TWO YEARS LATER. 



39 



surface, under such an atmosphere as we have been 

 supposing, would suffice to liquefy if not to solidify the 

 most subtle gases we are acquainted with. 



There cannot be any question therefore that the 

 spectroscopic observation of the sun has sufficed to throw 

 very great doubt indeed upon the theory that the corona 

 is due to a solar atmosphere. Or rather, we may fairly 

 say that that theory has been distinctly shown by 

 Dr. Frankland's laboratory researches to be untenable. 



*But then there remains the difficulty of explaining 

 what the corona really is. We know that it cannot be 

 a lunar atmosphere, because a number of very exact 

 observations have shown, beyond all possibility of 

 question, that the moon has no atmosphere of appre- 

 ciable extent, far less such an atmosphere as would 

 account for the corona. Again, the theory which was 

 put forward by De Lisle in the seventeenth century, 

 that the corona is caused by the diffraction of the sun's 

 rays as they pass by the moon, has been disproved by 

 the inquiries of Sir David Brewster. 



There is indeed another theory, which has 

 strangely enough been exhumed quite recently. Ac- 

 cording to this theory the solar corona is simply a 

 phenomenon belonging to our own atmosphere. The 

 theory was first mentioned though only to be sum- 

 marily rejected by Halley, and touched on somewhat 

 contemptuously by other astronomers. It explains the 

 corona as due to the illumination of the upper regions 

 of the air by the sun's rays. We know that if we hide 

 the sun with a globe or disc of any sort, a strong light 



