VOL. XLVIII.] PHILOSOPHICAL TRANSACTIONS. SQl 



supposed that tht'ir particles are of different magnitudes or densities : but if there 

 be any analogy between the refractive power and gravity, it will produce equal 

 velocities in all particles, whatever their magnitude or density be ; and so all sorts 

 of rays would be equally bent from their right-lined direction. 



It seems therefoie a more probable opinion, which others have advanced, that 

 the differently-coloured rays are projected with different velocities from the lumi- 

 nous body ; the red with the greatest, violet with the least, and the intermediate 

 colours with intermediate degrees of velocity ; for, on this hypothesis it is mani- 

 fest, that they will be differently refracted in the prismatic order, according to 

 observation. 



On supposition that the different refrangibility of light arises solely from the 

 different velocities of the rays before incidence, these velocities must be to each 

 other nearly as their sines of refraction. 



Their velocities in any given medium, suppose air, being once determined, 

 their velocities in any other may be easily discovered ; for they are to those in air 

 as the sine of incidence to the sine of refraction, when the ray passes from air 

 into the other medium. 



While the differently-coloured rays are supposed to move with one common 

 velocity, any pulse?, excited in the aethereai medium, must overtake them at 

 equal distances : and therefore the intervals of the fits of reflexion and trans- 

 mission, if they arise in this manner, as Sir Isaac conjectures, would be all equal : 

 but if the red move swiftest, the violet slowest, and the intermediate colours 

 with intermediate velocities, it is plain that the same pulses must overtake the 

 violet soonest, the other colours in their order, and last of all the red ; that is, 

 the inter\'als of the fits must be least in the violet, and gradually greater in the 

 prismatic order, agreeably to observation. 



Let c denote the velocity of the aethereai pulses, ?; the velocity of red light, 

 and u that of violet ; / and j the intervals of their fits, and d the distance be- 

 tween 1 succeeding pulses: it is plain, from the nature of Newton's hypothesis, 

 that z is to f^ as f to c — v. and again, o( to J as c — m to w : therefore, ex aequo, 



/ is to 7, as cii — vu to cu — 7 m, from which we have the equation c = . -•^- v 



•^ ' tu—jv 



vu. Therefore, as the proportion between the intervals of the fits in red and 

 violet, can be assigned by experiment, and the proportion of their velocities in 

 any medium also, the velocity of the aethereai pulses may be easily computed. 

 The velocities of the red and violet in air are nearly as 78 and TJ. In the ce- 

 lestial spaces they are less, but almost in the same proportion ; the intervals of 

 their fits are by experiment as 100 and 63. Whence, by the canon now laid 

 down, the velocity of the aethereai pulses in the celestial space, is found to be to 

 that of red light, as 79763 to 7 8000. As light moves from the sun to us, by 



