lled the _optic axis_ of the
crystal.

Hence, if a plate be cut from a crystal of Iceland spar perpendicular
to the axis, all rays sent across this plate in the direction of the
axis will produce but one image. But, the moment we deviate from the
parallelism with the axis, double refraction sets in. If, therefore, a
beam that has been rendered _conical_ by a converging lens be sent
through the spar so that the central ray of the cone passes along the
axis, this ray only will escape double refraction. Each of the others
will be divided into an ordinary and an extraordinary ray, the one
moving more slowly through the crystal than the other; the one,
therefore, retarded with reference to the other. Here, then, we have
the conditions for interference, when the waves are reduced by the
analyzer to a common plane.

Placing the plate of Iceland spar between the crossed Nicol prisms,
and employing the conical beam, we have upon the screen a beautiful
system of iris-rings surrounding the end of the optic axis, the
circular bands of colour being intersected by a black cross (fig. 45).
The arms of this cross are parallel to the two directions of vibration
in the polarizer and analyzer. It is easy to see that those rays whose
planes of vibration within the spar coincide with the plane of
vibration of _either_ prism, cannot get through _both_. This complete
interception produces the arms of the cross.

[Illustration: Fig. 45.]

With monochromatic light the rings would be simply bright and
black--the bright rings occurring at those thicknesses of the spar
which cause the rays to conspire; the black rings at those thicknesses
which cause them to quench each other. Turning the analyzer 90 deg. round,
we obtain the complementary phenomena. The black cross gives place to
a bright one, and every dark ring is supplanted also by a bright one
(fig. 46). Here, as elsewhere, the different lengths of the
light-waves give rise to iris-colours when white light is employed.

[Illustration: Fig. 46.]

[Illustration: Fig. 47.]

Besides the _regular_ crystals which produce double refraction in no
direction, and the _uniaxal_ crystals which produce it in all
directions but one, Brewster discovered that in a large class of
crystals there are _two_ directions in which double refraction does
not take place. These are called _biaxal_ crystals. When plates of
these crystals, suitably cut, are placed between the polarizer and
analyzer, the axes (A