HISTORY OF PHYSICAL ASTRONOMY.

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The darkness ■which prevails daring the total obscuration of the sun,does not appear to he equally profound, when the observations of differenteclipses are compared together; nor even at different places where thesame eclipse has been observed. This arises partly from the variablecondition of the atmosphere, and partly from the circumstance that duringan eclipse all places are not equally immersed in the moon’s shadow. Itis manifest that all those places which are situate close to the boundaryof the lunar shadow, are exposed in a greater degree to the light reflectedfrom the regions of the atmosphere upon which the sun is still shining,than are the places contiguous to the centre of the shadow. It was uponthis principle that Halley explained the fact that at London the obscurityduring the total eclipse of 1715, was less intense than that which pre-vailed in various other parts of England that were more deeply immersedin the lunar shadow* * * § .

A fact was noticed during the total eclipse of 1842, which deserves tobe mentioned. Sig. Piola states, that at Lodi, the darkness during thetotality of the eclipse was equal to that by which the stars of the secondmagnitude are usually discerned, and yet only those of the first magnitudewere visible f. A similar remark was made by M. Otto Struve, withrespect to the same eclipse. He states that at Lipesk the darknesssurpassed in a small degree that which reigns at St. Petersburg duringthe summer solstice; but that, while in the latter case the stars of thethird magnitude are usually discernible without any difficulty, in theformer case those of the first alone could be perceived J. Sig. Belliexplains this curious fact by reference to a physiological principle. Heremarks that during the short interval of total obscuration, the eye hasnot sufficient time to recover from the dazzling effect of the sun’s rays,and consequently is unable to take due advantage of the obscurity whichactually prevails §. This is, doubtless, the true explanation of the anomalyobserved on such occasions.

The suddenness with which day succeeds night, upon the reappearanceof the sun after undergoing a total eclipse, has been remarked by allpersons who have witnessed a phenomenon of this nature. The first rayof the sun darts forth from behind the moon’s limb with a velocity thathas been compared to the swiftness of an arrow’, a flash of lightning, orsome such emblem of extraordinary speed I]. According to Sig. Piola, sorapid was the effect produced by the reappearance of the sun on theoccasion of the total eclipse of 1842, that it might be said to have beennight, and in an instant it w'as day IT. The same observer states, that it

* Phil. Trans., 1715, p. 250. Sig. Piola has stated that persons stationed upon thehills around Brescia, during the total eclipse of 1842, enjoyed the beautiful prospect ofthe peaks of Rosa and Cimone brilliantly illuminated by the sun’s rays, while they them-selves were involved in the obscurity of the moon’s shadow. ( Gior.dell ’ 1st. Lomb.,p. 310.)

t Giorn. dell’ 1st. Lomb., tome iv., p. 341.

| Bibliotheque Universelle de Geneve, tome xliv., p. 368.

§ Giorn. dell ’ 1st. Lomb., tome iv., p. 341.

|| “ Instar fulgoris”—“instar sagittarum radii solis repente prorumpentes”■—“radiussolis instar sagiftas prevolavit”—“ radius admod um iliustris prorupit,” Sec., Sec. Such arethe terms employed by the Lutheran pastors of Sweden , in describing the reappearanceof the first rays of the sun on the occasion of the total eclipse of 1733.

f “ Talche si pote dire che in momento era notte e fu giorno ” ( Giorrtale dell J. 11.Jstituto del Lombardo, tome iv., p. 314). Attempts have been made to detect themotion of the moon’s shadow in the course of its passage over the surface of the earth.To effect such an object, however, is manifestly a very difficult matter, on account of the