DISPERSION

In Optics, a term used to denote the angular separation of the constituent rays of light when decomposed by the prism. In consequence of the unequal refrangibility of the different rays, a beam of light admitted through a small hole or slit in the shutter of a darkened room, and refracted by pa ing through a prism, forms an elongated image or spectrum; the red rays, which are the least refracted, occupying one end of the spectrum, and the violet rays, whicli suffer the greatest refraction, occupying the other end. The rays therefore, after refraction, are no longer parallel; so that the index of refraction (or the ratio of the sine of incidence to the sine of refraction) is different for each ray; and the difference of the indices for the extreme rays is called the dispersioji of the light. Sir Isaac Newton, who first examined the prismatic colours, was led by some imperfect experiments to suppose the dispersion proportional to the refraction; but it was soon discovered that although the colours in spectra formed by prisms of different substances are always arranged in the same order, they do not occupy the samp relative lengths, — a prism of flint gla , for example, giving proportionally le red and more violet than a prism of crown gla ; and that substances, for which the index of refraction of the middle ray of the spectrum is nearly the same, produce spectra oi different lengths, or different amounts of dispersion. It is on this property 351 DISSENTERS. namely, the Irrationality of the refractive and dipersive powers of different substances, that the methods of forming achromatic lenses depend: had the supposition of Newton been correct, it would have been impo ible to produce an image by refraction unaccompanied by thf* prismatic colours. See Achromatism. The difference between the indices of refraction of the extreme rays of the spectrum formed by a prism of any substance is called the coefficient of dispersion with respect to that substance, or simply the dispersion; and the dispersive power is the quotient which is obtained by dividing the coefficient of dispersion by the mean index of refraction diminished by unity. The mean index is that of the ray which corresponds to the middle of the spectrum. As these terms are of frequent occurrence in scientific works, we shall illustrate the definitions by an example. According to Sir D.Brewster, the index of refraction of diamond for the extreme violet ray is 2*467, and for the extreme red ray 2"411; the difference of these two indices is -056, which, therefore, is the coefficient of dispersion for diamond. Again, the mean index, or mean of the above two numbers, is 2-439, which diminished by unity becomes 1-439; therefore the dispersive power of diamond is -05G divided by 1-439, or -0388. For a table of the dispersive powers of a great number of different substances, see Brewster's Optics, in the Cabinet Cyclopedia; but we may remark, that as this table appears to have been constructed before the discovery of Fraunhofer's method of determining the indices or refraction by means of the dark rays in the spectrum, it rests on data subject to great uncertainty. See Refraction; also Light, Optics, and Spectrum, [s. 364]