Visual Diffusivity. 



19 



with the physical laws o£ conduction. The chief experi- 

 mentally established phenomena of the flicker photometer 

 find explanation in the assumption that the stimulus is 

 transmitted through a layer of matter having a coefficient 

 of diffusivity which is different for different colours and 

 varies with the intensity of the stimulus. In accordance 

 with our previous work, the diffusivity for coloured light is 

 a rectilinear function of the logarithm of the stimulus 

 intensity. For colourless or rod vision the diffusity is 

 constant. These relations are exhibited diagrammatically 

 in fig. 1. 



Fig. 1. 

















Bod vr's/o 



7 

















L 



Diagrammatic representation of brightness-dimisiyity 

 relation for red, bine, and rod vision. 



This conception of the visual process has not only served 

 to explain previously known facts in the realm of inter- 

 mittent vision, but has indicated others not heretofore 

 observed. The phenomena of flicker are, however, of such 

 a complex character that, satisfactory as this hypothesis has 

 been, it may have appeared to some to be insufficiently 

 supported so long as it played no part in explaining other 

 simpler phenomena. 



The present paper deals with the visual parallel to one of 

 the simplest cases of heat conduction, in which the diffusivity 

 of the conducting layer figures, namely that of instantaneous 



C2 



