HISTOEY OF PHYSICAL ASTKOXOMY.

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may be ascertained by a simple process of arithmetic. James Gregoryappears to have first suggested this mode of determining the distance ofa star from the earth. Huyghens , by applying it to Sirius, found thatthe distance of that star from the earth exceeded the distance of thesun 28,000 times. The same method subsequently attracted the atten-tion of various astronomers, among whom may be mentioned Chesaux,Lambert, Micliell, and Olbers , who all agree in assigning to the stars ofthe first magnitude a parallax less than 0".5, as the result of its application.

Wollaston found that the light of Sirius appeared equal to that of thesun when reflected from the surface of a sphere -Jjth of an inch in

diameter, and seen at the distance of 210 feet. Supposing the half of

the solar light to be lost during reflexion, he hence concluded that thelight of the sun was to that of the star as 20,000 millions to one.Hence, if we assume the star to be equal in magnitude and intrinsicsplendour to the sun, this result would imply that the distance of the starfrom the earth is 141,421 times greater than the distance of the sun,whence its parallax would amount to 1".8*. It has been ascertained,

however, from the observations of astronomers, that the parallax of

Sirius is in reality less than half a second. The conclusion thereforeis, that in magnitude the star greatly surpasses the sun.

Wollaston found by a similar method, that the light of the sun wasto the light of a Lyrae, as 180 thousand millions to one, and consequentlythat the light of a Lyrae was only one-ninth of that of Sirius. This wouldimply that a Lyne was three times more distant than Sirius, and that itsparallax was therefore only 0".6. Even this, however, considerably ex-ceeds the true value as recently determined by M. Struve.

It has been already mentioned, that the attempts of the earlier astro-nomers to detect an annual variation in the apparent places of the stars,depending on the motion of the earth in her orbit, was not attended withany success. Copernicus attributed the absence of all sensible indicationof parallax to the immense distance of the stars compared with the radiusof the terrestrial orbit. Tycho Brahe , although possessing instruments ofmuch more perfect construction than any hitherto employed for the pur-poses of astronomical observation, was unable to detect the slightest traceof an annual displacement in any of the stars. On the other hand, heestimated the stars of the first magnitude to have an apparent diameterof about 2' or 3'. He asserted, therefore, that if the earth really movedround the sun, it would necessarily follow that the stars of the firstmagnitude exceeded in dimensions the whole amplitude of the terrestrialorbit, a conclusion, which appeared to him to be palpably absurd, and fatalto the Copernican theory of the universe.

Galileo suggested a mode of investigating the parallax of the fixedstars, which has received more than one successful application in recenttimes. He remarked, that in those instances wherein two stars of un-equal magnitudes are apparently very near to each other, it may bepresumed that the smaller of the two stars, appears so only from itsbeing more distant, and therefore that its parallax may be considered asinsensible relative to the parallax of the larger star. He imagined,therefore, that by continuing to observe the position of the larger starwith respect to the smaller, throughout the course of the year, theparallax of the larger star might be detected-)-. This method obviously

* Phil. Trans., 1829, p. 19, et seq. + Opere di Galileo, tome iv., p. 272.

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