File:Dispersion of Light by Potassium Vapour (1910) (14766384472).jpg

Identifier: philtrans07330394 (find matches)
Title: Dispersion of Light by Potassium Vapour
Year: 1910 (1910s)
Authors: Bevan, P.
Subjects: Proceedings of the Royal Society of London Philosophical Transactions of the Royal Society
Publisher: Royal Society of London

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the Sellmeier formula representsthe facts. This may easily be observed by fixing the cross wires of thetelescope on this point for a low density of vapour, and then graduallyheating the tube to obtain denser vapour. The dispersion curve spreadsupwards and downwards, but the fine bright line between the absorptionlines always passes through the same point on the undeviated position of thespectrum. 220 Prof. P. V. Bevan. (June /, The curve of fig. 8 represents the dispersion in the neighbourhood of thered lines. It was obtained from a series of four photographs of the samekind as those of fig. 1. With the photograph of the dispersed linearspectrum was photographed the spectrum from a neon tube, and alsothe two potassium lines (absorption) at 7699 and 7666. The neon tube,in addition to the neon lines, showed some helium lines, of which 6678,7065, and 7282 proved useful in constructing a curve of wave-lengths. Theinstrument used was a simple spectrometer, with a flint-glass prism, so that
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7^ 73 74 75 76 7650 7700 78 Fig. 8.—Abscissa is wave-length; ordinate is deviation. the dispersion was small, and na great accuracy could be obtained in themeasurements. The actual measurements on a single photograph wereobtained by ruling fine lines parallel to the undeviated spectrum, and thenfinding the wave-length corresponding to the point at which the ruled linecut the dispersion curve. The deviations were thus fixed, and the wave-lengths corresponding to these were determined. The four photographs canbe regarded as representations of the dispersion curve on dijEferent scales—the photograph corresponding to less dense vapour is, of course, on a smaller 1910.) Dispersion of Light by Potassium Vapour, 221 scale than those corresponding to denser vapour, but it shows the dispersedlight nearer to the absorption lines. By drawing curves for each photograph,the relation of scale for the series can be found, and thus a factor obtainedfor each to reduce all to the same scale; The fa

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