or, its real diameter, magnitude, density,
distance, and the force of gravity on its surface. He says:

"I should not wonder if, considering all this, we were induced to
think that nothing remained to be added; and yet we are still very
ignorant in regard to the internal construction of the sun." "The
spots have been supposed to be solid bodies, the smoke of
volcanoes, the scum floating on an ocean of fluid matter, clouds,
opaque masses, and to be many other things." "The sun itself has
been called a globe of fire, though, perhaps, metaphorically." "It
is time now to profit by the observations we are in possession of.
I have availed myself of the labors of preceding astronomers, but
have been induced thereto by my own actual observation of the solar
phenomena."

HERSCHEL then refers to the theories advanced by his friend, Prof.
WILSON, of Glasgow, in 1774. WILSON maintained that the spots were
depressions below the sun's atmosphere, vast hollows as it were, at the
bases of which the true surface of the sun could be seen.

The essence of his theory was the existence of two different kinds of
matter in the sun: one solid and non-luminous--the nucleus--the other
gaseous and incandescent--the atmosphere. Vacant places in the
atmosphere, however caused, would show the black surface of the solid
mass below. These were the spots. No explanation could be given of the
_faculae_, bright streaks, which appear on the sun's surface from time
to time; but his theory accounted for the existence of the black
_nuclei_ of the spots, and for the existence of the _penumbrae_ about
these. The penumbra of a spot was formed by the thinner parts of the
atmosphere about the vacancy which surrounded the nucleus.

This theory of WILSON'S was adopted by HERSCHEL as a basis for his own,
and he brought numerous observations to confirm it, in the modified
shape which he gave to it.

According to HERSCHEL, the sun consisted of three essentially different
parts. First, there was a solid nucleus, non-luminous, cool, and even
capable of being inhabited. Second, above this was an atmosphere proper;
and, lastly, outside of this was a layer in which floated the clouds, or
bodies which gave to the solar surface its intense brilliancy:

"According to my theory, a dark spot in the sun is a place in its
atmosphere which happens to be free from luminous decompositions"
above it.

The two atmospheric