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Simultaneous visual contrast includes contrast in brightness, saturation, color, and emotional tone. It is dependent on several variables: the extent and distance from each other of the contrasting surfaces, the intensity of the light from the objects, the shade of color, and the degree of saturation. Within the limits of clear perception, the intensity of brightness, and probably also of color contrast, increases proportionally to the linear extent of the inducing part of the retina, or to the square root of the surface content. A less intensity of correspondingly larger extent does not change the strength of the contrast. Color contrast is best, when brightness contrast is excluded and there is a combination of medium degrees of saturation of the colors. Experiment shows that black and white are not sensations, but concepts, influenced by the relativity of the maxima of brightness and saturation, and the correction of sense-perceptions by previous experience. The effect of colored illumination in the case of slightly colored glasses is altered by an illusion of the judgment, previous knowledge leading to the incorrect supposition that the objects are seen in their usual colors. But with glasses of more saturated colors, two other factors assist recognition of the true color, fatigue of the retina and absence of any relation between the glass-color and other colors, it being only in the first moments that we compare the present with the normal illumination. Contrast has a large influence in the idea of polish, which is not given as sensation, but is a product of the combinations of sensations. Emotional contrast is not independent of contrast in brightness and quality, but it determines the aesthetic effect of the colors and their combinations as a result of simultaneous contrast of the sensations. The maximum emotional effect is reached when color, brightness, and saturation are properly contrasted. Other effects of contrast are light-induction — so far known as accompanying successive and contour contrasts — and, under some circumstances, the repression of an otherwise perfectly apparent difference. The absolute intensities and colors of objects are untrustworthy factors of the visual percepts, and it is to contrast that we owe the recognition and determination of the objects of the external world.