74 SPECIAL KESULTS IN THE URANOLOGICAL 



Our Berlin Astronomer, Bode, without being acquainted 

 with the earlier memoir of Wilson, developed, in his pecu- 

 liarly lucid and popular manner, perfectly similar views, in 

 his " Thoughts on the Nature of the Sun and the origin of 

 its spots" (" Gedanken liber die Natur der Sonne und die 

 Enstehung ihrer Flecken"). Bode had also the further 

 merit of having facilitated the explanation of the penumbra 

 by assuming, almost as in the anticipatory conjectures of 

 Cardinal Nicolaus of Cusn, an additional stratum of cloudy 

 vapour between the photosphere and the dark body of the 

 Sun. This hypothesis of two distinct envelopes leads to the 

 following inferences : if, in the smaller number of cases, an 

 opening is formed in the photosphere only, and not at the 

 same time in the inner vaporous stratum which is supposed 

 to be only imperfectly illuminated by the brighter outer one, 

 then this inner envelope will reflect towards the earth only 

 a very mitigated light, and thus there is produced a grey 

 penumbra without any black nucleus. But if in the tem- 

 pestuous meteorological processes taking place on the surface 

 of the Sun the opening penetrates both envelopes (i. e. both 

 the luminous and the cloudy one), then there appears in the 

 ash-coloured penumbra, a nucleus " shewing a more or less 

 intense blackness according to the character of the surface of 

 the body of the Sun at the part exposed by the opening" ( 472 ). 

 The shade round the nucleus is a part of the external surface 

 of the inner vaporous stratum, and as the latter, by reason of 

 the funnel shape of the whole excavation, has a smaller 

 opening than the photosphere, so the path of the rays which on 

 both sides pass along the edges of the interrupted strata, and 

 arrive at the eye of the observer, explains the difference first 

 perceived by Wilson to take place gradually in the relative 



