. 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,