122 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
Fig. 74), the direction of extinction of the plate to make an angle of 30' 
(0 = 30') with the principal plane of the lower nicol, and the principal plane 
of the upper nicol to include an angle of 89 10' with the lower nicol (</> = 89 
10')- On the 30' curve of Fig. 74 the ordinate for 89 10' is 0.104 and the 
relative intensity is therefore 0.104 per cent of the total intensity. 
100 
80 
70 
ec 
5C 
40 
30 

10' 
20V 
30' 
40* 
50' 
60* 
80' 
90' 
FIG. 73. Curves showing relative intensity of light emerging from upper nicol after 
transmission through polarizer, crystal plate, and analyzer, the positions of the crystal 
plate and also the analyzer ranging from o to 90. The abscissa values refer to angu- 
lar distances of the major ellipsoidal axis of the crystal plate and also of the plane of the 
analyzer from the plane of the polarizer. For curves I to IV the nicols are considered 
crossed 
(0~~) 
crystal plate alone rotated from o to 90. In curve I, sin* 
J; in curve III, K*= \; in curve IV, K o. Curve 
- d(y' a')*-K i; in curve II, 
V shows the relative intensity of the emerging light for different positions of the analyzer 
alone (8 = o, # ranging from o to 90). Curves calculated from the general equation (6) 
above. 
These figures are well adapted to show graphically certain facts which 
are evident from a mathematical consideration of the intensity formula. 
(i) If K=o, which occurs when the one wave is any number of whole wave- 
lengths ahead of the second, the crystal plate is dark and remains dark for 
