94 On the Laivs of Crystalline 



used in obtaining the formula, were erroneous. It is to M. Seebeck 

 that I am obliged for pointing out this curious circumstance. 

 In Poggendorff's Annals* he gave an abstract of my letter to 

 Sir David Brewster, and compared my results with his own 

 numerous and accurate experiments, both on the polarizing 

 angles of Iceland spar and on the angles of deviation. He 

 found that my formula represented the former class of ex- 

 periments as well as could be wished; but the theoretical 

 values of the deviations did not at all agree with his experi- 

 mental measures. These measures of the deviation he pub- 

 lished on this occasion; and, with their assistance, I traced 

 the error to its source, which was the relation among the press- 

 ures. The principle of vis vica was therefore introduced, instead 

 of that relation, and the theory became much simpler by the 

 change. I now obtained, for the deviation, a new expression, 

 which agreed with the experiments of M. Seebeck; but the 

 formula for the polarizing angle came out the very same as 

 before. This correction was made on the 6th of December, 

 and was published in the Philosophical Magazine^ on the first 

 of the present month. 



In the interval I have arrived at very elegant geometrical 

 laws, which can be easily remembered, and which embrace the 

 whole theory of crystalline reflexion. In enunciating these, it 

 will be convenient to draw our transversals always through the 

 same origin 0, which we shall suppose to be the point of inci- 

 dence, as this point is common to all the rays, whether incident, 

 reflected, or refracted ; and we may imagine wave planes to be 

 drawn through the origin, parallel to the plane of each wave, 

 so that every transversal will lie in its own wave plane. The 

 incident and reflected wave planes will be perpendicular to the 

 incident and reflected rays, but the two refracted wave planes 

 will in general be oblique to their respective rays. In the latter 

 case, a right line drawn through the origin perpendicular to 



* Annakn der Physik und Chemie, Vol. xxxvm. p. 276. 



t London and Edinburgh Philosophical Magazine, Vol. x. p. 43. (Supra, p. 84.) 



