20 Dr. H. E. Ives 



on 



heat sources. This, in the case of light, is of course the case 

 of single brief exposures. It is shown below that, in accord- 

 ance with our theon r , a difference of diffusivity should result 

 in a difference in the time of perception after exposure. This 

 predicted difference has been found both in the case of light 

 of one colour but different intensity and in the case of lights 

 of different colour and the same intensity. In addition, the 

 positive after-image has been indicated to be simply the 

 rod-vision impression delayed in transmission by the low 

 diffusivity peculiar to this kind of vision. Both the new 

 phenomena of differential rate of transmission for photopic 

 stimuli and the relation of the colourless after-image to 

 the photopic images have been subjected to quantitative 

 measurement with results supporting the theory. 



2. Application of the Idea of Diffusivity to Instantaneous 

 Light Sources. 



It is shown in treatises on heat conduction that the tem- 

 perature Y of a conducting slab of diffusivity K, at a 

 depth X, at a time t after the instantaneous application to 

 the surface of a quantity of heat Q is 



Now the time at which this is a maximum may be found 

 by differentiating (1) with respect to t. This gives 



' X 2 

 * m= 2K' (2) 



or the interval between reception and perception of a stimulus 

 is inversely as the diffusivity. 



The measurement of this interval is obviously not feasible, 

 but the difference between the intervals for two stimuli is 

 measurable. Several special cases may be treated : — 



1st Case. 



One colour, two intensities, Ij and I 2 ; I2 > Ii- 



«i= -,„t ., , (3) 



a lo<>' Ii -4- I 



t 2 = . \ ;, (4) 



2 alogI 2 + />' 

 h h ~ alogl^b" a\ogl 2 W '.■" ' (D> 



