Page:On the Rotation of Plane of Polarisation of Electric Waves by a Twisted Structure.djvu/6

Rh the receiver continuing to remain unaffected. From the mixture of positive and negative varieties, one set, say the negative, is now rapidly withdrawn, and an equal number of positive substituted. The receiver which has not been disturbed since its first adjustment is now found to respond, all the elements conspiring to produce rotation in the same direction. It will be seen that the two experiments are carried out under identical conditions.

In the above, we have electro-optic analogues of two varieties of sugar—dextrose and levulose. There is also the production of an apparently inactive variety by the mixture of two active ones.

It is to be noted that there is no polarity in the elements, in the sense we use the term in reference to, say, magnetic molecules. There is nothing to distinguish one end of the jute element from the other end; indeed a right-handed element would appear right-handed when looked at from either end. It thus happens that if the rotation is determined by the direction of the twist, two molecules of the same variety will always conspire, whether they are arranged as ab, cd, or, to take the extreme case, as ab, dc (with the second molecule reversed). The assumption of any particular arrangement of molecules is thus not necessary in explaining the rotation. The average effect produced by a large number of active elements interspersed in an inactive medium will thus be the same in all directions, and proportional to the number of molecules traversed by the ray. As there is no polarity in the molecule, a right-handed element will always produce the same kind of rotation, say, to the right of an observer travelling with the ray. The rotation produced when the ray is reversed by reflection will thus be in an opposite direction, and the two rotations will neutralise each other.

But if the molecules exhibit any polarity, that is to say, if the effects produced by the two ends of the same molecule are opposite, the resultant effect produced by a number of such molecules arranged in haphazard directions, will be zero. In order that the effects produced by the molecules may conspire, it is necessary that they should