are three chief characteristics of light sensations— color -tone, brightness, saturation—which correspond respectively to three chief moments—wave-length, Stimulation of the retina by ether waves gives rise to sensations of color and to colorless sensations (black, white, and gray—called sensations of brightness quality by Hering). There are three chief characteristics of light sensations—color-tone, brightness, saturation—which correspond respectively to three chief moments—wave-length, amplitude, purity—in the physical vibrations set up in the ether. (1) Color-tone (as red, orange, yellow, blue) corresponds in the first instance to wave-length. The longest wave (answering to red) has a length of about 700 µµ, (700 millionths of a millimeter); the shortest (which corresponds to violet), a length of about 400 µµ. Hence, in making an exhaustive enumeration of visual sensations, we must include the brightnesses, the color-tones, and the various saturations. The total has been estimated at about 33,000 sensation qualities. The retina is not equally sensitive at all points to stimulation by ether waves. When the illumination 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 invisible at the fovea. It is carried out by the mixture of spectral lights, of colored shadows, or reflections; of pigments, by the irradiation of neighboring colored spots (as in a mosaic); and by the rapid alternation of stimuli by means of revolving disks. There are three chief laws of color-mixture. (1) The law of complementaries: “For every color there can be found another complementary or antagonistic color, which if mixed with it in the right proportion gives a brightness quality (white or gray), and if mixed in any other proportion an unsaturated color of the tone of the stronger   ​ component.” Some of the complementary pairs are red and verdigris, orange and greenish-blue, yellow and blue, green and purple. The result is a gray which matches either of the component grays. The fact that this law does not hold when the illumination is diminished (failure of Newton’s law of color mixtures)—that the mixture which contains green will then be far too bright—constitutes the phenomenon known as the ‘extended’ Purkinje phenomenon (i.e. for colorless light). There are two principal theories of visual sensation, two hypothetical statements of the physiological conditions which underlie the various phenomena of color-vision. They are known as the Young-Helmholtz and the Hering theories. The Young-Helmholtz theory sets out from the laws of color mixture as derived by Newton. It posits the existence, in the visual apparatus, of three kinds of nerve fibres—a red fibre, a green fibre, and a blue or violet fibre-each of which if it were excited alone, would give its particular sensation. These three colors, red, green, and blue, are chosen as the primary colors because they give, by mixture, all the various colors and brightnesses at a relatively high degree of saturation (although approximately the same result may be obtained with other combinations of three). Any light stimulus is said to excite all three kinds of nervous elements, but in varying proportions. Red, e.g. excites the red fibre maximally and the green and blue fibres minimally. The color seen under any kind of stimulation depends upon the proportion in which the three kinds of nervous apparatus are affected. Yellow, e.g. results from a strong excitation of the red and green fibres and a weak excitation of the blue fibre. White is due to the equal excitation of all three fibres. Fig . 2. The Young-Helmholtz Theory. The letters R, O, Y, etc., designate the spectral colors. The three curves show roughly the relative values of the excitatory process, 1, in the red-sensitive fibres, 2, in the green-sensitive, and 3, in the violet-sensitive. The Hering theory says that there are three ‘visual substances’ which undergo metabolic changes under the influence of light rays. The Hering theory meets the various classes of facts more easily and with less recourse to subsidiary hypotheses than does the Young-Helmholtz theory. The latter, indeed, would appear to have survived rather in virtue of the authority of Helmholtz than by its intrinsic merits. At the same time, it cannot be said that the Hering theory is wholly adequate to all the minute variations of visual phenomena. Numerous other theories have been devised with a view to meeting the outstanding difficulties of visual sensation. Thus the Franklin theory supposes that at an early period of development the eye is sensitive only to luminosity and not to color; at this period it possesses only a gray-perceiving substance, present in all parts of the retina, which is affected by all luminous vibrations, but most markedly by those corresponding to the middle of the spectrum; stimulation of this substance produces the sensation of white of all shades. In the course of development of the eye, this gray-perceiving material becomes differentiated in the cones first into two different substances, which are especially affected by the two halves, respectively, of the spectrum; this is the stage of development which corresponds to the yellow-blue color system of the partially color-blind, and also to that of the middle zone of the normal retina. In the final stage of development, the yellow constituent becomes again differentiated into two substances which respond respectively to red and to green light. The mixed colors are produced by   ​ simultaneous stimulation of two or more color substances in varying proportions. Ebbinghaus, Grundzüge der Psychologie (Leipzig, 1897); Wundt, Human and Animal Psychology (New York, 1894); Helmholtz, Physiologische Optik (Leipzig, 1896); Hering, Lehre vom Lichtsinne (Vienna, 1878); Titchener, Experimental Psychology (New York, 1901);