3l6 PHILOSOPHICAL TUANSACTIONS. [aNNO J7'28. 



alteration of declination of each star from the observations themselves ; rather 

 choosing to depend on them in this article, because all that have yet been made 

 concur to prove, that the stars near the equinoctial colure, change their de- 

 clination at this time J^" or 2" in a year, more than they would do if the pre- 

 cession was only 50", as is now generally supposed. Mr. B. has likewise met 

 with some small varieties in the declination of other stars in different years, 

 which do not seem to proceed from the same cause, particularly in those that 

 are near the solstitial colure, which on the contrary have altered their declination 

 less than they ought, if the precession was 50". But whether these small 

 alterations proceed from a regular cause, or are occasioned by any change in the 

 materials, &c. of the instrument, he is not yet able fully to determine. 



From what has been premised, it will appear that the greatest alteration of 

 the apparent declination of y Draconis, on account of the successive propagation 

 of light, would be to the diameter of the little circle which a star would seem 

 to describe about the pole of the ecliptic, as 39" to 40".4. The half of this 

 is the angle acb. This therefore being 20".2, AC will be to ab, that is, the 

 velocity of light to the velocity of the eye, which in this case may be supposed 

 the same as the velocity of the earth's annual motion in its orbit, as 10210 to 1 ; 

 from whence it would follow that light moves, or is propagated as far as from 

 the sun to the earth, in 8"" 12^ 



It is well known, that Mr. Romer, who first attempted to account for an ap- 

 parent inequality in the times of the eclipses of Jupiter's satellites, by the hy- 

 pothesis of the progressive motion of light, supposed that it spent about 11 

 minutes of time in its passage from the sun to us: but it has since been con- 

 cluded by others, from the like eclipses, that it is propagated as far in about ^ 

 minutes. The velocity of light therefore deduced from the foregoing hypo- 

 thesis, is as it were a mean between what had at different times been determined 

 from the eclipses of Jupiter's satellites. 



These different methods of finding the velocity of light thus agreeing in the 

 result, we may reasonably conclude, not only that these phaenomena are owing 

 to the causes to which they have been ascribed ; but also, that light is propa- 

 gated, in the same medium, with the same velocity after it has been reflected, as 

 before : for this will be the consequence, if we allow that the light of the sun 

 is propagated with the same velocity, before it is reflected, as the light of the 

 fixed stars. And this will scarcely be questioned, if it can be made appear that 

 the velocity of the light of all the fixed stars is equal, and that their light 

 moves, or is propagated, through equal spaces in equal times, at all distances 

 from them : both which points appear to be sufficiently proved from the appar- 

 ent alteration of the declination of stars of dittcrent lustre ; for that is not 



