ETHER. 



capacity should be equal to the square of the index of 



Bohjonann* has found that this is very accurately true for the gases which 

 he bM examined. Liquids and solids exhibit a greater divergence from this 

 , but we can liardly expect even an approximate verification when we 

 bar* to compare the result* of our sluggish electrical experiments with the 

 *it*ntatiom of light, which take place billions of times in a second 



The undulatory theory, in the form which treats the phenomena of light 

 M UM motion of an elastic solid, is still encumbered with several difficulties!. 



The first and most important of these is that the theory indicates the possi- 

 bility of undulations consisting of vibrations normal to the surface of the wave. 

 The only way of accounting for the fact that the optical phenomena which 

 would arise from these waves do not take place is to assume that the aether 

 is incompressible. 



The next is that, whereas the phenomena of reflection are best explained 

 on the hypothesis that the vibrations are perpendicular to the plane of polari- 

 m fa* those of double refraction require us to assume that the vibrations are 

 in that plane. 



The third is that, in order to account for the fact that in a doubly re- 

 fracting crystal the velocity of rays in any principal plane and polarized in 

 that plane is the same, we must assume certain highly artificial relations among 

 the coefficients of elasticity. 



The electromagnetic theory of light satisfies all these requirements by the 

 ingle hypothesis | that the electric displacement is perpendicular to the plane of 

 polarization. No normal displacement can exist, and in doubly refracting crystals 

 the specific dielectric capacity for each principal axis is assumed to be equal 

 to the square of the index of refraction of a ray perpendicular to that axis, 

 and polarized in a plane perpendicular to that axis. Boltzmann has found 

 that these relations are approximately true in the case of crystallized sulphur, 



SUsk., 53 April, 1874. 



t 8w Prot Stoke* " Report on Double Refraction," British Ass. Report, 1862, p. 253. 

 Our 4* Aeon* der tcruykaaUing en brekimj van ftet licfU, Academisch Proefschrift door H. A. 



Ambrm, K. Tn dcr Zande, 1875. 



Uetor di Venchiedenheit dor Dielektricitatsconstante des krystallisirten Schwcfels nach verscliie- 

 " by Ludwig Boltzmann, Wiener Sitzb., 8th Oct, 1874. 



