292 Biographical Account of DR WILSON, 



which surrounds the nucleus, is nothing but the sloping sides of 

 the excavation reaching from the sun's general surface downward 

 to the nucleus or bottom." All this he has demonstrated by a 

 strict induction drawn from the following phases of the spots, as 

 they traverse the sun's disk. 



When a large well-formed spot, consisting of a dark nucleus, 

 and its surrounding umbra or dusky zone, is seen upon the 

 middle of the sun's disk, the zone is generally equally broad all 

 around ; but when the same spot verges near to the limb, that 

 side of the dusky zone which lies nest to the centre of the disk, 

 begins much sooner than the side diametrically opposite to turn 

 narrower, and at last disappears, while the other still remains 

 dilated and visible. And, in like manner, when a spot enters the 

 disk, by the sun's rotation, we see first the nucleus, and the up- 

 per and under sides of the shady zone or umbra, together with 

 that side of it nearest to the limb, whilst the side opposite is still 

 wholly invisible. But as the spot advances farther upon the 

 disk, that side of its dusky zone which lately was invisible, now 

 shews itself, and continues to enlarge more and more, till it be- 

 comes as broad as any other part surrounding the nucleus. 



These phases, which he found so very palpable when observ- 

 ing carefully the great solar spot in November 1769, and so very 

 frequent, though less obvious, in numberless other spots of a 

 smaller size, which for several years afterwards he examined, 

 prove in the clearest manner that the spots themselves are de- 

 pressions in the luminous matter of the sun, and lead to many 

 new and interesting ideas concerning the nature and constitution 

 of that stupendous body. 



But though he was the first astronomer to whose lot it fell 

 to remark these phenomena of the solar spots which have been 

 just now described, and to draw such important conclusions from 

 them, it appears that the celebrated Mr FLAMSTEAD, so far back 

 as the year 1676, had very nearly anticipated this discovery. 



