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.
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