[223] [Light is called polarized, which, having been once reflected or refracted, is incapable of being again reflected or refracted in certain positions of the second medium. Ordinarily, light which has been reflected from a pane of glass or any other substance, may be a second time reflected from another surface, and will also freely pass through transparent bodies. But if a ray of light be reflected from a pane of glass at an angle of 57°, it is rendered totally incapable of reflexion from the surface of another pane in some positions, whilst it will be completely reflected by it in others. If a plate of tourmaline, cut in the manner described above, or a Nichol’s prism be held between the eye and a candle, and turned slowly round in its own plane, no change will take place in the image of the candle; but if the plate or prism be fixed in a vertical position, on interposing another of the same kind between the former and the eye, parallel to the first, and turning it round slowly in its own plane, the image of the candle will be found to vanish and re-appear alternately at each quarter turn of the plate, varying through all degrees of brightness down to total or almost total evanescence, and then increasing again by the same degrees as it had before decreased. These changes depend upon the relative positions of the plates; when the longitudinal sections of the two plates are parallel, the brightness of the image is at its maximum; and when the axes of the sections cross at right angles, the image of the candle vanishes. Thus the light, in passing through the first plate of tourmaline, has acquired a property totally different from the ordinary light of the candle; the latter would penetrate the second plate equally well in all directions, whereas the altered light will only pass through it in particular positions, and is altogether incapable of penetrating it in others. The light is polarized by passing through the first plate or prism. Thus, one of the properties of polarized light is proved to be the incapability of passing through a plate of tourmaline perpendicular to it in certain positions, and its ready transmission in other positions at right angles to the former.] SPEAKING-TRUMPET.