HISTOBY OF PHYSICAL ASTRONOMY.

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ing, and that it would encircle the moon like a ring. I was, therefore,somewhat surprised and astonished at the splendid scene which now sosuddenly burst upon my view.”* At Lipesk, the brilliancy of the ringwas still more vivid. According to M. Otto Struve, it was so intense asto be barely supportable to the naked eye. So strong was the impressionwhich it produced upon the spectators of the eclipse, that many of themcould with difficulty be persuaded that the whole of the solar disk wasactually concealed behind the body of the moon j-.

These extraordinary variations in the brightness of the ring are doubt-less attributable to the more or less favourable condition of the atmosphereat the different places of observation. In connexion with this explanation,M. Arago has suggested, with great probability, that the intense brilliancywhich the ring exhibited at Lipesk, may have arisen from the compara-tively high altitude of the sun during the totality of the eclipse. As anillustration of this remark it may be stated that at Perpignan , where thering resembled the moon in lustre, the altitude of the sun at the time ofhis total immersion was 11° 59'; whereas at Lipesk the altitude of thesame body was 41° 19', when the totality of the obscuration took place.Now it is very manifest that the atmospheric medium of air through whichthe eclipse was observed, was much less favourable for discerning thebrightness of the ring in the former case, than it was in the latter

At Montpellier , there were many persons who asserted that the ringturned continually round its centre. We have seen that a similar remarkhad been already made with respect to the corona which appeared duringthe total eclipses of 1052 and 1778. At Lipesk, the light of the ringseemed to M. Otto Struve to be in a state of violent agitation. Mr. Bailystates, that the rays had a flickering appearance, somewhat like that whicha gas illumination might be supposed to assume, if formed into a similarshape.

The ring generally became visible a few seconds previous to the totalimmersion of the sun, and it continued to be perceived during an equalinterval of time subsequent to his reappearance. This circumstance sug-gested to M. Arago an interesting method of determining the intensity ofthe light of the ring relative to the light diffused throughout the atmo-sphere by the full sun. It is manifest, that at the instant when the ringfirst becomes visible, its light must exceed, in intensity, the atmosphericlight which appears around it. In order that an object may becomebarely visible, it is necessary that its brightness should exceed that of theground upon which it appears projected by a certain determinate quan-tity §. In this manner, then, the light of the ring becomes directly com-parable in intensity with the light diffused around it. Now, the atmo-spheric light around the ring varies in the direct ratio of the solar segment,which remains uncovered by the moon. Hence, knowing the magnitudeof the visible segment of the sun, it is easy to compare the light diffusedby it through the atmosphere with the light diffused by the full sun, andwhen this point has been ascertained, the relative intensities of the lightof the ring and the light diffused by the full sun become comparable also.Now, the magnitude of the solar segment may obviously be deduced from

* Mem. Ast. Soc., vol. xv., p. 4.

t Annuaire, 1846, p. 336. j: Ibid., p. 336.

§ At p. 381 of the Annuaire for 1846, M. Arago asserts, as the result of experiment,that in such a case the brightness of the object must exceed that of the ground uponwhich it is projected by a sixtieth part.