THE discovery. of the aberration of light was soon followed by an explanation according to the emission theory. The effect was attributed to a simple composition of the velocity of light with the velocity of the earth in its orbit. The difficulties in this apparently sufficient explanation were overlooked until after an explanation on the undulatory theory of light was proposed. This new explanation was at first almost as simple as the former. But it failed to account for the fact proved by experiment that the aberration was unchanged when observations were made with a telescope filled with water. For if the tangent of the angle of aberration is the ratio of the velocity of the earth to the velocity of light, then, since the latter velocity in water is three-fourths its velocity ill a vacuum, the aberration observed with a water telescope should be fourthirds of its true value.t * This research was carried out with the aid of the Bache Fund. t It may be noticed that most writers admit the sufficiency ot the explanation according to the emission theory of light; while in fact the difficulty is even greater than according to the undulatory theory. For on the emission theory the velocity of light !!lnst be greater in the water telescope, and therefore the angle of aberration should be less; hence, in order to reduce it to its Lrue value, we must make the absurd hypothesis that the motion of the water in the telescope carries the ray of light in the opposite direction