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HISTORY OF PHYSICAL ASTRONOMY.

intelligent observer of the eclipse at that city alludes—in a note contain-ing suggestions relative to the total eclipse of 1842—to a phenomenonwhich he says may indeed have been observed at other times and recorded,although he has no knowledge that it has been. On that occasion he noticedseveral minutes before the time of the nearest completion of the ring, thefine cusps of the sun’s unobscured crescent prolonged by a hair breadthline of brightness, totally diverse in colour and intensity from the sun's disk.As the cusps approached, the line or thread of light in advance of eachshot round the moon’s edge, between them rapidly, till, at a certain time,the threads from the two, met and joined in one—thus muting thecusps *.

The foregoing observations relative to the appearances noticed duringannular and partial eclipses of the sun, present a satisfactory agreementwith those made during total eclipses, in so far as they indicate theexistence of a solar atmosphere. With respect to the luminous arc seenaround the moon’s dark limb, it would appear in some cases to indicatethe ruddy streak of light which is usually perceived during total eclipsesof the sun a few seconds before and after the complete obscuration ; butin general the phenomenon in all probability represents a fragment ofthe luminous ring, rendered visible in consequence of the atmosphericlight diffused around it being enfeebled by the partial obscuration of thesolar disk. The specks or points of light observed on so many occasionsare manifestly analogous to those seen at Toulon and elsewhere during thetotal eclipse of July 8, 1842. The brush of light observed during theannular eclipse of 1838 is very instructive. Its direct emanation outwardsfrom a faint speck of light situate in the luminous arc, shows that it wasnot the consequence of any irregular scintillation, but rather that it wasoccasioned by some extraneous cause acting constantly in a determinatedirection, and totally independent of the brightness of the source fromwhich it seemed to issue. In fact, it can only be explained by supposingthe luminous speck to be a collection of nebulous matter, through whichthe rays of the sun had partially penetrated, so as to illuminate the par-ticles of the solar atmosphere situated beyond in the line of theirtransmission.

According to the usual theory of the physical constitution of the sun,that body consists of an opaque nucleus, surrounded by an atmosphere inwhich are suspended at different elevations two enveloping substances ofdissimilar physical properties, the lower envelope being imperfectlyluminous, but capable in a high degree of reflecting light, while, on theother hand, the upper envelope forms a resplendent canopy of clouds,which are luminous in themselves, and constitute the source of the lightdiffused in every direction by the sun. The observations of solar eclipseswould seem to indicate, that above the luminiferous envelope there existsa stratum of nebulous matter, which is visible only by means of reflectedlight. Various interesting questions present themselves for solution inconnexion with the admitted existence of such a stratum. In the firstplace, does this third envelope exercise an influence in the production ofany of the other phenomena which have been disclosed by observations onthe physical constitution of the sun? M. Arago has very ingeniouslysuggested that the solar spots which exhibit the aspect of a penumbrawithout a nucleus, may arise from the superposition of masses of this

Silliman’s Journal, vol. xlii., p. 396.