ARTICLE XI. 
Memoir on the Theory of Light. By M. A. L. Caucny. 
[Read at the Royal Academy of Sciences, May 31 and June 7, 1830*.] 
Part THE First. 
I FIRST+ gave, in the Hxercices de Mathématiques (third and 
fourth volumes), the general equations of equilibrium or of motion 
of a system of molecules acted upon by mutual forces of attrac- 
tion or repulsion, admitting that these forces were represented 
by functions of the distances between the molecules; and I 
proved that these equations (which include a great number of 
coefficients depending on the nature of the system) were reduci- 
ble in the case where the elasticity became the same in every 
direction to other formule which include but one coefficient, 
and which had been originally obtained by M. Navier. I also 
deduced from these equations those which determine the mo- 
tions of elastic plates and rods when the elasticity is supposed 
not to be the same in every direction; and I thus obtained for- 
mulz which comprehend, as particular cases, those which M. 
Poisson and some other geometricians had found on the oppo- 
site supposition. The remarkable agreement of these different 
formule, and of the laws which are deducible from them, with 
the observations of physicists, and especially the beautiful ex- 
periments of M. Savart, encouraged me to adopt the advice of 
some persons who urged me to make a new application of the 
general equations which I had given to the theory of light. 
Having followed this advice, I have been fortunate enough to 
arrive at results which I am about to detail in this Memoir, and 
which seem to me worthy of occupying for a moment the atten- 
tion of physicists and geometricians. 
* From Mémoires de l’Académie Royale des Sciences de UInstitut de 
France, t. x. 
+ The investigation which M. Cauchy here refers to, as establishing the 
principles which he assumes in this paper, are nearly the same as those of 
which an abstract was given by Prof. Powell in the L. and E. Phil. Mag., vol. 
vi. The reader may also refer to the same writer’s volume on the Undula- 
tory theory as applied to dispersion, &c. London, 1841, J. W. Parker ;—espe- 
cially the Introduction, p. xlii.; and a Note in L. E. and D. Phil. Mag. for 
November, 1841. 
