of Light and the Theory of a Quasi-labile yEther. 245 



in the Philosophical Magazine*, and a further discussion is 

 promised. For the electrical theory, the case of double refrac- 

 tion in perfectly transparent media has been discussed quite 

 in detail in the Amer. Journ. Sci.f; and the intensities of 

 reflected and refracted hght have been abundantly deduced 

 from the above conditions by various authors |. So far as all 

 these laws are concerned, the object of this paper will be 

 attained if it has been made clear that the two theories, in 

 their extreme cases, give identical results. The greater or 

 less degree of elegance, or completeness, or perspicuity, with 

 which these laws may be developed by different authors 

 should weigh nothing in favour of either theory. 



The nonmagnetic rotation of the plane of polarization, with 

 the allied phenomena in seolotropic bodies, lie in a certain 

 sense outside of the above laws, as depending on minute 

 quantities which have been neglected in this discussion. The 

 manner in which these minute quantities affect the equations 

 of motion on the electrical theory has been shown in a former 

 paper §, where these phenomena in transparent bodies are 

 treated quite at length. For the new theory, a discussion of 

 this subject is promised by Mr. Glazebrook. 



But the magnetic rotation of the plane of polarization, with 

 the allied phenomena when an seolotropic body is subjected to 

 magnetic influence, fall entirely within the scope of the above 

 equations and surface-conditions. The characteristic of this 

 case is that ^ and ^ are not self- conjugate ||. This is what 

 we might expect on the electric theory from the experiments 

 of Dr. Hall, which show that the operators expressing the 

 relation between the electromotive force and current are not 

 in general self-conjugate in this case. 



In the preceding comparison, we have considered only the 

 limiting cases of the two theories. With respect to the sense 

 in which the hmiting case is admissible, the two theories do 

 not stand on quite the same footing. In the electric theory, 

 or in any in which the velocity of the missing wave is very 

 great, if we are satisfied that the compressibility is so small as 

 to produce no appreciable results, we may set it equal to zero 



* Sir William Thomson, he. cit. E.. T, Glazebrook, loc. cit. 



t Vol. xxiii. p. 262. 



X Lorentz, ScMomilcli's Zeitschrift, vol. xxii. pp. 1-30 and 205-219 ; 

 vol. xxiii. pp. 197-210; Fitzgerald, Phil. Trans, vol. clxxi. p. 691 j J. J, 

 Thomson, Phil. Mag. [5] vol. ix.p.284; Ptayleigh, Phil. Mag. [5] vol. xii. 

 p. 81. Glazebrook, Proc. Camb. Phil. Soc. vol. iv. p. 155. 



§ Amer. Journ. Sci. vol. xxiii. p. 460. 



II See Amer. Journ. Sci. vol. xxv. p. 113. 



