larized
perpendicularly to this plane. If it be admitted that a photographic
impression results from the active force of the vibratory movement of
the ether, the question is, in fact, completely elucidated, and the
discrepancy is abolished in Fresnel's favour.

[Footnote 22: That is to say, he reflected the beam of polarized light
by a mirror placed at that angle. See Turpain, _Lecons elementaires de
Physique_, t. ii. p. 311, for details of the experiment.--ED.]

M.H. Poincare has pointed out, however, that we know nothing as to the
mechanism of the photographic impression. We cannot consider it
evident that it is the kinetic energy of the ether which produces the
decomposition of the sensitive salt; and if, on the contrary, we
suppose it to be due to the potential energy, all the conclusions are
reversed, and Neumann's idea triumphs.

Recently a very clever physicist, M. Cotton, especially known for his
skilful researches in the domain of optics, has taken up anew the
study of stationary waves. He has made very precise quantitative
experiments, and has demonstrated, in his turn, that it is impossible,
even with spherical waves, to succeed in determining on which of the
two vectors which have to be regarded in all theories of light on the
subject of polarization phenomena the luminous intensity and the
chemical action really depend. This question, therefore, no longer
exists for those physicists who admit that luminous vibrations are
electrical oscillations. Whatever, then, the hypothesis formed,
whether it be electric force or, on the contrary, magnetic force which
we place in the plane of polarization, the mode of propagation
foreseen will always be in accord with the facts observed.

Sec. 3. THE ELECTROMAGNETIC ETHER

The idea of attributing the phenomena of electricity to perturbations
produced in the medium which transmits the light is already of old
standing; and the physicists who witnessed the triumph of Fresnel's
theories could not fail to conceive that this fluid, which fills the
whole of space and penetrates into all bodies, might also play a
preponderant part in electrical actions. Some even formed too hasty
hypotheses on this point; for the hour had not arrived when it was
possible to place them on a sufficiently sound basis, and the known
facts were not numerous enough to give the necessary precision.

The founders of modern electricity also thought it wiser to adopt,
with regard to this science,