Spectrum

(spěk'trùm.) pl. SPECTRA. [L., a something seen.] (Opt.) If a pencil of solar light, SA (Fig. 605), is allowed to pa through a small aperture in the window shutter of a dark chamber, the pencil tends to form a round and colorle image of the sun at K; but if a flint-gla prism, arranged horizontally, be interposed in its pa age, the beam, on emerging from the prism, becomes refracted towards its base, and produces on a distant screen a vertical band, colored in all the tints of the rainbow, which is called the solar spectrum. In this spectrum there is, in reality, an infinity of different tints, which imperceptibly merge into each other, but it is customary to distinguish seven principal colors. These are violet, indigo, blue, green, yellow, orange, red; they are arranged in this order in the spectrum, the violet being the most refrangible, and the red the least so. They do not all occupy an equal extent in the spectrum, violet having the greatest extent, and orange the least. This experiment proves that common light is not homogeneous, but compounded of rays of various colors, which become separated by reason of their different refrangibilities. By means of a suitable contriv ance, the colored rays obtained from a sunbeam may be again collected into a beam of white light. If any of the colors of the S. be experimented on, it will be found that it cannot be decomposed; in other words, the original beam has been divided into homogeneous rays. [s. 844]