66 COSMOS. 



An ingenious observation, which has subsequently been 

 fully confirmed, made by the astronomer, Alexander Wilson, 

 of Glasgow, of a large solar spot, on the 22d of November, 

 1769, led him to an elucidation of the penumbras. Wilson 

 discovered that as a spot moved toward the Sun's margin, 

 the penumbra became gradually more and more narrow on 

 the side turned toward the center of the Sun, compared with 

 the opposite side. The observer, in 1774, very correctly con- 

 cluded,* from these relations of dimension, that the nucleus 

 of the spot (the portion of the dark solar body visible through 

 the funnel-shaped excavation in the luminous envelope) was 

 situated at a greater depth than the penumbra, and that the 

 latter was formed by the shelving lateral walls of the funnel. 

 This mode of explanation did not, however, solve the ques- 

 tion why the penumbrse were the lightest near the nuclei. 



The Berlin astronomer, Bode, in his work entitled " Thoughts 

 on the Nature of the Sun, and the Formation of its Spots" 

 (Gedanketi uber die I^atur dcr Sonne und.die Entstehung 

 Hirer Flecken), developed very similar views with his usual 

 perspicuity, although he was unacquainted with Wilson's ear- 

 lier treatise. He, moreover, had the merit of having facili 

 tated 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 vapor be- 

 tween the photosphere and the dark solar body. This hy- 

 pothesis of two strata leads to the following conclusions : If 

 there occur in less frequent cases an opening in the photo- 

 sphere alone, and not, at the same time, in the less transpar- 

 ent lower vaporous stratum, which is but faintly illumined by 

 the photosphere, it must reflect a very inconsiderable degree 

 of light toward the inhabitants of the Earth, and a gray pe- 

 numbra will be formed a mere halo without a nucleus ; but 

 when, owing to tumultuous meteorological processes on the 

 surface of the Sun, the opening extends simultaneously through 

 both the luminous and the cloudy envelopes, a nucleoid spot 

 will appear in the ash-gray penumbra, " which will exhibit 



* Alexander Wilson, Observations on the Solar Spots, writes as fol- 

 lows iu the Philos. Transact., vol. Ixiv., 1774, part i., p. 6-13, tab. i. : 

 " I found that the umbra, which before was equally broad all rouud the 

 nucleus, appeared much contracted on that part which lay toicard the 

 center of the disk, while the other parts of it remained nearly of the 

 former dimensions. I perceived that the shady zone or umbra, which 

 surrounded the nucleus, might be nothing else but the shelving sides 

 of the luminous matter of the Sun." Compare also Arago, in the Annit- 

 aire for 1842, p. 506. 



