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Pure color is seen in the spectrum of sunlight.

(87) The strongest sensation of color is gained in a darkened room, with a prism used to split a beam of sunlight into its various

wave lengths. Through a narrow slit there enters a straight pencil of light which we are accustomed to think of as white, although it is a bundle of variously colored rays (or waves of ether) whose union and balance is so perfect that no single ray predominates.

(88) Cover the narrow slit, and we are plunged in darkness. Admit the beam, and the eye feels a powerful contrast between the spot of light on the floor and its surrounding darkness. Place a triangular glass prism near the slit to intercept the beam of white light, and suddenly there appears on the opposite wall a band of brilliant colors. This delightful experiment rivets the eye by the beauty and purity of its hues. All other colors seem weak by comparison. Their weakness is due to impurity, for all pigments and dyes reflect portions of hues other than their dominant one, which tend to “gray” and diminish their chroma.

(89) But prismatic color is pure, or very nearly so, because the shape of the glass refracts each hue, and separates it by the length