124 The Luminiferous Medium, 



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 pro- 

 duced 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 is "repulsive," and weaker if it is "attractive." The 

 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 



