402 Royal Irish Academy: Prof. MacCullagh on a 
Physical Section of the British Association in Bristol (see Transactions 
of the Sections, p. 18), that a mechanical account of the phenomena 
still remained a desideratum which no attempts of mine had been 
able to supply. I am not sure that on the first occasion I stated 
the precise nature of these attempts, though I incline to think I 
did; but I have a distinct recollection of having done so on the 
second occasion, in reply to questions that were asked me by some 
members of the Association*. Now, my first attempt to explain 
those equations, which was made almost as soon as I discovered them, 
actually turned upon the very idea which about the same time found 
entrance into the mind of M. Cauchy—I mean the idea of an unsym- 
metrical arrangement of the ether. For as it was generally believed, 
at that period, that the hypothesis of «ethereal molecules symmetri- 
cally distributed had led, in the hands of M. Cauchy, to a complete 
theory of rectilinear polarization in crystals (see his Hwercices de 
Mathématiques, Cinquiéme Année, Paris, 1830, and the Mémoires de 
l Institut, tom. x. p. 293), the notion of endeavouring to account for 
the phenomena of elliptic polarization, by freeing the hypothesis from 
any restriction as to the distribution of the ether, would naturally 
occur to any one who was thinking on the subject, no less than to 
M. Cauchy himself. And though, for my own part, I never was 
satisfied with that theory, which seemed to me to possess no other 
merit than that of following out in detail the extremely curious, but 
(as I thought) very imperfect, analogy which had been perceived to 
exist between the vibrations of the luminiferous medium and those 
of a common elastic} solid (for it is usual to regard such a solid as a 
rigid system of attracting or repelling molecules, and M. Cauchy has 
really done nothing more than transfer to the luminiferous zther 
both the constitution of the solid and differential formulas of its vi- 
bration), still I should have been glad, in the absence of anything 
* At the period of this meeting, M. Cauchy’s letter on Elliptic Polariza- 
tion had been published for some months ; but I was not then aware of its 
existence. Indeed the letter appears not to have attracted any general 
notice: for the theory which it contains was afterwards advanced in En- 
gland as a new one, and M. Cauchy has been lately obliged to assert his prior 
claim to it, thrcugh the medium of Prof. Powell.—See notes, pp. 400, 405. 
+ The analogy was suggested by the hypothesis of transversal vibrations, 
which, when viewed in its physical bearing, was considered by Dr. Young 
to be “perfectly appalling in its consequences,” as it was only to solids 
that a “lateral resistance’’ tending to produce such vibrations had ever 
been attributed. (Supplement to the Encyclopedia Britannica, vol. vi. 
p- 862. Edinburgh, 1824.) He admits, however, that the question whether 
fluids may not “transmit impressions by lateral adhesion, remains com- 
pletely open for discussion, notwithstanding the apparent difficulties at- 
tending it.” As far as I am aware, Fresnel always regarded the ether as 
a fluid. M. Poisson affirms that it must be so regarded, and attributes its 
apparent peculiarities to the immense rapidity ofits vibrations, which does 
not allow the law of equal pressure to hold good in the state of motion 
(Annales de Chimie, tom. xliv. p. 432). M. Cauchy calls the zether a fluid, 
though he treats it as a solid. My own impression is, that the ether is a 
medium of a peculiar kind, differing from all ponderable bodies, whether 
solid or fluid, in this respect, that it absolutely refuses, in any case, to 
