PHYSIOLOGY OF COLOR VISION 465 



violet line which so many post- impressionists are using to outline objects 

 to which the} r desire, without shading, to give the impression of ro- 

 tundity, or more correctly, of projection. The effect of such a line is 

 perhaps best demonstrated in still life studies where its existence at the 

 edges of, say, a vase, will, when the picture is viewed at such a distance 

 that the line just disappears, cause the vase not only to stand forward 

 from its background but also make it appear rotund, as if shaded 

 towards the edges. The line is sometimes used in landscape pictures 

 with the object of holding the pattern together. These effects are most 

 marked when the object is painted in hues that are considerably re- 

 moved from blue on the chromatic circle, or are of much less saturation 

 (more removed towards neutrality). Similar effects can sometimes be 

 obtained by the use of a black line, but none of the flaring hues can be 

 successfully employed for making it. It is difficult to explain the action 

 of these outlines, indeed it is almost certain that several factors play a 

 role in producing the illusion which they produce. When the line is a 

 blue one and the prevailing hue of the color field which it borders tends 

 towards yellow a synthetic gray will result at a certain distance, thus 

 creating the impression that some space exists between the object and its 

 surroundings. When a black line separates two colored areas there 

 occurs a certain amount of irradiation on to it of the neighboring hues, 

 which therefore undergo a more or less sudden lowering of intensit3 r at 

 its edges, which becomes more and more pronounced towards the middle 

 of the line until the hues finally meet and partly overlap, thus pro- 

 ducing a certain amount of synthetic gray. This phenomenon of irra- 

 diation 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 

 focusing, 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 merely 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 satisfactory ex- 

 planation. These are not illusions of color but illusions of line, indeed 

 many of the latest post-impressionistic pictures are produced almost 

 entirely in black and white and the peculiar emotions which they arouse 

 depend on metaphysical processes whose explanation we can not under- 

 take to expound. Their aim is "to create an illusion of the fact" 

 rather than the fact itself; to write "a visual music which shall in 

 itself arouse the emotions." 



VOL. LXXXIII. — 32. 



