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HISTORY OF PHYSICAL ASTRONOMY.

evident, from a multitude of other considerations, that the moon does notpossess an atmosphere so extensive as that which the ring would assign toher. Assuming the ring which was visible during the total eclipse of 1806to be really due to such a cause, Ferrer calculated, from the apparentbreadth of the ring, that the atmosphere must have attained an elevationof 348 miles above the moon’s surface, whence it would seem to be fiftytimes more extensive than the earth’s atmosphere. He remarked, however,that the refraction of the lunar atmosphere, as indicated by observations ofoccultatious and eclipses, is so small, as to prove, beyond all doubt, that itsdensity is 1080 times less than the density of the terrestrial atmosphere®.The observations of the total eclipse of 1842, tend also to shew that if alunar atmosphere really exist, its effects are totally inappreciablef. It isimpossible, therefore, to avoid the conclusion that the luminous ringcannot proceed from such a cause.

Finding that the hypothesis of a lunar atmosphere was incapable of ac-counting for the phenomenon of the luminous ring, La Hire suggested,that it might be produced by the reflexion of the solar rays from the in-equalities of the moon’s surface contiguous to the edge of her disk, com-bined with their subsequent passage through the terrestrial atmosphere.In order to obtain an experimental illustration of this view of the originof the ring, he took a round unpolished stone of a yellowish colour, whichwas about two inches in diameter; and having suspended it in the air, atthe window of his chamber, so as to appear in the direction of the sun, heretired within the chamber, keeping his eye in the centre of the shadow,until the stone covered the sun and extended a little beyond it. Ob-serving the stone at this distance, he perceived around it a brightmargin of light which he ascribed to the reflexion of the solarrays from the asperities of its edge, and upon the same principlehe asserted that the luminous ring which appeared around the moonduring a total eclipse of the sun, might be accounted for j. Delisle, how-ever remarked, as a proof of the fallacy of this explanation, that the sameappearance would be produced if the stone was ‘perfectly smooth , nay, heeven asserted, that if instead of the stone, a piece of black pasteboard, cutout into any shape approaching to a circle was employed, there would ap-pear a similar line of light around its edges. It was clear, therefore, thatthe hypothesis of La Hire was untenable.

Delisle was the first who conjectured that the luminous ring might beoccasioned by the diffraction of the solar rays which passed near themoon’s edge. He exhibited an illustration of the effect so produced by

* Trans. Amer. Phil. Soc., vol. vi., p. 274.

-)• Louville has stated, that at London , during the progress of the total eclipse of 1715,the part of the solar disk contiguous to the eastern limb of the moon appeared to growcontinually paler, so as to announce beforehand that it was about to be eclipsed {Mem.Acad, des Sciences, 1715, p. 94). This circumstance was considered by him to afford avery convincing proof of the existence of an atmosphere about the moon. It is to be re-marked, however, that modem observations of eclipses have not indicated the slightest traceof a phenomenon analogous to that, which Louville asserts to have noticed. The luminousridges termed_/aeu/re, which occasionally appear on the surface of the sun, are especiallyfavourable for such observations, on account of their uniform aspect, and the consequentease with which any variation in their brightness, resulting from the interposition of thelunar atmosphere, or any change in their form due to the refraction of the solar rays inpassing through it, might be detected; but although M. Arago and his colleagues watchedseveral of such ridges with great attention during the progress of the total eclipse of1842, they were unable to discern the slightest change in their appearance as they ap-proached the moon’s limb ( Annuaire , 1846, p. 351).

J Mem. Acad, des Sciences, 1715, p. 161, et seq.