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



square pieces of thick copper. When the square pieces were adjusted 

 with coincident edges, the analyser and polariser were either exactly 

 parallel or exactly crossed. This improvement enabled me to carry 

 out successfully some of the more delicate experiments. In the 

 present course of investigation the sensitiver.ess of the receiver had 

 to be raised to a still higher extent, and it was found that the 

 polariser hitherto found efficient failed to produce complete polarisa- 

 tion, so that even when the polariser and the analyser were exactly 

 crossed the non -polarised portion of radiation was of sufficient 

 intensity to produce strong action on the receiver. 



In the paper " On the Selective Conductivity exhibited by some 

 Polarising Substances "* I described a book-form of polariser, when 

 an ordinary book was shown to produce polarisation of the trans- 

 mitted beam, the vibrations parallel to the pages being absorbed, and 

 those at right angles transmitted in a polarised condition. The 

 advantage of this form of polariser was that the extent to which the 

 rays were polarised depends on the thickness of the polarising 

 medium. The rays could thus be completely polarised by giving the 

 medium a sufficient thickness, this thickness being determined by the 

 intensity of the radiation used and the sensitiveness of the receiver. 

 The necessary thickness of the book-polariser may be materially 

 decreased by making the book consist of alternate leaves of paper and 

 tinfoil. The book being then strongly compressed, blocks of suitable 

 size are cut out to form the polariser and the analyser. Each of these 

 blocks is then enclosed in a brass cell, with two circular openings on 

 opposite sides for the passage of radiation. The size of the polariser 

 I use is 6 X 6 cm., with a thickness of 4'5 cm. ; the aperture is 

 4 cm. in diameter. These polarising cells I find to be quite efficient ; 

 when two such cells are crossed, the field is completely extin- 

 guished. 



Polarisation apparatus. B, the radiating box ; P, the pclariser ; A, the 

 analyser ; S, S', the screens ; It, the receiver. 



* * Eoy. Proc. Soc.,' vol. 60. 



