Samuel Molyneux was a gentleman of fortune much attached to science, and particularly to astronomy, who was living about this time at Kew. He was one of the few, moreover, who are not content merely to amuse themselves with a telescope, but had the ambition to do some real earnest work, and the courage to choose a problem which had baffled the human race for more than a century. The theory of Copernicus, that the earth moved round the sun, necessitated a corresponding apparent change in the places of the stars, one relatively to another; and it was a standing difficulty in the way of accepting this theory that no such change could be detected. In the old days before the telescope it was perhaps easy to understand that the change might be too small to be noticed, but the telescope had made it possible to measure changes of position at least a hundred times as small as before, and still no "parallax," as the astronomical term goes, could be found for the stars. The observations of Galileo, and the measures of Tycho Brahé, as reduced to systematic laws by Kepler, and finally by the great Newton, made it clear that the Copernican theory was _true_: but no one had succeeded in proving its truth in this particular way. Samuel Molyneux must have been a man of great courage to set himself to try to crack this hard nut; and we can understand the attraction which his enterprise must have had for Bradley, who had just lost the beloved colleague of many courageous astronomical undertakings. His co-operation seems to have been welcomed from the first; his help was invited and freely given in setting up the instrument, and he fortunately had the leisure to spend considerable time at Kew making the observations with Molyneux, just as he had been wont to observe with his uncle. I must now briefly explain what these observations were. There is a bright star [gamma] Draconis, which passes almost directly overhead in the latitude of London. Its position is slowly changing owing to the precession of the equinoxes, but for two centuries it has been, and is still, under constant observation by London astronomers owing to this circumstance, that it passes directly overhead, and so its position is practically undisturbed by the refraction of our atmosphere. [Sidenote: The instrument.] [Sidenote: Expected results.]