VISUAL SENSATION (Lai. visualis, relat ing to sight, from visits, sight, from riders, to see). Stimulation of the retina by ether waves gives rise to sensations of color and to colorless sensations (black, white, and gray—called sensa tions of brightness quality by Hering). There arc three chief characteristics of light sensa tions—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) rolor-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 Ap., .(700 millionths of a millimeter) ; the shortest corresponds to violet), a length of about 400 p.)4. Intermediate wave-lengths condition the color qualities orange, yellow, green, blue, etc. (2) Brightness corresponds to the ampli tude of the ether wave. A stimulus of wide am plitude gives rise to a sensation of great bright ness, i.e. a brightness that approaches white. Illack, which is as positive a sensation quality as red or white, is correlated II a stimulus of small amplithde or with the total absence of peripheral stimidation (as in a completely dark ened room). (3) ,Saturation depends upon the purity or homogeneity of the stimulus. A light of a single wave-length produces a sensation of a high degree of saturation: a light which includes ninny or all wave-lengths of the physical spec trum, a sensation of a low degree of saturation (as a yellowish or reddish white) or one whose saturation is zero (black, white, or gray). While this correlation of physical and psychical mo ments is a general one, it has, nevertheless, sev eral exceptions and modifications, the most im portant of which are considered below.
The colorless sensations (brightness qualities: Hering) form a one-dimensional system running through the grays from light to dark, with white at one end of the series and black at the other. The series may be represented by a ,straight line, at any point of which there is a brightness quality shading gradually on one side toward black, and on the other toward white. The
place in the series of any given brightness is determined, theoretically, by the number of just noticeably different qualities between it and one end or other of the series. or practically by the position of the qualities which most resemble it on either side. The capacity for discriminating these qualities is known as sensible discrimina tion (q.v.) for brightness. It has been found that for moderate brightnesses the relative dif ference limen, i.e. the ratio of a just noticeable difference to the absolute brightness value with which the observation is made, is practically constant at about 100 ; e.g. a gray of 100 photo metric units is just noticeably different from one of 101 units. At the two extremes of the bright ness series—the dead blacks and the brilliant whites—discriminability is much less. (See IN TENSITY OF SENSATION.) It has been estimated. on the basis of experimental work, that there are almost 800 different brightnesses.
The series of color-tones runs through the spectrum from red to violet and embraces also the purples. They may be conceived as lying around the periphery of a quadrilateral figure with red, yellow, green, blue at the four corners and the intermediates—oranges, yellow-greens, green-blues, purples, ete.—Iying along the sides. A quadrilateral form is chosen instead of a circle because there are distinctive changes at the four points indicated—more or less abrupt changes in 'direction.' The figure also provides for the fact that the ends of the spectrum re semble each other more than either end and the middle. Just noticeable differences have been worked out for color-tone as well as for brightness. Qualitative changes are ninth more rapid in some parts of the spectrum than in others. Sensible discrimination is greatest in yellow and blue green and least in red and violet. In yellow and blue-green one can, under favorable conditions, detect a change in color quality in a difference in wavelengths of less than mm.