64 



Mr. J. S. Dow on tlie Physiological 



dividing line between them appears to oscillate to and fro. 

 By altering the position of the pin P the width of this band 

 of flicker can be adjusted until it occupies exactly half the 

 field of view, i. e. has the same dimensions as either of the 

 coloured surfaces. We thus have the exact equivalent of 

 observing two coloured surfaces of equal size, either simul- 

 taneously, when we judge their intensity by the equality of 

 brightness method, or in rapid succession, when we judge 

 their intensity by the absence of flicker. A typical result 

 of varying the distance of the eye from the screen is shown 

 in fig. 3, the readings, with the eye at a distance of 20 cms., 

 being taken as unity in each case. 



Fig:. 3. 



Comparison of " Yellow-spot Effect " in Flicker and Equality 

 of Brightness Photometers. 

 Equality of Brightness. 



Actual Dimensions 

 of aperture and 

 appearance as 

 used on Equality 

 of Brightness 

 and Elicker 

 methods. 



£T> 





ro 



IE 



m 



m 



m 



O 



2 







Flicker. 





33 



■n 

 r~ 



ro 



m 



o 



m 



m 



^ 



n 



2 



m 



23 





& 



O 1-4 



3 1-2 



6 



1-0 















. j* 































^ 

















































FUCf<J^_^ 







































10 20 30 40 50 60 



Distance of Eye from aperture, centimetres. 



70 



The results of experiments on this matter were perplexing 

 and variable in many respects. But one point brought out 



