OP ARTS AND SCIENCES. 143 



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

 spectra of centre and limb are extremely minute, escaping all but the 

 most perfect instruments, and all methods which do not place them in 

 close juxtaposition. 



II. Certain lines, the thickest and darkest in the spectrum, notably 

 those of hydrogen, magnesium, and sodium, which appear with haze on 

 either side, in the spectrum of the centre of the solar disk, are deprived 

 of this accompaniment in that of the limb. 



III. Certain very fine lines (four observed) are stronger at limb. 



IV. Other very fine lines (two or three observed) are stronger at 

 centre. 



The ordinarily accepted theory of the origin of the Fraunhofer lines 

 fiiWs to explain the phenomena as observed. That is, if we suppose the 

 photosphere, whether solid, liquid, gaseous, or cloud-like, to yield a con- 

 tinuous spectrum which is modified only by the selective absorption of 

 a surrounding atmosphere, then the absorption must be greater at the 

 limb than at the centre of the solar disk ; and this must be true inde- 

 pendently of the thickness of that atmosphere, as well as of the form, 

 rough or otherwise, of the surface of the photosphere. This evident 

 consequence, pointed out in the first place by Forbes, nearly half a 

 century ago, cannot be avoided. There is but one way of maintain- 

 ing the theory and escaping Forbes's conclusion already quoted, and 

 that the course pursued by Kirchhoff in the original statement of his 

 theory of the solar constitution,* namely, by assuming that the depth 

 of the reversing atmosphere is not small compared to the radius of the 

 sun. But innumerable observations during the score of years which 

 have lapsed since that time prove that such a reversing atmosphere 

 must be very thin. The famous observation of Professor Young dur- 

 ing the total eclipse of 1870, when he saw appreciably all the Fraun- 

 hofer lines reversed, has naturally been received as the strongest 

 confirmation of Kirchhoff's views as to the locus of the origin of the 

 dark lines. But this very observation restricts the effective atmos- 

 phere (save for hydrogen and one or two other substances) to a depth 

 of not more than 2". Thus, singularly enough, the very observa- 

 tion which led to the firmest belief among spectroscopists in the cor- 

 rectness of Kirchhoff's view exposed, at the same time, its most 

 vulnerable point. 



Another theory of the solar constitution, that of Faye, assigns a 

 diflerent seat to the stratum producing the Fraunhofer lines, namely, 



* Untersuchungen iiber das Sonnenspectrum, (Berlin, 1862,) pp. 14, 15. 



