<608 Dr. H. Geiger and Mr. E. Marsden on the Laws oj 



rotating the microscope the a particles scattered in different 

 directions could be observed on the screen S. Although over 

 100 millicuries of radium emanation were available for the 

 experiments, the smallness of the effect for the larger angles 

 of deflexion necessitated short distances of screen and source 

 from the scattering foil. In some experiments the distance 

 between the source and scattering foil was 2*5 cm., and the 

 screen moved in a circle of 1*6 cm. radius, while in other 

 experiments these distances were increased. Observations 

 were taken in various experiments for angles of deflexion 

 from 5° to 150°. When measuring the scattering through 

 large angles the zinc-sulphide screen had to be turned 

 very near to the source, and the ft and <y rays produced a 

 considerable luminescence on it, thus making countings of 

 the scintillations difficult. The effect of the j3 rays was 

 reduced as far as possible by enclosing the source in a lead 

 box shown shaded in the diagram. The amount of lead was, 

 however, limited by considerations of the space taken up by 

 it, and consequently observations could not be made for 

 angles of deflexion between 150° and 180°. 



In the investigation of the scattering through relatively 

 small angles the distances of source and screen from the 

 scattering foil were increased considerably in order to obtain 

 beams of smaller solid angle. 



The number of particles scattered through different angles 

 was found to decrease extremely rapidly with increase of 

 /angle, and as it is not possible to count with certainty more 

 than 90 scintillations per minute or less than about 5 per 

 minute, measurements could only be made over a relatively 

 small range of angles at the same time. The number of 

 a particles scattered through large angles was first measured, 

 and as the emanation decayed it was possible to take mea 

 surements for smaller and smaller angles, and from the 

 known decay of the emanation measurements taken at dif- 

 ferent times could be corrected for the decrease of activity. 



Even when no scattering foil was used a few scintillations 

 were always observed on the screen. They were obviously 

 due to scattered radiation from the walls of the vessel and 

 from the edge of the diaphragm limiting the beam. The 

 effect was reduced as far as possible by lining the box with 

 paper and by using a substance of low atomic weight, viz. 

 aluminium, for the diaphragm. The number of stray a. par- 

 ticles was determined for different positions of the microscope 

 by removing the scattering foil so that the necessary cor- 

 . rections could be applied with certainty. 



In order to make the best use of the emanation available, 



