39^> A Short History of Astronomy [CH. Xlll. 



Plutarch, and discussed by Kepler (chapter vn., 145) 

 Several i8th century astronomers noticed a red streak along 

 some portion of the common edge of the s-un and moon, 

 and red spots or clouds here and there (cf. chapter x., 205). 

 But little serious attention was given to the subject till after 

 the total solar eclipse of 1842. Observations made then 

 and at the two following eclipses of 1851 and 1860, in the 

 latter of which years photography was for the first time 

 effectively employed, made it evident that the red streak 

 represented a continuous envelope of some kind surrounding 

 the sun, to which the name of chromosphere has been given, 

 and that the red objects, generally known as prominences, 

 were in general projecting parts of the chromosphere, though 

 sometimes detached from it. At the eclipse of 1868 the 

 spectrum of the prominences and the chromosphere was 

 obtained, and found to be one of bright lines, shewing that 

 they consisted of gas. Immediately afterwards M. Janssen, 

 who was one of the observers of the eclipse, and Sir 

 J. Norman Lockyer independently devised a method 

 whereby it was possible to get the spectrum of a prominence 

 at the edge of the sun's disc in ordinary daylight, without 

 waiting for an eclipse ; and a modification introduced by 

 Sir William Huggins in the following year (1869) enabled 

 the form of a prominence to be observed spectroscopically. 

 Recently (1892) Professor G. E. Hale of Chicago has 

 succeeded in obtaining by a photographic process a repre- 

 sentation of the whole of the chromosphere and prominences, 

 while the same method gives also photographs of faculae 

 (chapter VIIL, 153) on the visible surface of the sun. 



The most important lines ordinarily present in the 

 spectrum of the chromosphere are those of hydrogen, two 

 lines (H and K) which have been identified with some 

 difficulty as belonging to calcium, and a yellow line the 

 substance producing which, known as helium, has only 

 recently (1895) been discovered on the earth. But the 

 chromosphere when disturbed and many of the prominences 

 give spectra containing a number of other lines. 



The corona was for some time regarded as of the nature 

 of an optical illusion produced in the atmosphere. That it 

 is, at any rate in great part, an actual appendage of the sun 

 was first established in 1869 by the American astronomers 



