e beams transmitted by Iceland spar had two sides; and
from the analogy of this _two-sidedness_ with the _two-endedness_ of a
magnet, wherein consists its polarity, the two beams came subsequently
to be described as _polarized_.

We may begin the study of the polarization of light, with ease and
profit, by means of a crystal of tourmaline. But we must start with a
clear conception of an ordinary beam of light. It has been already
explained that the vibrations of the individual ether-particles are
executed _across_ the line of propagation. In the case of ordinary
light we are to figure the ether-particles as vibrating in all
directions, or azimuths, as it is sometimes expressed, across this
line.

Now, in the case of a plate of tourmaline cut parallel to the axis of
the crystal, a beam of light incident upon the plate is divided into
two, the one vibrating parallel to the axis of the crystal, the other
at right angles to the axis. The grouping of the molecules, and of
the ether associated with the molecules, reduces all the vibrations
incident upon the crystal to these two directions. One of these beams,
namely, that whose vibrations are perpendicular to the axis, is
quenched with exceeding rapidity by the tourmaline. To such vibrations
many specimens of the crystal are highly opaque; so that, after having
passed through a very small thickness of the tourmaline, the light
emerges with all its vibrations reduced to a single plane. In this
condition it is what we call _plane polarized light_.

[Illustration: Fig. 27.]

[Illustration: Fig. 28.]

A moment's reflection will show that, if what is here stated be
correct, on placing a second plate of tourmaline with its axis
parallel to the first, the light will pass through both; but that, if
the axes be crossed, the light that passes through the one plate will
be quenched by the other, a total interception of the light being the
consequence. Let us test this conclusion by experiment. The image of a
plate of tourmaline (_t_ _t_, fig. 27) is now before you. I place
parallel to it another plate (_t'_ _t'_): the green of the crystal is
a little deepened, nothing more; this agrees with our conclusion. By
means of an endless screw, I now turn one of the crystals gradually
round, and you observe that as long as the two plates are oblique to
each other, a certain portion of light gets through; but that when
they are at right angles to each other, the space common to both is a
s