HISTORY OF PHYSICAL ASTRONOMY.

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of 2' or o' * * * § ; but the telescope shewed this to be to a great extent an illu-sion, produced by the false light surrounding the star. Galileo attempted,by a very ingenious method, to get rid of the effects of irradiation inmeasuring the apparent diameter of « Lyne, and he so far succeeded inhis object as to assign to the star an apparent diameter of only 5" j-. Thiswas undoubtedly a much nearer approximation to the truth than any esti-mate that had been hitherto formed of the apparent magnitude of a star.

Horroclcs first remarked a phenomenon which furnishes a strikingproof of the extreme smallness of the apparent diameters of the stars.In company with his friend Crabtree, he witnessed an occultation of thePleiades by the moon, on the evening of the 3 0th of March, 1637. Assoon as the stars approached in succession the dark limb of the moon,they were in each instance observed to vanish instantaneously!. Horrocksjustly asserted, that if the whole of the light in each case had emanateddirectly from the body of the star, the latter ought to have disappeared bysensible gradations. He therefore concluded that the apparent diametersof the stars are mere points which are not capable of measurement.

Hevelius , by diminishing the aperture of his telescope, succeeded ingiving a round planetary appearance to the stars, and he determined theapparent diameters of the resulting disks by comparing them in succes-sion with Mercury, whose apparent diameter he had previously ascer-tained from observations of its transit across the sun’s disk. In thismanner he found the apparent diameter of Sirius to be 6" 21'"; that ofProcyon, 4" 58'", &c., &c. §. In 1717, J. Cassini observed Sirius with atelescope, the aperture of which he had similarly reduced, and by com-paring the round disk thus formed with the planet Jupiter , he concludedthat the apparent diameter of the star did not exceed 5"j|. Both He-velius and Cassini erred in supposing that the round appearance of starsseen in telescopes with reduced apertures was real. The phenomenon is,in fact, a spurious image produced by the diffraction of the light in passingthe contour of the aperture.

Halley did not fail to express his suspicion that the apparent diameterascribed by J. Cassini to Sirius was an optical fallacy, occasioned by thegreat contraction of the aperture of the object-glass IT. From the instan-taneous disappearance of stars on the occasion of their occultation by themoon, he concluded, as Horrocks had already done, that the apparentdiameters of the stars is excessively small. He considered that the appa-rent diameters of Spica Virginis and Aldebaran were in both cases lessthan 1". Michell supposed that the diameter of Sirius was even less than0".02. We thus see that the progress of research on the subject tendedinvariably to assign a less and less apparent diameter to the stars.

* Tycho Brahe estimated the apparent diameters of the stars of the first magnitude at2'; those of the second at i y ; those of the third at 1 T 1 2 '; those of the fourth at J'; thoseof the fifth at -J'; and those of the sixth at j'. ( Progymnasmata , p. 482.)

+ Opere de Galileo, tome iv., p. 259.

J “ Venus in Sole Visa,” p. 139. The merit of having originally deduced from thisphenomenon a proof of the extreme smallness of the apparent diameters of the stars hasbeen erroneously ascribed by some writers to Halley, upon the strength of a remark madeby that philosopher, in 1718, relative to the same subject. (Phil. Trans., 1718, p. 853.)

§ “ Mercurius in Sole Visus,” p. 92. Hevelius states on this occasion that he saw Siriuswith the naked eye, on the morning of October 2, 1661, when the sun had alreadyascended above the horizon.

II Mem. Acad, des Sciences, 1717, p. 258.

f Phil. Trans., 1720, p. 3.

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