270 CAUCHY ON THE THEORY OF LIGHT. 
which originally included the vibrating molecules. If the con- 
stitution of the elastic fluid is such that the velocities of propa- 
gation of plane waves become imaginary, this propagation can 
no longer be effected, and the body in which the ethereal fluid 
is comprised, will become what we term an opake body. If the 
body remain transparent, and if in this body the ewthereal fluid 
is distributed in such a manner that its elasticity continues the 
same in every direction around any given point, the three polar- 
ized rays, into which a ray of light generally subdivides itself, 
will be directed along the same right line ; and, as the velocity 
of the light will be the same in the two first rays, these will be 
confounded with each other; there will then remain but two 
polarized rays, one double, the other simple, having the same 
direction. Now, calculation shows that in the simple ray the 
light will be polarized in the direction spoken of; whilst, in the 
double ray, the light will be polarized perpendicular to this di- 
rection. If the initial vibrations of the luminous molecules are 
contained in a plane perpendicular to the direction in question, 
the simple ray will disappear, and the proper velocities of the 
molecules in the double ray will remain constantly directed 
along right lines parallel to the directions of the initial veloci- 
ties ; so that, to speak properly. there will be no more polariza- 
tion. Then also the velocity of propagation of the light will be 
equivalent to the velocity of propagation of a plane wave, and the 
same in every direction around each point. Now, the reduction 
of all the rays to a single one, and the absence of all polariza- 
tion in the media in which the light is propagated in every di- 
rection with the same velocity, being facts proved by experi- 
ment, we must conclude from what precedes, that in these media 
the proper velocities of the zthereal molecules are perpendicular 
to the directions of the luminous rays, and comprised in the 
plane waves. Thus the hypothesis, admitted by Fresnel, be- 
comes a reality. This able physicist, of whom science has been 
unfortunately deprived by a premature death, was then right in 
saying, that in ordinary light the vibrations are transversal, that 
is to say, perpendicular to the directions of the rays. The ideas 
of Fresnel upon this subject have, in truth, been strongly com- 
bated by an illustrious academician, in several articles contained 
in the Annales de Physique et de Chimie, and one of which relates 
to the motions of two superposed fluids. According to the 
author of these articles, the vibrations of the molecules in ether 
