F. E. Wright — Measurement of Extinction Angles. 363 



revolution of the upper nicol 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 revolved. Nevertheless, even in this instance the 

 former are the more sensitive methods and results attained by 

 their use are correspondingly more accurate. For K = l/2, fig. 

 5, the extinction curve for the crystal plate alone (nicols crossed 

 and plate only revolved) no longer coincides with that for 

 the upper nicol alone, but similar conclusions can be drawn as 

 to the relative sensitiveness of the two methods, the one involv- 

 ing the revolution of the crystal plate (while the nicols remain 

 crossed), and the second, the revolution of the upper nicol while 

 the crystal plate remains stationary. 



The amount of light 



Fig 



20 



4-5 



30' 

 15 

 0' 



JO 



.0 



















\ \ \ 



















\\ 













// 



\\ 



\\ x 



\ 









/ 







\\ 



x\ 



^ 



^-_ 





/Jf 





88° 8T ^0° °i\° 92° 



Fig. 6. — Differs frorn fig. 5 only in K, which is 1/4. The curves are 

 expressed by the formula 



Ij = 1/8 [ 4 + 3 cos 2(j) + cos 2 (<p - 20) ] 

 and the heavy curve by 



I x - 1/8 (1 + cos id) 



which is required to produce the sensation of light in the 

 human eye is different for different persons. But for a given 

 eye the limit of the actual sensation of monochromatic light is 

 fixed for any particular instant and may be represented by one 

 of the horizontal percentage lines of the figures. Let us assume 

 that for a source of monochromatic light of definite intensity 

 I, the limit for the sensation of light is "050 per cent of the 



