The Aether as an Elastic Solid. 167 



Analysis similar to that of Cauchy's and Green's Second 

 Theory of crystal-optics may be applied to explain the doubly 

 refracting property which is possessed by strained glass ; but 

 in this case the formulae derived are found to conflict with 

 the results of experiment. The discordance led Kelvin to 

 doubt the truth of the whole theory. "After earnest and 

 hopeful consideration of the stress theory of double refraction 

 during fourteen years," he said,* " I am unable to see how it 

 can give the true explanation either of the double refraction of 

 natural crystals, or of double refraction induced in isotropic 

 solids by the application of unequal pressures in different 

 directions." 



It is impossible to avoid noticing throughout all Kelvin's 

 work evidences of the deep impression which was made 

 upon him by the writings of Green. The same may be said 

 of Kelvin's friend and contemporary Stokes; and, indeed, it 

 is no exaggeration to describe Green as the real founder of 

 that " Cambridge school " of natural philosophers, of which 

 Kelvin, Stokes, Lord Eayleigh, and Clerk Maxwell were the 

 most illustrious members in the latter half of the nineteenth 

 century, and which is now led by Sir Joseph Thomson and 

 Sir Joseph Larrnor. In order to understand the peculiar 

 position occupied by Green, it is necessary to recall some- 

 thing of the history of mathematical studies at Cambridge. 



The century which elapsed between the death of Newton 

 and the scientific activity of Green was the darkest in the 

 history of the University. It is true that Cavendish and 

 Young were educated at Cambridge; but they, after taking 

 undergraduate courses, removed to London. In the entire 

 period the only natural philosopher of distinction who lived 

 and taught at Cambridge was Michell ; and for some reason 

 which at this distance of time it is difficult to understand 

 fully, Michell's researches seem to have attracted little or no 

 attention among his collegiate contemporaries and successors, 



* Baltimore Lectures (ed. 1904), p. 258. 



