tion: Fig. 29.]

[Illustration; Fig. 30.]

Let us push our test still further. By means of an endless screw, the
crystal can be turned ninety degrees round. The black image, as I
turn, becomes gradually brighter, and the bright one gradually darker;
at an angle of forty-five degrees both images are equally bright (fig.
30); while, when ninety degrees have been obtained, the axis of the
crystal being then vertical, the bright and black images have changed
places, exactly as reasoning would have led us to suppose (fig. 31).

[Illustration: Fig. 31.]

[Illustration: Fig. 32.]

Considering what has been already said (p. 114) regarding the
reflection of light polarized by transmission through tourmaline, you
will readily foresee what must occur when we receive upon a plate of
glass, held at the polarizing angle, the two beams emergent from our
prism of Iceland spar. I cause both beams to pass side by side through
the air, catch them on a glass plate, and seek to reflect them
upwards. At the polarizing angle one beam only is capable of being
thus reflected. Which? Your prompt answer will be, The beam whose
vibrations are horizontal (fig. 32). I now turn the glass plate and
try to reflect both beams laterally. One of them only is reflected;
that, namely, the vibrations of which are vertical (fig. 33). It is
plain that, by means either of the tourmaline or the reflecting glass,
we can determine in a moment the direction of vibration in any
polarized beam.

[Illustration: Fig. 33.]

As already stated, the whole of a beam of ordinary light reflected
from glass at the polarizing angle is polarized; a word must now be
added regarding the far larger portion of the light which is
_transmitted_ by the glass. The transmitted beam contains a quantity
of polarized light equal to the reflected beam; but this is only a
fraction of the whole transmitted light. By taking two plates of glass
instead of one, we augment the quantity of the transmitted polarized
light; and by taking _a bundle_ of plates, we so increase the quantity
as to render the transmitted beam, for all practical purposes,
_perfectly_ polarized. Indeed, bundles of glass plates are often
employed as a means of furnishing polarized light. It is important to
note that the plane of vibration of this transmitted light is at right
angles to that of the reflected light.

One word more. When the tourmalines are crossed, the space where they
cross each other is black. But