122 



Dr. W. Huggins. 



(&.) A solar spectrum, which shows that the incandescent solid or 

 liquid matter of the corona reflects to us light from the photosphere. 



(c.) A spectrum of bright lines, which is relatively faint and varies 

 greatly at different eclipses. We shall consider this spectrum more 

 particularly further on ; it is sufficient at this part of the argument 

 that we speak of this spectrum so far only as it tells us of gaseous 

 matter which accompanies the solid or liquid matter- 

 It is scarcely necessary to say that solid or liquid matter can exist 

 in the corona only in the form of discrete particles of extreme 

 minuteness. 



The corona must, therefore, consist of a fog, in which the particles 

 are incandescent, and in which the gaseous matter does not form a 

 continuous atmosphere. Some of the considerations we have already 

 had before us, make it evident that this coronal fog, except very near 

 the sun, must be of a degree of tenuity surpassing any experience we 

 possess from terrestrial experiments. In order to give some definite- 

 ness to our conceptions, let us suppose a single minute liquid or solid 

 particle in each cubic mile. A fog even so extremely attenuated, or 

 even much more so, would probably be fully sufficient to give rise to 

 the corona, under the enormous radiation to which it is subjected. 



(2.) The next question we have to consider is whether the matter 

 of the corona is of solar origin, or whether it comes upon the sun 

 from without. 



Two external sources of the coronal matter have been suggested, 

 and are widely held, namely : — (a) meteroids, and (6) the lost matter 

 of the tails of comets. 



(aS) The solar system is crowded with meteoroids revolving in all 

 kinds of erratic orbits, and we know that the earth encounters many 

 thousands of them every hour, but the sun is in a different position 

 with regard to such of these bodies as belong to our system. In 

 order to fall into the sun, the planetary meteoroids would have to be 

 thrown into it, through some disturbance of their orbits, produced by 

 planetary attraction, or by collision with- "other streams* unless we 

 admit a slow retardation of velocity produced by a resisting 

 medium. . 



There may be meteoroids which fall directly into the sun from 

 space. Mr. Denning's recent observations seem to show that the 

 solar system encounters meteor streams which) may be moving with 

 great velocity through space.* 



Many of these bodies may fall into the sun, but we have no know- 

 ledge of conditions which would ensure so steady an inflow of 

 meteoroids as would be needed to maintain the observed extent of 

 the corona in a state of permanence about the sun. 



(6.) The other suggestion which has been made regards the corona 



* " Month. "Not. R. Ast. S.," vol. xly, p. 931 See also subsequent papers. 



