Bradley
determined to enter on this research once again; he thought that by
using instruments of greater power, and by making measurements of
increased delicacy, he would be able to perceive and to measure
displacements which had proved so small as to elude the skill of the
other astronomers who had previously made efforts in the same
direction. In order to simplify the investigation as much as
possible, Bradley devoted his attention to one particular star, Beta
Draconis, which happened to pass near his zenith. The object of
choosing a star in this position was to avoid the difficulties which
would be introduced by refraction had the star occupied any other
place in the heavens than that directly overhead.
We are still able to identify the very spot on which the telescope
stood which was used in this memorable research. It was erected at
the house then occupied by Molyneux, on the western extremity of Kew
Green. The focal length was 24 feet 3 inches, and the eye-glass was
3 and a half feet above the ground floor. The instrument was first
set up on November 26th, 1725. If there had be any appreciable
disturbance in the place of Beta Draconis in consequence of the
movement of the earth around the sun, the star must appear to have
the smallest latitude when in conjunction with the sun, and the
greatest when in opposition.
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