﻿Colour Phenomena in Photometry. 121 



would settle down to another very consistent value, but 

 differing by perhaps 5 or even 10 per cent, or so from those 

 obtained before. This seems to suggest that extreme con- 

 sistency in reading is partly a matter of visual memory. 

 We recall the impression previously received by the eye and 

 involuntarily set the photometer, the next time, so as to 

 produce the same appearance of the field of view. 



The difference in sensibility of different eyes to a parti- 

 cular colour certainly introduces another disturbing factor in 

 observations made by different people. The author has not 

 met with any serious differences in judging the colour con- 

 trasts which ordinarily occur, but it might be supposed that 

 in such an extreme case as that quoted above considerable 

 divergences in judgment would exist, and have, indeed, been 

 recorded by Sir Wm. Abney, Professor Rood, and others. 



It must be noted, however, that the effect mentioned in (2) 

 must be eliminated, in order to make any satisfactory com- 

 parisons. All the figures given in this paper were obtained 

 by the author himself, though corroborative results from 

 other eyes were obtained in many cases. 



(2) This effect has been very completely dealt with by 

 Sir William Abney in his investigation on Colour Vision. 



It has long been known that the central portion of the 

 retina — " the yellow spot " — is much more sensitive to the 

 red end, and less sensitive to the blue end of the spectrum, 

 than the surrounding portion of the retina. 



It has been suggested that the " yellow spot/ 5 being yellow 

 in colour, will obstruct the blue rays, but will allow the 

 yellow rays to pass through practically unimpeded, to the 

 light-perceiving organs. This explanation, however, does not 

 explain why the differences observed are distinctly more 

 noticeable at low illuminations. 



There appears to be another physiological effect, which 

 will be referred to later. 



Suppose, now, that we compare a red and a green light 

 with a Joly photometer. An image of the illuminated blocks 

 is formed on the retina, and we adjust the position of the 

 photometer until the red and green appear equally bright. 



But if we now observe the photometer obliquely, or if we 

 observe it with the eye at a different distance away, the 

 image falls on a different part of the retina where the sensi- 

 bility to red and green may be different. Consequently the 

 red and green may no longer appear equally bright. 



And if a photometer with blocks of a different size were 

 used we might again come to a different conclusion ; for, 



