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



narrowed, or nearly obliterated in the spot spectrum by superposed metallic lines. 

 In the case of magnesium hydride, the gaps are not well displayed as such in the 

 photograph reproduced in Plate 13, but they form a prominent feature in the photo- 

 graph of the green fluting on Plate 12A, No. 3, which was given a relatively much longer 

 exposure. Among the many gaps common to the two spectra are 5180'9, 5179'4, 

 5177'2, and 5176'6. It follows, then, that such bright places in the spot spectrum 

 cannot be bright lines superposed on a truly continuous dark background, but simply 

 represent wave-lengths for which magnesium hydride is most transparent. Further 

 investigation is necessary to determine whether the possibly narrower gaps seen with 

 the most powerful instruments are explained in the same manner. 



It may be remarked that magnesium hydride is not the only probable compound 

 which is represented in the spot spectrum. HALE and ADAMS* have shown that the 

 flutings which I identified in the Third-type stars and have attributed to titanium 

 oxidef are a very prominent feature in the red region, and two additional flutings at 

 6382 and 6389 have been traced to calcium hydride (or, at least, to calcium in the 

 presence of hydrogen) by OLMSTED at the Mount Wilson laboratory.} The discovery 

 of these fluted spectra favours the view that spots are regions of local cooling, which 

 was previously suggested by the behaviour of different classes of metallic lines. 



It is probable also that the combined absorption of the thousands of lines belonging 

 to these spectra, extending as they do from the extreme red to the far ultra-violet, 

 contributes materially to the darkness of sun-spots, and absorption corresponding to 

 the "dark ground" occurring in negatives of these spectra will doubtless aid the 

 process of darkening. It cannot be concluded, however, that the absorption of the 

 substances now known to be present in spots is adequate to account entirely for the 

 darkening, as the spot spectrum is darker than the photospheric spectrum in places 

 where the known band spectra are feeble, as, for example, to the red side of the green 

 fluting of magnesium hydride at 5211, and in the ultra-violet region. 



A further contribution to the darkness might be made by the scattering of the 

 photospheric light in its passage through the vapours of the spot, or by reduced 

 emission from the underlying photosphere. These, however, would seem to be 

 inadmissible if the bright interruptions of the spot spectrum be really as bright as 

 the corresponding places in the ordinary photospheric spectrum. In visual obser- 

 vations the interruptions certainly appear to be of the same order of brightness as 

 the adjacent photospheric spectrum, but it is difficult to allow for contrast effects, 

 and I am glad to learn fcbat this point is under investigation by photographic methods 

 at Kodaikanal. 



* ' Astrophys. Jour.,' vol. 25, p. 76 (1907). 



t 'Roy. Soc. Proc.,'vol. 73, p. 219 (1904); vol. 79, p. 509 (1907). 'Monthly Notices, R.A.S.,' vol. 69, p. 508. 

 | ' Astrophys. Jour.,' vol. 27, p. 66 (1908). 



FOWLER, 'Trans. Internl. Union Sol. Res.,' vol. 1, p. 228 (1906); HALE, ADAMS, and GALE, 

 'Astrophys. Jour.,' vol. 24, p. 185 (1906). 



