i^HYSICAL AND LITERARY. 4t 



with different velocities from the luminous 

 body : the red, with the greateft ; violet, 

 with the leafi; ; and the intermediate colours, 

 with intermediate degrees of velocity : fofj, 

 upon this hypothelis, it is manifeft, that 

 they will be differently refracfted in the prif- 

 ipatic order j according to obfervation. Since* 

 according to Sir Ifaac Newton^ dod:rine of 

 refraction now generally received, the velo- 

 city of a ray, after entering any new mediutrii 

 is, to its former velocity, as the fme of inci- 

 4^nce to the fine of refradion '"^ ; if all the 

 colours move with equal fwifttiefs in any one 

 medium, their velocity will neceffarily be- 

 come unequalj upon entering a denfer medium^ 

 in the inverfe proportion of their fever al 

 fines of refrangibility : tho' we fuppofej 

 therefore, the fun's rays to be emitted with 

 one common velocity, it will follow that 

 their velocities are unequal in air, glafs, wa- 

 ter^ or any tranfparent body, whofe refra- 

 ftive denfity differs from that of the folar at- 

 mofphere '|-. This confideration is fufficient 

 to take off the appearance of impiobability 

 from our hypothejis. 



Vol. II. F 38. Oi< 



* Nenutoni PtincipU, lib. I. prop. 95. 

 "j- See below, qaery 3, 



