39*2 PHILOSOPHICAL TRANSACTIONS. [aNNO J 753. 



Dr. Bradley's accurate estimation, in 8"" 12% the pulses of the aethereal fluid 

 must be propagated through the same space in about 8™ 1*. 



Hence also may be determined, in known measures, the distance between two 

 succeeding aethereal pulses ; for </ = . 



On the hypothesis of the different velocities of different colours, we may un- 

 derstand, at least in general, the reason of the strange analogy, discovered by 

 Sir Isaac, between the intervals of the fits, and the spaces occupied by the se- 

 veral colours in the spectrum (a thing hitherto unexplained *) ; since, from the 

 velocities of the several rays, on which depend the intervals of the fits, as has 

 been now explained, arise likewise their several degrees of refrangibility. 



But, as it is of great consequence in philosophy, to distinguish between facts 

 and hypotheses, however plausible, the various refrangibility, reflexibility, and 

 inflexibility, of the different colours, and their alternate dispositions, at equal 

 intervals, to be reflected and transmitted, which are the whole ground-work of 

 the Newtonian system, are to be considered as undoubted facts, deduced from 

 experiment ; but that the velocities of different rays are different in the manner 

 now described, is no more than probable conjecture : and though this point 

 should be decided, by a method that we are now to propose, it would still re- 

 main uncertain, whether the fits of reflexion and transmission consists in an al- 

 ternate acceleration and retardation of the particles of light, or in something 

 else. For instance, it might be supposed, that every particle of light has 1 con- 

 trary poles, like a loadstone ; the one of which is attracted by the parts of bodies, 

 and the other repelled ; and that, besides their uniform rectilineal motion, the 

 particles of differently-coloured rays revolve in different periods round their 

 centres ; for thus their friendly and unfriendly poles being alternately turned 

 towards the surfaces of bodies, they might be alternately disposed to reflexion 

 and transmission, and that at different intervals, in proportion to the periods of 

 their rotation. Lastly, though it were proved, that the fits proceed from an al- 

 ternate acceleration and retardation of the particles of light, it would still be no 

 more than probable conjecture, that this is brought about by pulses excited in 

 the aethereal medium. Nay there are some circumstances in these phenomena, 

 that seem hardly intelligible by that hypothesis alone : as, why the intervals of 

 the fits are less in denser mediums ;-|- and why they increase so fast, and in so 

 intricate a proportion, according to the obliquity of incidence. ^ 



By Dr. Bradley's beautiful theory of the aberration of light, the stars appear to 

 be removed from their true place to a certain distance, according to the propor- 

 tion which the transverse motion of the spectator's eye bears to the velocity of 



* Compare Newt. Opt. Book i. part 'i, prop. 3, with Book ii. part 3, prop. l6. — Orig. 



t Newt. Opt. Book ii. part 3, prop. 17.— Orig. X P''op- ^5, ibidem. — Orig. 



