is.
Moreover, the rays of Schumann are, as we have seen, extraordinarily
absorbable,--so much so that they have to be observed in a vacuum. The
most striking property of the X rays is, on the contrary, the facility
with which they pass through obstacles, and it is impossible not to
attach considerable importance to such a difference.

Some attribute this marvellous radiation to longitudinal vibrations,
which, as M. Duhem has shown, would be propagated in dielectric media
with a speed equal to that of light. But the most generally accepted
idea is the one formulated from the first by Sir George Stokes and
followed up by Professor Wiechert. According to this theory the X rays
should be due to a succession of independent pulsations of the ether,
starting from the points where the molecules projected by the cathode
of the Crookes tube meet the anticathode. These pulsations are not
continuous vibrations like the radiations of the spectrum; they are
isolated and extremely short; they are, besides, transverse, like the
undulations of light, and the theory shows that they must be
propagated with the speed of light. They should present neither
refraction nor reflection, but, under certain conditions, they may be
subject to the phenomena of diffraction. All these characteristics are
found in the Roentgen rays.

Professor J.J. Thomson adopts an analogous idea, and states the
precise way in which the pulsations may be produced at the moment when
the electrified particles forming the cathode rays suddenly strike the
anticathode wall. The electromagnetic induction behaves in such a way
that the magnetic field is not annihilated when the particle stops,
and the new field produced, which is no longer in equilibrium, is
propagated in the dielectric like an electric pulsation. The electric
and magnetic pulsations excited by this mechanism may give birth to
effects similar to those of light. Their slight amplitude, however, is
the cause of there here being neither refraction nor diffraction
phenomena, save in very special conditions. If the cathode particle is
not stopped in zero time, the pulsation will take a greater amplitude,
and be, in consequence, more easily absorbable; to this is probably to
be attributed the differences which may exist between different tubes
and different rays.

It is right to add that some authors, notwithstanding the proved
impossibility of deviating them in a magnetic field, have not
renounced the idea of