the Theory of Light and Colours . 31 
returning in place of a condensation ; and this will perhaps be 
found most consistent with the phenomena. 
proposition v. 
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 tzvo 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. 
PROPOSITION VI. 
When an Undulation falls on the Surface of a rarer Medium , so 
obliquely that it cannot be regularly refracted , it is totally re- 
flected , , at an Angle equal to that of its Incidence. 
(Phil. Trans, for 1800, p. 128.) 
Corollary. This phenomenon tends to prove the gradual in- 
crease and diminution of density at the surface terminating two 
mediums, as supposed in hypothesis iv ; although Huygens 
has attempted to explain it somewhat differently. 
