G14 Dr. H. Geiger and Mr. E. Marsden on the Laivs of 



screen Z, which was temporarily placed at Z\ The magni- 

 tude of the effect may be judged from the following figures 

 obtained in a particular experiment with an aluminium 

 diaphragm : — The number of scintillations without both the 

 screen K and the scattering foil F was 60 per minute, whilst 

 by bringing the screen K into position the number was 

 reduced to 0*5 per minute. With the screen K in position 

 and an aluminium foil equivalent to 0*5 cm. air as scattering 

 foil, the number of scintillations was 14 per minute, or about 

 one quarter the effect without screen or scattering foil. 



Fig. 4. 



In the following table the results of an experiment with 

 gold foils are tabulated. Column I. gives the number of 

 toils and column II. the thicknesses expressed as the stopping 

 power of a particles in centimetres of air as determined ba- 

 ttle scintillation method. The figures given in column 111. 

 represent the number of scintillations observed on the zinc- 

 sulphide screen. These figures are corrected for the variation 

 of activity with time of the source. A slight correction has 

 been made due to the increase of scattering on account of 

 the decrease of velocity of the a, particles in passing through 

 the foils. The magnitude of this correction could be calcu- 

 lated from the results given in the last section of the present 

 paper, and amounted to 9 per cent, in this experiment for 

 the thickest foil used. The last column of the table gives 

 the ratio of the corrected number of scintillations to the 

 thickness. The values are constant within the limits of the 

 experimental error. The variations exhibited by the figures 

 are well within the probability errors, owing to the relatively 

 small number of scintillations which could be counted in the 

 time available. 



