Mr. Potter on Conical Refraction. 343 



Gentlemen, however hastily surveyed and imperfectly de- 

 scribed, this is still another proof in confirmation of the glacier 

 theory in Scotland, so lately promulgated by the great phi- 

 losopher of Neufchatel, to whom the geological world is so 

 deeply indebted, by revealing so bright a page in the dark 

 history of the past. W. Kemp. 



LVII. An Examination of the Phcenomena of Conical Re- 

 fraction in Biaxal Crystals. By R. Potter, Esq., B.A.* 

 THE verification of the two species of conical refraction, 

 which were discovered by Sir William Hamilton to be 

 results of Fresnel's analytical expression for the wave surface 

 in biaxal crystals, forms an epoch in the progress of the 

 conversion of the scientific world to a belief of the undula- 

 tory theory of light. Many waverers were confirmed in their 

 belief by so singular a coincidence of theory and experiment ; 

 and Professor Lloyd, who made the experiments, had a har- 

 vest of reputation from them, such as is seldom reaped in the 

 field of science. 



I joined with the scientific world, in the confidence which 

 it gave to Professor Lloyd's investigations; and although I 

 had seen the undulatory theory fail in so many important 

 cases, yet I believed that the true law of refraction in biaxal 

 crystals had been discovered by Fresnel, as certainly as that 

 of uniaxal crystals has been by Huyghens ; but in both cases 

 from wrong premises. It was from a desire to view these in- 

 teresting phaenomena, that I availed myself of an opportunity 



are sometimes found in the rock, but in places remote from running waters, 

 and where the form of the surface will not permit us to suppose that any 

 cascade could ever have existed. In other cases, a long, sinuous, dry, 

 water-worn gutter or channel is observed, the course of which runs across 

 instead of along the natural declivity of the ground. The study of the 

 glaciers has enabled Agassiz to find a key to these enigmatical phaenomena, 

 which had perplexed previous inquirers. Streams of water flow along the 

 surface of a glacier, and when one of these falls into a fissure which is open 

 to the bottom, if it forms a cascade, it cuts a round cavity in the rock with 

 the gravel and sand which it either finds there or carries down with it, as 

 some of our rivulets work out the hollows termed caldrons. If the gla- 

 cier is travelling downwards the cascade will travel with it, and convert 

 the round cavity into a long gutter; or supposing the water to reach the 

 bottom without falling in a cascade, still, in finding an issue below the gla- 

 cier, it will be compelled to follow the sinuous openings left by inequalities 

 in the bottom of the ice, and thus take a course at variance with the natu- 

 ral inclination of the surface. We liave here an explanation of the Creux 

 or holes, and the long water-worn gutters found in such unlikely situations, 

 which bear the local names of Lapiaz or Karren. These are chiefly 

 observed where the rock is soft, and are seldom visible on the granite. — 

 Scotsman. 

 * Communicated by the Author. 



