Now, according to the Hering theory of color, white, red, and yellow are the so-called "dissimilating" colors in the three pairs, white-black, red-green, and yellow-blue, corresponding to three hypothetical visual substances in the retina. These substances, that is, in undergoing a kind of chemical disintegration under the action of light-rays, are supposed to give the sensations white, red, or yellow respectively, and in renewing themselves again to give the sensations of black, green, and blue. The dissimilating process seems to bring about stronger reactions on the physiological side, as if it were a more exciting process. Thus it is found<1> that as measured by the increase in strength of the hand grip under the stimulation of the respective colors, red has particularly exciting qualities, but the other colors have an analogous effect, lessening, however, with the descent from red to violet. The pleasure in bright red, or yellow, for instance, may thus well be the feeling-tone arising in the purely physiological effect of the color. If red works like a trumpet call, while blue calms and cools, and if red is preferred to blue, it is because a sharp stimulation is so felt, and so preferred. <1> Ch. Fere, _Sensation et Mouvement_, 1887, p. 80. The question of the demands of the eye in color combination is still more complicated. It has been traditional to consider the complementaries black-white, red-green, blue-yellow, and the other pairs resulting from the mixtures of these as the best combinations. The physiological explanation is of course found in the relief and refreshment to the organs in successive alternation of the processes of assimilation and dissimilation, and objectively in the reinforcement, through this stronger functioning of the retina, of the complementary colors themselves. This tendency to mutual aid is shown in the familiar experiment of fixating for some moments a colored object, say red, and then transferring the gaze to a white or gray expanse. The image of the object appears thereon in the complementary green. Per contra, the most complete lack of contrast makes the most unpleasing combination, because instead of a refreshing alternation of processes in the retina, a fatiguing repetition results. Red and orange (red-yellow), or red and purple (red-blue), successively stimulate the red- process with most evil effect.