138 The Rev. S. Haughton o?i a Classification of Elastic Media, ^x. 



difficult to make them with sufficient accuracy. The present state of the wave 

 theory of light certainly suggests grave doubts as to tlie nature of the foundation 

 on which the whole system is based. We first assume the existence of an un- 

 known medium, whose existence must remain unproved and unprovable by us ; 

 then, from supposed properties of this unknown medium, we deduce the laws of 

 propagation, &c. Here a new difficulty arises; for we find several different theories 

 capable of explaining the laws of propagation, and explaining with more or less 

 exactness the most obvious of the laws of reflexion and refraction. How are we 

 to decide among these conflicting theories ? Are we to assume, with M. Cauchy, 

 that the observed laws of polarized light occupy, with respect to the mathematical 

 laws deduced from his theory, the same position that tlie laws of Kepler stand in 

 with respect to the more accurate laws of planetarj' motion? or are we to assume 

 that theory to be correct which agrees accurately with the common formulce 

 for reflected light, when it is well known that these formulaj themselves are 

 doubtful for highly refracting substances? It appears certain, that we do 

 not yet possess experimental knowledge sufficient to enable us to determine 

 which of the theories of light is correct, or whether any of them be so. In a 

 general point of view Professor Mac Cullagh's theory possesses an important 

 advantage, as compared with other theories. It contains no inexplicable normal 

 wave, and does not render this difficult subject still more intricate, by the in- 

 troduction of a useless vibration. It is greatly to be desired, that the attention 

 of experimentalists were directed to the necessity which exists for more accurate 

 and general researches into the laws of crystalline reflexion and refraction, and 

 that the surface of Fkesnel were placed upon a purely experimental basis. From 

 such researches, carefully conducted, might be deduced the geometrical laws of 

 double refraction, and a foundation be laid for a complete and positive theory 

 of the laws of polarized light. 



