484 THE POPULAR SCIENCE MONTHLY. 



tances all constituted so many riddles for astronomers till 1868. In 

 that year one of the great eclipses of the sun took place. We might 

 say that, at the very moment when the heavens had just suffered their 

 most precious secrets to be revealed, the star of day had deigned to 

 invite us to the study of his admirable structure. 



The eclipse was observed, and the result surpassed even the general 

 expectation. The nature of the protuberances was immediately recog- 

 nized, and a method was discovered that permitted the study of these 

 phenomena every day, without having to wait for the rare occasions 

 of eclipses. This method led in a short time to the discovery of the 

 chromospheric atmosphere, and this completed and explained the phe- 

 nomena of the protuberances. The first results of the spectroscopic 

 investigations may be stated thus : 



The sun of Herschel and Arago, formed of a central nucleus and 

 a luminous envelope, the photosphere, has an additional stratum formed 

 chiefly of incandescent hydrogen. This stratum, in immediate contact 

 with the photosphere, is very thin, being only from eight to ten sec- 

 onds thick ; it is the seat of small eruptions of metallic vapors rising 

 from the photosphere, in which sodium, magnesium, and calcium pre- 

 dominate. Frequently, however, principally at the time when the 

 sun-spots become abundant, there rise from the solar globe formidable 

 eruptions of hydrogen, which pass through this same envelope and rise 

 to a height of sixty thousand or ninety thousand miles. These erup- 

 tions are the protuberances of the total eclipses, the nature of which is 

 thus revealed and the forms explained. 



The corona and the phenomena exterior to it were the objects of 

 study in the next eclipses. In 1874 French observations showed that 

 the corona constituted a new solar atmosphere, a very rare one and 

 enormously extended, in which hydrogen still dominated, while it pre- 

 sented spectral conditions as yet unexplained. This atmosphere seemed 

 to borrow a part of the appearances of the protuberance-eruptions 

 which penetrate it and are extinguished in it. It also seemed prob- 

 able, and that opinion was expressed by the author of these observa- 

 tions, that the figure of the corona would vary with the condition 

 of external activity of the sun. At the times of the maximum of 

 spots, when the protuberance-eruptions were in the highest activity, 

 the coronal atmosphere would be intersected by numerous and rich jets 

 which would increase its extent and density, and change its aspect. 

 This opinion was confirmed by one of the observers of the last eclipse 

 in Egypt. 



I shall conclude this brief review of the methods of physical astron- 

 omy with a word upon an art which has recently brought a really 

 wonderful aid to all our scientific studies I mean photography. Con- 

 sidered in its old and primary object, the aim of photography is to fix 

 the images of the camera-obscura. Its aim, however, and its means 

 have been singularly extended. We have to consider here only 



