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CHAPTER V. 



THE AETHER AS AN ELASTIC SOLID. 



WHEN Young and Fresnel put forward the view that the 

 vibrations of light are performed at right angles to its direction 

 of propagation, they at the same time pointed out that this 

 peculiarity might be explained by making a new hypothesis 

 regarding the nature of the luminiferous medium ; namely, that 

 it possesses the power of resisting attempts to distort its shape. 

 It is by the possession of such a power that solid bodies are 

 distinguished from fluids, which offer no resistance to distortion; 

 the idea of Young and Fresnel may therefore be expressed by 

 the simple statement that the aether behaves as an elastic solid. 

 After the death of Fresnel this conception was developed in a 

 brilliant series of memoirs to which our attention must now be 

 directed. 



The elastic-solid theory meets with one obvious difficulty at 

 the outset. If the aether has the qualities of a solid, how is it that 

 the planets in their orbital motions are able to journey through 

 it at immense speeds without encountering any perceptible 

 resistance ? This objection was first satisfactorily answered by 

 Sir George Gabriel Stokes* (b. 1819, d. 1903), who remarked 

 that such substances as pitch and shoemaker's wax, though so 

 rigid as to be capable of elastic vibration, are yet sufficiently 

 plastic to permit other bodies to pass slowly through them. 

 The aether, he suggested, may have this combination of qualities 

 in an extreme degree, behaving like an elastic solid for vibrations 

 so rapid as those of light, but yielding like a fluid to the much 

 slower progressive motions of the planets. 



Stokes's explanation harmonizes in a curious way with 

 Fresnel's hypothesis that the velocity of longitudinal waves in 



* Trans. Camb. Phil. Soc., viii, p. 287 (1845). 



