In consequence of this symmetry the elasticity is the same in all
directions perpendicular to the axis, and hence a ray transmitted
along the axis suffers no double refraction. But the elasticity along
the axis is greater than the elasticity at right angles to it.
Consider, then, a system of waves crossing the crystal in a direction
perpendicular to the axis. Two directions of vibration are open to
such waves: the ether particles can vibrate parallel to the axis or
perpendicular to it. _They do both_, and hence immediately divide
themselves into two systems propagated with different velocities.
Double refraction is the necessary consequence.

[Illustration: Fig. 26.]

By means of Iceland spar cut in the proper direction, double
refraction is capable of easy illustration. Causing the beam which
builds the image of our carbon-points to pass through the spar, the
single image is instantly divided into two. Projecting (by the lens E,
fig. 26) an image of the aperture (L) through which the light issues
from the electric lamp, and introducing the spar (P), two luminous
disks (E O) appear immediately upon the screen instead of one.

The two beams into which the spar divides the single incident-beam
have been subjected to the closest examination. They do not behave
alike. One of them obeys the ordinary law of refraction discovered by
Snell, and is, therefore, called the _ordinary ray_: its index of
refraction is 1.654. The other does not obey this law. Its index of
refraction, for example, is not constant, but varies from a maximum of
1.654 to a minimum of 1.483; nor in this case do the incident and
refracted rays always lie in the same plane. It is, therefore, called
the _extraordinary ray_. In calc-spar, as just stated, the ordinary