As already stated, the whole of a beam of ordinary light reflected from glass at the polarizing angle is polarized; a word must now be added regarding the far larger portion of the light which is transmitted by the glass.  The transmitted beam contains a quantity of polarized light equal to the reflected beam; but this is only a fraction of the whole transmitted light.  By taking two plates of glass instead of one, we augment the quantity of the transmitted polarized light; and by taking a bundle of plates, we so increase the quantity as to render the transmitted beam, for all practical purposes, perfectly polarized.  Indeed, bundles of glass plates are often employed as a means of furnishing polarized light.  It is important to note that the plane of vibration of this transmitted light is at right angles to that of the reflected light.

One word more.  When the tourmalines are crossed, the space where they cross each other is black.  But we have seen that the least obliquity on the part of the crystals permits light to get through both.  Now suppose, when the two plates are crossed, that we interpose a third plate of tourmaline between them, with its axis oblique to both.  A portion of the light transmitted by the first plate will get through this intermediate one.  But, after it has got through, its plane of vibration is changed:  it is no longer perpendicular to the axis of the crystal in front.  Hence it will, in part, get through that crystal.  Thus, by pure reasoning, we infer that the interposition of a third plate of tourmaline will in part abolish the darkness produced by the perpendicular crossing of the other two plates.  I have not a third plate of tourmaline; but the talc or mica which you employ in your stoves is a more convenient substance, which acts in the same way.  Between the crossed tourmalines, I introduce a film of this crystal with its axis oblique to theirs.  You see the edge of the film slowly descending, and, as it descends, light takes the place of darkness.  The darkness, in fact, seems scraped away, as if it were something material.  This effect has been called, naturally but improperly, depolarization.  Its proper meaning will be disclosed in our next lecture.

These experiments and reasonings, if only thoroughly studied and understood, will form a solid groundwork for the analysis of the splendid optical phenomena next to be considered.

LECTURE IV.

  CHROMATIC PHENOMENA PRODUCED BY CRYSTALS IN POLARIZED LIGHT
  THE NICOL PRISM
  POLARIZER AND ANALYZER
  ACTION OF THICK AND THIN PLATES OF SELENITE
  COLOURS DEPENDENT ON THICKNESS
  RESOLUTION OF POLARIZED BEAM INTO TWO OTHERS BY THE SELENITE
  ONE OF THEM MORE RETARDED THAN THE OTHER
  RECOMPOUNDING OF THE TWO SYSTEMS OF WAVES BY THE ANALYZER
  INTERFERENCE THUS RENDERED POSSIBLE
  CONSEQUENT PRODUCTION OF COLOURS