VISUAL SENSATION. 182 VISUAL SENSATION. detect a change in color quality in a difference in wave-lensths of less than Under Fig. 1. THE COLOR PYRAMID. mm. 1,000,000 conditions (comparison of two spectra bit by bit) about 100 color- tones can be dis- criminated in the whole spectrum. It should be noted that language is mislead- ing in regard to the names of color quali- ties. The compound terms "yellow-green' and 'green-blue' in- dicate that these colors are less simple than red, green, or blue. This is not true. Any one of the 160 qualities is for introspection as simple and as ulti- mate as any other. The combination of color with bright- ness or, as it was stated above, the homogeneity or het- erogeneity of the stimulus, gives rise to a third moment, namely, saturation. Saturation denotes the likeness' or unlikeness of a color to gray. The relation of saturation to color- tone and to brightness is most easily shown by mixing, in various proportions (e.g. with revolving disks) a color (say, a red) with a gray of the same brightness value. Whatever the ratio of the two, the color-tone and the brightness remain constant, but the saturation, the richness, the purity, the 'depth,' changes. The more gray that is added the less saturated the color, until finally no color-tone remains and the saturation becomes zero. The whole number of visual qualities — brightness, color-tone, satu- ration — may be represented by a solid figure of the form of a double pyramid with a common base. The brightnesses will lie along the vertical axis of such a figure, the color-tones (at their maximal saturation) about the periphery of the base, and the various saturations along horizon- tal lines connecting the black-white axis with the peripliery. The liase will be tipped in such a manner "that yellow will be nearer white than blue is. This indicates that a saturated yellow is brighter tlian a saturated blue. Every point on or within the figiire will express, then, some definite color of a certain saturation and a cer- tain brightness, while the vertical axis itself rep- resents the series of colorless sensations. It is to be noticed that although change of saturation does not affect color-tone, it introduces, never- theless, new visual qualities. A red, or a green, or a violet that is washed over with gray is qualitatively ditferent from the original satu- rated color. Hence, in making an exhaustive enunieration of visual sensations, we nuist in- clude the brightnesses, the color-tones, and the varinus saturations. The total has been esti- mated at about .'i.'J.OOO sensation qualities. The retina is not equally sensitive at nil points to stimulation by ether waves. When the illu- mination is faint, the greatest sensibility to change of brightness is found a little outside of the fovea, or point of clearest vision. A very faint star, e.g. can be sensed there when it is in- visible at the fovea. Stimulation at and around the fovea gives all the color qualities (spectral colors and purples) ; farther out toward the pe- ripher.y only yellows and blues are visible ; farther out still, only gray, black, and white. As regards color sensitivity, therefore, the retina may be said to be divided into three zones, a black-white zone (outermost), a black- white and blue-yellow zone, and an inner black-white, blue-yellow, and red-green zone. Hence it is found that as a mixed color stimulus moves outward from the fovea the resulting color changes at certain points and finallj' goes over into a gray. The method of determining the zones is called ampimetry (with constant fixation at the centre of a black field, a colored object moving out and in, to and from the limits of the field), or perimetry (with con- stant fixation, the object moving in a similar manner along the arc of a great circle which lies in a hemispherical surface convex to the ob- server's eye ) . We have seen that color-tone depends upon wave-length. It also depends (1) upon ampli- tude (intensity of stimulus) to some extent, and ('2) upon the composition of stimulus (mixture of radiations of different wave-lengths ) . ( 1 ) If the solar spectrum be made very bright or very weak, the relations of the colors to each other change. In a very bright dazzling series of colors (got, e.g. by looking at the sun through colored glasses) all the colors but yellow and blue tend to change in tone. The green-blues and violet-blues pass over into blue, the yellow- greens, yellow-reds, and even the reds grow yel- lowish, and the greens become whitish. In the darkened spectrum (called the Purkinje spec- trum) it is the r«d, the green, and a bluish violet that spread at the expense of the other colors. At the same time the point of greatest brightness migrates from yellow to green, so that the red end of the spectrum grows dark faster than the violet end. The result is that blue, which at a moderate intensity looks darker than red, at a lower intensit.v grows lighter than red, before it disappears into gray. This shift of brightness in the darkened spectrum is known as Purkinje's pheiwmciwn, after the physiologist who first called attention to it. To produce these latter changes it is necessary that the in- crease or decrease in brightness shall have lasted some time, and that it extend over the entire visual field, in order that the eye may become adajited to the new intensity. (2) The production of (lilferent color-tones by the mixture of wave- lengths is known as eaUir-inixttire. It is carried out by the mixture of spectral lights, of colored shadows, or refiections; of pigments, by the irradiation of neighboring colored spots (as in a mosaic) : and by the rajiid alternation of stinuili by means of revolving disks. There are thr<'" ciiief laws of color-mixture. (1) The law of conqilenientaries: "For every color there can be found another complementary or antagonistic color, which if mixed with it in the right pro- portion gives a brightness quality (white or gray), and if mixed in any other proportion nn unsaturated color of the tone of the stronger
