68 ESSAYS ann OBSERVATIONS 
incidence? The exception, made by fome, 
of electrical light is founded on no lefs a 
miftake than confounding the luminous 
body with its light *. But the beft proof 
of this propofition is from the coincidence 
of the computations of the velocity of 
light, from the equation of the eclipfes 
of Jupiter’s fatellites,and the aberration of 
the fixed flars f. 
Quer. IL Is light emitted with the 
fame velocity, in whatever medium thelu- 
minous body be placed? Or, is it not ra- 
ther emitted with greater velocity in den- 
fer mediums, and that in proportion to their 
refraGtive powers? The fame argument 
from whence we gather in general the e- 
qual velocity of light emitted by all forts 
of luminous bodies, feems to prove the 
truth of the latter fuppofition. For, fince 
rays of any one colour, from the fan and 
a candle, for inftance, are equally refrac- 
ted by a furface of glafs or water, we 
* Muffchenbroeck’s Elementa Phyfices, late edition, 
in his chapter on electricity, 
el e y 
+ Eames’s Tranfact. vol. 6. &e, 
