12 Biographical Account ofDr Wilson, 



ing ground more and more among those most conversant in 

 astronomy, yet, like many other new discoveries, has not escap- 

 ed its share of opposition. This gave him occasion to publish, 

 in the Philosophical Transactions of London for 1783, the 

 second paper upon that subject, after a silence of near ten years, 

 wherein, upon the authority of many more observations made 

 in that interval, he obviates objections, and maintains the rea- 

 lity of his discovery with an entire conviction. The amount 

 ef it is, *' That the spots are cavities or depressions m that im- 

 mensely resplendent substance which invests the body of the 

 sun to a certain depth ; that the dark nucleus of the spot is at 

 the bottom of this excavation, which commonly extends down- 

 wards to a space equal to the semidiameter of our globe ; that 

 the shady or dusky zone which surrounds the nucleus, is no- 

 thing 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 next 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 

 upper and under sides of tlie 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 up- 

 on the disk, that side of its dusky zone which lately was invi- 

 sible now shows itself, and continues to enlarge more and more 

 till it becomes as broad as any other part surrounding the 

 nucleus. 



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

 serving 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 ex- 



