VISION 383 



the light of previous experience. In anatomical language, the 

 effect of sensations upon the personality depends upon the 

 paths which impulses follow in the brain, and the associations 

 which have been established by previous impulses which have 

 followed the same paths. The retina enables us to distinguish 

 tone and colour. By the variations in tone, the juxtapositions 

 of light and shade, we recognize form. All streams of impulses 

 which do not present tone- variations do not, that is to say, 

 reproduce the details of a scene are interpreted in terms of 

 colour. Every child discovers that the tedium of the intervals 

 during which it is proper that his eyes should be closed may 

 be relieved by pressing his knuckles against the lids. Although 



FIG. 31. SIMULTANEOUS CONTRAST. 



The shading of the two V's is exactly similar ; but the figure in half-tone on black appears 

 brighter than the figure in half-tone on a white ground. 



the world is shut out, a phosphene offers itself for his considera- 

 tion a yellow or white disc of irregular form with a red 

 margin, changing into lilac bordered with green, and then into 

 yellowish- green with a blue edge. Such, if my recollection 

 can be trusted, were the pictures which I used to see as a boy ; 

 but no adjustment of pressure calls them forth with anything 

 like the same vividness now. 



All the senses show a tendency to rebound after activity, 

 exhibiting contrast-phenomena ; but the contrasts of vision are 

 more marked and varied than those of the other senses, as 

 everyone who is curious in the observation of his own sensations 

 is aware. Negative after-images are generally referred to the 

 retina ; but various other kinds of after-image and contrast- 

 phenomena must be attributed to the judgments passed by the 



