common direction in the crystal, is proportional to the product of the 

 sines of the angles which that direction makes with the optic axes. 

 This is the celebrated law of M. Biot, to which he was led by ana- 

 logy, and which he afterwards found to agree with that previously 

 laid down by Dr. Brewster. 



Such, in their simplest form, are the principal features of the 

 Mechanical Theory of double Refraction, invented by the late M. 

 Fresnel — a theory which would do honour to the sagacity of Newton, 

 and which gives us ample reason to regret that the life of its author 

 was not longer spared, to enrich with further discoveries his favourite 

 science. Of him it may be said, as Newton said of Cotes — and 

 apparenth'^ with much greater reason — that if he had lived longer, 

 we should have known something at last of the laws of nature. 



Trinity College, Dublin, 

 Oclober 27, 1829. 



