Light. 235 



men," says this acute observer, "■ who were marching with every 

 appearance of muscular energy and with the most decided and 

 soldier-hke pace, suddenly complain that a thick veil was co- 

 vering their eyes : those organs, 'at first, for an instant haggard, 

 soon became immoveable : all the muscular apparatus of the 

 neck, and more particularly the sterno-cleido-masto-idean mus- 

 cles became rigid, and gradually riveted the head on the 

 right or left shoulder : this rigidity next extended to the trunk ; 

 the lower extremities tottered, and the unhappy victim fell upon 

 the snow, exhibiting, to complete the frightful picture, all the 

 symptoms of catalepsy or epilepsy." 



The Moniteur, of January 15, contained the following short 

 article, by M. Biot, '^'^ On the nature of the forces which produce 

 double refraction." 



"^ When a ray of light penetrates a crystal, the primitive form 

 of which is neither the regular octohedron nor the cube, we ob- 

 serve in general that it is divided into two fasciculi unequally re- 

 fracted. The one which we call the ordinarv fasciculus follows 

 the law of refraction discovered by Des Cartes, and which is 

 common to all crystallized and non-crystallized bodies : the other 

 follows a different and more complex law: it is called the extra- 

 ordinary fasciculus. 



" Huyghens has determined this last law by observation in 

 the rhomboidal carbonate of lime, vulgarly called Iceland spar, 

 and he has described it by a construction equally ingenious and 

 precise. By combining this fact with the general principles of 

 mechanics, as Newton has combined the laws of Kepler with the 

 theory of central forces ; M. Laplace deduced from it the general 

 expression of the velocity of the huTiinpus particles which com- 

 pose the extraordinary ray. This expression indicates that they 

 are separated from the others by a force which has emanated 

 from the axis of the crystal, and which in the Iceland spar is 

 found to be repulsive. 



*' It was generally supposed that this was the case in all the 

 other crystals endowed with double refraction. But new experi- 

 ments have proved to me that in a great number the extraor- 

 dinary ray is attracted towards the axis instead of being repelled. 

 So that with respect to this property the crystals onght to he. 

 divided into two classes ; the one I call double attractive refrac- 

 tion, and the other double repulsive refraction. Iceland spar 

 forms part of the latter: rock crystal is comprehended in the 

 former. Finally, it appeared to me, that the force, whether at- 

 tractive or repulsive, always emanates from the axis of the cry- 

 stal, 



