A. Wilson, of Glasgow, in 1769,[2] noticed a movement of the umbra
relative to the penumbra in the transit of the spot over the sun's
surface; exactly as if the spot were a hollow, with a black base and
grey shelving sides. This was generally accepted, but later
investigations have contradicted its universality. Regarding the cause
of these hollows, Wilson said:--

Whether their first production and subsequent numberless changes
depend upon the eructation of elastic vapours from below, or upon
eddies or whirlpools commencing at the surface, or upon the
dissolving of the luminous matter in the solar atmosphere, as clouds
are melted and again given out by our air; or, if the reader
pleases, upon the annihilation and reproduction of parts of this
resplendent covering, is left for theory to guess at.[3]

Ever since that date theory has been guessing at it. The solar
astronomer is still applying all the instruments of modern research to
find out which of these suppositions, or what modification of any of
them, is nearest the truth. The obstacle--one that is perhaps fatal to
a real theory--lies in the impossibility of reproducing comparative
experiments in our laboratories or in our atmosphere.

Sir William Herschel propounded an explanation of Wilson's observation
which received much notice, but which, out of respect for his memory,
is not now described, as it violated the elementary laws of heat.

Sir John Herschel noticed that the spots are mostly confined to two
zones extending to about 35 degrees on each side of the equator, and that a
zone of equatoreal calms is free from spots. But it was
R. C. Carrington[4] who, by his continuous observations at Redhill, in