292 Messrs. H. E. Ives and E, F. Kingsbury on the 



Let us consider the case of no colour difference. At the 

 condition of equality, with an ideal flicker photometer, no 

 flicker results, no matter what the speed of alternation. 

 The critical speed is therefore zero. As the equality point 

 is deviated from in either direction a flickering condition 

 is produced due to the alternation of the two unequal 

 illuminations. The speed necessary to make this flicker 

 disappear may be determined by the ordinary experimental 

 method for various known differences of illumination. When 

 sufficient points have thus been fixed, the lines joining them 

 enclose a space within which no flicker occurs. At any 

 given speed the limits of the no-flicker space indicate the 

 limits within which the flicker photometer setting must lie. 

 The sensibility of the flicker photometer may thus be indi- 

 cated graphically by the narrowness of this region. 



Let Ix be the illumination of one side of the flicker photo- 

 meter. 

 I 2 that of the other side. (We shall in this paper 

 distinguish between the actual illumination and the apparent 

 illumination, which latter refers to the appearance of the 

 disk when running, and is always I x angular opening. This 

 usage causes the difference of the constants in certain of the 

 equations common to the two papers.) 



Let us, as in the previous paper, deal with stimuli repre- 

 sented by simple sine curves. Then, as before, the ranges 

 of the two transmitted impressions at a depth X are 



-X\/— 

 \e 2K 5 K=diffusivityj 



-xv^ . • (1) 



I 2 e ~ Iv , co = speed 



The total resultant range is the difference between these or 



2K [Ii-I 2 ] (2) 



The fractional part this is of the whole, or the fractional 

 range is 



*- Wi \m : m 



Galling this fractional range a constant, 8, according to 

 our previous assumption, and solving for w, we obtain 



w = x 1 ^ 



