540 Dr. R. T. Glazebrook : 



view it is still possible that for the tiny motions involved in 

 the propagation of light the fluid may have rigidity. 



However, be this satisfactory or not, and the difficulty is one 

 which occurs in every elastic-solid theory of Optics, the result 

 remains that an elastic-solid • theory is not consistent with 

 the facts. The phenomena of reflexion and refraction at the 

 bounding surface of two media may be due either to a change 

 in density or to a change in rigidity. 



Green's theory of refraction assumes the change to be one 

 of density, the rigidity of the aether in all isotropic media is 

 the same; his theory of double refraction assumes this to 

 arise from a variation of the rigidity in different directions 

 within a crystal. 



These difficulties are clearly exposed in Stokes' Report to 

 the British Association in 1862, in which he also shows that 

 MacCullagb and Neumann's theory is impossible so long as 

 the potential energy of the aether when transmitting light is 

 assumed to be that of a strained elastic solid. If we suppose 

 the aether to differ from an ordinary elastic solid but to 

 possess what has been called rotational elasticity, in conse- 

 quence of which it opposes forces tending to cause molecular 

 twist to an extent proportional to the twist, then MacCullagh's 

 form of the potential energy is obtained and his conclusions 

 hold. From this point of view the matter has been developed 

 of late years by Larmor. 



The Report of 1862 deals with another matter, specially 

 interesting to myself, because in later years Stokes encouraged 

 me to pursue it. 



Up to that date the experiments to verify Huyghens' 

 construction for a uniaxial crystal had been of the roughest 

 character. Stokes devised a method of testing the construc- 

 tion to a very high degree of accuracy and carried it into 

 effect for Iceland spar. The results are very briefly referred 

 to ; they were published later, but hardly in greater detail, 

 at Lord Kelvin's urgent request, in the Proceedings of the 

 Royal Society. 



The outcome was that while for a uniaxial crystal at least 

 Huyghens' construction was undoubtedly true, no theoretical 

 basis could be given for it. 



It w r as left to Maxwell to carry the question a stage 

 further. He showed that the laws which regulate the propa- 

 gation of electric force in a crystal are identical with those 

 of light, while experiment proved that the velocity of light is 

 the same as that of an electric disturbance, and hence we 

 have the electromagnetic theory of light. 



It should be noted, however, that this theory, as Maxwell 



