.HISTORY OF PHYSICAL ASTRONOMY.

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important step in the progress of researches on the solar spots, and it re-commended itself more especially to the attention of astronomers, inas-much as it was not due to any arbitrary assumptions, hut ■was establishedby a rigorous process of inductive reasoning. With respect to the phy-sical cause which determines the generation of the spots, and the variousphenomena connected with them, Wilson candidly acknowledged his in-ability to advance an adequate explanation ; hut he propounded his viewson the subject in the form of queries, some of which are not unworthy ofattention. In order to account for the formation of the spots, he suggestswhether an elastic vapour might not be generated in the interior of thesun, which, by escaping at the surface, would diffuse itself in all directions,and, forcing a passage through the luminous envelope, would expose tothe eye of an observer the dark body of the sun. This hypothesis, heremarked, would satisfactorily explain the circumstance of the boundarybetween the nucleus and the penumbra being always distinctly visible andwell defined. When the action of the vapour diminished in intensity, theluminous matter, in obedience to the solar gravitation, would flow into theexcavation, causing it to contract equally on all sides, and thereby givingrise to the appearance presented by the diminution of a spot. In order toaccount for the gradual process by which the changes of the spots ap-peared to be effected, Dr. Wilson supposed the luminous matter of the sunto possess rather the consistency of a thick fog than the mobility of afluid or gaseous envelope. The penumbra indicated the sloping sides ofthe excavation, which were supposed to be less bright than the rest of thesurface, on account of a diminution of lustre experienced by the luminousmatter when it diffused itself over them. With respect to the faculae,Dr. Wilson imagined them to be, in all probability, merely phenomenaof light and shade, arising from tumultuous movements in the luminousmatter, which were occasioned by the intense action of the excavations.He adduced in support of this explanation, the remarkable fact, that thefaculee appear always in the vicinity of spots, and are never observed inthe polar regions of the sun.

The foregoing speculations of Dr. Wilson, on the origin of the solarspots, are very ingenious ; but as they involve the assumption of a gaseousvapour, of whose existence we have no proof, they are inadmissible intothe rank of physical truths deduced by legitimate reasoning from esta-blished facts, and, therefore, cannot be recognised as really forming a partof astronomical science. But apart from the objection that the principleto which the phenomena of the spots are ascribed is not a vera causa, thehypothesis of Wilson, w'hen submitted to close examination, will be foundto afford a very inadequate explanation of these phenomena. One impor-tant defect we shall mention, and it will be unnecessary to point outmore ; for unless a theory be capable of representing all the details of aphenomenon, it cannot be considered as a true embodiment of naturalfacts. Although the boundary between the nucleus and the penumbra issufficiently well accounted for, the case is very different with respect to theexterior boundary of the latter. It is impossible to conceive how a homo-geneous vaporous substance, such as the luminous matter of the sun isassumed to be composed of, could experience so striking a change oflustre as observation indicates, by a mere change in the inclination of itssurface to the visual ray. That the transition is abrupt and not gradualfrom the nucleus to the penumbra, and from the penumbra to the whollyluminous region around it, is a feature of the solar spots which has been