106 On the M OTIO N of LIG H T. 



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

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

 be reflected back again in the medium ; nor to mow that the 

 angle of reflection 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 direction* 



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

 and their thicknefs diminifhed, without end. The foregoing 

 demonflration will now be applicable to the motion of light 

 through refracting fubflances which are in motion, and which 

 act on it with forces, continually varying according to any law of 

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

 pofition on this fubject, that, 



If a ray of lights moving in any direction and with any velocity, 



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



final relative motion will be the fame as if the medium bad been at 



refl, and the light bad 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 acts upon it with attractive 

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

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

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

 pendicular relative velocity be greater than the fpecific velocity 

 of the medium, but will be reflected 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 demonflration 

 will apply to the motion of light through two contiguous 

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

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



refracting 



