oured to
build a bridge between the two domains. We have seen how Rubens showed
us calorific rays 60 metres long; on the other hand, MM. Lecher, Bose,
and Lampa have succeeded, one after the other, in gradually obtaining
oscillations with shorter and shorter periods. There have been
produced, and are now being studied, electromagnetic waves of four
millimetres; and the gap subsisting in the spectrum between the rays
left undetected by sylvine and the radiations of M. Lampa now hardly
comprise more than five octaves--that is to say, an interval
perceptibly equal to that which separates the rays observed by M.
Rubens from the last which are evident to the eye.

The analogy then becomes quite close, and in the remaining rays the
properties, so to speak, characteristic of the Hertzian waves, begin
to appear. For these waves, as we have seen, the most transparent
bodies are the most perfect electrical insulators; while bodies still
slightly conducting are entirely opaque. The index of refraction of
these substances tends in the case of great wave-lengths to become, as
the theory anticipates, nearly the square root of the dielectric
constant.

MM. Rubens and Nichols have even produced with the waves which remain
phenomena of electric resonance quite similar to those which an
Italian scholar, M. Garbasso, obtained with electric waves. This
physicist showed that, if the electric waves are made to impinge on a
flat wooden stand, on which are a series of resonators parallel to
each other and uniformly arranged, these waves are hardly reflected
save in the case where the resonators have the same period as the
spark-gap. If the remaining rays are allowed to fall on a glass plate
silvered and divided by a diamond fixed on a dividing machine into
small rectangles of equal dimensions, there will be observed
variations in the reflecting power according to the orientation of the
rectangles, under conditions entirely comparable with the experiment
of Garbasso.

In order that the phenomenon be produced it is necessary that the
remaining waves should be previously polarized. This is because, in
fact, the mechanism employed to produce the electric oscillations
evidently gives out vibrations which occur on a single plane and are
subsequently polarized.

We cannot therefore entirely assimilate a radiation proceeding from a
spark-gap to a ray of natural light. For the synthesis of light to be
realized, still other conditions must be comp