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we hope) with the rights, or even with the convenience of authors ; 

 I think myself allowed to enter more at large into the merits of the 

 award, and to lay before you some of the thoughts which the pe- 

 rusal of the present prize essay has suggested to my own mind. 



When ordinary light is reflected at the common boundary of 

 two transparent and uncrystallized media, as when we see (for ex- 

 ample) the reflexion of the sun in water, the reflected light differs 

 from the incident in both direction and intensity, according to laws 

 which were known to Euclid in so far as they regard direction, 

 but of which the discovery, in so far as intensity is concerned, was 

 reserved for the sagacity ofFresnel. In general, the laws which 

 regulate the changes of the direction of light have been found easier 

 of discovery than those which regulate its changes of intensity ; the 

 laws of the reflexions and refractions of the lines along which light 

 is propagated, than the laws of the accompanying determinations or 

 alterations of its planes of polarisation ; or, to express the same dis- 

 tinction in the language of the theory of undulations, it has been 

 found easier to assign the form of the waves which spread from any 

 origin of disturbance through any given portion of the elastic lumi- 

 niferous ether, than to assign the directions and relative magnitudes 

 of the vibrations which constitute those waves, and the laws which 

 regulate the changes of such vibrations, in the passage from one 

 medium to another. 



The laws which regulate such changes of vibration, produced 

 by reflexion and refraction, at the boundaries of crystallized 

 media, have been the special object of Mr. Mac Cullagh's investi- 

 gations, in the paper now before us. But in investigating them, he 

 has been obliged to consider also the laws which regulate the vi- 

 brations of the ether, in the interior of a crystallized body, and 

 not at its surface only ; the laws of the propagation as well as 

 those of the reflexion and refraction of light. His researches are 

 therefore connected intimately with a wide range of optical pheno- 

 mena ; and the hypotheses on which his formulae are founded, and 

 which seem to have their own correctness proved by the experi- 

 ments of many kinds with which they have been successfully com- 

 pared, though liable, of course, like every physical induction, to be 

 modified in some degree by future observation, appear to be en- 



