224 



Dr. W. Makower on tk 



thick brass plates, A and B, which were parallel and fixed 

 slightly less than a millimetre apart. The wire was held in 

 position by wax. The apparatus was placed in a magnetic 

 field sufficiently intense to deflect j3 rays, but too weak to 

 deflect a rays appreciably. Thus only a. rays could reach, 

 the Schumann plate P, which was 8*5 cm. long and 1*2 cm. 

 wide and situated 6'2 cm. below the wire. After a suitable 

 exposure the plate was removed and developed, and mounted 

 on a movable microscope stage in such a way that it could 

 be displaced parallel to its length. Cross-wires, suitably 

 placed in the eyepiece of the microscope, defined a definite 

 area within which the number o£ silver grains could be 

 counted, and by altering the position o£ the plate the number 

 o£ a particles reaching different sections of the film could 

 be compared. 



In order to deduce from such observations the variation 

 in the number of a particles with the distance from the 

 screen, it is necessary to apply certain corrections. For 

 consider the active wire W (fig. 2) at a distance WN 

 = 6'2 cm. from the photographic plate. The whole photo- 

 graphic effect must be confined between the points A and B, 



Fio-. 2. 



where WA = WB = the range in air of the a particles from 

 radium C. But quite apart from any stoppage of a particles 

 by air, the density of the silver grains on the photographic 

 plate at any point X will diminish as that point recedes 

 from N, both because WX continually increases and because 

 the obliquity of the rays to the photographic plate changes. 



