124 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
all angles of rotation as indicated by the heavy abscissa line of Fig. 78. (2) In 
case K = J, Fig. 77, the intensity curve for the crystal plate, coincides very 
closely with that for the rotating nicol. The extinction curves, moreover, 
for the crystal plate at different angles (0= 15', 30', 45', and i) with the 
principal plane of the lower nicol and for different positions of the upper 
nicol (<f> ranging from 88 to 92) are similar and lie close together, so that, 
in this particular case, methods involving the rotation of the upper nicol 
.40 
Fie. 75. Intensity curves for crystal plates at angular distances of o', 15', 30', 45', 
and i from plane of polarizer, the analyzer being revolved about axis 88 to 90 from 
plane of polarizer, sin* = d (y r o') K = J. Curves calculated from the formula 
Ti"i (4 4- cos 2<f>+3 cos 2 (0-20)]. 
As in Fig. 74, the heavy curve indicates the relative intensity of emergent light for 
different positions of the crystal plate (6 ranging from 88 to 92, or 2 to +2) under 
crossed nicols 
-) and K- 
2 
j. Calculated from the formula 7, = J (i cos 40). 
for the location of zero intensity directions are not greatly different in their 
degree of accuracy from those in which the nicols remain crossed and the 
crystal plate is rotated. Nevertheless, even in this instance the former are 
the more sensitive methods and results attained by their use are corre- 
spondingly more accurate. For K = %, Fig. 76, the extinction curve for the 
crystal plate alone (nicols crossed and plate only rotated) no longer coin- 
