POLARIZATION OF LIGHT is a peculiar affection of that agent to which the term has been applied because a ray of polarized light possesses properties which have relation to two opposite directions in a to the ray. We shall first describe some of the circumstances under which polarized light is formed, and the characters by which it is recognised as polarized, and afterwards explain the view which is taken of its nature in the theory of undula tions, the only theory which has given a simple and intelligible account of its complicated phenomena.
The affection of light now known as polarization was discovered by Huygens in the course of his researches on the laws of double refraction in Iceland spar. A rhomb of this crystal has the property of dividing a ray of light incident upon it into two rays, which are propagated in different directions within the crystal, and produce each an emergent ray parallel to the incident, so that the two emergent rays are separated laterally by a space proportional, cat ens paribus, to the thick ness of the rhomb, the direction of separation (supposing for simplicity the ray to be incident perpendicularly) being that of a line bisecting an obtuse angle of the rhombic face, and the intensity of the two emergent rays being under ordinary circumstances the same. Such a rhomb gives two images, of equal intensity, of an object on which it is placed. Now Huygens found that either of the pencils separated by a first rhomb is affected by a second rhomb quite differently from a pencil of common light. When the second rhomb is placed with its faces parallel to the corresponding faces of the first rhomb, so that the two are in the same relative position as if they formed parts of a larger block of the crystal, each pencil separated by the first rhomb furnishes only a single pencil in the second rhomb, the ordinary only an ordinary, and the extraordinary only an extraordinary, 'the two being separated on emergence as much as if they had been transmitted through a single block having a thickness equal to the sum of the thicknesses of the two rhombs. If now the second rhomb be turned round the common normal to the adjacent faces of the two rhombs, each pencil transmitted by the first rhomb is immediately divided into two by the second rhomb, the pencil (o) which suffered ordinary refraction in the first rhomb furnishing besides the ordinary pencil (o o) in the second a faint extraordinary pencil (o E), and the pencil (E) which suffered extraordinary refraction in the first rhomb furnishing besides an extraordinary pencil (E E) in the second a faint ordinary (s 0); the direction and amount of separation of the pencils o o and o E, and likewise of E 0 and E E, being those due to the azimuth and thickness of the second rhomb, while the direction and amount of separation of 0 0 and E o, and likewise of 0 E and E E, are those due to the first rhomb. On continuing to turn, the faint pencils o E and E 0 become
brighter and brighter, and the bright pencils 0 0 and E E fainter and fainter, until the rotation amounts to 90° when the original pencils o 0 and RE disappear altogether, the whole Of the light of o (not counting the small portion lost by reflection) now passing into 0 E, and the whole of the light of E into E O. On further turning, the pencils o 0, E z which vanished reappear, and appropriate to themselves more and more of the light of 0, E, respectively, until the rotation amounts to 180°, when the pencils 0 E, E o vanish, while 0 o, E E become as bright as at first, their lateral separation being now, however, that 'duo to the difference instead of the sum of the thicknesses of the rhombs. On continuing to turn, the same changes recur periodically, each pair of images vanishing alternately at every quarter of a revolution.
Now imagine the ordinary pencil 0 to be isolated by means of a screen, and presented for observation, the apparatus by which it was produced being hidden from the observer. By examining it with a rhomb of Iceland spar 110 could distinguish it at once from common light, which has no relation to any other direction in space than that of its propagation, whereas the pencil we have supposed possesses different properties with reference to different directions transverse to that of its propagation. By turning the rhomb with which he was furnished into either of the two opposite positions in which the extra ordinary pencil vanishes, the plane of crystalline symmetry in the rhomb would mark a plane determined solely by the properties of the pencil of light presented for observation, and which (as well as its rectangular plane) is a plane of symmetry in relation to those properties. The plane of crystalline symmetry passing through the normal to the face of the rhomb is evidently parallel to a line bisecting either obtuse angle of the rhombic face, and contains the crystallographic axis. Any plane passing through this axis in Iceland spar is called in optical language a principal plane, because all such planes are planes of optical, though six of them only of crystallographic, symmetry.