PHYSIOLOGY OF COLOR VISION KELLER AND MACLEOD. 739 



certain amount of synthetic gray. This phenomenon of irradiation 

 is well illustrated by comparing two squares of equal size, one being 

 black on a white field and the other white on a black field ; the white 

 square looks distinctly larger than the black one. The reason is that 

 the stimulus produced by white, mainly because of imperfect focus- 

 ing, spreads on the retina somewhat beyond the margin of its image. 

 In this account we have not essayed to explain all of the peculiar 

 effects which are produced by some of the most modern creations of 

 the so-called post-impressionists. We have merelj^ indicated some of 

 the physiological truths of color vision upon which certain of their 

 color illusions depend. To go further would require consideration of 

 many optical illusions for which at present there exists no satisfac- 

 toiy explanation. These are not illusions of color but illusions of 

 line ; indeed, many of the latest post-impressionistic pictures are pro- 

 duced almost entirely m black and white, and the peculiar emotions 

 which they arouse depend on metaphysical processes whose explana- 

 tion we can not undertake to expound. Their aim is " to create an 

 illusion of the fact " rather than the fact itself ; to write " a visual 

 nuisic which shall in itself arouse the emotions." 



