up to the present met with
failure.

The general form under which the corona appears to our eyes varies
markedly at different eclipses. Sometimes its streamers are many, and
radiate all round; at other times they are confined only to the middle
portions of the sun, and are very elongated, with short feathery-looking
wisps adorning the solar poles. It is noticed that this change of shape
varies in close accordance with that 11-1/4 year period during which the
sun spots wax and wane; the many-streamered regular type corresponding
to the time of great sunspot activity, while the irregular type with the
long streamers is present only when the spots are few (see Plate VII.,
p. 142). Streamers have often been noted to issue from those regions of
the sun where active prominences are at the moment in existence; but it
cannot be laid down that this is always the case.

No hypothesis has yet been formulated which will account for the
structure of the corona, or for its variation in shape. The great
difficulty with regard to theorising upon this subject, is the fact
that we see so much of the corona under conditions of marked
foreshortening. Assuming, what indeed seems natural, that the rays of
which it is composed issue in every direction from the solar body, in a
manner which may be roughly imitated by sticking pins all over a ball;
it is plainly impossible to form any definite idea concerning streamers,
which actually may owe most of the shape they present to us, to the
mixing up of multitudes of rays at all kinds of angles to the line of
sight. In a word, we have to try and form an opinion concerning an
arrangement which, broadly speaking, is _spherical_, but which, on
account of its distance, must needs appear to us as absolutely _flat_.

The most known about the composition of the corona is that it is made up
of particles of matter, mingled with a glowing gas. It is an element in
the composition of this gas which, as has been stated, is not found to
tally with any known terrestrial element, and has, therefore, received
the name of coronium for want of a better designation.

One definite conclusion appears to be reached with regard to the corona,
_i.e._ that the matter of which it is composed, must be exceedingly
rarefied; as it is not found, for instance, to retard appreciably the
speed of comets, on occasions when these bodies pass very close to the
sun. A calculation has indeed been made which would tend to show that