THE SUN'S SPOTS. . 371 



centre of the Sun's disc. If, as Laugier has frequently 

 remarked, the penumbra passes over the black nucleus 

 causing 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 

 powers 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, 

 referring only in one instance to Alexander Wilson. In 

 their general character, his views may be regarded as iden- 

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

 dimensions of the nucleus and the "penumbra (Philos. Transact. 

 1801, pp. 270, 318, tab. xviii. fig. 2), on the assumption of 

 an opening 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 the 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, gene- 

 rating only small luminous pores in the higher regions where 

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

 rounded by shallows or penumbraj, where the action is mort 

 tumultuous.'* 



The black spots, which are seldom round, almost always 



VOL. iv. a 



