420 



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 aw^arded the Copley Medal for the present year 

 to Professor MacCuHagh, 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 cry- 

 stallized media, embodied in his ' Memoire sur la double refraction.' 

 The object proposed by Professor MacCuUagh, 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 sether 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, in a 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 Lightf," 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 

 their passage through a double-refracting crystal ; and thus that the 

 forms of the rings, or isochromatic curves, which had previously been 

 deduced only by approximate methods, may be determined generally. 



The next paper of Professor MacCullagh is that " On the Double 

 Refraction of Quartz | ;" a subject which had engaged the attention, 

 successively, of Biot, Fresnel, and Airy. The first of these writers 

 had determined experimentally the laws of rotatory polarization, 

 which take place when a ray is transmitted along the axis of rock- 

 crystal ; and the second had shown that these laws were explained 

 by the interference of two circularly polarized rays, which are trans- 

 mitted along the axis with different velocities. The next step in this 

 curious subject was made by Mr. Airy, who examined the peculiar 

 phenomena of refraction by quartz in other directions, and showed 

 that they were accounted for by the supposition of two elliptically 

 'polarized rays, the ratio of the axes of these elliptical vibrations va- 



* Transactions of the Royal Irish Academy, vol. xvi. 

 t Ibid. vol. xvii. % Ibid. 



