NO. 1865 OUR knowledge; of the sun — HALE 357 



has recently been confirmed on Mount Wilson. Ha, therefore, is 

 the exceptional line, as its spectroscopic peculiarities also indicate. 

 We are at once reminded of the remarkable behavior of the hydro- 

 gen lines in the Wolf-Rayet stars, where Ha is sometimes bright 

 and the other hydrogen lines invisible or dark. Kayser has ex- 

 plained this condition of things by a simple application of the law 

 of radiation and absorption. But in the well-known variable star 

 Ceti, and others of its type, Ha and H^ are invisible, while Hy 

 and //8, and the more refrangible hydrogen lines, are bright. In 

 R Androinedce H^ is the chief bright line, while Ha is absent. More- 

 over, the bright line spectra of the nebulae contain H/3 and Hy, but 

 Ha, when visible at all, is very faint. Finally, such stars as y Cas- 

 siopcice show Ha and the other hydrogen lines with the same rela- 

 tive brightness they exhibit in a hydrogen tube. 



As the relative temperatures of the radiating and absorbing gases 

 may play a dominant part in determining the character of the spec- 

 tral lines, and therefore the appearance of the flocculi, the question 

 of their level in the solar atmosphere assumes greater importance 

 than ever. An attempt to photograph prominences at the Sun's 

 limb with the Ha line met with instant success, and brought out a 

 most interesting fact : a large prominence appeared at exactly the 

 point where a dark Ha flocculus was being carried over the limb by 

 the Sun's rotation. As the structure of the prominence closely re- 

 sembles that of the flocculus, it is very probable that the latter was 

 simply the prominence seen in projection on the disk, its darkness 

 being due to the fact that the temperature of the gas was low 

 enough to produce perceptible absorption. Most of the H8 image 

 of the prominence was very weak on the photograph, and thus the 

 absence of a corresponding dark H8 flocculus is readily accounted 

 for. Furthermore, a portion of the H8 prominence, which was as 

 bright as the corresponding portion of the Ha prominence, is clearly 

 shown as a dark flocculus on the H8 image of the disk. Hereafter 

 the Ha prominences, as well as the Ha flocculi, will be photographed 

 daily for comparison.^ 



' For an account of the discovery of vortices and magnetic fields associated 

 with Sun-spots, which resulted from work with the //« line soon after this 

 lecture was delivered, see Hale, "Solar Vortices," Contributions from the 

 Mount Wilson Solar Observatory. No. 26, Astrophysical Journal, September, 

 1908, and Hale. "On the Probable Existence of a Magnetic Field in Sun-spots," 

 Contributions from the Mount Wilson Solar Observatory, No. 30, Astro- 

 physical Journal, November, 1908. 



