Polarisation of Electric Waves by a Twisted Structure. 151 



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 rapidly withdrawn, and an equal number of positive substituted.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 experi- 

 ments 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. 



FIG. 2. Jute elements. 



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 afc, 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 inter- 

 spersed 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 pro- 

 duced by the molecules may conspire, it is necessary that they should 



