﻿526 The Hon. J. W. Strutt on Double Refraction. 



tion which is a circle 5 but the other is not a true ellipse. Within 

 a uniaxal crystal one ray always follows the ordinary law. 



In ordinary media the transversal vibrations can be propagated 

 without any tendency to produce dilatation (positive or negative). 

 But it is not so here. Suppose in our illustration that the centre 

 of the ellipsoid is constrained to move in a certain plane. We 

 should find two directions of possible vibration and two corre- 

 sponding periods, just as for light in a crystal. The question 

 presents itself, What in the latter case takes the place of the ex- 

 ternal constraint ? The resistance of the ather to compression — is 

 the answer. Any part of the aether during the passage of a trans- 

 verse wave over it tends (except in particular cases) to move nor- 

 mally ; but the tendency is shared by all the other parts in the 

 same sheet parallel to the wave-front. The motion, therefore, 

 cannot be actually performed, because it would involve a com- 

 pression of the medium, which by hypothesis requires an infinite 

 force. The pressure p, however, is not without effect ; for it 

 modifies the reflection and refraction when light enters a crystal, 

 and it is probably closely connected with the oblique propagation 

 of a ray in the interior. The actual direction of a ray is to be 

 found from the wave-surface, just as in FresnePs theory. 



I had got about as far as this in my original work when, on 

 reference to Professor Stokes's report, I was greatly surprised to 

 find allusions to a theory of double refraction mathematically, if 

 not physically, identical with that here advanced. After insist- 

 ing on the importance of precise measurements, he says : — " To 

 make my meaning clearer, I will refer to FresnePs construction, 

 in which the laws of polarization and wave-velocity are deter- 

 mined by the sections, by a diametral plane parallel to the wave- 

 front, of the ellipsoid 



a¥+% 2 +cV=l ; . . (11) 



where a, b, c denote the principal wave- velocities. The principal 

 semiaxes of the section determine by their direction the normals 

 to the two planes of polarization, and by their magnitude the re- 

 ciprocals of the corresponding wave- velocities. Now a certain 

 other physical theory which might be proposed leads to a con- 

 struction differing from FresnePs only in this, that the planes 

 of polarization and wave- velocities are determined by the section, 

 by a diametral plane parallel to the wave-front, of the ellipsoid 



r 2 y 2 z i 



^ + | + ? = l> • • (12) 



the principal semiaxes of the section determining by their direc- 

 tion the normals to the two planes of polarization, and by their 

 magnitudes the corresponding wave-velocities. The law that the 

 planes of polarization of the two waves propagated in a given di- 



