark should 
