410 Hastings — Optical Errors of the Human Eye. 



is extremely rare ; consequently there seems to be no escaping 

 the conclusion that similar visual impressions receive, not 

 merely unlike, but opposite interpretations in different indi- 

 viduals. Any acceptable theory must adapt itself to this funda- 

 mental fact. 



It seems to me that we can find a probable. explanation in 

 the following considerations. Imagine a number of red points 

 distributed in two parallel planes which lie at right angles to 

 their direction from the observer. If the eyes be fixed upon 

 one of the points in the nearer plane, the images of all the 

 other points in that plane will fall on pairs of congruent points 

 of the two retinas, but the images of the points on the more 

 remote plane, which will be quite sharp if the separation of 

 the planes is moderate, will fall on disparate points of the two 

 retinas. Now consider the case of red and blue points in a 

 single plane. Here, if the eyes be fixed upon one of the red 

 points, all the other red points will have their images on con- 

 gruent points of the right and of the left retina, while the 

 blue points, relatively very diffuse as compared to the images 

 of the more distant red points in the former case, will fall on 

 disparate points. So far there is a formal similarity between 

 the two cases, but it cannot be carried further : in the first case 

 a simple change of convergence of the axes of vision will 

 change the disparate points to congruent, and vice versa, while 

 in the second case no such change can produce such an effect ; 

 but a change of accommodation proper to blue light will do 

 so at once. The resemblances and differences are such, there- 

 fore, that it should hardly surprise us that certain persons 

 recognize the second case as only a confusion of visual images, 

 w T hen others interpret the effect as a sort of chromatic relief ; 

 nor is it astonishing that of the latter group, since there is 

 really no fundamental relation between the two phenomena, 

 some should imagine the red to be nearer than the blue points 

 and others invert the order. It is singular, however, that 

 Einthoven found those whom he observed to be distributed so 

 nearly equally among the three classes. 



The experiments of restricting the pupils by screens in front 

 of the eyes is by no means conclusive ; indeed, it is questionable 

 whether it has any bearing whatever upon the phenomenon. 

 The screens will, primarily, increase very greatly the separation 

 of the centers of the red and blue areas on the retinas which 

 represent the images of the points ; moreover, the areas them- 

 selves will be much decreased. This will become evident from 

 an inspection of fig. 1. But, as is easy to prove by a diagram, 

 any lateral change of the common point of fixation will pro- 

 duce a relative change in the position of the colored areas on 



