Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/144

 isolate the light of this particular particle from that of other luminous particles, we should doubtless not recognize in it any appearance of polarization. If we consider now the effect produced by the union of all the waves which emanate from the different points of a luminous body, we see that at each instant, at a definite point of the aether, the general resultant of all the motions which commingle there will have a determinate direction, but this direction will vary from one instant to the next. So direct light can be considered as the union, or more exactly as the rapid succession, of systems of waves polarized in all directions. According to this way of looking at the matter, the act of polarization consists not in creating these transverse motions, but in decomposing them in two invariable directions, and separating the components from each other; for. then, in each of them, the oscillatory motions take place always in the same plane."

He then proceeded to consider the relation of the direction of vibration to the plane of polarization. "Apply these ideas to double refraction, and regard a uniaxal crystal as an elastic medium in which the accelerating force which results from the displacement of a row of molecules perpendicular to the axis, relative to contiguous rows, is the same all round the axis; while the displacements parallel to the axis produce accelerating forces of a different intensity, stronger if the crystal "repulsive," and weaker if it is "attractive." Th distinctive character of the rays which are ordinarily refracted being that of propagating themselves with the same velocity in all directions, we must admit that their oscillatory motions are executed at right angles to the plane drawn through these rays and the axis of the crystal; for then the displacements which they occasion, always taking place along directions perpendicular to the axis, will, by hypothesis, always give rise to the same accelerating forces. But, with the conventional meaning which is attached to the expression plane of polarization, the plane of polarization of the ordinary rays is the plane through the axis: thus, in a pencil of polarized light, the