378 



Mr. H. E. Ives on a 



operated at the point of just-disappearance of colour 

 flicker, is 



log I t - log ^/("i^H-ftiH 



logI 2 + 6 2 ) 



2(a 1 logI 1 + 6 1 ) 



= log Ic 



log K\/ 



(a 2 log I 2 + b 2 ) -f- (a ± logli + ^Q. 



2(<2 2 log I 2 + b 2 ) 



(15) 



In making this substitution for S H two assumptions figure : 

 first, the one leading to equation (11), that the critical 

 speed of the mixture of the two compared colours may be 

 calculated to a close approximation by disregarding the dif- 

 ferent diffusivities of the two colours ; and second, that Sh is 

 constant over the range of intensities in question. If these 

 assumptions are true, then equation (14) states (Sb having 

 been found constant over wide range, as shown by the data 

 of fig. 6) that the plot of g>m against log I should be a 

 straight line, lying below the mean straight line given by 

 the critical frequencies for the two compared colours, and 

 inclined to it at such an angle that both intersect the axis of 

 abscissae at a common point. 



Fig. 8. 

 30 



§ 

 fa 20 



^ /Oh 











^^ 



^^^ 









Ioq brightness 



Upper points, mean critical frequencies tor red and green light. Lower 

 points, critical frequencies for red and green alternated against each 

 other. 



This relationship was tested out experimentally with the 

 result shown in fig. 8, where the upper, straight line is 



