THE SUN S COEONA. 
379 
it is most unfit that the consideration of what is worthiest of 
observation should be left to the very moment at which obser- 
vation is to begin. It cannot be but that a careful consider- 
ation beforehand of the probable nature of the phenomena he 
is to observe, of the circumstances which, if carefully noted, 
may resolve doubts, and of the special parts of the heavens to 
which his attention should be directed, will tend materially to 
increase the value of his observations. 
Indeed, it is only necessary to consider the records of former 
eclipses to see that this must be so. Without in any way 
slighting the observing powers of those who have handed down 
to us the records of great eclipses, it is impossible not to feel, 
as we consider what they actually accomplished, that they might 
readily have accomplished much more. We find the attention 
of each observer distracted between a variety of objects ; facts 
are recorded which do not tend, and could not possibly tend, to 
elucidate any of the questions of interest which have been at 
issue ; and, in fine, one record after another displays evidence 
of an eager anxiety to ascertain new truths, marred by a very 
imperfect recognition of the way in which that purpose could 
best be accomplished. 
In the first place, it will be well to enquire what lessons may 
be educed from observations already made upon the corona, and 
further, what light other observations or researches may throw 
upon our subject. 
There are three theories of the corona which have at various 
times been upheld by astronomers, and between which it will 
be well that we should endeavour to make a selection. 
These theories assign to the corona very different positions in 
space. One places the corona around the sun, the second 
around our moon, the third in our own atmosphere. According 
to the theory (of these three) which may be finally established, 
we shall have three very various degrees of magnitude and im- 
portance to assign to the corona. If it is a solar appendage, its 
extent exceeds that of any body within the solar system, save 
perhaps one or two of the most remarkable comets. If it is a 
lunar appendage, it sinks into relative insignificance, but still 
has an absolute volume far exceeding that ofi our own earth. 
If, lastly, it is brought within the confines of our own atmo- 
sphere, it is not merely reduced to proportions altogether insig- 
nificant, as well absolutely as relatively, but it no longer has 
any real existence as a substance of any sort, any more than the 
beam of light which shines through clouds can be regarded as 
an actually existent measurable mass. 
I shall take the second of these theories first in order, 
because it is the one we can most readily dispose of. 
To Halley it seemed an acceptable theory, that the corona is 
