_When an Undulation falls on the Surface of a rarer Medium, so
obliquely that it cannot be regularly refracted, it is totally
reflected, at an Angle equal to that of its Incidence._ (Phil. Trans.
for 1800, p. 128.)
_Corollary._ This phenomenon tends to prove the gradual increase
and diminution of density at the surface terminating two mediums, as
supposed in HYPOTHESIS IV; although HUYGENS has attempted to explain it
somewhat differently.