82 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
has described methods which are similar to those of Ce"saro and by means 
of which the direction of the acute bisectrix can be ascertained in the inter- 
ference figure from certain sections. 
DISPERSION OF THE OPTIC AXES. 
The methods for determining this feature (both the relative dispersion 
and the kind of dispersion) of biaxial minerals are described in detail in the 
standard text-books* and will not be discussed further than to note that the 
chromatic aberrations of the objective contribute a certain amount of color 
to the interference figure and tend to modify it slightly, a fact which should 
be considered in critical work. 
DISPERSION OF THE BISECTRICES. 
This feature can be determined occasionally in the interference figure, but 
parallel polarized light should be used for accurate work and the positions 
of extinction on a section parallel with the plane of the optic axes should be 
determined accurately for different colors. 
*See also E. v. Fedorow. Zcitschr. Kryst., 37, 143, 1902. 
