XV. 



A COMPARISON OF THE ELECTRIC THEORY OF LIGHT 

 AND SIR WILLIAM THOMSON'S THEORY OF A QUASI- 

 LABILE ETHER. 



[American Journal of Science, ser. 3, vol. xxxvn, pp. 139-144, February, 1889.] 



A REMARKABLE paper by Sir William Thomson, in the November 

 number of the Philosophical Magazine, has opened a new vista in 

 the possibilities of the theory of an elastic ether. Since the general 

 theory of elasticity gives three waves characterized by different 

 directions of displacement for a single wave-plane, while the pheno- 

 mena of optics show but two, the first point in accommodating any 

 theory to observation, is to get rid (absolutely or sensibly) of the 

 third wave. For this end, it has been common to make the ether 

 incompressible, or, as it is sometimes expressed, to make the velocity 

 of the third wave infinite. The velocity of the wave of compression 

 becomes in fact infinite as the compressibility vanishes. Of course 

 it has not escaped the notice of physicists that we may also get 

 rid of the third wave by making its velocity zero, as may be done 

 by giving certain values to the constants which express the elastic 

 properties of the medium, but such values have appeared impossible, 

 as involving an unstable state of the medium. The condition of 

 incompressibility, absolute or approximate, has therefore appeared 

 necessary.* This question of instability has now, however, been 

 subjected to a more searching examination, with the result that 

 the instability does not really exist "provided we either suppose the 

 medium to extend all through boundless space, or give it a faced 

 containing vessel as its boundary." This renders possible a very 

 simple theory of light, which has been shown to give Fresnel's laws 

 for the intensities of reflected and refracted light and for double 

 refraction, so far as concerns the phenomena which can be directly 

 observed. The displacement in an aeolotropic medium is in the same 

 plane passing through the wave-normal as was supposed by Fresnel, 



* It was under this impression that the paper entitled " A Comparison of the Elastic 

 and the Electric Theories of Light with respect to the Law of Double Refraction and the 

 Dispersion of Colors," [this volume pp. 223-231], was written. The conclusions of 

 that paper, except so far as respects the dispersion of colors, will not apply to the 

 new theory. 



