pear 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:
The two atmospheric layers, which will be of varying thickness about a spot, will
