Page:Popular Astronomy - Airy - 1881.djvu/202

188 are two of the irregularities of which it is necessary to take account in obtaining accurate results from observations; but there is a third, which is greater in its magnitude than nutation, and totally different in its nature. It is the aberration of light. It was long ago made out that vision is produced by something coming from the object to the eye, and that this something comes from the object to the eye with a definite velocity. Now, in consequence of this light coming from the object to the eye with a definite velocity; and in consequence of the earth's moving with a definite velocity; by the combination of these two things, there is produced a disturbance in the visible place of every object not connected with the earth which we look at. Perhaps one of the simplest ways of giving an idea of the effect of this combination, in relation to the aberration of light, will be to refer you to the chance experiment which suggested the theory of aberration to one of my predecessors (Dr. Bradley), by whom in fact, the aberration of light was discovered and reduced to law. He says, he was being rowed on the Thames, in a boat which had a small mast with a vane at the top. At one time the boat was stationary, and he observed, by the position of the vane, the direction in which the wind was blowing. The men commenced pulling with their oars, and he observed that, at the very time they commenced pulling, the vane changed its position. He asked the watermen what made the vane change its position? The men said they had often observed the same thing before, but did not pretend to explain the cause. Dr. Bradley reflected upon it, and was led by it to the theory of aberration of light. I may here offer a slight illustion of it, which every person may observe if he walks out on a rainy day. If you can choose a day when