384

HISTORY OF PHYSICAL ASTRONOMY.

the interval of time which elapses between the instant of the ring becoming visible, and that of total obscuration, for the time which the moontakes to cover the segment will at once afford an indication of its breadth.

It appears from the foregoing remarks, that the comparison of thelight of the ring with the light diffused in the atmosphere by the fullsun, depends upon the determination of the precise time which elapsesbetween the commencement of the visibility of the ring, and the instantof total obscuration. On this point, however, the observations do notagree sufficiently well with each other, to be of much utility to enquirersin resolving so delicate a question. At Montpellier , M. Petit perceivedthe ring, five or six seconds before the sun had totally disappeared behindthe dark body of the moon. At Salon it was seen by M. Largetau, fouror five seconds previous to the total immersion. Mr. Baily has remarked,that only three or four seconds were wanting to complete the total immer-sion of the sun when he first saw the ring.

The luminous ring was seen at some places that were situate actuallybeyond the limits of the lunar shadow, and where consequently a segmentof the solar disk continued visible, even during the time of greatest obscu-ration. In such cases it is manifest that the magnitude of the solar seg-ment visible, upon which the relative intensity of the light of the ring, asdeterminable by the foregoing method, depends, may he obtained by cal-culating the maximum phase of the eclipse at the place under considera-tion. M. Arago cites an interesting observation of this nature, made byM. DTIombre Firmas, at Alais, which was contiguous to the lunar shadow,but not actually involved in it. “ Every one,” says the last-mentionedindividual, “ remarked the circle of pale light which encompassed themoon when she almost entirely covered the sun*. It would appear thatthis is not the only instance in which the ring has been seen during apartial eclipse of the sun t.

The ring at first was only partially visible, being incomplete on theside of the solar disk which was still uncovered by the moon. As soon,however, as the total obscuration w r as effected, the ring appeared entire.“ A bright line,” says Mrs. Airy, “ seemed to form round the right sideof the moon before the disappearance, but not quite round, so that thering was not complete; but at the moment of the total disappearance, theends seemed suddenly to join and form the complete ring.”! At Milan,Sig. Majocchi, and his companions, perceived a fragment of the ring afew seconds before the total obscuration, situate in the region where thefirst contact of the two bodies took place ; and they continued to discern afaint trace of it in the opposite region, a short time after the reappearanceof the sun §. This tallies exactly with the observation just cited.

It was remarked that the ring first appeared brightest on the sideof the solar disk, which was just covered by the moon, but that previousto the close of the total obscuration, it w’as brightest at the part wherethe sun was about to reappear. This interesting fact is alluded to byseveral of the observers of the eclipse. There was no defined edgeto the ring; it changed sensibly, being brightest first on the left

* Annuaire, 1846, p. 339.

+ Thus in an account of the solar eclipse of November 27, 1722, communicated byan observer in America to the Royal Society, it is stated that at Barnstable, on CapeCod , there was but a little left of the sun, and that nearer the head of the Cape, therewas a ring of light quite round the moon (Phil. Trans. 1724, p. 69).

f Mem. Ast. Soc., vol. xv., p. 16. § Annuaire, 1846, p. 340.