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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 1835, 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 
