50 Mr. Mac Cutxiacu on Reflexion and Refraction. 
prevent any misconception as to the nature of the foundation on which it stands. 
In this theory, every thing depends on the form of the function V ; 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 rea- 
soning 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 Pro- 
ceedings of the Royal Irish Academy, vol. ii. p. 96. 
