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very evident, that they did not all change their Declination equally. I have before taken notice, that it appeared from Mr. Molyneaux's Observations, that y 'Draconis altered its Declination about twice as much as the fore-mentioned finall Star almost opposite to it; but examining the matter more particularly, I found that the greatest Alteration of Declination in these Stars, was as the Sine of the Latitude of each refpe&ively. This made me suSpedt that there might be the like Proportion between the Maxima of other Stars ; but finding, that the Observations of fome of them would not perfectly cor-refpond with such an Hypothesis, and not knowing, whether the fmall Difference I met with, might not be owing to the Uncertainty and Error of the Observations, I deferred the farther Examination into the Truth of this Hypothesis, till I should be furnished with a Series of Observations made in all Parts of the Year; which might enable me, not only to determine what Errors the Observations are liable to, or how far they may safely be depended upon; but alfo to judge, whether there had been any sensible Change in the Parts of the Instrument itself.
Upon these Considerations, I laid aside all Thoughts at that Time about the Caufe of the fore-mentioned Phenomena, hoping that I should the easier difcovei it, when I was better provided with proper Means to determine more precisely what they were.
When the Year was compleated, I began to examine and compare my Observations, and having pretty well Satisfied my SelS as to the general Laws of the Phenomena, I then endeavoured to find out the
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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