Visual Diffusivity. 21 



intensity the after-image moves in closer ; at rather lower 

 intensity the after-image appears merely as a grey tail to 

 the green and bine images ; while at the very lowest inten- 

 sities these latter become single grey images. 



An observation worthy of record, though probably not 

 new, is that the after-image when only slightly separated 

 from the bine primary image is distinctly the brighter of 

 the two. 



4. Measurements. 



(a) Instrumental conditions and constants. 



For the greater part of the measurements the rotating 

 sector disk was illuminated by a tungsten lamp of approxi- 

 mately 100 candle-power, mounted on an eight metre 

 photometer track to permit of a wide variation of illu- 

 mination by variation of distance. The brightness of the 

 opal glass without any colour screen was one candle per 

 square centimetre at the point marked on the arbitrary 

 logarithmic scale of fig. 7. The photometric transmission 



Tm. 7. 



•lb 



•10 



•05 

 sec. 









X 



X 



X 



- 



X 



X 







• 

 • 



.i 



• 



X 





/ 



/ 

 / 



• - , 



T B -T R 









Log b 



!^Ext" 



cm2 



Experimental results. Time-intervals between blue and red image 

 perceptions, and between after and red image perceptions. 



of the red and blue glasses used over the opal glass was 

 nearly the same, probably not far from five per cent, for the 

 high brightness conditions. The relative brightness of 



