pace of darkness (fig. 28). Our conclusion, arrived at prior to
experiment, is thus verified.

Let us now return to a single plate; and here let me say that it is on
the green light transmitted by the tourmaline that you are to fix your
attention. We have to illustrate the two-sidedness of that green
light, in contrast to the all-sidedness of ordinary light. The white
light surrounding the green image, being ordinary light, is reflected
by a plane glass mirror in all directions; the green light, on the
contrary, is not so reflected. The image of the tourmaline is now
horizontal; reflected upwards, it is still green; reflected sideways,
the image is reduced to blackness, because of the incompetency of the
green light to be reflected in this direction. Making the plate of
tourmaline vertical, and reflecting it as before, it is the light of
the upper image that is quenched; the side image now shows the green.
This is a result of the greatest significance. If the vibrations of
light were longitudinal, like those of sound, you could have no action
of this kind; and this very action compels us to assume that the
vibrations are transversal. Picture the thing clearly. In the one case
the mirror receives, as it were, the impact of the _edges_ of the
waves, the green light being then quenched. In the other case the
_sides_ of the waves strike the mirror, and the green light is
reflected. To render the extinction complete, the light must be
received upon the mirror at a special angle. What this angle is we
shall learn presently.

The quality of two-sidedness conferred upon light by bi-refracting
crystals may also be conferred upon it by ordinary reflection. Malus
made this discovery in 1808, while looking through Iceland spar at the
light of the sun reflected from the windows of the Luxembourg palace
in Paris. I receive upon a plate of window-glass the beam from our
lamp; a great portion of the light reflected from the glass is
polarized. The vibrations of this reflected beam are executed, for the
most part, parallel to the surface of the glass, and when the glass is
held so that the beam shall make an angle of 58 deg. with the
perpendicular to the glass, the _whole_ of the reflected beam is
polarized. It was at this angle that the image of the tourmaline was
completely quenched in our former experiment. It is called _the
polarizing angle_.

Sir David Brewster proved the angle of polarization of a medium to be
that particular