612 Dr. H. Geiger and Mr. E. Marsden on the Laws of 



of different thicknesses of metal foils is shown diagram-* 

 matically in fig. 3. It consists essentially of a source of 

 « radiation R, a diaphragm D, a scattering foil F, and a zinc- 

 snlphide screen Z on which the scattered a. particles were 

 observed. The main part of the apparatus was enclosed in 

 a cylindrical brass ring A, the ends of which were planed so 

 that they could be closed airtight by the two glass plates 

 B and C. The depth of the ring was 3*5 cm., and its- 

 internal and external diameters 5'5 and 7*5 cm. respectively. 

 Two holes were drilled through the glass plate B, one in the 

 centre and the other 1*65 cm. excentric. The source of 

 radiation R was placed directly against a sheet of mica, 

 which was waxed over and closed the opening E. By 

 placing the source outside the apparatus, any small amount 

 of emanation associated with it was prevented from entering, 

 the chamber and disturbing the measurements. 



Fi£. 3. 



By means of the diaphragm D a narrow pencil of a. par- 

 ticles could be directed on to the scattering foil. The 

 different foils were attached to the disk S and covered five of 

 six holes drilled through it at equal distances from its centre. 

 The uncovered opening was used to determine the natural 

 effect. The disk could be fitted on to the rod P, which was 

 fastened to the ground-glass joint M so that it could be 

 rotated and the different foils brought in front of the 

 diaphragm. The scattered a particles were observed by 

 means of a microscope on the zinc-sulphide screen Z fixed 

 inside the glass plate. 



