304 CAPTAIN L. DARWIN, DR. A. SCHUSTER, AND MR. E. W. MAUNDER 
two points which have a bearing on the speculations respecting the origin of the 
corona. 
It is generally accepted now, I believe, that the Sun cannot have an atmosphere of 
anything like the extent of this luminous appendage. The reasoning generally given, 
and which seems conclusive, is that the pressure on the Sun’s surface must, to judge 
from spectroscopic evidence, not exceed that on the surface of the Earth, and that an 
atmosphere comparable in size with the volume of the Sun would necessarily produce 
an enormous pressure. I do not know, however, whether attention has been drawn 
to the fact that, even if such a gaseous atmosphere existed, it could not be luminous 
to any great height unless it contained a great number of solid or liquid particles. 
The same laws of convective thermal equilibrium which regulate the decrease of tem¬ 
perature in the atmosphere act still more strongly on the surface of the Sun, and even 
taking the highest estimate which can reasonably be made of the Sun’s temperature, 
the gases rising in the Solar atmosphere would quickly fall in temperature below the 
point at which they can be luminous. The only way in which they can be kept 
luminous is by containing sufficient matter to absorb the radiation from the body of 
the Sun, or by some independent cause, such as friction between solid and gaseous 
matter, or electric discharges, 
If electric discharges are the cause of the luminosity, the matter through which the 
discharge takes place must either be supplied by the Sun or by something outside, 
such as streams of meteoric matter. The latter hypothesis has the great advantage 
that it may possibly account for the periodicity of Sun-spot phenomena, and it 
deserves, therefore, the attention of scientific men. Now, it occurred to me several 
years ago that if meteoric streams fixed in space were in any way coimected with the 
corona we should find some evidenpe of them in the general shape as seen from the 
Earth. During eclipses which take place about the same periods of the year the 
Earth and the orbits of these hypothetical meteor streams would occupy the same 
relative positions. We might then expect some periodicity in the shape of the corona 
depending on the time of the year. To my surprise, I found that, as far as the 
evidence went, it seemed indeed to point in that direction. In tlie report of the 
Eclipse of 1875 a paragraph was inserted, which, without laying any stress on the 
point, called attention to the similarity betvyeen that eclipse and the one which had 
taken place the previous year during the same month. The eclipses which have taken 
place since have added to the evidence, and it seems worth while, therefore, to draw 
attention to it, without however in the least wishing to imply that I consider the fact 
as proved or even as probable. 
The Solar corona often shows a rough .symmetry about its axis of rotation, but 
deviates from complete symmetry owing to one of the halves being broader than the 
other. Hence, it often appears in the form of a trapezium. Fig. 1 may serve to 
illustrate this. AB is the axis ; C, D, E, F the points of the longest rays of the 
corona, the distance DE being longer than the distance CF. Now’, during the 
