R.S., at the Royal Institution, on April 12,
1889.]
By LORD RAYLEIGH.
The principal subject of the lecture is the peculiar colored
reflection observed in certain specimens of chlorate of potash.
Reflection implies a high degree of discontinuity. In some cases, as
in decomposed glass, and probably in opals, the discontinuity is due
to the interposition of layers of air; but, as was proved by Stokes,
in the case of chlorate crystals the discontinuity is that known as
twinning. The seat of the color is a very thin layer in the interior
of the crystal and parallel to its faces.
The following laws were discovered by Stokes:
(1) If one of the crystalline plates be turned round in its own
plane, without alteration of the angle of incidence, the
peculiar reflection vanishes twice in a revolution, viz., when
the plane of incidence coincides with the plane of symmetry of
the crystal. [Shown.]
(2) As the angle of incidence is increased, the reflected light
becomes brighter and rises in refrangibility. [Shown.]
(3) The colors are not due to absorption, the transmitted light
being strictly complementary to the reflected.
(4) The colored light is not polarized. It is produced
indifferently, whether the incident light be common light or
light polarized in any plane, and is seen whether the reflected
light be viewed directly or through a Nicol's prism turned in
any way.
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