XII. Researches towards establishing a theory of the dispersion of light

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Abstract
The phenomena of prismatic dispersion, as originally discovered by Newton, and since examined throughout a vast range of transparent bodies by succeeding philo­sophers, especially Sir David Brewster, were principally considered with reference only to successive parts, or spaces, of the coloured spectrum, designated generally as the red, or violet, or mean rays. The increasing precision of modern science has been evinced in the elaborate and justly celebrated researches of M. Fraunhofer, who, availing himself of the dark and bright lines to mark and designate distinct points of the spectrum, by prismatic ob­servations for ten different media, solid and liquid, has determined in each the re­fractive indices for seven principal rays, thus always absolutely identifiable. We will use the term “definite rays” to signify the specific parts of the spectrum thus defined. As to the law of the phenomena, the first notion of a simple proportionality was soon disproved. The refrangibility was seen to vary considerably and irregularly for each ray and each medium ; and when Fraunhofer had assigned serieses of numbers as the accurate expressions of the varying refractive powers throughout the several spectra, the apparent absence of any law connecting these numbers was only ren­dered more palpable. All that could be said was, that the numbers increased from the red to the blue end of the scale, and in a different way in each medium.

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