THE ECLIPSE OF 1871. 121 



that the blue light does not emanate from the whole 

 of the inner corona, since the ill-defined nature of the 

 image affords reason for believing that its observed 

 extension was merely a question of eyesight. 



We have then and the result cannot but be re- 

 garded as one of the most important ever established 

 during eclipses the conclusion that surrounding the 

 sun to a depth of nearly two hundred thousand miles, 

 there is an envelope of hydrogen mixed with an element 

 capable of emitting the green light so often referred to 

 in the above description. 



But I am led to pause here, in order to inquire 

 what element it is which supplies the green light. 

 Now here we have a most interesting question to 

 consider. For the light of our own auroras shows 

 this very green line. Professor Young has tested the 

 matter in a way which prevents all possibility of doubt. 

 Using a spectroscope of almost unmatched power, he 

 could recognise no difference of position between the 

 green line of the aurora, the green line of the inner 

 corona, and a green line seen always in the spectrum 

 of iron. But of all elements in the universe iron seems 

 to be precisely the element which ought not to be 

 present, either in the regions whence comes the light 

 of our auroras, or in the inner corona of the sun. Iron 

 in the solid state might indeed be present from time 

 to time in the upper regions of our air, because iron is 

 nearly always present in meteorites, and meteorites are 

 always passing through the upper regions of the air in 

 greater or less numbers. But the green line, if it in 



