286 SPECIAL RESULTS IN THE TJRANOLOGICAL 



number of luminous points at the margin than at the centre, 

 we see that we cannot reckon on the compensation which, if 

 the Sun were a solid body, as a glowing iron ball, would take 

 place at the edges, between the effects of smallness of radiation- 

 angle, and the comprehension of a greater number of lumi- 

 nous points within the same angle of vision. If, then, there 

 were no additional circumstance to be taken into account, it 

 would follow that the gaseous self-luminous envelope, i. e. 

 the solar disk seen by us, should, in contradiction to the 

 indications of the polariscope, which show equal intensity of 

 light in the centre and at the limb, be brighter at the edges 

 than at the centre. That this is not so must be attributed 

 to the outermost opaque or imperfectly transparent vaporous 

 envelope or veil which surrounds the photosphere, and dims 

 the light from the centre less than the rays from the margins 

 which traverse the envelope by a longer path ( 483 ). Bouguer 

 and Laplace, Airy, and Sir John Herschel, are opposed to 

 the views taken by Arago : they hold the intensity of the 

 light of the edges to be less than that of the centre, and the 

 last-named of these distinguished physicists and astronomers 

 remarks ( 484 ), " granting the existence of such an atmo- 

 sphere" (or external vaporous envelope) " its form in obe- 

 dience to the laws of equilibrium must be that of an oblate 

 spheroid, the ellipticities of whose strata differ from each 

 other and from that of the nucleus. Consequently the 

 equatorial portions of this envelope must be of a thickness 

 different from that of the polar, density for density, so that 

 a different obstacle must be thereby opposed to the escape 

 of heat from the equatorial and polar regions of the Sun." 

 Arago is at the present moment occupied with experiments, 



