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it would ftill remain uncertain, whether the fits of 
reflexion and tranfmiflion confift in an alternate acce- 
leration and retardation of the particles of light, or in 
fomething elfe. For inftance, it might be fuppofed, 
that every particle of light has two contrary poles, 
like a loadftone ; the one of which is attracted by 
the parts of bodies, and the other repell’d ; and that, 
befides their uniform redilineal motion, the particles 
of differently-colour'd rays revolve in different periods- 
round their centers ; for thus their friendly and un- 
friendly poles being alternately turned towards the 
furfaces of bodies, they might be alternately difpofed 
to reflexion and tranfmiflion, and that at different 
intervals, in proportion to the periods of their rota- 
tion. Laftly, tho’ it were proved, that the fits do 
proceed from an alternate acceleration and retardation 
of the particles of light, it would ftill be no more 
than probable conjedure, that this is brought about 
by pulfes excited in the asthereal medium. Nay there 
are fome circumftances in thefe phasnomena, that 
feem hardly intelligible by that hypothefis alone : as, 
Why the intervals of the fits are lefs in denfer me- 
diums (<?) ; and, Why they increafe fo faft, and in 
fo intricate a proportion, according to the obliquity 
of incidence (b). 
10. By Dr. Bradley’s beautiful theory of the aber- 
ration of light, the flats appear to be removed from 
their true place to a certain diftance, according to the 
proportion which the tranfverfe motion of the fpe&a- 
torV eye bears to the velocity of light. It is plain 
therefore. 
(a) Newt. Opt. Book II, Part 3. Prop. 17. 
(£) Prop. 15. ibidem. 
