violet line which so many post-impressionists are using to outline objects to which they desire, without shading, to give the impression of rotundity, 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 removed 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 intensity at its edges, which becomes more and more pronounced towards the middle of the line until the hues finally meet and partly overlap, thus producing a 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 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 explanation. 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 undertake 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."
