On the Corona of the Sun. 



133 



is of the same nature throughout, but that there is often so much 

 more matter, in other words, the coronal fog is so much denser, within 

 5' to 10' of the limb, that under the effects of projection, and when seen 

 by the eye or with a very low power, this part of the corona appears 

 to be marked off from the corona beyond. It is possible that a clue 

 to the real state of things may be found in the photographs of the corona 

 of 1878. When these are examined the long equatorial rays seen by 

 eye, can be traced a little distance beyond the bright corona, but it is 

 found that the corona, as a whole, is not drawn out at this part, so as 

 to extend to several solar diameters, but that these great extensions 

 consist of rays or streamers coming out from the general coronal 

 mass, something in the way in which fainter rays often extend from 

 the principal tail of a comet. They may be due to a similar cause, 

 namely, the electric repulsion acting upon particles which are more 

 completely under its sway, either from their less specific gravity, or 

 their more highly electrified condition. The consideration presents 

 itself, whether in this state of things, we have onlj' an extreme case 

 of the conditions always present in the corona, which gives rise to the 

 appearances which have suggested the distinction of an " inner 

 corona " and an " outer corona." 



There is another question which awaits consideration, namely, 

 whether the corona rotates with the sun. It seems obvious that if the 

 corona is due to a supply of matter and to forces coming from the sun, 

 then the coronal structure and the degree of extension, which are pro- 

 duced by them, at any part of the sun, would continue to be produced 

 by these agencies at that part of the sun, and in that sense the corona 

 would rotate. In the case of the more distant and diffused parts, the 

 rotation could scarcely be of one and the same material object, any 

 more than in the sweep of a comet's tail at perihelion, the corona 

 being constantly renewed and reformed over each part of the solar 

 surface. If we suppose the corona to come under the influence of an 

 external force as that of a planet, then we should expect the ray 

 drawn out towards it, or the rift formed opposite to it, to continue to 

 be directed to this external object, and to be independent of the solar 

 rotation. The subpermanence of any great coronal form, therefore, 

 would probably have to be explained by the maintenance for some 

 time of the conditions upon which the form depends, and not by an 

 unaltered identity of the coronal matter ; as in the case of a cloud 

 over a mountain top, or of a flame over the mouth of a volcano. 



We have to consider another question : What becomes of the coronal 

 particles ? Are they carried away from the sun, as the matter of the 

 tails of comets is lost to them ; or do they return to the sun ? 



The results of eye observations, as well as of the taking of photo- 

 graphs with different exposures, have shown that there is great pro- 

 bability that the corona has not an outer boundary, but that it is lost 



