Theory of the Flicker Photometer. 293 



In fig. 1 are shown calculated values of co, for the simplest 

 possible photometric condition, namely that where the total 

 illumination is maintained constant. This means that we 

 are here free from any questions as to the rate of change of 

 diffusivity or of the Fechner fraction with the illumination. 



4C 









Fig. 



1. 































30 



NX 















u 









\\ 



\ 







A 



% 



/ 











\ 



v 



[ 



/ 



k/p 



¥~ 















\ 







i 









10 





















































\ 



J 







i 



I 



The sensibility characteristics of the Flicker Photometer as exhibited 

 by the critical frequency of disappearance of flicker for various ratios 

 of the two compared lights. Calculated curves for the case of total 

 illumination maintained constant. Various values of the Fechner 

 fraction (5). 



Curves are shown for three values of 8, namely, *01, *001, 

 and "0001. An experimental value for o> has been taken 

 for the limiting conditions, and the value used for the 

 diffusivity solved for from the equation (4). 



The figure shows with what extraordinary rapidity the 

 critical speed rises as the two illuminations become unequal. 

 Thus a variation of only about one per cent, causes as great 

 a change in the critical frequency as did fifty per cent, in 



