154 } Royal Society. 
stance. He was an eminent chemist, and had furnished two papers 
to our Transactions*. 
It is now, Gentlemen, time for me to perform the most agreeable 
part of the duty which falls to the lot of a President on your Anni- 
versafy—that of giving the Medals awarded by the Council. As we 
have not the pleasure of seeing here today Mr. MacCullagh, I 
shall beg Mr. Wheatstone, as his friend, to transmit his Medal to 
that gentleman. 
Mr. WHEATSTONE. 
It gives me great satisfaction to be the organ of the Council of 
the Royal Society in bestowing on your friend Mr. MacCullagh the 
Copley Medal. It is needless for me to dilate on the profound ma- 
thematical skill and exemplary diligence with which he has explained 
the laws of the undulatory theory of light. Philosophers more able 
than myself to appreciate their merits, have given their testimony to 
the great value of his discoveries, and to the elegant means that he 
has employed. It is the sincere wish of us all, that these labours 
may be followed by others as important to science and as honourable 
to the University of Dublin; an University that numbers Mr. Mac- 
Cullagh among the most eminent of her sons. 
The Council have awarded the Copley Medal for the present year 
to Professor MacCullagh, for his researches connected with the wave- 
theory of light, contained in the Transactions of the Royal Irish 
Academy. The grounds on which they have made this award are 
the following. One of the most important steps made in the physi- 
cal theory of light, since it was first promulgated by Huygens, is, 
undoubtedly, Fresnel’s discovery of the laws of refraction by ery- 
stallized media, embodied in his ‘ Mémoire sur la double réfraction.’ 
The object proposed by Professor MacCullagh, in his first paper +, 
was to simplify and to develope that theory. He has shown in this 
paper, that the elastic force of the luminiferous ether may be repre- 
sented, in magnitude and direction, by means of an ellipsoid, whose 
semiaxes are the three principal refractive indices of the medium ; 
and he has thence deduced, ina geometrical form, the leading results 
of Fresnel’s theory. This ellipsoid is closely related to the genera~- 
ting ellipsoid of Fresnel; and by the aid of these relations, Professor 
MacCullagh has demonstrated, in a very simple manner, the truth of 
Fresnel’s construction of the wave-surface, the demonstration of 
which had been left imperfect by its author. 
In Mr. MacCullagh’s next paper, entitled ‘Geometrical propositions 
applied to the Wave-theory of Light, {” he has examined the proper- 
ties of a surface, which he calls the surface of indices, and which had 
presented itself likewise in the researches of M. Cauchy and Sir 
William Hamilton; and he has shown that it affords a general and 
exact construction for the interval of retardation of the two rays in 
* Lord Vivian, the Earl of Macclesfield, and Mr. Gage Rokewode, with 
other deceased Fellows of the Society, were also noticed in the President’s 
Address.—En1r. 
+ ‘l'ransactions of the Royal Irish Academy, vol. xvi. 
{ Ibid. vol. xvii. 
