436 On the Selective Conductivity of Polarising Substances. 



It should, however, be borne in mind that the selective absorption 

 exhibited by a substance depends, also, on the vibration frequency of 

 the incident radiation. I have drawn attention to the peculiarity of 

 tourmaline which does not exhibit double absorption of the electric 

 ray to a very great extent. The specimen I experimented with is, 

 however, one of a black variety of tourmaline, and not of the semi- 

 transparent kind generally used for optical work. 



Though the experiments already described are not sufficiently nume- 

 rous for drawing a general conclusion as to the connection between 

 double absorption attended with polarisation, and double conductivity, 

 there is, however, a large number of experiments I have carried out 

 which seem to show that a double-conducting structure does, as a rule, 

 exhibit double absorption and consequent polarisation. Out of these 

 experiments I shall here mention one which may prove interesting. 

 Observing that an ordinary book is unequally conducting in the two 

 directions parallel to and across the pages I interposed it, with its 

 edge at 45, between the crossed polariser and analyzer of an electro- 

 polariscope. The extinguished field of radiation was immediately 

 restored. I then arranged both the polariser and the analyzer vertical 

 and parallel, and interposed the book with its edge parallel to the 

 direction of electric vibration. The radiation was found completely 

 absorbed by the book, and there was not the slightest action on the 

 receiver. On holding the book with its edge at right angles to the 

 electric vibration, the electric ray was found copiously transmitted. 

 An ordinary book would thus serve as a perfect polariser of the 

 electric ray. The vibrations parallel to the pages are completely 

 absorbed, and those, at right angles transmitted in a perfectly polar- 

 ised condition. January 28, 1897.] 





