42 Mr. MacCuUagh o?i the Laws of Crystalline Befcxion, 



cannot on this occasion pass by without notice Mr. Professor 

 Don, of Kin<);'s College, London, whom we regard as occu- 

 pying a very elevated station among the botanists of this coun-, 

 try, and second only to Dr. Brown in the number of valuable 

 contributions with which he has enriched the Transactions of 

 the Linnjean Society, all tending to advance and to illustrate 

 the system of Jussieu. 



[To be continued.] 



XL Qn the La-ws of Crystalline Beflexion. By James Mac- 

 Cull a gh. Fellow of Trinity College^ Duhlin* 



TN a Number of PoggendorfiT's Annalen (No. 6, for J836,) 

 ^ which reached Dublin late in November, there are some re- 

 marks 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, 

 foUowinfT 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. 1 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 undertook to re;Solve, is one that has not been attempted by 

 any other person, although the want of a solution has long 

 been felt. The difficulties which we have to deal with, in en- 

 tering upon this problem, are not mere mathematical difficul- 

 ties, but difficulties arising from the want of first principles ; 

 and, in physical questions of this kind, where we must, at the 

 outset, have recourse to conjecture, in order to supply the very 

 principles of our reasoning, it can hardly be expected that the 

 whole truth should be divined at once. I think, however, 



* Communicated bv the Autlior. 



