1893.] the Electric and Luminiferous Medium. 445 



crystalline material structures seem to prohibit the occurrence of 

 such asymmetry as these terms would indicate, except to the very 

 small extent evidenced by the hemihedral faces of quartz crystals. 

 The influence on the optical medium of this asymmetric arrangement 

 of the molecules must be very much smaller still, for the rotatory 

 terms are in all media exceedingly minute compared with the ordinary 

 dispersional terms. The form of these rotatory terms in the energy- 

 function is at once definitely assigned by our condition of perfect 

 fluidity of the medium, both for crystals and for rotational liquids 

 such as turpentine, and this form is the one usually accepted, on 

 MacCullagh's suggestion, as yielding a correct account of the 

 phenomena. 



When dispersional terms are included in the energy function, our 

 continuous analysis is not any longer applicable to the problem of 

 reflexion ; the conditions at the interface are altogether too numerous 

 to be satisfied by the available variables. There is in fact discon- 

 tinuity at the interface in the discrete molecular structure, such as 

 could not be representable by a continuous analysis. But if we 

 proceed by the method of rays, and assume that there is a play of 

 surface forces which do not absorb any energy, while they adjust the 

 dispersional part of the stress, it appears that reflexion is independent 

 of dispersion. 



The treatment of the problem of reflexion by Fresnel involves a 

 different direction of vibration of the light, and different surface 

 conditions, from MacCullagh's. It is of interest to remark that this 

 theory may be stated in a dynamically rigorous form, provided the 

 medium to which it refers possesses the properties of the labile 

 elastic-solid a3ther of Lord Kelvin ; and Fresnel's own account of his 

 analysis of the problem becomes more intelligible from such a stand- 

 point. 



The mention of the phenomena of magnetic rotational quality will 

 introduce us to the next division of the subject, that of the inclusion 

 of electric and magnetic phenomena in the domain of the activity of 

 this primordial medium. 



The problem of the aether has been first determinedly attacked 

 from the side of electrical phenomena by Clerk Maxwell in quite 

 recent times; his great memoir on a * Dynamical Theory of the 

 Electromagnetic Field ' is of date 1864. It is in fact only compara- 

 tively recently that the observation of Oersted, and the discoveries 

 and deductions of Ampere, Faraday, and Thomson had accumulated 

 sufficient material to allow the question to be profitably attacked 

 from this side. Even as it is, our notions of what constitute electric 

 and magnetic phenomena are of the vaguest as compared with our 

 ideas of what constitutes radiation, so that Maxwell's views involve 

 difficulties, not to say contradictions, and in places present obstacles 



