THE ADJUSTMENT OF THE PETROGRAPHIC MICROSCOPE. 
6l 
eye estimates coordinate directions much more readily than diagonal direc- 
tions. The practice of placing the adjustment screws on the rotating stage 
instead of above the objective is wrong. The part of the optical system 
which is not in adjustment is the objective and not the stage. The axis of 
rotation of the stage should form the starting-point for the adjustment of 
the whole instrument and should always remain fixed in its position. To 
this axis the ocular, condenser, and objective should be adjusted, and since 
the ocular and condenser remain practically stationary while the objectives 
are changed constantly the only logical point of adjustment is above the 
objective. 
(10) Verniers to read the stage circles are attached both to the rotating 
connection between the nicols and to the body of the microscope; they 
read directly to 3' of arc. 
FIG. 40. In this figure, the point D is the gnomonic projection point of the point 
P on the sphere or of the direction CP in the crystal. The distance MD is evidently r 
tan p, r being the radius of the sphere and p the angle MCD. Similarly, the distance 
CE in the sterographic projection (Fig. 41) is r tan - and CF in the orthographic 
2 
projection (Fig. 42) is r sin p. 
THE ADJUSTMENT OF THE PETROGRAPHIC MICROSCOPE. 
The properly adjusted petrographic microscope should satisfy the follow- 
ing requirements: (i) The optical system should be centered and its axis 
should contain the center of rotation of the stage. (2) The nicols should 
be accurately crossed. (3) The cross-hairs of the ocular should be parallel 
with the principal planes of the nicols. 
(i) The first condition involves (a) centering of the objective by the 
usual method; (b) centering of the condenser by observing its image in the 
eye-point of the ocular. In this adjustment it is assumed that the draw- 
tube and the substage are in alinement. This can be tested by focussing 
