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HISTORY OF PHYSICAL ASTRONOMY.

1842, would serve to throw some light upon this interesting phenomenon.On this occasion, however, astronomers were disappointed. In someinstances a faint resemblance to beads of light was seen previous to thetotal immersion, but, generally speaking, the phenomenon was totallyinvisible. Even Mr. Baily, whose principal motive for proceeding to Italy was to verify, by means of this eclipse, his previous observations, wasunable to discern any trace of the phenomenon.

The observations of the annular eclipse of October 0, 1847, afforded apartial verification of the phenomenon witnessed by Mr. Baily in 1836.Captain Jacob, who observed the eclipse at Bombay, states, that when theannulus was about being formed, the moon’s limb seemed to be united withthat of the sun by a dark ligament about 1' in breadth, which became moreelongated as the moon advanced upon the sun. The moon’s limb wasperfectly well defined, except at the part where the ligament was perceived.This phenomenon lasted about three or four seconds, when the ligamentat length suddenly gave way, and the annulus was seen complete. Thephenomenon exhibited at the dissolution of the annulus was somewhatdifferent. When the moou’s limb approached very close to that of thesun, a portion of it, about 30° in extent, suddenly floived over in dark lines,leaving bright spaces between. This appearance lasted only about twoseconds *.

The same eclipse was observed at several stations in Europe , but no de-cisive indication of the beads or dark lines which exhibited so striking an ap-pearance during the annular eclipse of 1836, was obtained in any instance.

The question relative to the physical cause of the phenomena abovementioned has naturally excited considerable speculation, but it cannotbe said that any definitive conclusion has yet been arrived at upon thishead. That they are mainly illusions of an optical nature is generallyadmitted by all enquirers, but the modus operandi seems to be involvedin much obscurity. The most probable hypothesis of their origin is thatwhich refers them to the influence of irradiation. The enlargement ofthe apparent magnitudes of the celestial bodies, arising from this cause,has been already alluded to. The observations of solar eclipses tend es-pecially to corroborate the truth of this remark. It has been alreadymentioned that Kepler adduced the appearance presented during the solareclipse of 1590, as affording an obvious proof of the influence of irradiation.The extraordinary agility with which, according to Wyberd, the moonthrew herself upon the solar disk on the occasion of the total eclipse ofMarch 29, 1652, cau only be accounted for by supposing that the sun wasfringed with a border of spurious light which disappeared all at once assoon as the last portion of his limb was immersed. In his account of theannular eclipse of Feb. 18, 1737, Maclaurin states that the breadth of theannulus appeared much greater to the naked eye than could have beenexpected from the difference of the semi-diameters of the sun and moon.“ This appearance,” he remarks, “ proceeded chiefly, I suppose, from thelight encroaching on the shade, as is usual; but whatever was the cause,everybody seemed surprised that the moon appeared so small upon thedisk of the sun.”f

The enlargement produced by irradiation is also indicated by a remarkof Mr. Fullarton, who, in a description of the same eclipse as it appearedat Crosby, near Ayr, states that the annulus was of a uniform breadth,

Monthly Proc. Ast. Soc., vol. viii., p. 27.

f Phil. Trans. 1737, p. 183.