THE SUN S SPOTS. 389 



the angle of radiation and the comprehension of a larger 

 number of luminous points at the same visual angle. The 

 self-luminous gaseous envelope, i. e., the solar disc visible 

 to us, must therefore (in opposition to the indications of the 

 polariscope, which shows the margin and the centre to be of 

 equal intensity ), appear more luminous in the centre than at 

 the margin. The cause of this discrepancy has been ascribed 

 to the outermost and less transparent vaporous envelope sur- 

 rounding the photosphere, which diminishes the light from 

 jie centre less than that of the marginal rays on its long 

 passage through the vaporous envelope. 84 Bouguer, Laplace, 



* Arago, in the Memoires des sciences mathem. et phys- 

 de I'Institut de France, annee 1811, partie i. p. 118; 

 Mntthieu, in Delambre, Hist, de lAstr. au dixhuitieme siecle, 

 pp. 351, 652. Fourrier, Eloye de William Herschel, in the 

 Mem. de I'Institut, torn. vi. annee 1823 (Par. 1827), p. Ixxii. 

 It is alike remarkable and corroborative of the great unifor- 

 mity of character in the light of the Sun, whether emanating 

 from its centre or its margins, that, according to an ingenious 

 experiment made by Forbes, during a solar eclipse in 1836, 

 a :-pectrum formed from the circumferential rays alone was 

 identical both in reference to the number and position of the 

 dark lines or stripes intersecting it, with the spectrum arising 

 from the entire solar light. When, therefore, rays of certain 

 refrangibility are wanting in solar light, they have probably 

 not passed into the Sun's atmosphere, as Sir David Brewster 

 conjectures, since the circumferential rays produce the same 

 dark lines when they shine through a much thicker medium. 

 (Forbes, in the Comptes rendus, torn. ii. 1836, p. 576.) I will 

 append to this note all the facts that I collected in the year 

 1847, from Arago's MSS. : 



" Des phenomenea de la polarisation coloree donnent la 

 certitude que le bord du Soleil a la meme intensite de lumiere 

 que le centre ; car en pla9ant dans la polariscope un segment 

 du bord sur un segment du centre, j'obtiens (comme effet 

 complementaire du rouge et du bleu) un blanc pur. Dans 

 un corps solide (dans une boule de fer chaufiee au rouge) le 



H 2 



