riir. si\ g spots. 79 



ous, self-luminous envelope. We thus possess a material phys- 

 ical analysis of the photosphere. 



The same instrument has, however, also led to the conclu- 

 sion that the intensity of the light of the Sun is not greater 

 in the center of the disk than at its margins. When the 

 two complementary colored images of the Sun — the red and 

 blue — are so arranged that the margin of the one image falls 

 on the center of the other, perfect white will be produced. 

 If the intensity of the light were not the same in the different 

 parts of the Sun's disk — if, for example, the center were more 

 luminous than the margin, then the partial covering of the 

 images in the common segments of the blue and red disk 

 would not exhibit a pure white, but a pale red, because the 

 blue rays would only be able to neutralize a portion of the 

 more numerous red rays. If, moreover, we remember that 

 in the gaseous photosphere of the Sun, in opposition to that 

 which occurs in solid or liquid bodies, the smallness of the 

 angle at which the rays of light emanate does not cause their 

 number to diminish at the margins, and as the same angle 

 of vision embraces a larger number of luminous points at the 

 margins than in the center of the disk, we could not here 

 reckon upon that compensation which, were the Sun a lu- 

 minous iron globe, and consequently a solid body, would take 

 place between the opposite effects of the smallness of the an- 

 gle of radiation and the comprehension of a larger number 

 of luminous points at the same visual angle. The self-lumin- 

 ous gaseous envelope, i. e., the solar disk visible to us, must 

 therefore (in opposition to the indications of the polariscope, 

 which shows the margin and the center to be of equal intens- 

 ity) appear more luminous in the center than at the margin. 

 The cause of this discrepancy has been ascribed to the outer- 

 most and less transparent vaporous envelope surrounding the 

 photosphere, which diminishes the light from the center less 

 than that of the marginal rays on its long passage through 

 the vaporous envelope.* Bouguer, Laplace, Airy, and Sir 



* Arago, in the M&moires des Sciences Matkdm. et Phys. de V Imtilut 

 de France, annee 1811, partie i., p. 118; Matthieu, in Delambre, Hist, 

 de V Astr. au dix-huitieme siccle, p. 351, 652 ; Fourrier, Eloge de William 

 Herschel, in the Mim. de V Institut, torn, vi., annee 1823 (Par., 1827), 

 p. lxxii. It is alike remarkable and corroborative of the great uniform- 

 ity of character in the light of the Sun, whether emanating from its cen- 

 ter or its margins, that, according to an ingenious experiment made by 

 Forbes, during a solar eclipse in 1836, a spectrum formed from the cir- 

 cumferential rays alone was identical both in reference to the number 

 and position of the dark lines or stripes intersecting it, with the spec- 



