HISTORY OF PHYSICAL ASTRONOMY.

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a micrometer the distance between tlie star and another very small star,situate about 43" from it, repeating the operation at different seasonsthroughout the year. The result of this enquiry went to assign a parallaxof 0".261 to the brighter of the two stars.

Shortly afterwards, the existence of an annual variation of sensibleamount in the apparent position of a star, depending on the motion ofthe earth in her orbit, was established beyond all doubt, in two instances,by the independent labours of two contemporary astronomers. The stars,whose absolute distances have been thus ascertained, are 61 Cygni anda. Centauri, the parallax of the former having been determined by Bessel,and that of the latter by the late Professor Henderson, of Edinburgh . Itis impossible, within the limits to which we are confined, to do anythingelse than merely bestow a passing glance on the important researchesof these astronomers on this occasion.

The star 61 Cygni, besides being doublets remarkable for an unusuallylarge proper motion. It appears, in fact, from the observations of astro-nomers, to be transported through space with an annual motion of rathermore than 5" of arc. This circumstance induced Bessel to suspect thatits parallax must be very considerable, and he resolved accordingly toascertain its amount. He chose for this purpose two very small stars,one at the distance of ll'.S from the centre of the line joining the twocomponent members of 61 Cygni, and the other at the distance of 7'. 7from the same point; and the establishment of the parallax of the stardepended on the detection of an annual variation of these distances, asdetermined by observation at different seasons of the year. The distanceswere ascertained by means of a magnificent heliometer, which Fraun-hofer, of Munich , had recently executed for the Observatory of Kiinigs-berg, and which, from the principle of its construction, was eminentlyadapted for micrometric measurements on so large a scale. The observa-tions were commenced in the month of October, 1837, and were continuedtill March, 1840. By a subsequent discussion of them, in which everyimaginable cause of disturbance was taken into careful consideration, andits effect rigorously calculated, Bessel arrived at the final conclusion thatthe parallax of the star amounts to 0".3483. This result has received acomplete confirmation from the recent researches of M. Peters, who, froma series of zenith distances of the star, determined at the Observatory ofPulkowa during the vears 1842-3, has found its parallax to be equal toOT349.

It has been mentioned, that the other astronomer who succeeded inestablishing by unequivocal evidence the existence of a sensible parallaxin one of the fixed stars, was the late Professor Henderson, of Edinburgh .The double star, a Centauri, which was the object of his researches onthat occasion, is one of the brightest sidereal objects in the southern he-misphere . His investigation was founded on the zenith distances of thestar as observed by himself at the Cape of Good Hope , with a muralcircle, during the years 1832-3. These observations were made with theview of establishing the mean declination of the star. It was only froma consideration of the large proper motion of the star, which amountsannually to 3".6 of .arc, that he w r as induced several years afterwards toinstitute an enquiry into its parallax. The result at which he arrivedconfirmed his previous suspicion, the star having been found by him tohave a parallax of 1".16. The researches of Mr. Maclear, the successorof Henderson at the Cape of Good Hope , have completely verified the