THE SUN'S SPOTS. 369 



lope enable us from time to time to see the mountain summits of 

 the non-luminous body of the Sun. These constitute the Hack 

 nuclei in the centre of the Sun's spots." The ash-coloured 

 penumbra? surrounding these nuclei had not then been ex- 

 plained. 



An ingenious observation, which has subsequently been 

 fully confirmed, made by the astronomer, Alexander Wilson 

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

 1 769, led him to an elucidation of the penumbra?. Wilson 

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

 the penumbra became gradually more and more narrow on 

 the side turned towards the centre of the Sun, compared 

 with the opposite side. The observer, in 1774, very cor- 

 rectly concluded," 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 en- 

 velope) 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 question why the penumbra? 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" (Gedanlcen uber die Natur der Sonne und die 



u Alexander Wilson, Observations on the Solar Spots, 

 writes as follows in the Philos. Transact, vol. Ixiv. 1774, 

 part i. pp. 6-13, tab. i. : " I found that the umbra, which 

 before was equally broad all round the nucleus, appeared 

 much contracted on that part which lay towards the centre of 

 the disc, whilst 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 Annuaire pour 1842, p. 506. 



