Straggling of a Particles. 223 



of view of the microscope to determine the number of 

 a particles which impinged on that area. 



The main difficulty of the method consists in obtaining 

 plates capable of development without showing under the 

 microscope a number of blackened grains, even when the 

 plate has not been exposed to light or any radioactive source; 

 but it was found that this difficulty could be overcome by 

 using Schumann plates, which possess the additional advan- 

 tage of having such thin sensitive films that an a particle 

 striking normally or at a small angle to the normal usually 

 meets only a single silver grain in its passage through the 

 film. Moreover, when viewed even under a high-power 

 microscope, all the silver grains, being nearly in one plane, 

 are simultaneously in focus, a circumstance which greatly 

 facilitates counting. To obtain accurate results the grains 

 affected by the a particles must not be too close together, 

 and it is important not to over-develop the plates, for if the 

 silver grains become too large, several neighbouring grains 

 may be fused into a single large particle, and if this takes 

 place the counting becomes inaccurate. 



Fig. 1 (a). 



Fig. 1 (6). 



N 



W 

 B 



The experimental arrangement used is shown in fig. 1 (a) 

 and (7>), drawn in plan and section. The thin platinum wire 

 W, rendered active by exposure for several hours to radium 

 emanation and freed from adhering emanation in the usual 

 way by heating, was passed through holes bored in the two 



!■ 





