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CauSe of them. I was already convinced, that the apparent Motion of the Stars was not owing to a Nutation of the Earth’s Axis. The next Thing that offered itself, was an Alteration in the Direction of the Plumb-line, with which the Instrument was constantly rectified ; but this upon Trial proved insufficient. Then I considered what Refraction might do, but here alfo nothing satisfactory occurred. At last I conjectured, that all the Phenomena hitherto mentioned, proceeded from the progreffive Motion of Light and the Earth’s annual Motion in its Orbit. For I perceived, that, if Light was propagated in Time, the apparent Place of a fixt Object would not be the same when the Eye is at Reft, as when it is moving in any other Direction, than that of the Line pasting through the Eye and Object; and that, when the Eye is moving in different Directions, the apparent Place of the Object would be different.
C I considered this Matter in the fol
lowing Manner. I imagined C A to be a Ray of Light, falling perpendicularly upon the Line BD ; then if the Eye is at reft at A, the Object muft appear in the Direction A C, l whether Light be propagated in Time \ or in an Instant. Bat if the Eye is \ moving from B towards A, and Light \ is propagated in Time, with a Velo-\ city that is to the Velocity of the \ Eye, as C A to B A; then Light mov-jj *nS fr°m C to A, whilft the Eye moves from B to A, that Particle, of
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it, by which the Object will be discerned, when the Eye in its Motion comes to A, is at C when the Eye is atB. Joining the Points B,C, I supposed the Line CB, to be a Tube (inclined to the Line B D in the Angle D B C ) of such a Diameter, as to admit of but one Particle of Light; then it was eafy to conceive, that the Particle of Light at C (by which the Object must be seen when the Eye, as it moves along, arrives at A) would pafs through the Tube B C, if it is inclined to B D in the Aqgle D B C, and accompanies the Eye in its Motion from B to A 5 and that it could not come to the Eye, placed behind such a Tube, if it had any other Inclination to the Line BD. If instead of supposing CB fo Small a Tube, we imagine it to be the Axis of a larger; then for the same Reafon, the Particle of Light at C, could not pals through that Axis, unless it is inclined toBD, in the Angle CBD. In like manner, if the Eye moved the contrary way, from D towards A, with the fame Velocity ; then the Tube must be inclined in the Angle B DC. Although therefore the true or real Place of an Object is perpendicular to the Line in which the Eye is moving, yet the visible Place will not be fo, since that, no doubt, must be in the Direction of the Tube ; but the Difference between the true and apparent Place will be (cateris paribus) greater or lefs, according to the different Proportion between the Velocity of Light and that of the Eye. So that if we could suppose that Light was propagated in an Instant, then there would be no Difference between the real and visible Place of an Object, altho’ the Eye were in Motion, for in that case, A C being infinite with Refpect to A B, the Angle A C B (the i Du-