6S6 



Mr. C. Gr. Barkla on Secondary 



after passing through another rectangular aperture in a lead 

 screen placed several centimetres from the side of the box 

 and parallel to it, the X-rays were not intercepted by any- 

 solid body for a considerable distance — about two metres — 

 but passed merely through air. 



To detect the secondary radiation, a special form of electro- 

 scope was used. It consisted of a case (as shown in fig. 1), 



Fig. 1. 



with four sides of stout brass. One end G w T as of glass, and 

 at the opposite end a face of any desired material and thick- 

 ness could be placed. As in C. T. E. Wilson's* experiments 

 on Spontaneous Ionization, the gold-leaf and brass plate to 

 which it was attached were suspended in this case by a bead 

 of sulphur S, which was fixed to the lower end of a vertical 

 brass rod R. This passed axially through a cylindrical brass 

 neck N, whose lower end was movable along grooves in a 

 * Boy. Soc. Proc. vol. lxviii. pp. 151-161 (1901). 



