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letited in the i^.)This thereSore being AC will
be to AB, that is, the Velocity of Light to the Velocity of the Eye (which in this Cafe may be supposed the Same as the Velocity of the Earth’s annual Motion in its Orbit) as ioxio to One, from whence it would follow, that Light moves, or is propagated as far as from the Sun to..the Earth in 8' ix".
It is well known,that Mr. Romer, who first attempted to account for an apparent Inequality in the Times of the Eclipfes oS Jupiter's Satellites, by the Hypothesis of the progressive Motion of Light, SuppoSed that it Spent about n' Minutes oS Time in its Passage from the Sun to us: but it hath Since been concluded by others from the like Eclipfes, that it is propagated as far in about 7 Minutes. The Velocity of Light therefore deduced from the foregoing Hypothesis, is as it were a Mean: betwixt what had at different times been determined from the Eclipfes of Jupiter's Satellites.
Thefe different Methods of finding the Velocity of. Light thus agreeing in the Refulr, we may reafonably conclude, not only that thefe Phenomena are owing; to the Causes to which they have been ascribed; but alfo, that Light is propagated (in the fame Medium), with the fame Velocity after it hath been reflected as before: for this will be the Consequence, if we allow that the Light of the Sun is propagated with the fame Velocity, before it is reflected, as the Light of the fixt Stars. And 1 imagine this will scarce be questioned, if it can be made appear that the Velocity of the Light of all the fixt Starsis equal, and that their Light moves or is propagated through equal Spaces in equal Tiroes, at all Distances from them: both which points (as I apprehend) are Sufficiently proved from the apparent Alteration
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ration of the Declination of Stars of different Lustre ; for that is not sensibly different in Such Stars as Seem rear togetaer, though they appear of very different Magnitudes. And whatever their Situations are (if I proceed according to the foregoing Hypothesis) I find the Same Velocity of Light from my Observations of fmail Stars of the fifth or sixth, as from thofe of the Second and third Magnitude, which in all Probability are placed at very different Distances from us. The fmall Star,for Example, before fpoken of, that is almost opposite to y 'Draconis (being the 3 5tli Camelopard. Hevelii in Mr. Flamfteed’s Catalogue) was 19" more Northerly about the Beginning of March than in September. Whence 1 conclude, according to my Hypothesis, that the Diameter of the little Circle described by a'Star in the Pole of the Ecliptick would be 40", z.
The last Star of the great Bear’s-tail of the id Magnitude (marked » by Bayer) was 3 6" more Southerly about the Middle of January than in July. Hence the Maximum, or greatest Alteration of Declination of a Star in the Pole of the Ecliptick would be 40", 4, exactly the fame as was before found from the Observations of y 'Draconis.
TbeStar of the 5th magnitude in the Head of Perfeus marked r bv Bayer, was ^^|| more Northerly about the End of December than on the 29th os July following. Hence the Maximum would be 41". This Star is not bright enough to be seen as it passes over my Zenith about the End of June, when it should be according to the Hypothesis farthest; South. But because 1 can more certainly depend upon the greatest Alteration of Declination of thofe Stars, which I have frequently observed about the Time? when they become ftatio-a nary.