94 Mr. 0. S. Hastings's Theory of 



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 Fraun- 

 hofer lines fails to explain the phenomena as observed. That 

 is, if we suppose the photosphere, whether solid, liquid, gaseous, 

 or cloud-like, to yield a continuous 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 independently 

 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 maintaining 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 with 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 obser- 

 vation of Professor Young during the total eclipse of 1870, 

 when he saw appreciably all the Fraunhofer 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 atmo- 

 sphere (save for hydrogen and one or two other substances) 

 to a depth of not more than 2 // . Thus, singularly enough, 

 the very observation which led to the firmest belief among 

 spectroscopists in the correctness of KirchhofF's view, exposed 

 at the same time its most vulnerable point. 



Another theory of the solar constitution, that of Faye, 

 assigns a different seat to the stratum producing the Fraun- 

 hofer lines, namely the photosphere itself. Regarding the 

 principal radiation of the sun as coming from solid or liquid 

 particles floating in a gaseous medium, the cloud -like stratum 



* JJntersuchungen ilber das Sonnenspectrum, Berlin, 1862, pp. 14, 15. 



