In a former paper I have shown that, of the various hypotheses which might be made to explain the diminished velocity of light in transparent matter, only one can be reconciled with the observed laws regulating the intensity of polarized light scattered in different directions from an assemblage of particles whose diameters do not exceed a small fraction of the wave-length. We are forced to suppose that the difference between media which is the cause of refraction is a dynamical and not a statical difference, that the rigidity or force with which the aether resists distortion is absolutely invariable. In this view there is nothing novel. Fresnel distinctly adopts it in the investigation of his celebrated formulae for the intensities of reflected light; and, what is more important, Green's rigorous mechanical theory of reflection is based on the same assumption. Cauchy also, to whom much of the credit really due to Green has been transferred, starts from the principle of continuity of movement, which asserts that in the passage from one medium to another there is no break in the continuity of the values, either of the displacements or of their differential coefficients. I believe that Cauchy has nowhere explained the ground or significance of his principle; but it is easy to see that to assume the continuity of strain is equivalent to asserting a complete continuity of statical properties, so that, as has been pointed out by Haughton, Cauchy's theory is essentially the same as Green's.