919 Biographical Account of Dr» WilsoH, [Nov. 



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 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 follow- 

 ing 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 disc, 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 disc, by the sun's rotation, we see first the nucleus, and the 

 upper 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 disc, that side of its dusky zone which lately was invisible, 

 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 observ-» 

 irig 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 

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

 new and interesting ideas concerning the nature and constitu- 

 tion of that stupendous body. 



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

 remark these pheenomena 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 disco-f 

 very. For one day when observing a spot of considerable size 

 near the sun's limb, he actually beheld this appearance of the 

 dusky zone which belongs to the nucleus, finding it almost 

 wholly deficient on that side which respected the centre of the 

 disc; and this too when the distance of the spot from the limb 

 corresponded very nearly with that which Dr. Wilson found to 

 be so constant in his observations. Mr. Flamstead was then 

 indeed viewing his spot in peculiar circumstances, and the most 

 favourable of all to perfect vision of the sun, as, by the inters 

 vention of a mist, he was enabled to use his telescope without 

 the help of tinged glass put before his eye. The following is 

 his account of thi* remarkable observation, in which, by the 



