[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.]