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LX. On the Application of Sir William Thomson's Theory 

 of a Contractile ^Ether to Double Refraction, Dispersion, 

 Metallic Reflexion, and other Optical Problems. By R. T. 

 Glazebeook, M.A., F.R.S.* 



IN any isotropic elastic solid there are, in general, two 

 velocities of wave-propagation — one for normal waves, 

 given by \/A/p in Green's notation ; the other, given by 

 V-B,'/o, for transverse waves; and when any system of waves 

 foils on the bounding surface of two such media both these 

 disturbances are set up. Since light-waves are entirely 

 transverse, and do not give rise to normal waves possessing, 

 at any rate, more than a very small fraction of the energy of 

 the incident waves, it follows, as was shown by Green, that 

 the ratio A/B is, for the aether, either extremely large or ex- 

 tremely small. If the surfaces of the solid at a finite distance 

 from the origin be free, it is necessary, in order that the equi- 

 librium position may be one of minimum potential energy, 

 that A— JB should be positive, and hence Green supposed 

 that A was very large and the aether incompressible. This 

 view has generally been accepted by English writers on 

 optical subjects. 



In his paper " On the Reflexion and Refraction of Light/' 

 in the last number of this Magazine, Sir William Thomson, 

 however, has shown that, hi provided ice suppose the medium to 

 extend all through boundless space or give it a fixed containing 

 vessel as its boundary," the conditions for stability in the aether 

 are satisfied if we suppose that neither A nor B is negative. 

 Under these circumstances it is not necessary that A should 

 be greater than |B, it is sufficient that A should be zero or 

 positive. Such a medium, according to Sir William Thomson, 

 is afforded us by homogeneous airless foam held from collapse 

 by adhesion to a bounding vessel which may be infinitely 

 distant all round, and for this medium A is zero, i. e. the 

 medium is incapable of transmitting normal waves. On this 

 hypothesis as to the nature of the aether it is possible to 

 suppose that the absence of the normal wave is because A is 

 zero, not because it is infinite. Sir William Thomson has, in 

 in his paper just referred to, treated the problem of reflexion 

 and refraction on this supposition ; the object of the present 

 communication is to consider double refraction and other allied 

 problems. 



In my Report on Optical Theories, presented to the British 

 Association at Aberdeen (B. A. Report, 1885, p. 179), when 

 discussing the equations which are given by certain theories 



* Communicated by the Author. 



