382 FOURTH REPORT 1834. 



suppositions were made to account for the phenomena of polari- 

 zation in biaxal crystals. 



Such was the state of the theory of double refraction when the 

 subject was taken up by Fresnel. The law of refraction, we 

 have seen, whether in the theory of emission or in that of waves, 

 was intimately connected with and dependent on the law of ve- 

 locities ; so that, considered as a physical question, the problem 

 resolved itself into the determination of the latter. With the 

 exception, however, of the reasonings of Young respecting the 

 form of the wave-surface in a medium compressed or dilated in 

 a given direction*, no attempt had been made to deduce the 

 velocity of the extraordinaiy ray from the principles of either 

 theory. Indeed, the general law of the velocities was itself un-, 

 known, even as an experimental fact, although an important 

 relation between the velocities of the two pencils had been dis- 

 covered by the labours of Sir David Brewster and M. Biot. But 

 this was not all. It was evident that no physical theory of double 

 refraction could be regarded as complete, which did not at the 

 same time account for the attendant phenomenon oi polarization. 

 In this branch of the subject, however, nothing had been accom- 

 plished ; and all that had been said in explanation of the pheno- 

 menon of polarization did not go further than some vague spe- 

 culations as to its cause. The theory of Fresnel to which I now 

 proceed, — and which not only embraces all the known pheno- 

 mena, but has even outstripped observation, and predicted con- 

 sequences which were afterwards fully verified, — will, I am per- 

 suaded, be regarded as the finest generalization in physical science 

 which has been made since the discovery of universal gravitation. 



Fresnel -f- sets out from the supposition that the elastic force 

 of the vibrating medium is, in general, different in different 

 directions. This is, in fact, the most general supposition 

 that casi be made ; and whether we suppose that the vibrating 

 medium is the ether Avithin the crystal, or that the molecules 

 of the body itself partake of the vibratory movement, there 

 will be obviously such a connexion and mutual dependence 

 of the parts of the solid and those of the medium in question, 

 that we cannot hesitate to admit for the one what has been al- 

 ready established on the clearest evidence for the other if. Now 

 if a disturbance be produced in a medium so constituted, and 



• Quarferhj Review, vol. ii. 



f " Memoire siiv la Double Refraction," Mem. Inst., torn. vii. 



X M. Savart has shown that the elasticity of crystals, determined bj^ means 

 of their sonorous vibrations, is, in general, different in difTercnt directions. The 

 optic axis of Iceland spar is the axis of least elasticity : that of rock crystal is 

 the axis of greatest elasticity. 



