SOLAR LIGHT. 83 



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

 bright than the Sun. The degree of illumination of the nu- 

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

 by reflection from the walls of the opened photosphere, the 

 interior atmosphere from which the penumbrse are generated, 

 and by the light of the strata of our terrestrial 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 toward us, the 

 near arid darkest nuclei presented a light brownish-gray ap- 

 pearance.* The admirable observer, Counselor 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 dis- 

 tances from the threads, I was unfortunately unable to make 

 any comparison between the different intensities of the light, 

 although Mercury's disk almost touched the nearest dark 

 spot. Professor Henry, of Princeton, North America, had al- 

 ready 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-electricai apparatus.! 



Whether rays of heat differ from rays of light by a differ- 

 ence in the lengths of the transversal vibrations of ether, or 

 whether they are identical with rays of light, but that a cer- 

 tain velocity in the vibrations which generates very high tem- 

 peratures is requisite to excite the impression of light in our 

 organs, the Sun, as the main source of light and heat, must 

 nevertheless be able to call forth and animate magnetic forces 

 on our planet, and more especially in the gaseous strata of 

 our atmosphere. The early knowledge of thermo-electricai 

 phenomena in crystallized bodies (such as tourmaline, bora- 

 cite, and topaz), and Oersted's great discovery (1820) that 

 every conducting body charged with electricity exerts a defin- 

 ite action on the magnetic needle during the continuation of 

 the electrical current, afforded practical evidence of the cor- 

 relation of heat, electricity, and magnetism. Basing his de- 

 ductions on the idea of such an affinity, Ampere, who ascribed 



* Madler, Astr., p. 81. 



t Philos. Mag., ser. iii., vol. xxviii., p. 230; and Poggend., Annalen, 

 bd. Ixviii., p. 101. 



