_When an Undulation is transmitted through a Surface terminating
different Mediums, it proceeds in such a Direction, that the Sines of
the Angles of Incidence and Refraction are in the constant Ratio of the
Velocity of Propagation in the two Mediums._
(BARROW, Lect. Opt. II. p. 4. HUYGENS, _de la Lum._ cap. 3. EULER,
_Conj. Phys._ Phil. Trans. for 1800, p. 128. YOUNG’s Syllabus. Art.
382.)
_Corollary_ 1. The same demonstrations prove the equality of the angles
of reflection and incidence.
_Corollary_ 2. It appears from experiments on the refraction of
condensed air, that the ratio of the difference of the sines varies
simply as the density. Hence it follows, by Schol. I. Prop. I. that
the excess of the density of the ethereal medium is in the duplicate
ratio of the density of the air; each particle cooperating with its
neighbours in attracting a greater portion of it.