VISUAL SENSATION. 184: VISUAL SENSATION. neous stimulation of two or more color substances in varying proportions. The Hering theory is for- mulated in static, not in genetic, terms ; but if we put a genetic interpretation upon it, we must say that for it. too, the black-white substance is the earliest, the blue-yellow the next, and the red- green the last to appear in the course of evolu- tion. Such an interpretation is borne out espe- cially by the facts of color-blindness and of in- direct vision, as understood by Hering and his school. Another important theory is that of Wundt, who posits only two excitatory processes, a chro- matic and an achromatic, and regards the former as a multiform, the latter as a uniform, plioto- chemical process. Wuiidt thus gives up alto- gether the idea of primary 'components' of visual sensation — of the three ultimate colors of Helra- holtz or the four of Hering — and substitutes for these ■component theories' a theory of gradations within a single process. It may be remarked that Wundt's recognition of a separate achro- matic process antedates the formulation of Her- jng's theory. The independence of the achromatic process is accepted also by Von Krics, the author of another theory, which, while based on that of Helmholtz as regards the function of the retinal cones, sees in the rods special organs of achro- matic vision. BiBLiOGE.PHY. Ebbinghaus, Grundzuge der Psi/chologie (Leipzig, 1897): Wundt, Human and Animal Psychology (New York, 1894) ; Helmholtz, Fhysiulogische Optik (Leipzig, 1890) ; Hering, Lchre rom Liehtsinne (Vienna, 1878) ; Titchener, Experimental Psychology (New York, 1901) ; Kiilpe, Outlines of Psychology (New York, 1895) : Von Kries, OesichtsempfindungcH (1882) ; Kcinig, Gcsammeltc Abhandlungcn CHc physiologischen Optik (Leipzig, 1903). See After-Image: Co.ntrast; Color-Blixdxess; Weber's Law; Quality; Intensity of Sen- sation ; PSYCHOLOGICAI. APPARATUS. VISUAL SENSATION — The Color-Tri- angle. The fact that there are four colors — red, yellow, green, and blue — which are of distinctive character, and that the other colors all look like combinations (color-blends) of some two adja- cent colors of this group, is well represented in the color-quadrilateral (shown in Fig. 1 in the preceding firticle). That does not, how- ever, give proper representation to certain other phenomena of color-vision— these can only be expressed diagrammatically by means of a color- triangle. For instance, yellow and blue, at opposite ends of a diagonal of the figure, are complementary colors, but red and green are not complementary — rod is complementary to green- blue, and green to purple (red-blue), while red and green when mixed together give yellow. So, in a general way, the non-saturnted colors may be regarded as represented by points within the square, but no exact one-to-one correspondence between points and colors is possiI)Ie in this dia- gram; the first principle of the phuie is violated in the beginning, for the shortest series of dis- tinguishable colors is not in a straight line. It is only necessary, however, to substitute a tri- angle for a square, with red, green, and blue at its vertices, white in the centre, and yellow on the line connecting green and red (Fig. 1) to Bceure a good diagrammatic representation of the whole color scheme, including the large body of fact compreliended under the term color- mixture. (For ditl'erent intensities, this plane figure must be made the basis of a double pyra- mid "with intense white and black at the two apices.) The underlying principle which makes this possil)le is, in the first place, that color sensations constitute a twofold continuum (in- tensity being kept constant ) , since they vary by just perceptible degrees in both color- tone and saturation. The further condition that is satis- fied in the actual representation is (among Green Yello Blue Fig. 1. THE COLOR-TEIANOLE (DIAGHAMMATIC). others) that all colors that can be got by the mixture of any two are on a line joining the points which represent those two, and nearer to the point representing the color that they most resemble. Thus if a certain color c (Fig. 1) can be composed out of two parts of one color a and one part of another color h, then it must be rep- resented on the color plane by a point on the line ab twice as far from b as from a; this is the point which would also be the centre of gravity of weights of two pounds and of one pound sus- pended respectively at the points a and b. All complementary colors, when mixed in the right proportion, make white light: hence all such colors are on opposite ends of lines which pass tlirough a common point, W. Any color can be made up (with some loss of saturation) out of mixtures of a certain three, red, green, and blue (or violet). Hence the general outline of the figure is a triangle. The fact that these mixtures lack something of saturation requires the assumed fundamental colors to be taken at least outside of the actual spectral color-line (Fig. 2).
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THE ACTITAL COI.OR-TRIANOLE. The color triangle of Fig. 1 expressed (as do all the color diagrams of Hering) the vague idea of the representation of the color continuum by the points of a plane. Clerk-Maxwell was the first
