110
determined; and the measure of the vis viva of the altered light, relatively to its standard intensity, is then taken as the luminosity of the color in question.
Maxwell Color Discs.—Discs having each a single color, and slit radially so that one may be made to lap over another to any desired extent. By rotating these on a spindle, the effect of combining certain colors in varying proportions can be studied.
Micron.—The millionth part of a metre, or 123400 of an English inch. The term has been formally adopted by the International Commission of Weights and Measures, representing the civilized nations of the world, and is adopted by all metrologists.
Orange.—A reddish yellow color, of which the orange is the type.
Vision, Persistence of.—The continuance of a visual impression upon the retina of the eye after the exciting cause is removed. The length of time varies with the intensity of the light and the excitability of the retina, and ordinarily is brief, though the duration may be for hours, or even days. The after image may be either positive or negative, the latter when the bright part appears dark and the colored parts in their corresponding contrast colors. It is because of this persistence that, for example, a firebrand moved very rapidly appears as a band or circle of light.
Photometer.—An instrument used to measure the intensity of light. Specifically, to compare the relative intensities of the light emitted from various sources.
Pigment.—Any substance that is or can be used by painters to impart color to bodies.
Pink.—A red color of low chroma, but high luminosity, inclining toward purple.
