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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-
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ference between the true and visible Place) vanishes. But if Light be propagated in Time (which I presume will readily be allowed by most of the Philosuphers of this Age) then it is evident from the foregoing Considerations, that there will be always a Difference between the real and visible Place of an Object, unless the Eye is moving either directly towards or from the Object. And in all Cases, the Sine: of the Difference between the real and visible Place of the Object, will be to the Sine of the visible Inclination of the Object to the Line in which the Eye is moving, as the Velocity of the Eye to the Velocity of Light.
If Light moved but i ooo times faster than the Eye, and an Object (fuppoSed to be at an infinite Distance) was really placed perpendicularly over the Plain in which the Eye is moving, it follows from what hath been already said, that the apparent Place of such an Object will be always inclined to that Plain, in an Angle of 89° ^6't ; 10 that it will constantly appear 3' > from its true Place, and seem fo much less inclined to the Plain, that way towards which the Eye tends. That is, if A C is to AB (or AD) as 1000 to one, the Angle ABC will be 89° 56' *, and A C B = 3' *, and B C D = i A C B = 7'. So that according to this Supposition, the visible or apparent Place of the Object will be altered 7', if the Direction of the Eye’s Motion is at one time contrary to what it is at another.
If the Earth revolve round the Sun annually, and the Velocity of Light were to the .Velocity of the Earth’s Motion in its Orbit (which I will at present suppose to be a Circle) as 1000 to one; then tis easy