and 1000 miles in thickness. It is cooler than the
underlying photosphere, and is composed of glowing gases. Many of the
elements which go to make up our earth are present in the reversing
layer in the form of vapour.

The _Chromosphere_, of which especial mention has already been made in
dealing with eclipses of the sun, is another layer lying immediately
upon the last one. It is between 5000 and 10,000 miles in thickness.
Like the reversing layer, it is composed of glowing gases, chief among
which is the vapour of hydrogen. The colour of the chromosphere is, in
reality, a brilliant scarlet; but, as we have already said, the
intensely white light of the photosphere shines through it from behind,
and entirely overpowers its redness. The upper portion of the
chromosphere is in violent agitation, like the waves of a stormy sea,
and from it rise those red prominences which, it will be recollected,
are such a notable feature in total solar eclipses.

[Illustration: FIG. 10.--A section through the Sun, showing how the
prominences rise from the chromosphere.]

The _Corona_ lies next in order outside the chromosphere, and is, so
far as we know, the outermost of the accompaniments of the sun. This
halo of pearly-white light is irregular in outline, and fades away into
the surrounding sky. It extends outwards from the sun to several
millions of miles. As has been stated, we can never see the corona
unless, when during a total solar eclipse, the moon has, for the time
being, hidden the brilliant photosphere completely from our view.

The solar spectrum is really composed of three separate spectra
commingled, _i.e._ those of the photosphere, of the reversing layer, and
of the chromosphere respectively.

If, therefore, the photosphere could be entirely removed, or covered up,
we should see only the spectra of those layers which lie upon it. Such a
state of things actually occurs in a total eclipse of the sun. When the
moon's body has crept across the solar disc, and hidden the last piece
of photosphere, the solar spectrum suddenly becomes what is technically
called "reversed,"--the dark lines crossing it changing into bright
lines. This occurs because a strip of those layers which lie immediately
upon the photosphere remains still uncovered. The lower of these layers
has therefore been called the "reversing layer," for want of a better
name. After a second or two this reversed spectrum mostly vanishes, and
an altered spectrum