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.
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