306 Professor Jagadis Chunder Bose [Jan. 29, 



the radiation is completely absorbed, none being transmitted. I now 

 hold the piece with its length perpendicular to the electric vibration ; 

 the radiation is now copiously transmitted. Crysotile is thus seen 

 to act as a perfect electric tourmaline. 



Anisotropic Conductivity exhibited by certain Polarising Substances, 



In a polarising grating, the electric vibrations perpendicular to 

 the bars of the grating are alone transmitted, the vibrations parallel 

 to the grating being absorbed or reflected. In a grating we have a 

 structure which is not isotropic, for the electric conductivity parallel 

 to the bars is very great, whereas the conductivity across the bars 

 (owing to the interruptioDS due to spaces) is almost nothing. We 

 may, therefore, expect electric vibrations parallel to the bars to pro- 

 duce local induction currents, which would ultimately be dissipated 

 as heat. There would thus be no transmission of vibrations parallel 

 to the grating, all such vibrations being absorbed. But owing to the 

 break of metallic continuity, no induction current can take place 

 across the grating ; the vibrations in this direction are, therefore, 

 transmitted. From these considerations we see how non-polarised 

 vibrations falling on a grating would have the vibration components 

 parallel to the direction of maximum conductivity absorbed, and those 

 in the direction of least conductivity transmitted in a polarised con- 

 dition. 



I have shown that nemalite and crysotile polarise by selective 

 absorption, the vibration perpendicular to their length being trans- 

 mitted, and those parallel to their length being absorbed. Bearing 

 in mind the relation between the double conductivity and double 

 absorption, as exhibited by gratings, I was led to investigate whether 

 the directions of the greatest and least absorptions in nemalite and 

 crysotile were also the directions of maximum and minimum conduc- 

 tivities respectively. I found the conductivity of a specimen of 

 nemalite in the direction of absorption to be about fourteen times the 

 conductivity in the direction of transmission. In crysotile, too, the 

 directions of the greatest and least absorption were also the directions 

 of maximum and minimum conductivities. 



It must, however, be noted that the substances mentioned above 

 are bad conductors, and the difference of conductivity in the two 

 directions is not anything like what we get in polarising gratings. A 

 thin layer of nemalite or crysotile will, therefore, be unable to pro- 

 duce complete polarisation. But by the cumulative effect of many 

 such layers in a thick piece, the vibrations which are perpendicular 

 to the direction of maximum conductivity are alone transmitted, the 

 emergent beam being thus completely polarised. 



' A double-conducting structure will thus be seen to act as a polariser. 

 I have here an artificial electric tourmaline, made of a bundle of parallel 

 capillary glass fibres. The capillaries have been filled with dilute 

 copper sulphate solution. A simple, and certainly the most handy, 



