PHYSICAL anv LITERARY. 45 
ding to obfervation, Since, according to 
Sir Ifaac Newton’s do@trine of refra@tion 
now generally received, the velocity of 
a ray, after entering any new medium, is 
to its former velocity, as the fine of inci- 
dence to the fine of refraction*; if all the 
colours move with equal fwiftnefs in any 
one medium, their velocity will neceflarily 
become unequal, upon entering a denfer 
medium, in the inverfe proportion ‘of their 
feveral fines of refrangibility: Tho’ we 
fuppofe, therefore, the fun’s rays to be e- 
mitted with the common velocity, it will 
follow that their velocities are unequal in 
air, glafs, water, or any tran{parent body, 
_ whofe refractive denfity differs from that 
of the folar atmofpheref. This confide- 
ation is fufficient to take off the appear- 
ance of improbability from our hypo- 
thefis. 
38. ON fuppofition that the different 
refrangibility of the rays of light arifes 
folely from their different velocities before 
inci-=- 
* Newtoni Principia, lib. 1. prop, 95, 
+ See below, query 3. 
