THE SUN'S SPOTS. 67 



more or less blackness, according as the opening occurs op- 

 posite to a sandy, rocky, or aqueous portion of the surface of 

 the Sun's disk.* The halo surrounding the nucleus is fur- 

 ther a portion of the outer surface of the vaporous stratum ; 

 and as this is less opened than the photosphere, owing to the 

 funnel-shaped form of the whole excavation, the direction of 

 the passage of the rays of light, impinging on both sides on 

 the margins of the interrupted envelope, and reaching the 

 eyes of the observer, occasions the difference, first noticed by 

 Wilson, in the breadth of the opposite sides of the penumbra, 

 which appears after the nucleoid spot has moved away from 

 the center of the Sun's disk. If, as Laugier has frequently 

 remarked, the penumbra passes over the black nucleus, caus- 

 ing it wholly to disappear, this obscuration must depend on 

 the closing of the opening not of the photosphere, but of the 

 vaporous stratum below it. 



A solar spot, which was visible to the naked eye in the year 

 1779, fortunately directed William Herschel's superior pow- 

 ers of observation and induction to the subject which we have 

 been considering. We possess the results of his great work, 

 which treats of the minutest particulars of the question in a 

 very definite manner, and in a nomenclature established by 

 himself. His observations appeared in the Philosophical 

 Transactions for 1795 and for 1801. As usual, this great 

 observer pursued his own course independently of others, re- 

 ferring only in one instance to Alexander Wilson. In their 

 general character, his views may be regarded as identical 

 with those of Bode, and he bases the visibility and dimensions 

 of the nucleus and the penumbra (Philos. Transact., 1801, 

 p. 270, 318, tab. xviii., fig. 2) on the assumption of an open- 

 ing in two envelopes, while he assumes the existence of a 

 clear and transparent aerial atmosphere (p. 302) between the 

 vaporous envelope and the dark body of the Sun, in which 

 clouds that are either wholly dark, or only faintly illumined 

 by reflection, are suspended at a height of about 280 to 320 

 geographical miles. William Herschel seems, in fact, also 

 disposed to regard thft photosphere as a mere stratum of 

 unconnected phosphorescent clouds of very unequal surface. 

 According to his view, " an elastic fluid of unknown nature 

 rises from the crust or surface of the dark solar body, gener- 

 ating only small luminous pores in the higher regions where 

 the action is weak, and large openings, with nuclei, sur- 



* Bode, in the Beschdfligungen der Berlinischen Gesellschaft Nalur- 

 forsckenderFreunde,bd. ii., 1776, p. 237-241, 249. 



