ot abandon an hypothesis entirely
different from that of radiant matter. They continued to regard the
cathode radiation as due to particular radiations of a nature still
little known but produced in the luminous ether. This interpretation
seemed, indeed, in 1894, destined to triumph definitely through the
remarkable discovery of Lenard, a discovery which, in its turn, was to
provoke so many others and to bring about consequences of which the
importance seems every day more considerable.

Professor Lenard's fundamental idea was to study the cathode rays
under conditions different from those in which they are produced.
These rays are born in a very rarefied space, under conditions
perfectly determined by Sir W. Crookes; but it was a question whether,
when once produced, they would be capable of propagating themselves in
other media, such as a gas at ordinary pressure, or even in an
absolute vacuum. Experiment alone could answer this question, but
there were difficulties in the way of this which seemed almost
insurmountable. The rays are stopped by glass even of slight
thickness, and how then could the almost vacuous space in which they
have to come into existence be separated from the space, absolutely
vacuous or filled with gas, into which it was desired to bring them?

The artifice used was suggested to Professor Lenard by an experiment
of Hertz. The great physicist had, in fact, shortly before his
premature death, taken up this important question of the cathode rays,
and his genius left there, as elsewhere, its powerful impress. He had
shown that metallic plates of very slight thickness were transparent
to the cathode rays; and Professor Lenard succeeded in obtaining
plates impermeable to air, but which yet allowed the pencil of cathode
rays to pass through them.

Now if we take a Crookes tube with the extremity hermetically closed
by a metallic plate with a slit across the diameter of 1 mm. in width,
and stop this slit with a sheet of very thin aluminium, it will be
immediately noticed that the rays pass through the aluminium and pass
outside the tube. They are propagated in air at atmospheric pressure,
and they can also penetrate into an absolute vacuum. They therefore
can no longer be attributed to radiant matter, and we are led to think
that the energy brought into play in this phenomenon must have its
seat in the light-bearing ether itself.

But it is a very strange light which is thus subject to magnetic
act