98 On the MO TIO N of LIGHT. 



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

 motion of an elaftlc 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 refradlion of light. I confefs 

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

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

 that the propagation of the tremulous motion of an elaftic fluid 

 is totally inconfiftent with thofe fadls in vifion where no refrac- 

 tion or refleiStion is obferved. But I fliall referve my objedlions 

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

 ciety a mechanical examination of this hypothefis, and I fliall 

 admit for the prefent that MrEuLER's explanation of refradlion 

 and refletflion is juft. It is an eflTential propofition in this hy- 

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

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

 fradlion, and therefore that light is retarded when it is refra<fled 

 toward the perpendicular. It feems a neceflary confequence 

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

 tending from the refradling body. I fliall, therefore, confider 

 what effects muft refult from the combination of this retarda- 

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

 allow, I fliall confider what viiW be the efFeds produced on the 

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

 fo diflferent 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, profeflx)r 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 fuggefl;ed a method of inveftigation, exceedingly ingenious, 

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

 be obferved in the planet Mercury, when in his greateft; elonga- 

 tions from the fun. 



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