[Footnote 1: Abstract of the Friday evening lecture delivered by
Lord Rayleigh, F.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. [Shown.]
(5) The spectrum of the reflected light is frequently found to
consist almost entirely of a comparatively narrow band. When the
angle of incidence is increased, the band moves in the direction
of increasing refrangibility, and at the same time increases
rapidly in width. In many cases the reflection appears to be
almost total.
[Illustration: FIG. 1 GENERAL SCHEME
FIG. 2 DETAIL OF LAZY-TONGS]
In order to project these phenomena a crystal is prepared by cementing
a smooth face to a strip of glass whose sides are not quite parallel.
The white reflection from the anterior face of the glass can then be
separated from the real subject of the experiment.
A very remarkable feature in the reflected light remains to be
noticed. If the angle of incidence be small, and if the incident light
be polarized in or perpendicularly to the plane of incidence, the
reflected light is polarized in the _opposite_ manner. [Shown.]
Similar phenomena, except that the reflection is white, are exhibited
by crystals prepared in a manner described by Madan. If the crystal be
heated beyond a certain point the peculiar reflection disappears, but
returns upon cooling. [Shown.]
In all these cases there can be little doubt that the reflection takes
place at twin surfaces, the theory of such reflection (_Phil. Mag._,
Sept., 1888) reproducing with remarkable exactness most of the
features above described. In order to explain the vigor and purity of
the color reflected in certain crystals, it is necessary to suppose
that there are a considerable number of twin surfaces disposed at
approximate equal intervals. At each angle of incidence there would be
a particular wave length for which the phases of the several
reflections are in agreement. The selection of light of a particular
wave length would thus take place upon the same principle as in
diffraction spectra, and might reach a high degree of perfection.
In illustration of this explanation an acoustical analogue is
exhibited. The successive twin planes are imitated by parallel and
equidistant disks of muslin (Figs. 1 and 2) stretched upon brass rings
and mounted (with the aid of three lazy-tongs arrangements) so that
there is but one degree of freedom to move, and that of such a
character as to vary the interval between the disks without disturbing
their equidistance and parallelism.
The source of sound is a bird call, giving a pure tone of high pitch
(inaudible), and the percipient is a high-pressure flame issuing from
a burner so oriented that the direct waves are without influence upon
the flame (see _Nature_, xxxviii., 208; Proc. Roy. Inst., January,
1888). But the waves reflected from the muslin arrive in the effective
direction, and if of sufficient intensity induce flaring. The
experiment consists in showing that the action depends upon the
distance between the disks. If the distance be such that the waves
reflected from the several disks co-operate,[2] the flame flares, but
for intermediate adjustments recovers its equilibrium. For full
success it is necessary that the reflective power of a single disk be
neither too great nor too small. A somewhat open fabric appears
suitable.
[Footnote 2: If the reflection were perpendicular, the interval
between successive disks would be equal to the half wave-length,
or to some multiple of this.]
It was shown by Brewster that certain natural specimens of Iceland
spar are traversed by thin twin strata. A convergent beam, reflected
at a nearly grazing incidence from the twin planes, depicts upon the
screen an arc of light, which is interrupted by a dark spot
corresponding to the plane of symmetry. [Shown.] A similar experiment
may be made with small rhombs in which twin layers have been developed
by mechanical force after the manner of Reusch.
The light reflected from fiery opals has been shown by Crookes to
possess in many cases a high degree of purity, rivaling in this
respect the reflection from chlorate of potash.
The explanation is to be sought in a periodic stratified structure.
But the other features differ widely in the two cases. There is here
no semicircular evanescence, as the specimen is rotated in azimuth. On
the contrary, the colored light transmitted perpendicularly through a
thin plate of opal undergoes no change when the gem is turned round in
its own plane. This appears to prove that the alternate states are not
related to one another as twin crystals. More probably the alternate
strata are of air, as in decomposed glass. The brilliancy of opals is
said to be readily affected by atmospheric conditions.
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