To play the part, for example, with the Hertzian waves,
which a mirror 1 millimetre square plays with regard to light, would
require a colossal mirror which would attain the size of a
myriametre[25] square.
[Footnote 25: I.e., 10,000 metres.--ED.]
The efforts of physicists have to-day, however, filled up, in great
part, this interval, and from both banks at once they have laboured 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.
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