40 ESSAYS snp OBSERVATIONS 
with refpect to the intimate nature of 
light (in like manner as his fyftem of u- 
niverfal gravitation is independent of all 
hypothefes concerning the caufe of gra- 
vity). In his optics, he lays down his 
‘difcoveries at full length, without ever in- 
quiring whether light confifts in vibrati+ 
ons propagated through a fluid, or of par- 
ticles projected in ftraight lines from the 
luminous body: And, in his queries, where 
he touches this matter *, he feems to be 
more pofitive in rejecting the hypothefis 
of vibrations, than in eftablifhing any o- 
ther. 
32. BuT Euler likeways advances a new 
notion with refpect to the origin of co= 
lours in opaque bodies, which is intirely 
inconfiftent with the principal part of Sir 
Ifaac Newton’s doctrine. He fuppofes, 
that coloured bodies reflect the fun’s in- 
cident white light from their anterior fur- 
face; but, that the particular fpecies of 
light, by which they appear coloured, is 
properly emitted by the parts of the body: 
For inftance, he imagines that vermilion 
does 
* Newt, Opt, quer, 28, and 2¢, 
