We may divide the causes which we might conceive concerned in the production of aberration into three: — (1), the motion of the earth; (2), the velocity of light; (3), the change in the direction of the light coming to the earth. Professor Challis has shown that a certain apparent displacement of a star would result from the first two causes; and as this happens to be the whole displacement observed (neglecting a quantity which may be considered insensible), so that there is none left to be attributed to the third cause, he says that he has explained aberration, assuming merely the first two causes. It is evident that the two senses attached to the words, to explain a phænomenon, are quite different. According to the sense in which I used the words, the explanation of the absence of any change in the final direction of the light would have to be included in any theory which professed to explain aberration by means of the first two causes only. In the present communication I have used the words in my own sense, for I believe that there is no impropriety in it; but Prof. Challis may, if he pleases, consider the object of my July paper to have been the explanation, not of aberration, but of the absence of any change in the final direction of the light. Whichever of these results be arrived at, the other readily follows.
