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.