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 faculæ, 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 penumbræ 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 layers, which will be of varying thickness about a spot, will account for all the shades of darkness seen in the penumbra. Ascending currents from the solar surface will elevate certain regions, and may increase the solar activity near by, and will thus give rise to faculæ, which Herschel shows to be elevated above the general surface. It will not be necessary to give a further account of this theory. The data in the possession of the modern theorist is a thousand-fold that to be derived from Herschel's observations, and, while the subject of the internal construction of the sun is to-day unsettled, we know that many important, even fundamental, portions of his theory are untenable. A remark of his should be recorded, however, as it has played a great part in such theories:
"That the emission of light must waste the sun, is not a difficulty that can be opposed to our hypothesis. Many of the operations of Nature are carried on in her great laboratory which we cannot comprehend. Perhaps the many telescopic comets may restore to the sun what is lost by the emission of light."
Arguments in favor of the habitability of both sun and moon are contained in this paper; but they rest more on a metaphysical than a scientific basis, and are to-day justly forgotten.
Researches on the Motion of the Sun and of the Solar System in Space.
In 1782 Herschel writes, in regard to some of his discoveries of double stars: