﻿520 The Hon. J. W. Strutt on Double Refraction, 



however, no difficulty in showing that their hypothesis is as in- 

 consistent with the phenomena of regular reflection as it is with 

 those of diffraction from small particles, as I may perhaps ex- 

 plain in detail on another occasion. But there is one argument 

 urged by them against the rival view which deserves the greatest 

 attention. How, they ask, can double refraction be accounted 

 for if the elastic forces brought into action by a given deforma- 

 tion of the aether are the same in all cases ? It is well known 

 that all the theories of double refraction hitherto given by 

 Fresnel and his followers assume expressly that within a doubly 

 refracting medium the elasticity varies in different directions. 

 How is it possible, in investigating the laws of reflection from 

 the surface t)f isotropic media, to suppose that the statical con- 

 dition of the sether is invariable, and then, when we come to 

 double refraction, to turn round and say that in them the sether 

 has a rigidity dependent on the direction of displacement ? I 

 am not surprised at the importance attached by MacCullagh and 

 Neumann to this objection. Fresnel and Green's investigations 

 of reflection are indeed absolutely inconsistent with the received 

 views as to the cause of double refraction. We find ourselves 

 then in this position : either we must give up Green's theory of 

 reflection, which is the only one hitherto proposed, or easily con- 

 ceivable, capable of meeting the facts of the case ; or else we 

 must abandon the ideas of Fresnel as to the mechanical cause of 

 double refraction. 



MacCullagh and Neumann were consistent, though, as I be- 

 lieve, consistently wrong. They rejected the hypothesis of a 

 constant rigidity and variable density as incompatible with the 

 existence of double refraction. How, indeed, conceive a density 

 different in different directions ? 



Fresnel and Green were inconsistent. The latter has given 

 two rigorous theories of double refraction* which differ from 

 one another in important points, but agree in this, that neither 

 of them can be reconciled with his explanation of reflection ; 

 for both assume that the forces which resist displacement within 

 a crystal yary according to the direction of displacement. Pre- 

 cisely the same remark applies to the investigations of Cauchy. 



It will readily be anticipated that, having the strongest grounds 

 for believing that the rigidity of the sether is constant whether it 

 be free as in vacuum or entangled with the molecules of matter, 

 I adopt the latter of the two alternatives already mentioned, and 

 look in another direction for the explanation of double refraction . 

 In taking a step which may seem retrograde, I would remark that 

 we are not abandoning a theory in itself very complete or satis- 

 factory. FresnePs explanation of double refraction will always 

 * Camb. Phil. Trans. 1837. Green's ' Math. Papers.' 



