r 269 ] 
fatellite, except in quantity. It may indeed be obferved, 
that thefe emerfions are feen fooner or later thro' te- 
lefcopes of different lengths, and by eyes of different 
goodnefs : and it may therefore be alleged, that there 
is a certain quantity of time elapfed between the very 
firft emerfion of the fatellite, and the inftant when 
it is perceived by the very beft eye, aflifted by the 
belt telefcope ; and that, during this interval, the 
fucceffion of colours, above-mentioned, is perform’d. 
But our author, in confequence of his hypothecs, 
fays, that this fucceffion of colours may be per- 
ceived for the fpace of 3 1 " after the firft emerfion of 
the fatellite ; and I am fully fatisfied, from repeated 
obfervations, that the quantity of time elapfed from 
the very firft emerfion of the fatellite, till it is per- 
ceived by a good eye, afiifted by a good telefcope, 
can amount only to a very few feconds. So that, 
upon the whole, we may conclude, that it does not 
appear, by the obfervations of the emerfions of the 
firft fatellite of Jupiter, that the rays of different 
colours move with different degrees of velocity. 
But our author’s conclufion, that, if the rays of 
light emitted from Jupiter's fatellites, at the time of 
their immerfion and emerfion, fhould not be found 
of different colours, the rays of all colours emitted 
from luminous bodies will have one common velo- 
city, feems only to hold good, on a fuppofition, that 
light is propagated by a continued motion, in the 
manner of a projectile. 
Dr. Knight, in his treatife on attraction and re- 
pulfion (Prop. 69.) has confider’d the propagation 
of light, as performed by vibrations in an elaftic 
fluid, in the fame manner as found is produced by 
vibrations 
