82 The Hon. J. W. Strutt on the Reflection of 



(that the vibrations in the two media* when resolved parallel to 

 the surface of separation are equal) and the conclusion are ap- 

 proximately true. Fresnel did not contemplate the possibility 

 of a change of phase which, as we now know from Jamm's ex- 

 perimentsf, accompanies reflection in one, if not in both of the 

 principal cases. 



Green's important work " On the Laws of Reflection and Re- 

 fraction of Light at the Common Surface of two non- crystallized 

 Media," was read before the Cambridge Philosophical Society on 

 December 1 1, 1837, and published in the Transactions for 1838 J. 

 In this paper, which has never received on theContinent the atten- 

 tion which it deserves, Green investigates the equations of motion 

 of an elastic medium, setting out, as we should now say, from the 

 principle of energy. By Lagrange's method he deduces both 

 the general equations applicable throughout the interior, and 

 the conditions which must be satisfied at the surface of sepa- 

 ration of two media. The statical properties of an isotropic 

 medium are defined by two constants A and B, the second ex- 

 pressing the rigidity, and the first depending, though not in the 

 simplest manner, on the compressibility. For the luminiferous 

 sether it is shown that A must be indefinitely great, or that the 

 medium resists change of volume with an infinite force. During 

 motion the inertia of the medium comes into play, and a con- 

 stant expressing the density must be added to the two statical 

 constants already mentioned. In all this there seems to be 

 nothing to which exception can be taken, unless it be to the as- 

 sumption (expressly stated by Green) that the sphere of sensible 

 action of the molecular forces, or, as I should prefer to say, the 

 range of the mutual influence of the parts of the medium, is in- 

 sensible in comparison with the length of the wave, and that the 

 transition from the one state of things to the other at the bound- 

 ing surface is so rapid that it may be treated as abrupt. 



But in the application to the question of reflection further 

 assumptions are made whose significance has been strangely 

 misunderstood. When light passes from air into a denser me- 

 dium, it propagates itself slower than before in the ratio of /^ : 1, 

 but this consideration alone is not sufficient to lead to a definite 

 solution. From the equation 



Pi I* 9 

 we can infer nothing as to the relation between B and B y , which 



* No account being taken of surface-waves, 

 f Ann. de Chimie, t. xxix. p. 31. 



X Reprinted in 'Green's Mathematical Papers,' edited by Ferrers. 

 MacMillan and Co. 1871. 



