( 105 ) 



The difference between the maximum and minimum amounted to 

 about 20''/„. 



I iiad tried to examine the same question by a somewhat different 

 metiiod. A pencil of Röntgex rays passed through a tube in the 

 direction of its axis, without touching the wall of the tube. A photo- 

 graphic fdm, bent cylindrically, covered the inner wall of the tube 

 in order to investigate whether the secondary rays emitted by the 

 air particles showed a greater action in one direction than in another. 

 I obtained a negative result and communicated this fact to Barkla, 

 who advised me to take carbon as a \'ery strong radiator for secon- 

 dary rays. I then made the following arrangement. 



s. 



> 



s. 



s. 



A. 



Ph 



R 



A 'JlS 



-n 



p 



c 



LA 



^-F 



F'i-i 



Let /Si (fig. 1) be the front side of a thick-walled leaden box, 

 in which the Röxtgen bulb is placed; S.^ and S^ brass plates 

 10 X 10 cm. large and 4 m.m. thick. Their distance is 15 cm. and 

 they are immovably fastened to the upper side of an iron beam. In 

 the middle of these plates apertures of 12 m.m. diameter were made. 

 A metal cylinder A is fastened to the back side of S^\ a brass tube 

 B provided with two rings R^ and R^ slides into it ^). 



An ebonite disk E in which a carbon bar is fastened fits in 

 tube B. This bar is 6 cm. long and has a diameter of 14 m.m. At 

 one end it has been turned off conically over a length of 2 cm. 



1) Fig. 1 and 2 are drawn at about half their real size. 



