470 



DR. CHARLES G. BARKLA ON POLARISED RONTGEN RADIATION. 



the primary beam in such .1 position that lines joining corresponding points were 

 vertical. Both were screened from the primary radiation, but secondary radiation 

 proceeding in a vertical direction from the radiator in the primary beam passed 



Earth 



Hfr 



Cs__ 



-* *_ 





A, 



Fig. 1. Showing position of bulb giving a maximum deflexion of electroscope Aj and a 



minimum of electroscope Aj. 



through the apertures and entered an electroscope, A 2 , through a thin paper and 

 aluminium face. In the primary beam, at a distance of about 14 centims. beyond the 

 secondary radiator and 21 centims. from screens S 3 , was placed another thick lead 

 screen, S 5 . In this was a small circular aperture through which a narrow pencil of 

 primary radiation passed into an electroscope B beyond. 



Electroscopes A, and B were similar, each consisting of a case (as shown in fig. 2), 

 with four sides of stout metal one brass, the other zinc. One end, G, was of glass, 

 and the opposite end consisted of a thin sheet of paper covered with aluminium leaf. 

 The gold-leaf and copper-wire 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, K. 

 This passed axially through a cylindrical brass neck, N, the whole of the suspension 

 being like that used by C. T. R. WILSON in his ' Experiments on Spontaneous 

 lonization of Air.'* Connexion between the rod and the insulated wire and gold-leaf 

 could be made by means of a light spring which was attached to the rod at its upper 

 end and which, when set in vibration, made momentary contact with the insulated 

 portion of the electroscope, the capacity of which was very small. The deflexion of 

 the gold-leaf was observed through a microscope with graduated eye-piece, which was 

 fixed just outside a small glass window, M, in one side of the case. 



* 'Roy. Soc. Proc.,' vol. 68, pp. 151-161, 1901. 



