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HISTORY OF PHYSICAL ASTRONOMY.

remarked by all astronomers who have observed these phenomena, fromthe period of their discovery down to the present day. “ This want ofgraduation,” says Sir John Herschel , “ this sharply-marked suddennessof transition, is altogether opposed to the conception of a susceptibility ofindefinite and easy mixture, in the luminous, non-luminous, and semi-

luminous constituents of the solar envelope. There is no gradual

melting of the one shade into the other—spot into penumbra—penumbrainto full light. The idea conveyed is more that of a successive withdrawalof veils, the partial removal of definite films, than the melting away of amist, or the mutual dilution of gaseous media.” *

In adverting to the views of Dr. Wilson on the solar spots, it would beunjust to attach to them a degree of importance beyond what they reallypossessed in the author’s own estimation. No philosopher could appre-ciate more fully than he has done, the essential distinction that exists be-tween conclusions drawn from an examination of the real phenomena ofnature, and all mere speculations in physical science, however ingenious orprobable. In a paper on the same subject, which >was subsequently com-municated by him to the Royal Society , and which was written principallywith the view of combating certain objections urged by Lalande against hisexplanation of the spots being excavations in the luminous matter of thesun, we find incidentally thrown out the following series of conjecturesrespecting the origin of the spots :—“ Whether their first production andsubsequent numberless changes depend upon the eructation of elasticvapours from below, or upon eddies or whirlpools commencing at the surface,or upon the dissolving of the luminous matter in the solar atmosphere, asclouds are melted and again given out by our air; or, if the reader pleases,upon the annihilation and reproduction of parts of this resplendent cover-ing, is left for theory to guess at.” j- Some of these surmises, when at-tentively considered, will be found to be by no means destitute of probability.

Allusion has been made to an objection urged by Lalande againstWilson’s explanation of the solar spots. The French astronomer, con-ceiving that the supposition of their being excavations, was incompatiblewith certain observations of Cassini and La Hire, came to the conclusionthat they were rather protuberances existing on the solar surface j. Inorder to account for their formation, he supposed that the luminous fluidwhich covered the dark body of the sun occasionally subsided, laying barethe projecting eminences. The nuclei of the spots represented the latter,while the penumbrse indicated the surrounding shallows. Without allud-ing to the numerous defects of this theory, a fatal objection to it is, thatthe penumbra shades insensibly into the luminous region around it. Thisconclusion, as we have repeatedly had occasion to remark, is totally atvariance with observation.

In 1776 Bode published a theory of the solar spots. He supposed thesun to be surrounded by two atmospheres. One of these was opaque, the other,which was the exterior atmosphere, was luminous. The appearance of aspot was occasioned by a partial opening formed in the luminous atmo-sphere, which disclosed to the eye of an observer the dark substratumbeneath. The luminous matter was wholly withdrawn only at the centreof the aperture, occasioning the appearance indicated by the nucleus of thespot. Towards the sides it merely diminished in thickness, and in conse-

* Results of Astronomical Observations at the Cape of Good Hope .

f Phil. Trans., 1783, p. J62. J Mem. Acad, des Sciences, 1776.