ATMOSPHERE OF THE SUN — DESLANDRES. 343 



the necessity of studying and examining this with care. For 

 nearly 20 years I have studied the entire atmosphere of the sun, and 

 to-day I place before you the most recent results which have been 

 brought to light relative to the upper layers of the solar envelope 

 until recently unexplored. 



1. THE ATMOSPHERE SEEN DURING ECLIPSES NEAR THE EXTERIOR EDGE 



OF THE SUN. 



The atmosphere of the sun is first revealed to man about the edge 

 of the disk during total eclipses. It then forms a luminous ring 

 that stands out from the now dark background of the sky surround- 

 ing the lunar disk, equally black. Stretching out beyond the moon 

 and the solar edge, it consists of two distinct portions: One, the 

 narrow, brilliant, rose-colored chromosphere, with its prominences, 

 also rose colored; the other, the fainter and more extensive corona. 

 In what immediately follows we shall consider especially the chromo- 

 sphere and the prominences. This luminous ring, visible at eclipses, 

 is ordinarily hidden by the much more brilliant illumination of 

 our sky. The screen which masks it is luminous; in order to annul 

 this screen the English astronomer, Sir Norman Lockyer, in 1866, 

 was among the first to have recourse to the spectrum, supposing what 

 seemed probable, that the solar atmosphere is gaseous. This was 

 one of those strokes of genius that have since become so fruitful. 



The eclipse of 1868 showed, indeed, that the rose-colored promi- 

 nences are composed almost wholly of incandescent hydrogen which, 

 under the influence of the electric spark, emits radiations already 

 well known in the laboratory, and especially an intense red ray, 

 designated as Ha. After the eclipse, Janssen in the Indies and 

 Lockyer in England rediscovered the chromosphere and prominences 

 of the eclipse with the assistance of the spectroscope and this bright 

 red line. This result was justly received with enthusiasm, for this 

 method, at once simple and fertile, has now been employed for 40 

 years in daily observations of the chromosphere and the positions 

 and forms of the prominences. This study is even more captivat- 

 ing than that of the spots, for the prominences have the most varied 

 and rapidly changing forms. They appear at all latitudes and 

 follow the same 11-year period as the spots, although it is true the 

 duration of the maximum is longer. 



The spectroscopic study of the solar border, carried on at ordi- 

 nary times or, still better, at eclipses, has brought us knowledge not 

 only of the chemical composition of the chromosphere, but also the 

 minimum height to which each vapor extends as estimated by the 

 length of the corresponding line in the spectrum. 



Speaking generally, vapors of low atomic weight rise to the 

 greatest heights; such is the case with hydrogen and helium. With 



