G20 Dr. H. Geiger and Mr. E. Marsden on the Laws of 



thickness in comparison with metal foils. The micas were 

 attached to a cardboard disk which could be rotated to bring 

 the different sheets successively in position. The a particles 

 were scattered by a foil of gold or silver, of stopping power 

 about 3 mm. of air, which was attached to a rod passing 

 through the ground glass 1ST. This made it possible to turn 

 the foil away from the main beam during an experiment in 

 order to test the natural effect. The disk S, in this case, 

 rotated in a plane very close to the glass plate C and carried 

 sheets of mica of different thicknesses. By rotating the 

 ground-glass joint the micas could be placed directly in 

 front of the zinc-sulphide screen, making it possible to test 

 the homogeneity of the a particles after they had been 

 scattered. 



The results are given in Table VII. Column I. gives the 

 number of mica sheets which were interposed in addition to 

 the mica window, and column II. the ranges of the a particles 

 incident on the scattering foil. The values of the velocities v 

 were calculated from these ranges R by use of the formula 

 v 3 = aR previously found by one of us *. The relative values 



Table YTI. 

 Variation of Scattering with Velocity. 



I. 



II. 



III. 



IV. 



v. 



Number of 



Range R, of 

 a particles 

 after leaving 

 mica. 



Relative 



Number N of 





sheets of 

 mica. 



values of 

 1/**. 



scintillations 

 per minute. 



Nv-i. 







fr5 



1-0 



247 



25 



1 



4-76 



1-21 



290 



24 



2 



4-05 



1-50 



33-4 



22 



3 



3-32 



1-91 



41 



23 



4 



2-51 



2-84 



81 



28 



5 



1-84 



4-32 



101 



23 



6 



1-04 



9-22 



255 



28 



of l/t? 4 are given in column III. The number of scintillations 

 per minute N are entered in column IV., and in column V. 

 relative values of N x v 4 are given. Over the range examined 

 the number of scintillations varies in the ratio 1 : 10, while 

 it will be seen that the product Nr 4 remains sensibly 

 constant. Several experiments were made, and in every case 



* H. Geiger, Roy. Soc. Proc. A. vol. lxxxiii. p. o06 (1910). 



