Polarization Flicker Photometer. 



367 



Fig. 6 shows experimental values obtained to test the 

 equation, using the new photometer. It will be seen that 

 the theory is fully confirmed. Equation (3) is hence the 

 general expression, of which the relation for light against 

 darkness (12=0), as found by Porter, is a special case. 



Fig. 6 



AC 



30 



8 

 £ 20 



o 



10 













y 























-^j^ 



M 



t^- — 



gg 



*r 



- — - " " 



__ _ — * — " 





I 2 



Locj. br/qhtness 

 Critical speed brightness relations for various ratios of brightness 

 of two fields. 



The value of 8 called for by these data is about *0001, 

 which is much smaller than the figure fitting the data of the 

 previous section. The measurement of 8 and its character- 

 istic of variability call for separate discussion. 



5. Measurement of the Brightness Difference Discrimination 

 Fraction for Fluctuating Impressions. 



A determination of the critical speeds for two different 

 ratios of the fields furnishes all the necessary data for finding 

 the value of 8. 



Denoting the two different critical speeds and brightness 

 ratios by the subscripts a and b, we obtain from (3) 



log 8 = log 2 — 





(4) 



