146 Prof. J. C. Bose. On the Rotation of Plane of 



a, volt, but slowly rose again to nearly its original value as the film 

 evaporated. This shows that the electrification was not a temporary 

 one of the glass surface, for that would not have returned to a 

 definite value. Heating the plate by radiation or washing the glass 

 with benzol caused the potential to rise further, but in no case was 

 the potential quite so high as when the plate was first formed. An 

 even more convincing proof that the potential measured was really 

 that of the sodium, was found in the fact that the sensibility of the 

 apparatus was such as would be given by a plate the size of the 

 sodium. If the electrification had been on the whole surface of the 

 glass, the sensibility, on account of the larger surface, wonld have 

 been at least ten times as great as that observed. 



85. The experiments described in this chapter show that (i) 

 when two metals are coated with the same non-conductor, such as 

 wax or glass, their potential is not sensibly different from that of the 

 bare metals in air ; (ii) that temperature-variation still takes place, 

 though air be excluded. These results seem to prove that gaseous 

 films play no essential part in the phenomenon. 



4< On the Rotation of Plane of Polarisation of Electric Waves 

 by a Twisted Structure." By JAGADIS CHUNDER BOSE, 

 M.A., D.Sc., Professor of Physical Science, Presidency 

 College, Calcutta. Communicated by Lord RAYLEIGH, 

 F.R.S. Received February 1 4, Read March 10, 1898. 



In my previous papers* I have given accounts of the double 

 refraction and polarisation of electric waves produced by various 

 crystals and other substances, and also by strained dielectrics. An 

 account was there given of the polarisation apparatus with which 

 the effects were studied. In the present investigation effects had 

 to be studied which were exceedingly feeble. The apparatus had, 

 therefore, to be made of extreme sensitiveness ; but the secondary 

 disturbances became at the same time more prominent, and the great 

 difficulty experienced was in getting rid of these disturbances. 



In one of my communications I alluded to the fact that these 

 secondary disturbances are to a great extent reduced when the radia- 

 tors are made small. The advantage of a large radiator is the com- 

 parative ease with which the receiver can be adjusted to respond to 

 the waves, but this advantage is more than counterbalanced by the 

 increased difficulty with the stray radiation and other disturbances. 



* "On the Polarisation of the Electric Kay by Double-refracting Crystals," 

 * Journal of the Asiatic Society of Bengal,' May, 1895, and "On a New Electro- 

 polariscope," ' The Electrician/ December 27, 1895. 



