217 



supposed only that the vibrations parallel to the separating surface, 

 but not that the vibrations perpendicular to the same surface were 

 equivalent. 



And here I may be permitted to state, what indeed cannot fail 

 to be remembered by many here, that when the British Association 

 for the Advancement of Science met in this city, about three years 

 ago, (in August, 1835), a communication was made by Mr. Mac 

 Cullagh to the Mathematical and Physical section, *' on the Laws 

 of Reflexion and Refraction at the Surface of Crystals," which 

 embodied nearly all the principles or hypotheses that I have now 

 recited, and of which an abstract was printed in the London and 

 Edinburgh Philosophical Magazine for October, 1835, having 

 indeed been published even earlier (in September, 1835) by 

 Mr. Hardy here. The only supposition, which was not either 

 formally stated or clearly indicated in this abstract, was that of 

 the preservation of the vis viva ; instead of which principle of 

 Fresnel, Mr. Mac Cullagh was, at one time, inclined to employ a 

 relation between pressures, proposed by M. Cauchy. Since, there- 

 fore, the leading principles of the new theory of Reflexion and 

 Refraction were all made known by Mr. Mac Cullagh so early as 

 the August of 1 835, were printed in Dublin in the September of 

 that year, and in London in the October following, it will not, per- 

 haps, be attributed solely to national partiality if we claim for him 

 the priority of discovery on this curious and important question, 

 notwithstanding that a very valuable and elaborate memoir on the 

 same subject, embodying the same results, was communicated, in 

 December, 1835, to the Academy of Sciences at Berlin, by M. Neu- 

 mann, and was published in 1837, before the publication (though 

 after the reading) of that essay of Mr. Mac Cullagh, to which the 

 present prize is awarded. 



It is, however, an interesting circumstance, and one which is 

 adapted to increase our confidence in these new laws of light, that 

 they should have been independently and almost simultaneously 

 discovered in these and in foreign countries ; and it will not, I 

 trust, be supposed that I desire to depreciate M. Neumann's admira- 

 ble essay, if having recalled some facts and dates which bear upon 

 the question of priority, I proceed to point out a few of the features 



