HISTORY OF PHYSICAL ASTRONOMY.

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ascensions, amounting to somewhere between l m and l| m . It has beenproved by the observations of subsequent astronomers, that such a dis-placement could not have arisen from the effects of parallax. The samemay be said respecting a series of results which Horrebow deduced at asubsequent period from the observations of Roemer, and which hepublished with great confidence, as affording incontestable evidence of themotion of the earth, in his work entitled “ Copernicus Triumphans .”

It has been already mentioned, that, in an attempt to establish theexistence of a sensible parallax in y Draconic , Bradley was conductedto his immortal discovery of the aberration of light. This importantresult put an end to the anomalous irregularities which continued toaffect the observations of astronomers, ever since they became sufficientlyaccurate to render sensible the displacement arising from that cause. Itwas now evident that the parallax of the fixed stars was in all casesvastly smaller than it had hitherto been supposed to be. Bradley was ofopinion that if the parallax of y Draconis had amounted even to 1", hisobservations with the zenith sector could not fail to have detected itsexistence *.

Allusion has been made to the method proposed by Galileo for deter-mining the parallax of a star. The invention of the micrometer, aboutthe middle of the seventeenth century, tended very much to enhance thepractical utility of that method. It was probably this circumstancewhich induced James Gregory to recommend its application in a letterto Collins, the Secretary of the Royal Society , dated June 24, 1673f.Gregory does not seem to have been aware that Galileo had alreadysuggested the same method. Huyghens appears to have been the personwho first reduced it to practice. He mentions, in one of his works, thathe in vain attempted to detect any traces of an annual change in therelative position of the two stars composing the middle star in the tail ofthe Great Bearj (f Ursa Majoris). In the following century Dr. Longendeavoured to detect a similar variation in the relative positions ofseveral double stars, but all his efforts were fruitless?. It is to beremarked, with respect to the stars upon which he made his observations,that the component objects in each case were nearly equal in apparentmagnitude, whence it might be presumed that they were equally distantfrom the earth. The absence of an indication of change in their relativepositions did not, therefore, necessarily imply the non-existence of a sen-sible parallax.

The parallax of the fixed stars was one of the earliest subjects whichengaged the attention of Sir William Herschel ||. He did not fail toperceive the many peculiar advantages which the method of Galileo offered, and, with a view to its practical application, he selected a greatnumber of double stars, which, in consequence of the inequality of thecomponent members, in every case appeared to be well adapted for thatpurpose. His labours on this occasion are memorable in the history ofastronomy, on account of their having been instrumental in enabling himto establish the existence of a physical connexion between the bodies com-posing Double Stars .

About the beginning of the present century, the celebrated Piazziendeavoured to ascertain the parallax of some of the principal stars bymeans of their declinations, as observed with the famous altitude and

* Phil. Trans., 1728, p. 637. t Birch, Hist. Roy. Soc., vol iii. p. 225.

1 Cosmotheoros, p. 134. § Astronomy, vol.' i., p. 322, Camb. 1742.

Phil. Trans., 1792, p. 82, et seq.