a region of partial darkness, beyond which is excessive light. They
have motion of their own over the surface--motion rotating about an
axis, upward and downward about the edges. They change their
apparent shape as the sun carries them across its disk by axial
revolution, being narrow as they present their edges to us, and
rounder as we look perpendicularly into them (Fig. 35).

[Illustration: Fig. 35.--Change in Spots as rotated across the Disk,
showing Cavities.]

These spots are also very variable in number, sometimes there being
none for nearly two hundred days, and again whole years during which
the sun is never without them. The period from minimum to maximum
[Page 91] of spots is about eleven years. We might look for them
again and again in vain this year (1878). They will be most numerous
in 1882 and 1893. The cause of this periodicity was inferred to be
the near approach of the enormous planet Jupiter, causing
disturbance by its attraction. But the periods do not correspond,
and the cause is the result of some law of solar action to us as yet
unknown.

These spots may be seen with almost any telescope, the eye being
protected by deeply colored glasses.

Until within one hundred years they were supposed to be islands of
scoriæ floating in the sea of molten matter. But they were depressed
below the surface, and showed a notch when on the edge. Wilson
originated and Herschel developed the theory that the sun's real
body was dark, cool, and habitable, and that the photosphere was
a luminous stratum at a distance from the real body, with openings