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