the tourmaline, both beams are transmitted with equal facility by the
spar. The two beams, in short, emergent from the spar, are polarized,
their directions of vibration being at right angles to each other.
When, therefore, the light is first polarized by reflection, the
direction of vibration in the spar which coincides with the direction
of vibration of the polarized beam, transmits the beam, and that
direction only. Only one image, therefore, is possible under the
conditions.

You will now observe that such logic as connects our experiments is
simply a transcript of the logic of Nature. On the screen before you
are two disks of light produced by the double refraction of Iceland
spar. They are, as you know, two images of the aperture through which
the light issues from the camera. Placing the tourmaline in front of
the aperture, two images of the crystal will also be obtained; but now
let us reason out beforehand what is to be expected from this
experiment. The light emergent from the tourmaline is polarized.
Placing the crystal with its axis horizontal, the vibrations of its
transmitted light will be horizontal. Now the spar, as already stated,
has two directions of vibration, one of which at the present moment
is vertical, the other horizontal. What are we to conclude? That the
green light will be transmitted along the latter, which is parallel to
the axis of the tourmaline, and not along the former, which is
perpendicular to that axis. Hence we may infer that one image of the
tourmaline will show the ordinary green light of the crystal, while
the other image will be black. Tested by experiment, our reasoning is
verified to the letter (fig. 29).

[Illustration: Fig. 29.]