CONTRAST (Fr. contrastc, from ML. con trastarc, to withstand, from Lat. contra, against -1- stare, to stand). The enhancement of the dif ference between objects or attributes of objects which results from their juxtaposition or im mediate comparison. Contrast has been eau played, very loosely, as a principle of explanation in psychology: and we cannot even now say that the whole field of 'contrast phenomena' has been adequately explored. It is, clearly, no explana tion of a given fact to refer it to a general law of contrast. any more than it is an explanation of a mental connection to refer it to the 'associa tion of ideas.' We must know, in each particu lar case, the conditions under which the fact is determined and the connection takes shape.
(1) One chapter of the psychophysics of con trast may, however, be considered closed. This is the chapter that deals with the contrast of brightness and colors. If we lay two black strips upon a gray ground, and then push a white cardboard under one of them, so that the white projects on either side, we see at once that the black on white is blacker than the (same) black on gray. If we lay a gray disk upon a red ground of the same brightness, and stand so far away that the contour of the disk is eliminated, we see a bluish-green in place of the gray disk; and the color is most intensive along the line of junction of disk and ground. These experiments may be varied in litany ways.
The principal laws of contrast, as determined by such experiments, are five in number: (a) Contrast always takes the direction of greatest opposition. A white induces a black, a color its complementary color. (See VISUAL SENSA TION.) (b) The more saturated the color (the redder the red, the purer the white), the greater is the contrast effect. (c) The contrast effect is greatest along the line of junction of the con trasting surfaces (marginal contrast). (d) The more nearly alike two colors are in bright ness (both bright, or both dull) , the better will be the contrast effect. And (e) the more nearly homogeneous the contrasting surfaces (the less distinction there is at the line of junction),. the better will be the contrast effect. Helmholtz sought to explain these facts, in purely psycho logical terms, as deceptions of judgment; we see aright, but estimate wrongly; and \Vundt ac cepts the explanation for certain of the phe nomena. The psychological theory, however,
breaks down when one attempts to carry it out in detail. There can he no doubt whatsoever that, as Hering maintains, contrast of color and brightness is a matter of direct physiological con ditioning, the result of the interaction of excita tions within the visual organ, a symptom or expression of the functional unity of the retina.
(2) Whether contrast occurs in the perceptual sphere; whether, e.g. 'great' and 'small' contrast aii do blue and yellow, is a much-disputed ques tion. There are certain optical illusions (see linusioNs) which seem to be most easily 'ex plained' by reference to a law of space contrast. In its most direct form, the theory of spatial contrast affirms that "the space sensation of a stimulated retinal point is a function pri marily of the position of the point stimu lated. but also of the space sensations of the neighboring retinal elements" (Loeb) ; the formula that holds of sensation is thus applied, without modification, to the sphere of visual space-perception. it has been said. again. that lines of different direction exercise 'contrastive' influences upon the movements of regard, i.e. upon the movements made by the eyes as we attentively follow the course of the lines (Hey mans). And, lastly, it has been asserted that figures of the same size, by like figures of another size, look larger `by contrast' if the surrounding figures are smaller, and smaller if these are larger ( \Vilna). Most of these illusions can, however, be otherwise ex plained; and, despite the array of authorities, the fact seems to be that a given perception is never really changed by contrast with other per ceptions. 'Perceptual contrast' consists rather in a feeling. When we see a middle-sized loan by the side of a dwarf, we are surprised at his height; when we see him by the side of a giant, we are disappointed in his height. Our percep tion of the man's height is not affected by the neighborhood of smaller and larger men; the contrast that we 'feel' is felt, literally, as sur prise or disappointment.