in optics (in Encycl.), refers the reader to the article OPTICS, p. 17, 136, 173. It should have referred him to OPTICS, p. 17, and 251—256.

ABERRATION of the Visual Ray, is a phenomenon, of which, though some account of it has been given in the Encyclopedia (see ABERRATION, in astronomy; and the article ASTRONOMY, p. 337.), one of the most candid of our correspondents requires a fuller explanation. If such an explanation be requisite to him, it must be much more so to many others; and we know not where to find, or how to devise, one which would be more satisfactory, or more familiar, than the following by Dr Hutton.

"This effect (says he) may be explained and familiarized by the motion of a line parallel to itself, much after the manner that the composition and resolution of forces are explained. If light have a progressive motion, let the proportion of its velocity to that of the earth in her orbit be as the line BC to the line AC; then, by the composition of these two motions, the particle of light will seem to describe the line BA or DC, instead of its real course BC; and will appear in the di-Aberration section AB or CD, instead of its true direction CB.

So that if AB represent a tube, carried with a parallel motion by an observer along the line AC, in the time that a particle of light would move over the space BC, the different places of the tube being AB, ab, cd, CD; and when the eye, or end of the tube, is at A, let a particle of light enter the other end at B; then when the tube is at ab, the particle of light will be at e, exactly in the axis of the tube; and when the tube is at cd, the particle of light will arrive at f, still in the axis of the tube; and, lastly, when the tube arrives at CD, the particle of light will arrive at the eye or point C, and consequently will appear to come in the direction DC of the tube, instead of the true direction BC: and so on, one particle succeeding another, and forming a continued stream or ray of light in the apparent direction DC. So that the apparent angle made by the ray of light with the line AE is the angle DCE, instead of the true angle BCR; and the difference BCD, or ABC, is the quantity of the aberration."

ABERRATION of the Planets, is equal to their geocentric motion, or, in other words, to the space which each appears to move as seen from the earth, during the time that light employs in passing from the planet to the eye of the observer. Thus the sun's aberration in longitude is constantly 20', that being the space actually moved by the earth; but apparently by the sun in 8 minutes and 7 seconds, the time in which light passes from the sun to the earth. If then the distance of any planet from the earth be known, the time which light employs in passing from the planet to the earth must likewise be known; for as the distance of the sun is to the distance of the planet, so is 8 minutes and 7 seconds to that time; and the planet's geocentric motion in that time is its aberration, whether it be in longitude, latitude, right ascension, or declination. See ASTRONOMY in this Supplement.