ace confirms the
evidence which the spots give us of the sun's rotation.

Our proofs of this rotation are still further strengthened by the
Doppler spectroscopic method of observation alluded to in Chapter XI. As
was then stated, one edge of the sun is thus found to be continually
approaching us, and the other side continually receding from us. The
varying rates of rotation, which the spots and faculae give us, are duly
confirmed by this method.

[Illustration: PLATE VI. PHOTOGRAPH OF A SUNSPOT

This fine picture was taken by the late M. Janssen. The granular
structure of the Sun's surface is here well represented. (From
_Knowledge_.)

(Page 135)]

The first attempt to bring some regularity into the question of
sunspots was the discovery by Schwabe, in 1852, that they were subject
to a regular variation. As a matter of fact they wax and wane in their
number, and the total area which they cover, in the course of a period,
or cycle, of on an average about 11-1/4 years; being at one part of this
period large and abundant, and at another few and small. This period of
11-1/4 years is known as the sun spot cycle. No explanation has yet been
given of the curious round of change, but the period in question seems
to govern most of the phenomena connected with the sun.

II. REVERSING LAYER.

This is a layer of relatively cool gases lying immediately upon the
photosphere. We never see it directly; and the only proof we have of its
presence is that remarkable reversal of the spectrum already described,
when during an instant or two in a total eclipse, the advancing edge of
the moon, having just hidden the brilliant photosphere, is moving across
the fine strip which the layer then presents edgewise towards us. The
fleeting moments during which this reversed spectrum lasts, informs us
that the layer is comparatively shallow; little more indeed than about
500 miles in depth.

The spectrum of the reversing layer, or "flash spectrum," as it is
sometimes called on account of the instantaneous character with which
the change takes place, was, as we have seen, first noticed by Young in
1870; and has been successfully photographed since then during several
eclipses. The layer itself appears to be in a fairly quiescent state; a
marked contrast to the seething photosphere beneath, and the agitated
chromosphere above.

III. THE CHROMOSPHERE.

The Chromosphere--so called from the Greek [chroma] (_chroma_), which
signifies _col