lied with. During a
luminous impression, the direction and the phase change millions of
times in the vibration sensible to the retina, yet the damping of this
vibration is very slow. With the Hertzian oscillations all these
conditions are changed--the damping is very rapid but the direction
remains invariable.

Every time, however, that we deal with general phenomena which are
independent of these special conditions, the parallelism is perfect;
and with the waves, we have put in evidence the reflexion, refraction,
total reflexion, double reflexion, rotatory polarization, dispersion,
and the ordinary interferences produced by rays travelling in the same
direction and crossing each other at a very acute angle, or the
interferences analogous to those which Wiener observed with rays of
the contrary direction.

A very important consequence of the electromagnetic theory foreseen by
Maxwell is that the luminous waves which fall on a surface must
exercise on this surface a pressure equal to the radiant energy which
exists in the unit of volume of the surrounding space. M. Lebedeff a
few years ago allowed a sheaf of rays from an arc lamp to fall on a
deflection radiometer,[26] and thus succeeded in revealing the
existence of this pressure. Its value is sufficient, in the case of
matter of little density and finely divided, to reduce and even change
into repulsion the attractive action exercised on bodies by the sun.
This is a fact formerly conjectured by Faye, and must certainly play a
great part in the deformation of the heads of comets.

[Footnote 26: By this M. Poincare appears to mean a radiometer in
which the vanes are not entirely free to move as in the radiometer of
Crookes but are suspended by one or two threads as in the instrument
devised by Professor Poynting.--ED.]

More recently, MM. Nichols and Hull have undertaken experiments on
this point. They have measured not only the pressure, but also the
energy of the radiation by means of a special bolometer. They have
thus arrived at numerical verifications which are entirely in
conformity with the calculations of Maxwell.

The existence of these pressures may be otherwise foreseen even apart
from the electromagnetic theory, by adding to the theory of
undulations the principles of thermodynamics. Bartoli, and more
recently Dr Larmor, have shown, in fact, that if these pressures did
not exist, it would be possible, without any other phenomenon, to pass
heat from a c