ON THE SOLAR CORONA. 763 



and 1883 were projected on the screen. Attention was called to the 

 equatorial extension seen in the photograph taken in 1878, and to the 

 suggestion which had been put forward that this peculiar character 

 was connected with the then comparative state of inactivity of the 

 sun's surface, at a period of minimum sun-spot action, especially as an 

 equatorial extension was observed in 18G7.] 



It is now time that something should be said of the probable na- 

 ture of the corona. 



Six hypotheses have been suggested : 



1. That the corona consists of a gaseous atmosphere resting upon 

 the sun's surface and carried round with it. 



2. That the corona is made up, wholly or in part, of gaseous and 

 finely divided matter which has been ejected from the sun, and is in 

 motion about the sun from the forces of ejection, of the sun's rotation, 

 and of gravity and possibly of a repulsion of some kind. 



3. That the corona resembles the rings of Saturn, and consists of 

 swarms of meteoric particles revolving with sufficient velocity to pre- 

 vent their falling into the sun. 



4. That the corona is the appearance presented to us by the un- 

 ceasing falling into the sun of meteoric matter and the debris of 

 comets' tails. 



5. That the coronal rays and streamers are, at least in part, mete- 

 oric streams strongly illuminated by their near approach to the sun, 

 neither revolving about nor falling into the sun, but permanent in 

 position and varying only in richness of meteoric matter, which are 

 parts of eccentric comet orbits. This view has been supported by 

 Mr. Proctor, on the ground that there must be such streams crowding 

 richly together in the sun's neighborhood. 



6. The view of the corona suggested by Sir William Siemens in 

 his solar theorv. 



It has been suggested, even, that the corona is so complex a phe- 

 nomenon that there may be an element of truth in every one of these 

 hypotheses. Anyway, this enumeration of hypotheses, more or less 

 mutually destructive, shows how great is the difficulty of explaining 

 the appearances which present themselves at a total solar eclipse, and 

 how little we really know about the corona. 



An American philosopher, Professor Hastings, has revived a prior 

 and altogether revolutionary question : Has the corona an objective 

 existence ? Is it anything more than an optical appearance depending 

 upon diffraction ? Professor Hastings has based his revival of this 

 long-discarded negative theory upon the behavior of a coronal line 

 which he saw, in his spectroscope, change in length east and west of 

 the sun during the progress of the eclipse at Caroline Island. His 

 view appears to rest on the negative foundation that Fresnel's theory 

 of diffraction may not apply in the case of a total eclipse, and that 

 at such great distances there is a possibility that the interior of the 



