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 complied with. During a
luminous impression, the direction and the phase change millions of
times in the vibration sensible to the retina, yet the damping of this
vibration is very slow. With the Hertzian oscillations all these
conditions are changed--the damping is very rapid but the direction
remains invariable.
Every time, however, that we deal with general phenomena which are
independent of these special conditions, the parallelism is perfect;
and with the waves, we have put in evidence the reflexion, refraction,
total reflexion, double reflexion, rotatory polarization, dispersion,
and the ordinary interferences produced by rays travelling in the same
direction and crossing each other at a very acute angle, or the
interferences analogous to those which Wiener observed with rays of
the contrary direction.
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