all respects, except that one is the perversion of the other, like its image in a looking-glass. One of them, however, say , has a shorter period of rotation than the other. If the motion is entirely due to the forces called into play by the displacement, this shews that greater forces are called into play by the same displacement when the configuration is like A than when it is like . Hence in this case the left-handed ray will be accelerated with respect to the right-handed ray, and this will be the case whether the rays are travelling from to or from to .
This therefore is the explanation of the phenomenon as it is produced by turpentine, &c. In these media the displacement caused by a circularly-polarized ray calls into play greater forces of restitution when the configuration is like than when it is like . The forces thus depend on the configuration alone, not on the direction of the motion.
But in a diamagnetic medium acted on by magnetism in the direction , of the two screws and , that one always rotates with the greatest velocity whose motion, as seen by an eye looking from to , appears like that of a watch. Hence for rays from to the right-handed ray will travel quickest, but for rays from to the left-handed ray will travel quickest.
815.] Confining our attention to one ray only, the helix has exactly the same configuration, whether it represents a ray from to or one from to . But in the first instance the ray travels faster, and therefore the helix rotates more rapidly. Hence greater forces are called into play when the helix is going round one way than when it is going round the other way. The forces, therefore, do not depend solely on the configuration of the ray, but also on the direction of the motion of its individual parts.
816.] The disturbance which constitutes light, whatever its physical nature may be, is of the nature of a vector, perpendicular to the direction of the ray. This is proved from the fact of the interference of two rays of light, which under certain conditions produces darkness, combined with the fact of the non-interference of two rays polarized in planes perpendicular to each other. For since the interference depends on the angular position of the planes of polarization, the disturbance must be a directed quantity or vector, and since the interference ceases when the planes of polarization are at right angles, the vector representing the disturbance must be perpendicular to the line of intersection of these planes, that is, to the direction of the ray.
