Scientific Lectures. 275 



tlie dark and solid central body of the sun, and whose tumultuous 

 agitation sometimes discloses some mountain summit which appears 

 as the black nucleus of the spot. Tliis suggestive idea formed, how- 

 ever, only a first step toward the solution of the problem, the honor 

 of which belongs to a Scotch astronomer, Prof. Alex. "Wilson, of 

 Glascow, one hundred years later. Wilson observed that the penum- 

 bra or grayish border which surrounds the dark nucleus, and which 

 is generally of about equal width on all sides when the spot is near 

 the middle of the sun's disc, always became narrower on the side 

 nearest the middle of the sun, when the spot approached the circum- 

 ference. Hence he inferred in 1773, that the spots were funnel-shaped 

 apertures in the luminous envelope, which disclosed the dark body 

 of the sun at the bottom, and whose shelving sides constituted the 

 penumbra. A moment's inspection of the diagram will show how 

 such an aperture would at the middle of the disc exhibit the full 

 size of the nucleus, D E, and an equable border, represented by the 

 apparent breadth at C D and E Fof the inclined sides of the opening ; 

 but that when it is nearer the circumference, as at A B or G H, the 

 nearest side becomes foreshortened, as does also the nucleus, though 

 to a less extent, while the farthest side of the aperture is presented ' 

 much more fully to the view. 



If, therefore, the appearances be as stated by Wilson, the inference 

 seems irresistable that the spots are openings in the glowing envelope 

 of the sun. To make sure of the facts in the case, the directors of 

 the Kew Observatory have carried out an extensive examination of 

 all the drawings and photographs of solar spots, which they could 

 make available, and the enormous preponderance of observations in' 

 f-ivor of this theory, must be considered as definitely settling the 

 question. Mr. De La Rue also suggested the use of the stereoscope 

 to decide whether the spots are actually cavities or depressions, with 

 the same result. But this argument, although a strong one, can 

 hardly be deemed conclusive — for our senses are tricky guides, and 

 things are not always what they seem. 



But why should the sloping sides of such a cavity manifest that 

 diminution of the solar luster which the penumbra exhibits ? Because 

 the darker body of the sun would be partially seen through it, would 

 be our first reply ; but a moment's thought will show that were tliis 

 the true explanation, the penumbra would exiiibit different degrees 

 of luminosity, and shade gradually away from full brightness at its " 

 circumference to darkness at its inner margin. This is not the 



