the Constitution of the Sun, 93 



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. 



4101-2 is a very hazy line, so represented by Angstrom ; but at 

 limb it 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 observed 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 ex- 

 hibits a very strong general absorption, with a very slightly 

 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 

 examination has persuaded me that the spectrum of a spot 

 differs from that of the unbroken photosphere just as the 

 spectrum of the limb differs 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 strength- 

 ened at limb by an examination of the spot-spectrum, 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 same nature as those producing the Fraunhofer lines. 

 Familiar experiments teach, however, that as the density of a 

 gas increases, the change in the character 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 sharpness. 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 terrestrial sources. Of these, Prof. Young has 

 discovered E 1; b 3 , and b i} with others. 



My own observations can be arranged very simply in classes, 



and will then better lend themselves to theoretical discussion. 



I. The most important fact of all is that the differences in 



the two spectra of centre and limb are extremely minute, 



