Polarisation of Electric Waves by a Twisted Structure. 149 



The diagram explains the general arrangement of the apparatus, 

 mounted on an optical bench. The spark gap of the radiator is 

 horizontal. The polariser, with the leaves vertical, is placed on a 

 shelf attached to a screen of thick brass plate 35 x 35 cm. In the 

 centre of the plate there is a circular opening 4 cm. in diameter ; 

 this aperture may be varied by a series of diaphragms. There is a 

 second similar screen with a shelf for the analyser, which is placed 

 with th.e leaves horizontal. Behind the analyser is the receiver. 



In the space between the brass plates is placed the substance to be 

 examined. Previous tests are made to see whether all disturbing 

 causes have been removed. The sensitiveness of the receiver is 

 occasionally tested by interposing one's fingers at 45 between the 

 crossed polariser and analyser; this should, by partially restoring 

 the field, produce strong action, provided the receiver is in a fairly 

 sensitive condition. 



Care should be taken that there are no metallic masses between 

 the screens, as reflection from metals is found to produce " depolari- 

 sation," the rajs being then elliptically polarised. The substance to 

 be examined should not, for very delicate experiments, be held by the 

 hand, owing to the disturbing action of the fingers. It is preferable 

 to have the substances supported on stirrups made of thin paper. 

 The above are some of the main precautions to be taken in carrying 

 out the following experiments, where the effects to be detected are 

 very small and therefore likely to be masked unless all disturbing 

 causes are carefully excluded. 



I have in a previous communication made mention of the double 

 refracting property of fibrous substances like jute. The field is 

 restored when a bundle of jute is placed at 45 between the crossed 

 polariser and analyser. There is, however, 110 depolarisation effect 

 when the axis of the bundle is parallel to the direction of the ray. 



I now took three similar bundles, A, B, and C, of parallel fibres of 

 jute 10 cm. in length and 4'5 cm. in diameter. No change was made 

 in the bundle A, which was kept as a test one. The bundles B and 

 C were then twisted, B in a right-handed direction and C in a left- 

 handed direction. 



The interposition of the untwisted bundle A between the crossed 

 polariser and analyser did not produce any effect, but strong 

 action was produced in the receiver when the bundles, twisted to the 

 right or to the left, were so interposed. It thus appeared as if the 

 twisted structures produced an optical twist of the plane of polari- 

 sation. 



The further experiments to be described below may be of some 

 interest in connection with the optical rotation produced by liquids. 

 Here two different classes of phenomena may be distinguished : 



(1) The rotation induced by magnetic field ; this rotation among 



