THE sun's spots. 81 



than in the center. The last named of these distinguished 

 physicists and astronomers expresses himself as follows, in 

 reference to this question.* " Now, granting the existence 

 of such an atmosphere, its form, in obedience to the laws of 

 equilibrium, must be that of an oblate spheroid, the elliptic- 

 ities of whose strata differ from each other and from that of 

 the nucleus. Consequently, the equatorial portions of this 



according to the ratio of the cosine of the angle ; but in the same ratio^ 

 the greater number of the material points emit a feebler light, in con' 

 sequence of their obliquity. The ratio of the angles is naturally the same 

 for a gaseous sphere ; but since the obliquity does not produce the same 

 amount of diminution in gases as in solid bodies, the margin of the gas- 

 eous sphere would be more luminous than its center. That which we 

 term the luminous disk of the Sun is the gaseous photosphere, as I have 

 proved by the entire absence of every trace of polarization on the mar- 

 gin of the disk. To explain the eqtiality of intensity indicated by the 

 polariscope for the margin and the center, we must admit the existence 

 of an outer envelope, which diminishes (extinguishes) less of the light 

 which comes from the center than from the marginal rays having a 

 longer way to traverse before they reach the eye. This outer envel- 

 ope forms the whitish corona of light observed in total eclipses of the 

 Sun. The light which emanates from solid and liquid incandescent 

 bodies is partially polarized when' the rays observed form an angle of 

 a few degrees with the surface from whence they emerge ; but there 

 is no sensible evidence of polarization when incandescent gases are 

 seen in the polariscope. This experiment proves, therefore, that solar 

 light does not emanate from a solid mass or an incandescent liquid. 

 Light is not engendered solely on the surface of bodies ; but a portion 

 originates within the substance itself, even when the experiment is 

 made with platinum. Light, therefore, is not produced by the decom- 

 position of the ambient oxygen. The emission of polarized light from 

 liquid iron is an effect of refraction during its passage toward a medium 

 of lesser density. Wherever there is refraction, a small amount of po- 

 larized light must be produced : gases do not emit polarized light, be- 

 cause their strata do not possess the requisite amount of density. When 

 the Moon is followed through all its phases, it will be found to afford 

 evidences of polarization, excepting at the full moon, and the days im- 

 mediately preceding and following it. It is more especially during 

 the first and last quarters that the unequal (mountainous) surface of 

 our satellite presents suitable inclinations for the polarization of solar 

 light by reflection." 



* Sir John Herschel, Astron. Observ. made at the Cape of Good Hope, 

 $ 425, p. 434; Outlines of Astr., $ 395, p. '234. Compare Fizeau and 

 Foucault, in the Comptes Rendus de V Acad, des Sciences, t. xviii., 1844, 

 p. 860. It is remarkable enough that Giordano Bruno, who was burned 

 eight years before the invention of the telescope, and eleven years be- 

 fore the discovery of the spots of the Sun, should have believed in the 

 rotation of the Sun upon its axis. He considered, on the other hand, 

 that the center of the Sun was less luminous than the edges. Owing 

 to an optical deception, he believed that he saw the disk turn round, 

 and the whirling edges expand and contract. (Jordauo Bruno, par 

 Christian Bartholmess, tom. ii., 1847, p. 367.) 



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