Theory of the Flicker Photometer*. 295 



speed-data recorded in an earlier paper *, for red and green 

 light,, using the experimental values o£ fig. 2. This is, there- 

 fore, a point which should receive careful attention in the 

 future design of apparatus to be used in large colour- 

 difference work, such as the determination of spectral 

 luminosity curves. 



Fiff. 2. 



**u 







1 

















i 



30 



1 



&S 





















1 





\ 



fc 



v 







/ 



7 



& 



> 









20 







\ 



\\ 







> 



fr 

















V 





/ 





S 











10 









\ 



1 



'/ 



/ 



















\ 













! 













1 



'/ 











i " 

 l i 



if 



•2 4 

 1-0 10 



•6 

 1-0 



■8 1-0 I 

 10 10 1 



2 







1-4 16 1 

 10 10 1 



8 

 



2 



1 



zz 



10 





I, 









■ 







1 





-— - 



IT" 













, . i 



Sensibility characteristics of the flicker photometer. Calculated curves 

 for various values of h for the case where one illumination is constant, 

 the other variable. Experimental values are shown by dots. 



The second point to be noted is that the experimental 

 values agree most nearly with those calculated from a very 

 small value of the Fechner fraction, namely -0001. The 

 actual value of this fraction for simultaneously presented 

 juxtaposed surfaces is between '0025 and "01. Why is this 

 small value apparently called for ? 



* "Spectral Luminosity Curves obtained by the Equality of Bright- 

 ness Photometer and the Flicker Photometer under Similar Conditions," 

 Ives, Phil. Mag. July 1912, p. 149. 



