120 



Dr. W. Huggins. 



especially between photographs taken during the same eclipse at 

 places many hundreds of miles apart. 



At the same time a very small part of the light which is seen 

 about the eclipsed sun must be due to diffusion by our atmo- 

 sphere of the coronal light itself, especially of the very bright part 

 near the sun's limb ; and we have an indication of the amount of this 

 diffused light from the apparent illumination of the dark moon, 

 where the effects of diffusion will be most strongly present. During 

 some eclipses the part of the sky where the sun and moon are may 

 be faintly illuminated by light reflected from those regions of the 

 atmosphere near the horizon which are still in direct sunlight. 



It may be well to mention the principal hypotheses which have 

 been suggested in explanation of the corona. 



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, or received 

 by it, and which 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 ring 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 of the debris of the 

 tails of comets. 



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

 meteoric 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 on 

 the ground that there must be such streams crowding richly together 

 in the sun's neighbourhood. 



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

 his solar theory.* 



The sun must be surrounded by a true gaseous atmosphere of 

 relatively limited extent, but there are several considerations which 

 forbid us to think of a solar atmosphere, in the proper sense of the 

 term, that is of a continuous mass of gas held up by its own elasti- 



* Since this lecture was read my attention has been called to the papers by Pro- 

 fessor O. Reynolds, "On the Tails of Comets, the Solar Corona, and the Aurora 

 considered as Electric Phenomena," and "On an Electrical Corona resembling the 

 Solar Corona," in vol. v, 3rd Ser., "Lit. and Phil. Soc," Manchester, pp. 44^56, and 

 pp. 202-209. Professor Reynolds considers the solar corona to be a species of that 

 action known as the electric brusb, and to be well represented by discharging elec- 

 tricity from a brass ball in a partially exhausted receiver. 



