is left to view. Taking into consideration the rate
at which the moon is moving across the face of the sun, and the very
short time during which the spectrum of the reversing layer lasts, the
thickness of that layer is estimated to be not more than a few hundred
miles. In the same way the last of the three spectra--namely, that of
the chromosphere--remains visible for such a time as allows us to
estimate its depth at about ten times that of the reversing layer, or
several thousand miles.

When the chromosphere, in its turn during a total eclipse, has been
covered by the moon, the corona alone is left. This has a distinct
spectrum of its own also; wherein is seen a strange line in the green
portion, which does not tally with that of any element we are acquainted
with upon the earth. This unknown element has received for the time
being the name of "Coronium."

CHAPTER XIII

THE SUN--_continued_

The various parts of the Sun will now be treated of in detail.

I. PHOTOSPHERE.

The photosphere, or "light-sphere," from the Greek [phos] (_phos_),
which means _light_, is, as we have already said, the innermost portion
of the sun which can be seen. Examined through a good telescope it shows
a finely mottled structure, as of brilliant granules, somewhat like rice
grains, with small dark spaces lying in between them. It has been
supposed that we have here the process of some system of circulation by
which the sun keeps sending forth its radiations. In the bright granules
we perhaps see masses of intensely heated matter, rising from the
interior of the sun. The dark interspaces may represent matter which has
become cooled and darkened through having parted with its heat and
light, and is falling back again into the solar furnace.

The _sun spots_, so familiar to every one nowadays, are dark patches
which are often seen to break out in the photosphere (see Plate V., p.
134). They last during various periods of time; sometimes only for a few
days, sometimes so long as a month or more. A spot is usually composed
of a dark central portion called the _umbra_, and a less dark fringe
around this called the _penumbra_ (see Plate VI., p. 136). The umbra
ordinarily has the appearance of a deep hole in the photosphere; but,
that it is a hole at all, has by no means been definitely proved.

[Illustration: PLATE V. THE SUN, SHOWING SEVERAL GROUPS OF SPOTS

From a photograph taken at the Royal Observatory, Greenwich. The
cro