EXTINCTION ANGLES. 
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FIG. 74. Intensity curves for crystal plates making angles o', 5', 10', 15', 20', 25', 30', 
40', 45', 50', and i with plane of polarizer. Analyzer revolved about axis 88 to 92 
from lower nicol plane, sin* d (y' a')=K=i. 
Curves calculated from formula (6) which, for K i, becomes / = \[i +cos 2(020)]. 
The heavy curve indicates the relative light intensity under crossed nicols (#=-) 
for different positions of the crystal plate (6 ranging from 88 to 92 or 2 to +2) 
and K = i. Calculated from the formula 2/! =i cos 46. 
