HISTORY OF PHYSICAL ASTRONOMY.

227

hypothesis'of their origin advanced in the foregoing passage, and suggeststhe expediency of a continued prosecution of attentive observations of thespots. By these means it is to be hoped that some light will be thrownupon the real nature of these mysterious but singularly interesting pheno-mena, which, in its turn, will lead to a more accurate knowledge of thephysical constitution of the sun.

We have mentioned that Sir William Herschel , in his theory of thesolar spots, supposes the solid nucleus of the sun to be effectuallyscreened from the luminous clouds of the superior regions by the interpo-sition of the planetary clouds which possess the property of reflecting therays tliatfall upon them. That the clouds of the lower regions shine only byreflection, he infers from their constantly exhibiting a faint and uniformcolour; and that they form a continuous envelope round the nucleus of thesun, appears to him equally manifest from his being enabled, by means ofhis powerful telescopes, to discover them through the minute fissures bywhich every part of the luminous surface of the sun is intersected *. Heconsiders, therefore, that, notwithstanding the circumstance of the lumi-nous atmosphere of the sun being the source from which light and heatare dispensed in all directions, the solid nucleus of that body may be con-stantly maintained at a moderate temperature. Proceeding upon this viewof the physical constitution of the sun, he comes to the conclusion that thecentral body of the planetary system is eminently fitted to be the abode ofanimated beings, and that in fact it offers the example of a magnificent self-luminous planet. This opinion has been ably controverted by one of themost illustrious philosophers of the present age.-—“ It is indeed incon-ceivable,” says Sir David Brewster , “ that luminous clouds, yielding toevery impulse, and in a state of perpetual change, could be the depositoryof that devouring flame, and that insupportable blaze of light, which areemitted by the sun; and it is still more inconceivable that the feeblebarrier of planetary clouds could shield the subjacent mass from the de-structive elements that raged above.” According to the opinion of theeminent authority just quoted, the heat and light of the sun derive theirorigin from two distinct sources. The luminous rays are emitted from thephosphorescent mantle which constitutes the exterior envelope of the sun,while the calorific rays proceed wholly from the opaque nucleus. Ourlimits will not allow us to notice the ingenious arguments which the authorof this explanation adduces in its support j-. Unfortunately our know-ledge of the nature of both heat and light is so obscure, that every attempthitherto made to form an hypothesis respecting the physical constitutionof the sun, appears to be beset with insuperable difficulties.

It is remarkable that, at so recent a period as that of the discovery ofthe solar spots, astronomers were unacquainted with any method bywhich the sun might be viewed without injury to the eyes. Appian,an astronomer of the fifteenth century, had indeed pointed out the utilityof coloured glasses for this purpose, but the hint appears to have beenentirely lost sight of hy his successors. Fabricius observed the solarspots either by introducing the sun’s rays through a small aperture intoa dark room, and projecting his image on a piece of white paper, or byviewing him directly through thin clouds and vapours when he was nearthe horizon. He recommends all those who purpose making direct obser-vations of the spots to admit to the eye at first only a small portion of the

* Phil. Trans., 1802, p. 294.

t See Brewster’s Edin. Encyclop., Art. “Astronomy.”

Q 2