Dr. Young’s Lecture on 
3 ° 
rectilinear propagation of undulations, Newton has made no 
reply ; perhaps because of his own misconception of the nature of 
the motions of elastic mediums, as dependent on a peculiar law 
of vibration, which has been corrected by later mathematicians. 
(Phil. Trans, for 1800, p. 116.) On the whole, it is presumed, 
that this proposition may be safely admitted, as perfectly con- 
sistent with analogy and with experiment. 
PROPOSITION IV. 
When an Undulation arrives at a Surface which is the Limit of 
Mediums of different Densities , a partial Refection takes place , 
proportionaterin Force to the Difference of the Densities. 
This may be illustrated, if not demonstrated, by the analogy , 
of elastic bodies of different sizes. “ If a smaller elastic body 
“ strikes against a larger one, it is well known that the smaller 
“ is reflected more or less powerfully, according to the diffe- 
“ rence of their magnitudes : thus, there is always a reflection 
“ when the rays of light pass from a rarer to a denser stratum 
« of ether; and frequently an echo when a sound strikes 
“ against a cloud. A greater body striking a smaller one, pro- 
“ pels it, without losing all its motion : thus, the particles of a 
“ denser stratum of ether, do not impart the whole of their 
« motion to a rarer, but, in their effort to proceed, they are 
« recalled by the attraction of the refracting substance with 
« equal force ; and thus a reflection is always secondarily pro- 
« duced, when the rays of light pass from a denser to a rarer 
«' stratum/’ (Phil. Trans, for 1800. p. 127.J But it is not ab- 
solutely necessary to suppose an attraction in the latter case, 
since the effort to proceed would be propagated backwards 
without it, and the undulation would be reversed, a rarefaction 
