PROPOSITION V.
Reading edition
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.