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HISTORY OF PHYSICAL ASTRONOMY.

of liglit which usually appears around the moon during a total eclipse ofthe sun *.

Tycho Brahe , misled by an erroneous determination of the apparentdiameters of the sun and moon, came to the conclusion that a total eclipseof the sun was impossible, and that, in fact, all central eclipses of thatbody were necessarily annular. In accordance with this view of thesubject, he contended that the eclipse of 1500, observed at Coimbra byClavius, as well as the eclipse of 1508, observed at various places in theNorth of Europe , both of which were reputed to have been total, were inreality annular. It has been ascertained, however, that in each of theseinstances, the apparent diameter of the moon was less than that of thesun; and, therefore, it follows that during a short interval of time thesun must have been totally concealed from observation.

In 1001 there happened an eclipse of the sun, which appears, beyondall doubt, to have been annular in Norway. Longomontanus asserts,upon the authority of Fossius, Bishop of Bergen, that the fishermen onthe neighbouring coast perceived with great admiration the whole body ofthe moon projected upon the sun, leaving uncovei'ed a uniform ring ofthe solar disk, about a digit and a half in breadthf. An annular eclipsewas also observed by Bouillaud, in the year 1G39J.

The first annular eclipse respecting which we possess a detailed account,was one which happened on February 18, 1737. It was observed inScotland by the celebrated mathematician, Maclaurin , who communicatedto the Royal Society an interesting paper respecting it, which appears inthe volume of the Transactions of that body for the same year§. Theannular eclipses which have since been observed are those of 1 748, 1704,1791, 1820, 1831, 1836, 1838, and 1847.

We now proceed to notice, in detail, the phenomena which generallycharacterise solar eclipses, and to give a brief account of the various specu-lations that have been propounded respecting their physical origin.

It has been universally remarked that during the progress of eclipsesthe colour of the sky undergoes a change. Halley, in his account of thetotal eclipse of 1715, speaks very explicitly upon this point. “Whenthe eclipse,” says he, “was about ten digits (that is, when about five-sixthsof the solar diameter were immersed), the face and colour of the skybegan to change from perfect serene azure blue to a more dusky lividcolour, intermixed with a tinge of purple, and grew darker and darker tillthe total immersion of the sun.’jj The observers of the total eclipse ofJuly 8, 1842, all concur in asserting that the colour of the sky underwenta remarkable change during the progress of the obscuration; but theydiffer materially in the details which they furnish respecting the pheno-menon. M. Arago remarks that these discordances are mainly attributableto physiological causes connected with the organ of vision; and as it isimpossible to arrive at an adequate appreciation of the influence of thesecauses, he considers that any enquiry founded upon the statements of thevarious observers would be altogether useless, more especially as they arefor the most part destitute of precision. He, therefore, undertakes simplyto investigate the effect which the progress of the eclipse tends to produceupon the colour of the atmospheric light, in any particular region, without

Ad Vitellionem Paralipomena, p. 299.

f Astronomia Danica, lib. i., cap. 9.§ Phil. Trans., 1737, p. 177, et seq.

| Astronomia Philolaiea, lib. ii., p. 210.|| Ibid., 1715, p. 247.