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 which determines their degree of difference from a gray of the same brilliance. Synonymous terms, as used by various writers, are “purity” (Rood, ', 32; Nutting, ', 139; Abney, ', 4) and “chroma” (Munsell, ', 12-14; Titchener, , 62-63).

(d) Auxiliary Terms: The term chromaticity may be used to characterize a color qualitatively without reference to its brilliance. Chromaticity is determined by hue and saturation together, a gray being specified by the statement that it has no chromaticity.

(e) Interdependence of the Attributes: All colors except absolute black exhibit brilliance, but grays have zero saturation, and hence no hue. All colors which exhibit a hue must also exhibit saturation, and vice versa.

(f) Species of Colors. Colors can be classified into chromatic and achromatic species, according as they do or do not exhibit hue, respectively. The former may be designated briefly as chromas (including colors of all finite degrees of saturation) and the latter as grays (including black and white).

Median gray (=“mid-gray”) is the middle member of a series of grays in which each member differs from its immediate neighbors by the least perceptible difference, and of which black and white are the terminal members. This gray furnishes the most practicable reference point for the achromatic as well as for the chromatic series of colors.

Median colors are all colors equivalent in brilliance to median gray, including the latter.

Tints and shades are colors, including grays and chromas, which are respectively lighter or darker than median gray.

D. .—The psychologically primary colors are those which are necessary and sufficient, in minimum number, for the description of all colors by introspective analysis.