EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 175 



when the space intervening between the plates is filled 

 with a substance of intermediate refractive power, another 

 phenomenon predicted by theory and verified by experi- 

 ment as Sir John Herschel has described. There is hardly 

 a limit to the number of other complicated effects of 

 the interference of rays of light under different circum- 

 stances which might be deduced from the mathematical 

 expressions, if it were worth while, or which, being 

 previously observed can be explained, as in an interesting 

 case observed by Sir John Herschel and explained by 

 AiryP. 



By a somewhat different effort of scientific foresight, 

 Fresnel discovered that any solid transparent medium 

 might be endowed with the power of double refraction by 

 mere compression. For as he attributed the peculiar re- 

 fracting power of crystals to the unequal elasticity in 

 different directions, he inferred that unequal elasticity, 

 if artificially produced, would give similar phenomena. 

 With a powerful screw and a piece of glass, he then pro- 

 duced not only the colours due to double refraction, but 

 the actual duplication of images. Thus, by a great scien- 

 tific generalisation, are the apparently unique properties 

 of Iceland spar shown to belong to all transparent sub- 

 stances under certain conditions q . 



All other predictions in optical science are, however, 

 thrown into the shade by the theoretical discovery of 

 conical refraction by the late Sir W. II . Hamilton, of 

 Dublin. In investigating the passage of light through 

 certain crystals, Hamilton found that Fresnel had slightly 

 misinterpreted his own formulas, and that, when rightly 

 understood, they indicated a phenomenon of a kind never 

 witnessed. A small ray of light sent into a crystal of 

 arragonite in a particular direction, becomes spread out 



P Airy's, 'Mathematical Tracts/ 3rd edit. p. 312. 

 <i Young's 'Works/ vol. i. p. 412. 



