LET. XIX. AND ABERRATION OF LIGHT. ZZI

Let A B represent a part of the earth’s annual pi. Vin.orbit, and C B a ray of light, falling from a star 6 °' 6 "perpendicularly upon the line B A : Then if theeye be at rest at B, the object will appear in thedirection B C, whether light be propagated intime, or in an instant; but if the eye be movingfrom A towards B, and light be propagated witha velocity that is to the velocity of the eye, asC B to AB, that particle of it, by which theobject will be discerned when the eye comes to

B, will be at C when the eye is at A. The star,therefore, will appear in the direction A C; andas the earth moves through the equal parts ofits orbit A a, a b, b c, &c. the light, coming fromthe star, will move through the equal divisionsC i k, k I, &c. and the star will appear succes-sively in the directions As, bf, cg, &c. whichare parallel to the former AC; so that whenthe eye comes to B, the object will be seen inthe direction B D.

If the line A C be a tube of such a diameteras to admit but one particle of light at a time,it is easy to perceive, that the particle of light at

C, by which the object must be seen, when theeye, as it moves along, arrives at B, would passthrough the tube A C, if it were inclined toA B, in the angle A B C, and accompanied theeye in its motion from A to B; and that it couldnot come to the eye through such a tube, if ithad by any other inclination to the line B I).

And