288 SPECIAL RESULTS IN THE UEANOLOGICAL 



moon. The degree of illumination of the nuclei of the solar 

 spots as seen by us, (i. e. of the dark body of the Sun illumi- 

 nated by reflection from the sides of the opening in the 

 photosphere and from the inner vaporous envelope which 

 produces the penumbras, and by the light of the terrestrial 

 atmospheric strata through which we look), has been shown 

 in a very remarkable manner by some observations made 

 during transits of Mercury. Compared with the Planet, 

 whose dark nocturnal, or unilluminated side is then turned 

 towards the earth, the darkest nuclei of spots in its vicinity 

 appeared of a light brownish grey ( 487 ). An excellent ob- 

 server, Hofrath Schwabe, of Dessau, had his attention par- 

 ticularly drawn to this difference between the darkness of 

 the planet and of the nuclei of the solar spots, on the occa- 

 sion of the transit of Mercury on the 5th of May, 1832. 

 When observing in Peru the transit of the same planet, which 

 took place on the 9th of November, 1802, I unfortunately 

 was so much occupied with noticing the distances from the 

 wires, that the comparison of the disk with dark solar spots 

 which it almost touched, escaped me. That the spots 

 radiate sensibly less heat than the other portions of the 

 Sun's disk, was shown as early as 1815, by Professor Henry, 

 of Princeton in the United States, by means of very deli- 

 cate experiments, in which the image of the Sun and that of 

 n large spot were projected on a screen, and the difference 

 of temperature was measured by a thermo-electric appa- 

 ratus ( 488 ). 



Whether the calorific are distinguished from the luminous 

 rays by different lengths in the transverse undulations of the 

 ether, or whether they are identical with the luminous rays, 

 but only excite in our organs the sensation of light at a 



