PORTION OP THE COSMOS. THE SUN. 273 



non-employment of which had greatly contributed to occasion 

 Galileo's loss of sight. 



As elicited by actual observation after the discovery of 

 the solar spots, I find the earliest and most definite expres- 

 sions as to the necessity of assuming the Sun to be a dark 

 globe surrounded by a luminous envelope (photosphere)., from 

 the pen of Dominique Cassiniinl671( 470 ) According to him 

 the solar disk which we see is "a luminous ocean sur- 

 rounding the solid and dark nucleus of the Sun ; tumultuous 

 movements taking place in the luminous envelope allow 

 us from, time to time to see the mountain summits of the 

 non-luminous body of the Sun itself. They are the black 

 nuclei in the centre of the solar spots/' The ash-coloured 

 penumbras surrounding the nuclei still remained without 

 any attempt at explanation. 



An ingenious, and since often confirmed observation, made 

 by Alexander Wilson, the Astronomer of Glasgow, on a 

 large solar spot on the 22d of November, 1769, led him to 

 an explanation of the penumbras. Wilson discovered that 

 as a spot moves towards the Sun's limb, the penumbra on 

 the side towards the centre of the Sun becomes gradually 

 narrower and narrower as compared wtth that on the 

 opposite side. He inferred, very justly, from the ratios of 

 these dimensions, that the nucleus of the spot (the part of 

 the dark body of the Sun becoming visible through the 

 funnel-shaped excavation of the luminous envelope), is 

 situated deeper than the penumbra, and that the penumbra is 

 formed by the steep declivities or side walls of the funnel ( 471 ). 

 This mode of explanation, however, offered no reply to the 

 question why the penumbra should be lightest near the 

 dark nucleus ? 



