PORTION OF 1HE COSMOS. THE SUN. 275 



breadths of the opposite sides of the penumbra, as the distance 

 of the nucleus from the centre of the Sun's disk increases. 

 When, as Laugier has more than once remarked, the penumbra 

 spreads over the black nucleus itself, so that the latter dis- 

 appears altogether, the cause is that the opening of the inner 

 cloudy envelope is closed, whilst that in the photosphere 

 remains open. 



A solar spot visible in 1779 to the naked eye fortunately 

 led the genius of* William Herschel, happy alike in observa- 

 tion and combination, to the subject now before us. The 

 results of his great examination, in which the details of 

 several cases are treated according to a very definite nomen- 

 clature established by himself, are given in the Philosophical 

 Transactions of 1795 and of 1801. He proceeds as usual in 

 his own manner, and merely names Alexander Wilson once. 

 His view is in its generality identical with that of Bode ; his 

 interpretation of the visibility and dimensions of the nucleus 

 and the penumbra (Phil. Trans. 1801, p. 270 and 318, 

 Tab. xviii. fig. 2), is based on the assumption of an opening 

 in two envelopes ; but besides these he places between the 

 envelope and the body of the Sun (p. 302), a clear and 

 transparent atmosphere, in which dark clouds (or at least 

 only faintly illuminated by reflection) are suspended at a con- 

 siderable height, as three hundred (English) geographical 

 miles. Wm. Herschel seems, indeed, inclined to believe the 

 photosphere also to be only a stratum of unconnected phos- 

 phoric clouds with very uneven surfaces. It seems to him 

 that an elastic fluid of an unknown nature rises from the crust 

 or surface of the dark body of the Sun, occasioning in the 

 upper region , when it acts most feebly, only small pores or 

 punctures, and when it acts most energetically and tempes- 



