154 The Aether as an Elastic Solid. 



Thus if BjA denote the modulus of /A, we have 



p\ 



if>l 



This expression represents the ratio of the intensity of the 

 transverse reflected wave to that of the incident wave. It 

 does not agree with Fresnel's tangent- formula : and both on this 

 account and also because (as we shall see) this theory of reflexion 

 does not harmonize well with the elastic-solid theory of crystal- 

 optics, it must be concluded that the vibrations of a Greenian 

 solid do not furnish an exact parallel to the vibrations which 

 constitute light. 



The success of Green's investigation from the standpoint of 

 dynamics, set off by its failure in the details last mentioned, 

 stimulated MacCullagh to fresh exertions. At length he succeeded 

 in placing his own theory, which had all along been free from 

 reproach so far as agreement with optical experiments was 

 concerned, on a sound dynamical basis ; thereby effecting that 

 reconciliation of the theories of Light and Dynamics which had 

 been the dream of every physicist since the days of Descartes. 



The central feature of MacCullagh's investigation,* which 

 was presented to the Eoyal Irish Academy in 1839, is the intro- 

 duction of a new type of elastic solid. He had, in fact, concluded 

 from Green's results that it was impossible to explain optical 

 phenomena satisfactorily by comparing the aether to an elastic 

 solid of the ordinary type, which resists compression and 

 distortion ; and he saw that the only hope of the situation was 

 to devise a medium which should be as strictly conformable to- 

 dynamical laws as Green's elastic solid, and yet should have 

 its properties specially designed to fulfil the requirements of 

 the theory of light. Such a medium he now described. 



If as before we denote by e the vector displacement of a 

 point of the medium from its equilibrium position, it is well 



* Trans. Roy. Irish Acad. xxi. : MacCullagh's Coll. Works, p. 145. 



