XVII. The spectrum of magnesium hydride

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Abstract
In a paper communicated to the Royal Astronomical Society in June, 1907, I gave a preliminary account of my identification of a number of hazy “band lines” in the spectra of sun-spots with lines composing the green fluting in the spectrum of magnesium hydride. Previous observations had suggested that the origin of these lines was most likely to be traced by the examination of fluted spectra, but numerous measurements of the flutings of substances such as vanadium and titanium, the lines of which are specially prominent in the spot spectrum, gave only negative results. In December, 1906, however, Mr. Newall published an account of some observations of sun-spots in which reference was made to a probable dark fluting beginning at 5210.2 or 5211.0, and this observation recalled the principal fluting of magnesium hydride with which I had long been familiar. No sufficiently precise measurements of the components of the fluting were then available for a satisfactory comparison, but new determinations of wave-lengths, based on a large-scale photograph taken for the purpose, left no doubt as to the identity of the stronger components with the principal band lines of the spots in the region considered. It seemed probable that several hundreds of these band lines would eventually be accounted for by magnesium hydride, and that the problem presented by the highly complex spot spectrum would be greatly simplified if the part due to the flutings could be completely eliminated. The present paper gives an account of a more complete investigation of the magnesium hydride spectrum, and includes a table of wave-lengths for the three principal flutings, which it is believed will be of service to those who are engaged in the reduction of photographs of the sun-spot spectrum.

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