[ 2 7 ° 1 
vibrations in the air : and he thinks, that it is as eafy 
to conceive how the velocities of the particles of light 
may be different, and yet take up equal times in pro- 
pagating their motion from one to another through a 
given lpace, as to explain how founds of different 
tones move with equal velociies. In accounting for 
both, he fhews, that, in a feries of particles, which 
mutually repel each other, the greater their velocity, 
the nearer they will approach other, in communi- 
cating their motions from one to another ; and con- 
sequently each of them muff move thro’ a greater 
fpace in fo doing : wherefore the fame time may be 
fpent in propagating a fucceffive motion thro’ a feries 
of particles, whofe velocity is greater, if each parti- 
cle has to move thro’ a greater fpace, as is fpent, 
where the velocity of each particle is lefs, but is con- 
tinued thro’ a lefs fpace. The dilemma, to which 
our author’s reafoning feems to have reduced the 
dodtrine of refrangibility, may therefore be confider’d 
as a probable argument for adopting this hypothecs 
of the propagation of light thro’ an elaftic medium. 
XXXIX. 1 he Cafe of the Operation for the 
Empyema, fuccefsfully performed by Jofeph 
Warner, F. R. S. and Surgeon to Guy V 
Hofpital. 
Read June 28 , the 1 9 of March 1751, I did 
myfelf the honour of communi- 
cating to the Royal Society the cafe of John Hines, 
on whom I had performed the operation for the 
empyema 
