724 
MR. J. LARMOR ON A DYNAMICAL THEORY OF 
Seebeck. He arrived at a complete solution of this problem, and one characterized 
by that straightforward simplicity which is the mark of all theories that are true 
to Nature; but he was not able to imagine any mechanical model by which the 
properties of his energy-function could be realized. In another connexion, in vindi¬ 
cating his equations for the rotatory polarization of quartz^ against a theory of 
Cauchy’s leading to different results, he howmver expresses himself on such a 
question, as follows.! “For though, in my Paper, I have said nothing of anv 
mechanical investigation, yet as a matter of course, before it was read to the 
Academy, I made every effort to connect my equations in some way with mechanical 
jDi'inciples ; and it was because I had failed in doing so to my own satisfaction, that I 
chose to publish the equations wdthout comment, as bare geometrical assumptions, 
and contented myself with stating orally .... that a mechanical account of the 
phenomena remained a desideratum which no efforts of mine had been able to 
supply.*’ And again, “ though for my own part I never was satisfied with that theory 
[of Cauchy], which seemed to me to possess no other merit than that of following 
out in detail the extremely curious, but (as I thoaght) veiy imperfect analogy which 
had been perceived to exist between the vibrations of the luminiferous medium and 
tliose of a common elastic solid, .... still I should have been glad, in the absence 
of anything better, to find my equations supported by a similar theory, and their 
form at least countenanced by a like mechanical analogy.” 
9. After trying an empirical alteration of Cauchy’s equations for the stress in his 
medium,]; vPich sufficed to satisfy Brewster’s observations on reflexion from ciystals, 
but did not agree with subsequent observations of a different kind by Seebeck, 
MacCullagh was finally led to results which were in keeping vdth aU the experi¬ 
ments by means of the principles § that (i) the displacements in the incident and 
reflected waves, compounded as vectors, are geometrically equivalent at the interface 
to the displacements in the refracted waves, compounded in the same manner, and 
(ii) there is no loss of energy involved in the act of reflexion and refraction. This 
agreement was obtained, provided he took the displacement to be in the plane of 
polarization of the light, and the density of the aether to be the same in all media. 
Shortly before, and unknown to MacCullagh, F. E, Neltmaxk|| had based the 
solution of the problem of reflexion on the very same principles ; and he had as early 
as 1833, ascertained that his results agreed vdth Seebeck’s experiments, though 
MacCullagh had priority in publication. He began by applying to the problem of 
reflexion the equations of motion of an elastic solid, as then imperfectly understood 
in accordance with the prevalent theory of Navier and Poisson ; he recognized that 
* MacCullagh, “ On the Laws of the Double Refraction of Quartz,” ‘ Trans. Roy. Irish Acad.,’ 
1836 ; ‘ Collected Works,’ p. 63. 
t MacCullagh, ‘ Proc. Roy. Irish Acad.,’ 1841 ; ‘ Collected Works,’ pp. 198, 200. 
X MacCullagh, “ On the Laws of Reflexion from Crystallized Surfaces,” ‘ Phil. Mag.,’ vol. 8, 1835. 
§ MacCullagh, “On the Laws of Crystalline Reflexion,” Dec. 13, 1836; ‘Phil. Mag.,’ vol. 10, ISoT. 
II F. E. Neumann, ‘Abhandl. der Rerliner Akad.,’ 1835, pp. 1-116. 
