98 On the MOTION of LIGHT. 



in it. He fuppofes, that vifion is produced by the tremulous 

 motion of an elaflic fluid which he calls aether, and which he 

 fuppofes to pervade all bodies. He attempts to {how that the 

 propagation of this tremulous motion is analogous to the ap- 

 pearances in the reflection and refraction of light. I confefs 

 that I cannot admit his reafonings on this fubject to be agreeable 

 to the principles of mechanics ; and I am decidedly of opinion, 

 that the propagation of the tremulous motion of an elaflic fluid 

 is totally inconuftent with thofe facts in vifion where no refrac- 

 tion or reflection is obferved. But I fhall referve my objections 

 till another opportunity, when I propofe to fubmit to this So- 

 ciety a mechanical examination of this hypothefis, and I fhall 

 admit for the prefent that Mr Euler's explanation of refraction 

 and reflection is jufl. It is an effential proportion in this hy- 

 pothetical theory, that the velocities of the incident and re- 

 fracted light are proportional to the fines of incidence and re- 

 fraction, and therefore that light is retarded when it is refracted 

 toward the perpendicular. It feems a neceflary confequence 

 that, in this cafe, the particles of aether are actuated by forces 

 tending from the refracting body. I fhall, therefore, confider 

 what effects muft refult from the combination of this retarda- 

 tion with the motion of the refracting body. If time will 

 allow, I fhall confider what will be the effects produced on the 

 motion of light by the motion of the vifible object. Thefe are 

 fo different in the two hypothefes, that it is very probable that 

 fome natural appearance may be found which will give us an 

 opportunity of determining whether either of thefe hypothefes 

 is to be received as true. Dr Robert Blair, profeffor of 

 Aftronomy in this Univerfity, has pointed out a cafe of this 

 kind, in the rotation of the planet Jupiter round its axis, and 

 has fuggefted a method of invefligation, exceedingly ingenious, 

 and which feems to promife fuccefs. I think that another may 

 be obferved in the planet Mercury, when in his greater! elonga- 

 tions from the fun. 



I 



