ossible, after numerous fruitless trials, to obtain
stationary waves analogous to those which, in the case of sound, are
produced in organ pipes. The marvellous application M. Lippmann has
made of these waves to completely solve the problem of photography in
colours is well known. This discovery, so important in itself and so
instructive, since it shows us how the most delicate anticipations of
theory may be verified in all their consequences, and lead the
physicist to the solution of the problems occurring in practice, has
justly become popular, and there is, therefore, no need to describe it
here in detail.

Professor Wiener obtained stationary waves some little while before M.
Lippmann's discovery, in a layer of a sensitive substance having a
grain sufficiently small in relation to the length of wave. His aim
was to solve a question of great importance to a complete knowledge of
the ether. Fresnel founded his theory of double refraction and
reflexion by transparent surfaces, on the hypothesis that the
vibration of a ray of polarized light is perpendicular to the plane of
polarization. But Neumann has proposed, on the contrary, a theory in
which he recognizes that the luminous vibration is in this very plane.
He rather supposes, in opposition to Fresnel's idea, that the density
of the ether remains the same in all media, while its coefficient of
elasticity is variable.

Very remarkable experiments on dispersion by M. Carvallo prove indeed
that the idea of Fresnel was, if not necessary for us to adopt, at
least the more probable of the two; but apart from this indication,
and contrary to the hypothesis of Neumann, the two theories, from the
point of view of the explanation of all known facts, really appear to
be equivalent. Are we then in presence of two mechanical explanations,
different indeed, but nevertheless both adaptable to all the facts,
and between which it will always be impossible to make a choice? Or,
on the contrary, shall we succeed in realising an _experimentum
crucis_, an experiment at the point where the two theories cross,
which will definitely settle the question?

Professor Wiener thought he could draw from his experiment a firm
conclusion on the point in dispute. He produced stationary waves with
light polarized at an angle of 45 deg.,[22] and established that, when
light is polarized in the plane of incidence, the fringes persist; but
that, on the other hand, they disappear when the light is po