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X. ON THE LAWS OF CRYSTALLINE REFLEXION. 



[From the Philosophical Magazine, VOL. x., 1837.] 



IN a Number of PoggendorfFs Annalen (No. 6, for 1836), which 

 reached Dublin late in November, there are some remarks by 

 M. Seebeck on a Paper of mine which appeared in the last 

 February Number of this Journal (vol. viii. p. 103). That 

 Paper contains a general theory of reflexion at the surfaces of 

 crystallized media ; and M. Seebeck, in comparing the results 

 with his own experiments, has fully confirmed some of my 

 formulae, while he has shown that others are defective. I have 

 therefore been obliged to revise my theory, and I have ascer- 

 tained that it was vitiated by the introduction of a certain re- 

 lation among the quantities denominated pressures, which, 

 following the example of M. Cauchy, I had supposed to be 

 concerned in the problem. This relation I had observed to 

 hold in the case of singly refracting media, and I concluded, 

 without any other reason, that it would hold good generally. 

 But though it led to the correct formula for the polarizing 

 angles in different azimuths, it was nevertheless arbitrary and 

 unfounded ; and therefore it is now banished entirely from the 

 investigation, the place which it occupied being supplied by the 

 natural and simple law of the preservation of vis viva, while 

 everything else remains as before. I hope the imperfection of 

 my first essay will be excused, when it is considered that the 

 erroneous proposition bears but a small proportion to the whole 

 theory; and, moreover, that the general problem, which I under- 

 took to resolve, is one that has not been attempted by any other 



