370 COSMOS. 



Entstehung ihrer Fleclten), developed very similar views with 

 his usual perspicuity, although he was unacquainted with Wil- 

 son's earlier treatise. He moreover had the merit of having 

 facilitated the explanation of the penumbra, by assuming, 

 very much in accordance with the conjectures of Cardinal 

 Nicolaus de Cusa. the existence of another cloudy stratum of 

 vapour between the photosphere and the dark solar body, 

 This hypothesis of two strata leads to the following conclu- 

 sions : If there occur in less frequent cases an opening in 

 the photosphere alone, and not at the same time in the less 

 transparent lower vaporous stratum, which is but faintly 

 illumined by the photosphere, it must reflect a very incon- 

 siderable degree of light towards the inhabitants of the 

 Earth, and a grey penumbra will be formed a mere halo 

 without a nucleus ; but when, owing to tumultuous meteor- 

 ological processes on the surface of the Sun, the opening 

 extends simultaneously through both the luminous and 

 the cloudy envelopes, a micleoid spot will appear in the 

 ash-grey penumbra, " which will exhibit more or less black- 

 ness, according as the opening occurs opposite to a sandy, 

 rocky, or aqueous portion of the surface of the Sun's disc." 1S 

 The halo surrounding the nucleus is further 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 



13 Bode, in the Beschaftigungen der Berllnischen Gesell- 

 schaft Naturforschender Freunde, Bd. ii. 1776, pp. 237-241, 

 249. 



