394 COSMOS. 



William Herschel, assuming the intensity of the whole 

 light of the Sun at 1,000, estimated the average light of the 

 penumbrse at 469, and the black nuclei at 7. According to 

 this estimate, which is certainly very conjectural, a black 

 nucleus would yet possess 2.000 times more light than the 

 full Moon, since the latter, according to Bouguer, is 300,000 

 less bright than the Sun. The degree of illumination ol 

 the nuclei visible to us, i. e. of the dark body of the Sun 

 illumined by reflection from the walls of the opened pho- 

 tosphere, the interior atmosphere from which the penumbrae 

 are generated, and by the light of the strata of our ter- 

 restrial atmosphere through which we see it, has been 

 strikingly manifested on the occasion of several transits 

 of Mercury. When compared with the planet, whose 

 dark side was turned towards us, the near and darkest 

 nuclei presented a light brownish-grey appearance. 81 The 

 admirable observer, Councillor Schwabe, of Dessau, was 

 particularly struck by this difference of blackness between 

 the planet and the nuclei, in the transit of Mercury on 

 the 5th of May, 1832. On the occasion of my observing 

 the transit of this planet in Peru, on the 9th of November, 

 1802, in consequence of being engaged in measuring the 

 distances from the threads, I was unfortunately unable to 

 make any comparison between the different intensities of the 

 light, although Mercury's disc almost touched the nearest 

 dark spot. Professor Henry, of Princeton, North America, 

 had already shown, by his experiments in 1815, that the 

 Sun's spots radiate a perceptibly less heat than those portions 

 on which there were no spots. The images of the Sun and 

 of a large spot were projected on a screen, and the differences 

 of heat measured by means of a thermo-electrical apparatus. 26 



28 Madler, Aatr. p. 81. 



29 Philos. Mag. ser. iii. vol. xxviii. p. 230; and Poggend. 

 Annalen, bd. Ixviii. p. 101. 



