530 Mr. K. T. Glazebrook on the Application of the 



of the section of the ellipsoid by the wave-front. According 

 to Fresnel, P is the direction of vibration in the ray R. 

 According to the theory of the present paper this is not the 

 case. 



Let P Y be the tangent at P to the ellipse P N R, and Y 

 perpendicular on P Y. P Y is the trace on the plane P N R 

 of the tangent-plane to the ellipsoid ; and this plane is per- 

 pendicular to the plane PNR, so that X is the normal to 

 the tangent-plane at P. According to our theory, Y is the 

 direction of vibration, and moreover Y is perpendicular 

 to R. 



Now experiment* shows us that FresneFs construction for 

 the velocity is very closely indeed approached to ; and hence 

 A must be, if not actually zero, very small indeed. We 

 have, however, no exact experimental evidence on the direc- 

 tion of vibration in a crystal ; and it would be extremely 

 difficult to devise an experiment which would decide between 

 Fresnel's result and that of the theory now suggested. So 

 far, then, as experimental evidence is concerned we may claim 

 that the theory here given is in very close accordance with 

 our present results. It has moreover the extreme advantage 

 of basing the laws of double refraction on variations of the 

 property of the aether, on which ordinary reflexion and refrac- 

 tion almost certainly depend. 



Refraction occurs because the optical density of the aether 

 is different in different media ; double refraction, because in 

 a crystal the optical density is different in different directions. 



It remains now to consider what is meant by the optical 

 density of the sether, and how it can vary in different media, 

 or in different directions in the same medium. The phe- 

 nomena of aberration and the other optical effects produced 

 by the motion of transparent bodies are more easily explicable 

 if we suppose the actual density of the aether as well as its 

 rigidity to be the same in all bodies. Let us make this 

 assumption for the present. Now the motion of the aether 

 within a transparent body is not free ; in addition to the 

 forces arising from its own rigidity there must be others 

 arising from the action of the transparent matter ; and though 

 we are ignorant of the nature of this action we can show, 

 remembering that light-waves travel through the medium 

 with a velocity which is independent of the amplitude, that 

 the forces resolve themselves into two sets. One of these 

 makes its appearance in such a way as to be equivalent to an 

 increase in the density of the aether, while the other is 



* Stokes, Proc. Roy. Soc. ; B. A. Report, 1862. Glazebrook, Phil. 

 Trans. 1879, part i. ; 1879, part ii. Hastings, Silliman's Journal. 



