Astronomical and Nautical Collections. 199 



and the second surface of the plale of air comprehended 

 between these glasses. But before we enter into the detail 

 of the explanation, it will be necessary to establish a prin- 

 ciple with regard to the reflection of light, which will be 

 required for this purpose. 



When an agitation is propagated in a medium of uniform 

 elasticity and density, it never returns to the same point ; 

 and while it is communicated perpetually to new strata of the 

 medium, it leaves no traces behind, but the strata which it 

 has passed remain in absolute rest ; in the same manner as a 

 ball of ivory, which strikes another of equal magnitude, 

 communicates all its motion to this second ball, and remains 

 at rest after the stroke. But the effect is not the same 

 when the second ball is either larger or smaller than the first, 

 for in either case the first ball continues to move after the 

 stroke. When the second ball is greater than the first, the 

 new velocity of the first is in a direction contrary to that of 

 its former motion ; and when smaller, the first ball continues 

 to move in the same direction as before, so that the new velo- 

 cities of the first ball, after the stroke, must be marked 

 by contrary signs in the two cases. This comparison may 

 assist us in understanding the consequence of the wrrival of 

 an undulation at the surface of contact of two elastic me- 

 diums of different densities : the infinitely thin stratum of the 

 first medium, which is in contact with the second, and which 

 may be compared to the first ball, does not remain at rest 

 after having put the contiguous stratum of the second medium 

 in motion, on account of the difference in their masses, and a 

 reflection takes place : but the new velocity which belongs 

 to the stratum of the first medium, after the stroke, and 

 which is communicated successively to the neighbouring 

 strata behind it, must have its sign changed accoi*dingly as 

 the stratum of the second medium is more or less dense 

 than that of the first. This important proposition, which 

 Dr. Young deduced from the considerations which have been 

 here explained, has also been derived by Mr. Poisson from 

 the formulas which he has demonstrated by means of a rigor- 

 ous and refined analysis. When applied to the reflection of 

 light, it enables us to infer that, accordingly as an unduktion 



