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XVII. The Spectrum of Magnesium Hydride. 



Bi/ A. FOWLER, A.R.C.S., F.R.A.S., Assistant Professor of Physics, Imperial College 



of Science and Technology, South Kensington. 



Communicated ly Prof. H. L. CALLENDAR, M.A., LL.D., F.R.S. 

 Received June 18, Read June 24, 1909. 



[PLATES 12, 13.] 



Introductory. 



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, t 

 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 52irO,| 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 



* ' Monthly Notices,' vol. 67, p. 530 (1907). 

 t 'Trans. Internl. Union Solar Research,' vol. 1, p. 217 (1906). 

 } ' Monthly Notices,' vol. 67, p. 170 (1906). 

 VOL. CCIX. A 457. 22.10.09 



