MICROSCOPICAL PETROGRAPHY 487 
determined easily and without complicated apparatus on favorable 
plates or grains 0.02 mm. in diameter and over. It may be of 
interest to indicate briefly a few of the methods which experience 
has shown to be most satisfactory in actual work. 
ADJUSTMENT OF THE PETROGRAPHIC MICROSCOPE 
In these methods it is assumed that the microscope is properly 
adjusted, otherwise the results obtained may be seriously in error. 
The simplest method for testing the adjustment of the petro- 
graphic microscope is probably the following: (t) Remove from 
the microscope all lenses—ocular, objective, and condenser—and 
point it directly at the sun whose rays are parallel and so intense 
that a rotation of less than 1’ of one of the nicols from the position 
of total extinction is readily discernible. When the nicols are 
accurately crossed the sun appears as a dim disk in the dark back- 
ground. (2) Test the adjustment of the cross-hairs of the ocular 
to the principal planes of the crossed nicols by observing under the 
microscope (fitted with ocular and centered objective but not with 
condenser, and pointed directly at the sun) a cleavage plate of 
some mineral, as anhydrite, celestite, or anthophyllite, which 
shows parallel extinction. The cross-hairs of the ocular should 
then be parallel with the cleavage edge of the plate in its position 
of total extinction. (3) Insert the condenser and note that it is 
properly centered when the image of the substage diaphragm 
occupies the center of the eye-circle of the ocular. In this plan 
of adjustment the assumption is properly made that the draw-tube 
and the substage are in alignment and that. the optical elements 
are correctly mounted—mechanical details which are satisfactorily 
met by modern instrument-makers. 
THE OPTICAL PROPERTIES OF MINERALS 
Passing now to the optical and crystallographical features on 
which mineral diagnosis under the microscope is based, we shall 
consider first their nomenclature. Both theory and experience 
have shown that for monochromatic light the variation in the 
optical properties of a given mineral with the direction can be 
adequately expressed and defined in the most general case by 
