15*2 Prof. J. C. Bose. On the Production of a " Dark 



take up a special arrangement like the disposition of molecules in a 

 magnetised rod. It is seen that in this case the rotations of the 

 direct and the reflected rays are in the same direction, and the 

 resultant rotation is therefore doubled. There is some analogy 

 between the action of such polarised molecules and of substances 

 which, when placed in a magnetic Afield, rotate the plane of polar- 

 isation. 



" On the Production of a ' Dark Cross ' in the Field of Electro- 

 magnetic Radiation/' By JAGADIS CHUNDER BOSE, M.A., 

 D.Sc., Professor of Physical Science, Presidency College, 

 Calcutta. Communicated by Lord RAYLEIGH, F.R.S. 

 Received February 14, Read March 10, 1898. 



A circular piece of chilled glass when interposed between crossed 

 nicols produces a dark cross. A similar effect is produced by 

 crystals like salicine where there is a radial disposition of the 

 principal planes. 



I have been able to detect a similar phenomenon in the field of 

 electric radiation by the interposition of an artificial structure 

 between the crossed polariser and analyser. 



I have in a previous communication described the polarisation 

 produced by the leaves of a book. For the following experiment, a 

 long strip of paper was rolled into a disc. A roll of Morse's tape 

 serves the purpose very well. The diameter of the disc is 14 cm. 

 and its thickness 2 cm. It will be observed that here we have a 

 single axis passing through the centre, and that all planes passing 

 through the centre are principal planes. 



The effect produced by the interposition of the structure may be 

 studied by keeping the disc fixed and exploring the different parts of 

 the field by means of the detector; or the detector may be kept 

 fixed (opposite the analyser) and the disc may be moved about so 

 that the different parts of the field may successively be brought to 

 act on the detector. This latter plan was adopted as being simpler 

 in practice. 



The arrangement of the apparatus is the same as in fig. 1 of my 

 paper " On the Rotation of Plane of Polarisation of Electric Waves 

 by a Twisted Structure." The polariser is vertical and the analyser 

 horizontal. The paper disc is interposed between the screens with 

 its plane at right angles to the direction of the ray. 



The receiver is fixed on the prolongation of the line (which I shall 

 call the axis), joining the centres of the polariser and the analyser. 



On the supposition that the interposition of the disc produces a 

 dark cross, the arms of the cross (with the particular arrangement 



