142 PROCEEDINGS OF THE AMERICAN ACADEMY 



4919 ±, a faint line slightly stronger at limb. 



(F) 4860.6 is much cleaner, more free from haze at limb. 



4702.3 seems cleaner at limb. 

 4340.0 cleaner at limb. 



4226.4 shows less haze at limb. 



o 



4101.2 is a very hazy line, so represented by Angstrom, but at limb 

 i is practically free from haze, — a striking difference. 



4045 is slightly less hazy at limb. 



Other differences have been recorded, but only these have been ob- 

 served more than once each. 



Any theory of the sun, worthy of attention, must not only explain 

 the above-described phenomena, but also others better known, and as 

 yet not accounted for satisfactorily. Of these the most noteworthy is 

 the spectroscopic appearance of a spot and its penumbra. As is well 

 known, such a spectrum exhibits a very strong general absorption, 

 with a very sliglitly modified elective absorption. A few faint lines 

 appear in the spot spectrum which are not otherwise seen ; and a few 

 faint lines of the ordinary spectrum are strengthened. A careful ex- 

 amination has persuaded me that the spectrum of a spot differs from 

 that of the unbroken photosphere, just as the spectrum of the limb dif- 

 fers from that of the centre of the disk, save that the variations are 

 more pronounced. Indeed, I could have considerably extended the 

 list of lines strengthened at liml< by an examination of the spot spec- 

 trum, where the variations appeal to the eye more clearly. 



The accepted theory of the spots attributes the phenomenon to the 

 absorption of the solar light by cooler, denser gases of the sarae nature 

 as those producing the Fraunhofer lines. Familiar experiments teach, 

 however, that, as the density of a gas increases, the change in the char- 

 acter of its radiation is shown in its spectrum by the broadening of 

 its distinctive spectral lines, which at the same time grow more ill 

 defined. Therefore it follows that, according to the law connecting 

 radiation and absorption, dark lines produced by such a gas must also, 

 under similar conditions, show increased breadth and diminished sharp- 

 ness. That no such changes are to be recognized, is a fatal objection 

 to the theory. 



Another class of unexplained phenomena is the duplicity of certain 

 lines of the solar spectrum, lines which are single in the spectra of ter- 

 restrial sources. Of these Professor Young has discovered £^, b^, and 

 b^, with others. 



My own observations can be arranged very simply in classes, and 

 will then better lend themselves to theoretical discussion. 



