

IX.] 



THE CHROMOSPHERE. 



Ill 



from the fainter outer atmosphere 

 as seen in eclipses on the one 

 hand, and the white photosphere 

 on the other. 



It was soon found that this 

 continuous ocean, this continuous 

 outer shell of the sun, varied 

 considerably in thickness from 

 time to time, and it was also 

 found that the lines seen in the 

 spectra of other substances be- 

 sides hydrogen, some of which at 

 present we know nothing of, 

 others of which we now think we 

 know a great deal of, also appeared 

 side by side with the lines of 

 hydrogen. Sometimes the spec- 

 trum of the chromosphere is full 

 of lines. In almost all cases, how- 

 ever, we find that these lines are 

 never so long as the hydrogen 

 line, from which we gather that 

 the depth of the layer of the 

 solar atmosphere measured for us 

 by the length of the magnesium 

 lines, to take a case, is much less 

 than that of the hydrogen one. 

 I should further add here that 

 when the sun is moderately active 

 and can be well observed, as in a 

 fine climate like that in Italy, 

 this magnesium layer can be de- 

 tected all round the sun, so that 

 we have in the chromospheric 



