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 1/23400 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. PRIMARY COLORS.--See Colors, primary. PURE COLOR.--A color produced by homogeneous light. Any very brilliant or decided color. PURPLE.--A color formed by the mixture of blue and red, including the violet of the spectrum above wave length 0.417, which is nearly a violet blue, and extending to, but not including, crimson. RAINBOW.--A bow or an arc of a circle, consisting of the prismatic colors, formed by the refraction and the reflection of rays of light from drops of rain or vapor, appearing in the part of the heavens opposite to the sun. RED.--A color more or less resembling that of blood, or the lower end of the spectrum. Red is one of the most general color names, and embraces colors ranging in hue from aniline to scarlet iodide of mercury and red lead. A red yellower than vermilion is called scarlet. One much more crimson is called crimson red. A very dark red, if pure or crimson, is called maroon; if brownish, chestnut or chocolate. A pale red--that is, one of low CHROMA and high LUMINOSITY--is called a pink, ranging from rose pink or pale crimson to salmon pink or pale scarlet. VENETIAN RED.--An important pigment used by artists, somewhat darker than brick red in color, and very permanent. RETINA.--The innermost and chiefly nervous coat of the posterior part of the eyeball.