It must, of course, be remembered that in those early days the art of constructing the astronomical telescope was very imperfectly understood.  The only known method for getting over the peculiar difficulties presented in the construction of the refracting telescope, was to have it of the most portentous length.  In fact, Bradley made several of his observations with an instrument of two hundred and twelve feet focus.  In such a case, no tube could be used, and the object glass was merely fixed at the top of a high pole.  Notwithstanding the inconvenience and awkwardness of such an instrument, Bradley by its means succeeded in making many careful measurements.  He observed, for example, the transit of Mercury over the sun’s disc, on October 9th, 1723; he also observed the dimensions of the planet Venus, while a comet which Halley discovered on October the 9th, 1723, was assiduously observed at Wanstead up to the middle of the ensuing month.  The first of Bradley’s remarkable contributions to the “Philosophical Transactions” relates to this comet, and the extraordinary amount of work that he went through in connection therewith may be seen from an examination of his book of Calculations which is still extant.

The time was now approaching when Bradley was to make the first of those two great discoveries by which his name has acquired a lustre that has placed him in the very foremost rank of astronomical discoverers.  As has been often the case in the history of science, the first of these great successes was attained while he was pursuing a research intended for a wholly different purpose.  It had long been recognised that as the earth describes a vast orbit, nearly two hundred million miles in diameter, in its annual journey round the sun, the apparent places of the stars should alter, to some extent, in correspondence with the changes in the earth’s position.  The nearer the star the greater the shift in its apparent place on the heavens, which must arise from the fact that it was seen from different positions in the earth’s orbit.  It had been pointed out that these apparent changes in the places of the stars, due to the movement of the earth, would provide the means of measuring the distances of the stars.  As, however, these distances are enormously great in comparison with the orbit which the earth describes around the sun, the attempt to determine the distances of the stars by the shift in their positions had hitherto proved ineffectual.  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.