_When an Undulation arrives at a Surface which is the Limit of Mediums
of different Densities, a partial Reflection takes place, proportionate
in 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 difference 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, propels 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 produced, when the rays of light pass
from a denser to a rarer stratum.” (Phil. Trans. for 1800. p. 127.) But
it is not absolutely 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 returning in
place of a condensation; and this will perhaps be found most consistent
with the phenomena.