3g4 PHILOSOPHICAL THANSACTIONS. [aNNO 1753. 



telesco[)es of different lengths, and by eyes of different goodness : and it may 

 therefore be alleged, that there is a certain quantity of time elapsed between the 

 very first emersion of the satellite, and the instant when it is perceived by the 

 very best eye, assisted by the best telescope ; and that, during this interval, the 

 succession of colours above-mentioned is performed. But our author, in conse- 

 quence of his hypothesis, says, that this succession of colours may be perceived 

 for the space of 3'2' after the first emersion of the satellite ; and Mr. S. was fully 

 satisfied, from repeated observations, that the quantity of time elapsed from the 

 very first emersion of the satellite, till it is perceived by a good eye, assisted by a 

 good telescope, can amount only to a very few seconds. So that, on the whole, 

 we may conclude, that it does not appear, by the observations of the emersions 

 of the first satellite of Jupiter, that the rays of different colours move with dif- 

 ferent degrees of velocity. 



But our author's conclusion, that, if the rays of light emitted from Jupiter's 

 satellites, at the time of their immersion and emersion, should not be found of 

 different colours, the rays of all colours emitted from luminous bodies will have 

 one common velocity, seems only to hold good, on a supposition that light i§, 

 propagated by a continued motion, in the manner of a projectile. )■.> 



Dr. Knight, in his treatise on attraction and repulsion, prop. 69, has con- 

 sidered the propagation of light, as performed by vibrations in an elastic fluid, 

 in the same manner as sound is produced by vibrations in the air : and he thinks 

 that it is as easy to conceive how the velocities of the particles of light may be 

 different, and yet take up equal times in propagating their motions from one to 

 another through a given space, as to explain how sounds of different tones move 

 with equal velocities. In accounting for both, he shows, that in a series of par- 

 ticles, which mutually repel each other, the greater their velocity, the nearer 

 they will approach each other, in communicating their motions from one to an- 

 other ; and consequently each of them must move through a greater space in so 

 doing : therefore the same time may be spent in propagating a successive motion 

 through a series of particles, whose velocity is greater, if each particle has to 

 move through a greater space, as is spent where the velocity of each particle is 

 less but is continued through a less space. The dilemma, to which our author's 

 reasoning seems to have reduced the doctrine of refrangibility, may therefore be 

 considered as a probable argument for adopting this hypothesis of the propagation 

 of light through an elastic medium. 



XXXIX. The Case 0/ the Operation for the Empyema, successfully perjormed 

 by Joseph Warner, F. R. S., and Surgeon to Guys Hospital, p. 270. 

 John Collier, aged 17, was admitted into Guy's Hospital on the 10th of May, 

 1753, on account of a complaint in his chest, which he had laboured under for 



