io6 On the M on: 10 N of LIGH T. 



incident light is not greater than the fpecific velocity of the me- 

 dium, it will not emerge from the refrading ftratum, but will 

 be refleded back again in the medium ; nor to {how that the 

 angle of refledlion is equal to the angle of incidence, and that 

 the relative velocity of the reflected light is equal to that of the 

 incident light, and that the relative perpendicular velocity of 

 the reflected light is equal to that of the incident light, but in 

 the oppofite diredlion. 



Lajlly, Let the number of the partial ftrata be augmented, 

 and their thicknefs diminiflied, without end. The foregoing 

 demonftration will now be applicable to the motion of light 

 through refrading fubftances which are in motion, and which 

 ail on it with forces, continually varyin'^ according to any law of 

 the diftances ; and it may be received as the fundamental pro- 

 pofition on this fubjedi, that, 



If a ray of lights moving in any dire^ion and with any velocity, 

 meet with the furface of a refraSling medium, which is in motion, its 

 fmil relative motion will ie the fame as if the medium had been at 

 rejl, and the light had approached it with the fame initial relative 

 motion. 



It is eafy to fee that what has been faid about the motion of 

 light within a medium which a(5ls upon it with attractive 

 forces, will apply to the motion of light which is without a 

 medium that adls upon it with repelling forces. In fuch a cafe, 

 the light will not be refradled into the medium, unlefs its per- 

 pendicular relative velocity be greater than the fpecific velocity 

 of the medium, but will be refle<5Ied with an equal relative ve- 

 locity, and at an equal relative angle on the other fide of the 

 perpendicular. 



It is alfo eafy to fee that the foregoing demonftration 

 will apply to the motion of light through two contiguous 

 mediums. For there will be a refratfling ftratum, where the 

 light will be aiFeded by the fum or the difference of the 



refracfting 



