PROPOSITION VI.
Reading edition
When an Undulation falls on the Surface of a rarer Medium, so bliquely 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.