( 185 ) 



XV. ON TEE OPTICAL LAWS OF ROCK-CRYSTALS. 



[Proceedings of the Royal Irish Academy, VOL. i. p. 385. Read Jan. 13, 1840.] 



PROFESSOR MAC CULLAGH made a communication respecting 

 the optical Laws of Rock-crystal (Quartz). 



In a Paper read to the Academy in February 1836, and 

 published in the Transactions (VoL. xvn. p. 461),* he had shown 

 how the peculiar properties of that crystal might be explained, 

 by adding to the usual equations of vibratory motion certain 

 terms depending on differential co-efficients of the third order, 

 and containing only one new constant C. This hypothesis, 

 which was very simple in itself, not only involved as conse- 

 quences all the laws that were previously known, but led to 

 the discovery of a new one the law, namely, by which the 

 ellipticity of the vibrations depends on the direction of the ray 

 within the crystal. He was not able, however, to account for 

 his hypothesis, nor has it since been accounted for by anyone. 



But the theory developed in the Paper which he read at 

 the last meeting of the Academy now enables him to assign, 

 with a high degree of probability, the origin of the additional 

 terms above mentioned, and, if not to account for them mecha- 

 nically, at least to advance a step higher in the inquiry. In 

 that theory it was supposed (and the supposition holds good 

 in all known crystals, except quartz), that tLe molecules of the 

 ether vibrate in right lines, the displacements remaining always 

 parallel to each other as the wave is propagated; and it was 

 shown that the function V, by which the motion is determined, 



* Supra, p. 63. 



