376
HISTORY OF PHYSICAL ASTRONOMY.
was universally remarked by the spectators of the eclipse at Lodi, thatthe light emitted by the sun previous to the total obscuration, produced amuch less dazzling effect than that which distinguished his emersion frombehind the dark body of the moon. In the former case, the spectatorwas enabled to look upon the sun for several minutes previous to the totalobscuration, without experiencing any inconvenience; in the latter case,it was impossible for the naked eye to withstand the violent impact of thefirst rays of the sun. This fact had been already noticed by Halley, onthe occasion of the total eclipse of 17 L5. The English astronomer hadsuggested two distinct causes, to whose combined operation it might beascribed. One of these was of a physiological nature; the other impliedthe existence of a lunar atmosphere. He remarked, in the first place,that previous to the total obscuration, the pupil of the eye might he verymuch contracted by viewing the sun, and, consequently, the organ ofvision would be less liable to suffer from the effulgence of the light thanat the instant of emersion, when the pupil had again expanded. Secondly,lie suggested that, as the eastern margin of the moon, at which the sundisappeared, had been exposed for a fortnight to the direct action of thesolar rays, the heat generated during this period might cause vapours toascend in the lunar atmosphere, which, by their interposition between thesun and the earth, would have the effect of tempering the effulgence ofthe solar rays passing through them. On the other hand, the westernmargin of the moon, at which the sun reappeared, had just experienced anight of equal length, during which the vapours suspended in the lunaratmosphere, had been undergoing a course of precipitation upon themoon’s surface under a process of cooling. In this case, therefore, thesolar rays would meet with less obstruction in passing through the lunaratmosphere, and, consequently, it was reasonable to suppose that theywould produce a more intense effect*.
It is clear that both causes above mentioned might conspire together inproducing the observed effect. As, however, there are no reasons forsupposing that the moon possesses an atmosphere capable of exercisingany appreciable influence, it is probable that the true explanation is to besought in the different dimensions of the pupil of the eye, at the com-mencement and the end of the total obscuration.
The luminous ring that appears around the moon is one of the mostinteresting features of a total eclipse of the sun. The earliest allusion toit is probably to he found in the passage of the Life of Ajwllonius, alreadycited, wherein the author mentions that the death of the emperorDomitian had been previously announced by a total eclipse of the sun.“In the heavens,” says Pbilostratus, “there appeared a prodigy of this
immense velocity with which the shadow sweeps over any particular place of observation.Halley calculated that the shadow of the eclipse of 1715 passed over England at therate of fifty-nine geographical miles in a minute {Phil. Trans., 1715, p. 260). Mr.Airy has mentioned in his account of the total eclipse of 1842, that he endeavoured todetect the progress of the lunar shadow as it passed over the immense plain of Lom-bardy , but that his efforts were unsuccessful. He adds, however, that Messrs. Plana andForbes felt assured that they saw the darkness travel over the country. The only remarkof a similar nature which the author has met with in the accounts of former eclipses iscontained in the following extract from Duillier’s paper on the total eclipse of 1706 :—“ A little before the total obscuration, the country on the west side did already seemovercast with darkness; and after the total obscuration, the darkness was seen to leave usmore and more, and to fly eastward ” {Phil. Trans., 1706, p. 2248).
* Phil. Trans., 1715, p. 248.