Thus, in the case of vision the conventional “color pyramid” expresses the various relations between the different elementary colors and the different degrees of brightness. Red, yellow, green, and blue occupy the corners of the base of a double pyramid. The upper apex represents white and the lower apex black. On the side between red and yellow are found the various oranges which result from mixing red and yellow light in varying proportions. On the remaining sides are represented the combinations of yellow and green, green and blue, blue and red. Along the vertical axis range the different grays. Cross sections of the pyramid indicate, at different levels, the result of mixing the different colors with these grays, thus yielding the tints and shades of the colors. Along the base, the colors which are at the extreme ends of any diagonal passing through the center are complementary,—they neutralize each other when mixed and under other circumstances each tends to induce the other by contrast. The visual manifold may thus be adequately schematized on a three-dimensional figure. In a similar way the various tones, in the case of hearing, may be arranged along a one-dimensional line, which represents the tonal scale. Is it possible to arrange in any such systematic way the elementary taste qualities so as to indicate their relationship to each other? Before suggesting such a diagram it will be well to have in mind just what relationships the various taste qualities do as a matter of fact display. _Taste Mixtures and Compounds_