1 84 On Crystalline Reflexion and Refraction. 



be necessary to prevent any misconception as to the nature of 

 the foundation on which it stands. In this theory, everything 

 depends on the form of the function Y ; and we have seen that, 

 when that form is properly assigned, the laws by which crystals 

 act upon light are included in the general equation of dynamics. 

 This fact is fully proved by the preceding investigations. But 

 the reasoning which has been used to account for the form of 

 the function* is indirect, and cannot be regarded as sufficient, in 

 a mechanical point of view. It is, however, the only kind of 

 reasoning which we are able to employ, as the constitution of 

 the luminiferous medium is entirely unknown. 



* Since this Paper was read to the Academy, I have found that the form of the 

 function V is more general than it would seem to be from the mode in which it is 

 here deduced ; and I have obtained from it a theory of the Total Reflexion of Light. 

 For a sketch of this theory, see the Proceedings of the Royal Irish Academy, 

 VOL. n. p. 96 (supra, p. 187). 



