ties.
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
ray is the most refracted. One consequence of this merits a passing
notice. Pour water and bisulphide of carbon into two cups of the same
depth; the cup that contains the more strongly refracting liquid will
appear shallower than the other. Place a piece of Iceland spar over a
dot of ink; two dots are seen, the one appearing nearer than the other
to the eye. The nearest dot belongs to the most strongly refracted
ray, exactly as the nearest cup-bottom belongs to the most highly
refracting liquid. When you turn the spar round, the extraordinary
image of the dot rotates round the ordinary one, which remains fixed.
This is also the deportment of our two disks upon the screen.

Sec. 5. _Polarization of Light explained by the Wave Theory_.

The double refraction of Iceland spar was first treated in a work
published by Erasmus Bartholinus, in 1669. Huyghens sought to account
for this phenomenon on the principles of the wave theory, and he
succeeded in doing so. He, moreover, made highly important
observations on the distinctive character of the two beams transmitted
by the spar, admitting, with resigned candour, that he had not solved
the difficulty, and leaving the solution to future times. Newton,
reflecting on the observations of Huyghens, came to the conclusion
that each of th