462 MR. A. FOWLER ON THE SPECTRUM OF MAGNESIUM HYDRIDE. 



(unless at special times and places}* for its production. If it be not due to a compound 

 of magnesium with hydrogen, at any rate it occurs with special facility in the presence 

 of hydrogen, and ought to occur in the sun if the temperature were not too high." 



There is no evidence of the presence of the fluting as a whole in the chromospheric 

 spectrum, and as the first head line at 521 I'll does not agree accurately with either 

 of YOUNG'S lines near its position, it must be concluded that magnesium hydride does 

 not appreciably contribute to the spectrum of the chromosphere. In the sun-spot 

 spectrum, however, the green fluting of magnesium hydride is strongly marked, and 

 at least the stronger components of the yellow-green and blue flutings are also easily 

 traced in the Mount Wilson and Kodaikanal photographs. In the spot spectrum the 

 lines are somewhat hazy, and belong to the class called " band lines " or " umbra lines " ; 

 in the regions where they occur they appear to be the more conspicuous of the 

 multitude of lines into which the dark background of the spot spectrum is resolved 

 when high dispersion is employed, t 



Complete series of measurements of the band lines of the spot spectrum are not yet 

 available. A few of them have been noted from visual observations by MAUNDER, 

 MITCHELL, and myself, and 127 were tabulated between 5212 and 5032 by HALE and 

 ADAMS from the earlier photographs of the spot spectrum^ taken at Mount Wilson. 

 Even the latter catalogue, however, is far from complete, although it contributed 

 largely to the first identification of magnesium hydride in the spot spectrum. Under 

 these circumstances, an exhaustive comparison of the two spectra cannot yet be given, 

 and indeed such a comparison is more appropriate to a detailed discussion of the sun- 

 spot spectrum than to the present paper. 



A convincing demonstration of the presence of magnesium hydride in spots, however, 

 is afforded by the photographs reproduced in Plate 13, showing the spectra in the 

 region 5212-5062. The photographs of the sun and sun-spot spectra were taken by 

 Mr. EVERSHED at the Solar Observatory, Kodaikanal, India, and were kindly placed 

 at my disposal by the Director, Mr. C. MICHIE SMITH ; the exposure given to the 

 sun-spot was about six times as long as that for the sun. The magnesium hydride 

 photograph was taken in the second order of the 10-feet concave grating, and is 

 presented as a negative in order to represent an absorption spectrum. It will be seen 

 that, apart from the interference caused by metallic lines, all the details of magnesium 

 hydride are reproduced in the spot spectrum. 



Effective comparisons of the spot with the yellow -green and blue flutings cannot 



* The italics are mine. A. F. 



t YOUNG, 'Amer. Jour, of Sci.,' 3rd series, vol. 26, p. 333 (1883). A useful summary of subsequent 

 observations is given by HALE and ADAMS in ' Astrophys. Jour.,' vol. 23, p. 30 (1906). 



t 'Astrophys. Jour.,' vol. 23, p. 36 (1906). 



Lines at 5169-96 and 5169-42 in the photograph of magnesium hydride, which do not appear in the 

 spot spectrum, are "ghosts" of ij and b- 2 . Certain other lines in this photograph, as 5197-57 and 5187 -26, 

 are relatively too strong in consequence of ghosts which are nearly coincident with them. 



