METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
intersection of the great circle PK with the diameter OC, the trace of the 
plane of vibration of the lower nicol. 
In plotting the angles corresponding to the coordinate readings of the 
double-screw micrometer ocular, it should be noted that these angles apply 
to small circles, the angle for each micrometer-screw reading indicating the 
position, from the center of the projection, of a certain horizontal or vertical 
small circle. The intersection of the horizontal and vertical small circles 
thus obtained from the two micrometer-screw readings for a particular 
point of the interference figure determines the location of that point in the 
projection. 
The reason for adopting this graphical method in preference to the Becke 
method is apparent when the factors underlying the formation of an inter- 
ference figure are considered. An interference figure is obtained by passing 
a cone of convergent polarized light-waves through a crystal plate and 
FIG. 89. 
FIG. 90. 
observing the interference phenomena as they appear in the rear focal plane 
of an objective of short focus when examined through an analyzer. In the 
course of their passage through the microscope, the light- waves emerging 
from the lower nicol (polarizer) may be considered practically parallel, 
plane polarized waves. In transmission through the condenser lens system, 
their directions of propagation are changed ^nd they emerge from it in a 
sharply divergent cone; but their lines of vibration have remained in the 
same plane except for the slight rotatory effects of the surfaces of the con- 
denser lenses, which for the moment may be disregarded. That this is the 
case is tacitly assumed in all microscopic work, since the rotatory effects pro- 
duced by the condenser and objective lens systems alone, on the plane of 
polarization of transmitted light-waves, are practically negligible and the 
field appears approximately dark under crossed nicols. Thus in Fig. 89, if 
the direction Z' be the axis of the optical system of the microscope, Y'Z' the 
plane of vibration of the entering waves, and P the direction of propagation 
of one of these waves after refraction, its direction of vibration will then be 
along T, at right angles to P and in the original plane of vibration. This 
same direction of vibration, OT (O being the center of the sphere of projec- 
tion), obtains for any other point P' in the polar plane to T. A wave 
