364 F. E. Wright — Measurement of Extinction Angles. 



total intensity and represented by second horizontal line above 

 the base line of fig. 5. 



Then the curve for the crystal alone shows that for all points 

 below that line, i. e., between 89° 04' and 90° 56', the crystal 

 will appear absolutely dark and on a single determination an 

 error of nearly ±1° may be made. If, however, the crystal 

 plate remain stationary, and the upper nicol be revolved 

 through small angles from its normal, crossed position ((j> =88° 

 to 92°), it is evident from the figure that if, for example, the 



Fig. 7. 



tL\J 1 







, n V / 



.1 U \ 7 



v / 



\ / 







u g8° m° qo° qi° % 



Fig. 7. — In this particular case K is considered — and the general 

 formula reduces to 



I - y (1 + cos 20) 



which is independent of 6. In other words, if the thickness of the plate be 



2rrt 

 such that sin 2 — (y' — a') = 0, or the emerging waves are any number of 



whole wave lengths apart, total interference takes place and the plate is 

 dark under crossed nicols for every angle of revolution about its normal 

 axis. 



crystal pi ate is 30' distant from its position of true extinction 

 and still dark under crossed nicols so far as the eye of the 

 observer can detect, the differences in intensity between the 

 field and crystal plate for different angles of revolution of the 

 upper nicol (measured by the ordinate intercepts between the 

 curve 0' and 30' of figure), are of such a character that at the 

 point where the illumination of the field can just be observed 

 (88° 43') the intensity of illumination of the crystal is more than 

 twice as great (-106 per cent instead of *05 per cent), whereas 



