794 Mr. J. Crosby Chapman on some 



electroscopes became approximately equal, showing that the 

 solid angle subtended by each electroscope at R 2 was the same. 



General Method. 

 The ratio : — 



intensity of horizontal tertiary radiation 

 intensity of vertical tertiary radiation ' 



when carbon is used as a tertiary as well as a secondary 

 radiator, can be taken as a measure of the polarization of the 

 beam. When this ratio is unity, there is no polarization, 

 and the greater this ratio becomes, the larger is the 

 polarization in the beam. It was this ratio, determining the 

 polarization, which was measured in the experiments. 



Suppose that in a particular case this ratio has been 

 determined when the secondary beam has passed directly 

 from E^ to R 2 . If, now, at XX there is placed a substance 

 which is capable of rotating the plane of polarization of 

 X-rays, after transmission through this substance, we should 

 expect each portion of the beam to be rotated through an 

 angle which is a function of its wave-length and the distance 

 traversed in the substance. If Rontgen radiation is light of 

 an exceptionally short wave-length, then, while there would 

 be considerable polarization in the incident secondary beam, 

 after transmission the polarization of the beam would be 

 destroyed, for something amounting to a random rotation of 

 the various constituents would have taken place. If, how- 

 ever, as seemed possible, only a few of the electrons of the 

 rotating agent were effective, then a much smaller change 

 in the state of polarization might be expected. 



With the idea of investigating this possibility of there 

 being a considerable rotation, the following experiments 

 were made. Three substances were examined : — 



(a) Iron in a magnetic field. 



(b) Quartz. 



(c) Sugar solution. 



Experiments on Iron in a Magnetic Field. 



The magnetic field was produced by means of a coil of 

 wire consisting of 200 turns, through which currents up to 

 4 amperes could be passed. In the centre of the magnetic 

 field so produced and perpendicular to it was placed the 

 thin sheet of iron, the power of rotation of which it was 

 required to investigate. The pure iron, specially rolled by 

 Messrs. Johnson and Matthey, was of thickness '008 cm., 

 which enabled a considerable portion of the X-ray beam 

 used in the experiment to be transmitted. The iron mounted 



