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



These figures 2-6 are well adapted to show graphically cer- 

 tain facts which are evident from a mathematical consideration 

 of the intensity formula. (1) If K -= 0, 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 all 

 positions of revolution, as indicated by the heavy abscissa line 



Fig. 5. 



.30' 



+ 5' 



Z % 







.0 



















\ 

















V 















\ 



\v 



N \ 













/ 





\ X ] 



\ \ 











/ 





s 





\ 



\ 





' / 



/// 









v \ 











88° gq° qo° qi° qz 



Fig. 5. — This figure differs from the two preceding figures only in the 

 value of K, which is 1/2. The curves were calculated from the formula 



Ii = 1/4 ( 2 + cos 20 + cos 2(0-20) ). 

 The heavy curve from the formula 



I, = 1/4 (1 + cos 4.6). 



of fig. 7. (2) In case K == 1/4, £g. 6, the intensity curve for 

 crystal plate, coincides very closely with that for the revolving 

 nicol. The extinction curves, moreover, for the crystal plate 

 at different angles (6 = 15 7 , 30', 45 7 , and 1°) with the principal 

 plane of the lower nicol and for different positions of the upper 

 nicol ((f) ranging from 83° to 92°) are similar and lie close 

 together, so that, in this particular case, methods involving the 



