Several observers about this time discovered a motion in the stars which they could not account for. Picard, Roemer, and Flamsteed all found that the position of Polaris varied at different times of the year, and Flamsteed fancied that the change was due to parallax. In or about 1725, a gentleman of the name of Molyneux, then resident at Kew, employed Graham to make him a parallactic telescope, so called from its object, namely, to discover the change of place in the stars occasioned by the earth's change of position in her orbit. This is described in Smith's 'Optics,' book iv., cap. 7, and more fully in Mr, Molyneux's own words in Bradley's Miscellaneous Works and Cor respondence,' p. 93. It consisted of a telescope 25 feet long, with a short cross axis at the upper end, just at the place of the object-glass. This axis was so adjusted on its supports that the telescope moved in the meridian. A plumb-line hung on one end of the axis and passed over a dot in a plate fixed on the tube near the eye-end. The tele scope was drawn forwards in the meridian by a string and weight passing over a fixed pulley, and pushed back by an antagonist-screw, which had a divided head and index. To make the observation the plumb-line is first made to bisect the dot by the screw, and the division read off; then the star is bisected also by moving the screw, when the division is again read off. The revolutions and parts through which the screw is moved measures the angular distance of the star from a line in the heavens, which continues fixed. if there is no change in the relation of the object-glass and wires to the axis and dot.
Observations were made at Kew for some time with this instrument by Molyneux, Graham, and Bradley ; and in 1727 Bradley had a zenith sector constructed by Graham with which he made his celebrated discoveries of aberration and notation. There is a short description of this instrument by Maskelyne in the first volume of his Greenwich Observations.' p. 9, which Rigaud has reprinted, with some memoranda by Bradley, in the Miscellaneous Works,' ecc.; but Bradley himself gave no description in his Memoir on Aberration (` Phil. Traria.' vol. xxxv., p. 637), and in his following Memoir on Notation (` Phil. Trans.', vol. xlv., p. 1) satisfied himself by referring to the description of a sector on a similar construction. (`Ddgre du Mdridien entre Paris et Amiens,' 1740.) This last-mentioned sector was made by Graham for the measurement of the degree in Lapland, and afterwards employed in the remeasurement of Picard's arc. We have now a full and minute description of Bradley's instrument, with numerous plates, in a work entitled ' Operations for the Verification and Extension of the Abb6 de la Caille's Arc of the Meridian,' by Thom'as Maclear, Esq., pp. 67.81, published by order of the Lords of the Admiralty. This account is by the astronomer-royal, who directed the alterations required to fit the sector for the field. The principal parts of this instrument, as originally
made by Graham, are a telescope with a short sector attached to the eye-end, and a short cross or transit-axis to the object-end, which causes it to move in the meridian when properly adjusted. A plumb-line passes over a fine dot at the extremity of the upper axis, and beats on the divided sector below, that is, it almost touches the dot above and arc below, but still hangs perfectly free. To prevent any disturbance from the wind, the plumb-line is screened by a tube, and the bob hangs in water that it may sooner come to rest. To make the telescope describe the meridian correctly, as well as to get a proper fixing for the clamp and micrometer-screw, another are is fixed to the wall, and the telescope carries a frame with rollers at its eye-end, and is thus kept in contact with the fixed arc before and behind. A clamping apparatus, which slides along the fixed arc, and can be attached to it by screws in any position, carries a fine screw with a micrometer head. which pushes the telescope by acting on a piece of hardened steel, while the teles cope resists either by gravity or by a counterpoise weight. There are numerous parts and contrivances for different adjustments, which will be easily understood from Mr. Airy's account. In making the observa tion, the telescope is first to be set, or nearly so, to the star, the bisection of the upper dot is verified, and then a division below is bisected by carrying the screw one way, forward, for instance, and the micrometer head is to be read off. When the star is in the centre of the field, it is bisected by carrying the screw still forward, and the micrometer is again read off. Finally, the screw is still to be carried forward till the next division is bisected, and the micrometer read off. A simple proportion will give the quantity, which is to be added to the first reading, or subtracted from the second reading, in order to get the reading corresponding to the star.* Bradley's sector as originally made was not reversible, and was therefore only fit for measuring differences or variations. When, after its removal to Greenwich, actual zenith distances were required, the instrument was shifted across the room, from the east to the west side, where a second fixed arc, &c. were ready to receive it. We need not say that this was ,very objectionable, as such an operation, take what care you may, is always liable to derange the relation of th parts of the instrument, and so to lead to untrue conclusions. Even while it was at Wanstead, and considered to be immoveable, a alight change seems to have taken place, which has been investigated by Dr. Busch. In the' Reduction of the Observations made by Bradley at Kew and Wanstead,' Oxford, 1832. In remounting it, the astronomor.royal has given the property of reversion very satis factorily, and Mr. Maelears observations with it are of extraordinary accuracy.