Reflexion of the ft- Particles by Matter. 863 



is after the first reflexion, we have, calling x the percentage 

 of reflected rays after the first, the series 



I + 2pI + 2pxl + 2px*I+ ... &c. 

 =I[l+2p(l+*+«M-«»+ ...&c.)] 



\^} 



or 



336 



=ioo[ 



1 + 



2x70-9 



]■ 



from which # = 40 per cent. 



Hence the percentage of reflected rays drops from 70*9 for 

 the first reflexion to an average value of 40 for the following 

 reflexions. This indicates that the slower ft rays are less 

 easily reflected than the swifter, unless the variation of the 

 ionization with the velocity is large. 



The apparatus used in the multiple reflexion experiments 

 consisted virtually of two ionization chambers. The parallel 

 plates A and B (fig. 5) were connected together and to a 



Fur. 5. 



?0 8fi 'TT ■£*/?» 



£/tr?TH 



source of potential and had equal size openings a and b of 

 4 cms. diameter ; C was another plate with a little larger 

 opening c and was separated from A and B by means o£ 

 sulphur. G acted as an electrode, and was connected to a 

 quadrant electrometer, a and b were covered with very thin 

 aluminium foils so that the /3-particles could readily pass 

 through them without suffering much in absorption or re- 

 flexion. The active material attached to a piece of mica or 

 aluminium to absorb the a-rays was placed at c. The distance 

 between A and B was varied from 1 mm. to 1 cm. in order 

 to see if the change of the solid angle of the issuing rays 

 within the present size vessel produced variations in the 

 multiple reflexion. Between 1 mm. and 3 mm. the readings 

 were concordant, but with a greater distance the ratio for 

 the multiple reflexion to the initial ionization decreased. 

 Hence the smaller distances were used in these experiments. 



