C in ] 
had formerly ftudied myfelf, I firft examined whe- 
ther any notice had been taken of the things com- 
mon to both of the works, and to what degree the 
two authors agreed together. 
The book I fpeak of is a treatife of optics, com- 
pofed by Mr. le Marquis de Courtivron, of the 
French Academy of Sciences. It was publifhed in 
iyy2, and prefented the fame year to the Royal So- 
ciety and confequently has the anteriority of date 
with regard to Mr. Melvil’s paper, which was' read 
only the 8th of March of i / ^3 . But I am far from 
imagining that he would have neglected citing Mr. 
de Courtivron, if he had had any notice of his book. 
As I do not doubt but the Royal Society would 
have ordered fome mention of Mr. de Courtivrr n 
upon the fame fubjedt, if an account, fuffic’ently cir- 
cumftantiated, of his book had been read before that 
illuftrious body, I hope a fhort expofition of the 
queftion will not be amifs. 
Both of thefe gentlemen thought of accounting 
for the difference of refrangibility, by the difference 
of velocity in the rays of light ; which, if it really 
agreed with the obfervations, would give a great 
fimplicity to the theory of refradtion, as reducing it 
under the fame laws as the theory of gravity : 
whereas in the hypothefis, wherein the particles of 
light are endowed with tendencies different from one 
another, one is obliged to multiply the properties of 
matter. 
Meffieurs de Courtivron and Melvil went fo far 
the fame way, as to examine, whether the immer- 
fions and emerfions of Jupiter’s fatellites could not 
5 F afford 
