232 Prof. J. A. McClelland on 



Effect of the Position of the Plate. 



With the apparatus described it is easy to detect a secondary 

 radiation which travels in all directions from the part of the 

 plate struck by the primary rays. The first point investigated 

 was whether the amount of secondary radiation depended on 

 the angle of incidence of the primary rays, and on the angle 

 that the testing-tube T made with the plate ; whether, in fact, 

 there was anything of the nature of " reflexion" of the pri- 

 mary rays. This is a point which has been investigated by 

 several observers using cathode rays as the primary rays; and 

 while some observers have found a well-marked maximum of 

 secondary radiation in the direction obeying the law of 

 reflexion, others have not obtained such a result. 



The point was first tested by keeping the direction of the 

 primary rays and that of the tube T constant and at right 

 angles, as in the figure, while the lead plate P was tilted ; 

 and this experiment showed a well-marked maximum when 6 

 was 45°. 



The following observations were made, the secondary 

 radiation being expressed in an arbitrary scale. 



Secondare Radiation. 

 = 221° 73 



= 45° 100 



6 = 6U° 71 



The observed numbers are reduced so as to express the 

 maximum by 100. 



The plate P was then set so that </> was 45°, and this angle 

 was kept constant, the direction of the primary rays only being 

 changed. We then get as follows : — 



Secondary Radiation. 



= 25° 93 



(9=45° 100 



6 = 65° 97 



= 80° 92 



In the third case, the primary rays and the plate are both 

 tilted, keeping the angle between them constant and equal to 

 45°, while the angle <£ changes. We have then : — 



Secondary Radiation. 



<£ = 20° 84 



<£ = 45° 100 



</> = 70° 98 



The first experiment shows a decided maximum when 



