60 



Mr. J. S. Dow on the Physiological 



and cones over the retina. At the centre of the retina only 

 cones appear to exist ; portions of the retina more remote 

 from the fovea are covered by a mixture of rods and cones, 

 while the extreme peripheral portions of the retina contain 

 rods only. It will be seen therefore, that when the bright- 

 ness of two surfaces, illuminated by the aid of red and green 

 light respectively, is compared, the result will depend very 

 largely on the proportions of rods and cones on the portion 

 of the retina on which the image of them is received. It 

 therefore occurred to the author that it would be of interest 

 to obtain curves connecting the distance away of the eye 

 from the photometer- surfaces so illuminated, and the apparent 

 relative brightness of the red and green, with a series of 

 different illuminations of the photometer. This experiment 

 was carried out in exactly the same manner as those of a 

 similar description detailed in the paper before the Physical 

 Society in 1906. 



Fig. 1. 



Showing- Effect of varying distance of Eye from Joly Photometer at 



various illuminations of the photometer-blocks. (" Equality of 



Brightness " Method.) 



60 



O 



4-0 



-2 30 



H 



a 

 Q 



o 



.2 2-0 



P3 



1-0 











^ 





























JS^" 











••wo ui* 















20 40 60 80 100 



Distance of Eye from blocks of Photometer, centimetres. 



The results are shown in fig. 1, and seem to bear out 

 completely the suggestion that, whatever be its explanation, 



