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 angle at which the refracted and reflected rays inclose a right angle.[17] The polarizing angle augments with the index of refraction.  For water it is 521/2 deg.; for glass, as already stated, 58 deg.; while for diamond it is 68 deg..

And now let us try to make substantially the experiment of Malus.  The beam from the lamp is received at the proper angle upon a plate of glass and reflected through the spar.  Instead of two images, you see but one.  So that the light, when polarized, as it now is by reflection, can only get through the spar in one direction, and consequently can produce but one image.  Why is this?  In the Iceland spar as in the tourmaline, all the vibrations of the ordinary light are reduced to two planes at right angles to each other; but, unlike 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.