50 ESSAYS AMD OBSERVATIONS 



immutability of fimple rays by the fecond 

 effefts of refraction. 



47. As it is of great confequeiice in philo- 

 fophy to diftinguilTi between fads and hypo-^ 

 thefes, however plaufible ; it ought to be ob- 

 ferved, that the various refrangibihty, reflexi- 

 bility, and inflexibility of the feveral colours, 

 and their alternate difpofitions at equal inter- 

 vals to be reflected and tranfmitted, which 

 are the whole ground-work of the Newto- 

 man fyftem, are to be confidered as certain 

 fadis deduced from experiment: but whether 

 the velocities of the different rays are exactly 

 equal, or different in the manner now de- 

 fcribed, is no more than probable conjefturej 

 and, tho' this point (liould be decided by a 

 method propofed afterwards, it would flill 

 continue uncertain, whether the fits of re- 

 flexion and tranfmiflion are occalioned by an 

 alternate acceleration and retardation, of the 

 motion of light, or in fomc other manrrer*. 



And, 



* For inftance, it might be fuppofed, that every particle 

 bf light has two contrary poles, like a load-ftone ; the one 

 of which is attradled by the parts of bodies, and the other 

 repelled ; and that, befides their uniform reftilineal motion, 

 the particles of differently-coloured rays revolve in different 

 periods round their center: for thus, their friendly and un- 



friendl)!' 



