DR. CHARLES O. BARKLA ON POLARISED RONTGEN RADIATION. 469 



in a direction perpendicular to the axis of the kathode stream. For the electrons 

 being projected in approximately parallel straight lines from the kathode to the 

 antikathode, there is probably at the antikathode a greater acceleration along the 

 line of propagation of the kathode rays than in a direction at right angles, conse- 

 quently in a beam of X-rays proceeding in a direction perpendicular to that of the 

 kathode stream it might reasonably be expected there would be greater electric 

 intensity parallel to the stream than in a direction at right angles to that. 



Such a beam was used as the primary radiation, and the intensity of secondary 

 radiation proceeding in a direction perpendicular to that of the projwigation of the 

 primary l>eam from different radiators placed successively in the primary beam was 

 studied by means of an electroscope. 



In preliminary experiments the intensity of secondary radiation in a direction per- 

 pendicular to the axis of the primary beam was compared with that in a direction 

 making a small angle with the axis of the primary beam, while the bulb was turned 

 round that axis. 



The intensity in the second direction named, according to theory, should not 

 vary. Using this to standardize the intensity of primary radiation, the intensity of 

 the other was found to reach a maximum when the direction of the kathode stream 

 was perpendicular to that of propagation of the secondary beam and a minimum when 

 these two were parallel. 



In other experiments the intensity of secondary radiation in a direction perpen- 

 dicular to the axis of the primary beam was compared with that of a small pencil of 

 the primary beam itself when the bulb was rotated as before. The results were 

 similar to those in the final experiment, the arrangement of apparatus in which will 

 be described more fully. It was as follows : 



t/ 



An X-ray bulb was contained in a large lead-covered box, in one side of which was 

 a rectangular aperture, C,, through which a beam of Rontgen rays passed. The size 

 of this aperture was adjustable by lead shutters, S,, placed just outside. Large 

 screens, S,, of thick sheet-lead were placed at a distance of 25 centima from this 

 aperture and parallel to the side of the box, so that the width of aperture between 

 them was also adjustable. The primary beam studied was that passing through the 

 second rectangular aperture. Beyond S, were two parallel lead screens, S 3 and S 4 , 

 8 centima apart, placed in vertical planes perpendicular to the screens Si and S 

 Each contained a square aperture, C 3 and C 4 , 5 centima square, in positions so that 

 lines joining corresponding points were approximately |>erpendicular to the planes of 

 both. No primary rays were incident upon them, but secondary rays proceeding from 

 a radiator in the primary beam passed through both apertures. Those passing through 

 the second aperture, C 4 , were approximately horizontal and perpendicular to the 

 direction of propagation of the primary beam. They then entered an electroscope, A,, 

 immediately behind the aperture C 4 through a thin paper and aluminium face. 

 Similar lead plates containing square apertures were placed in horizontal planes above 



