U N" RAYS IN SOLAR RADIATION 25 



placed, containing a phosphorescent substance, 

 say calcium sulphide, which has been pre- 

 viously exposed for a short time to solar rays. 

 If, now, on the path of the solar rays, which are 

 supposed to reach the tube through the wood, 

 a sheet of lead, or the hand simply, is inter- 

 posed, even at a great distance from the tube, 

 the phosphorescent glow is seen to diminish ; 

 when the obstacle is removed, the glow re- 

 appears. The extreme simplicity of this experi- 

 ment will incite many persons, I hope, to 

 repeat it. The only precaution one need take 

 is to operate with a feeble preliminary phos- 

 phorescence (note 9). It is advantageous to 

 arrange permanently a sheet of black paper, 

 so that the interposition of the screen does not 

 change the background on which the tube 

 stands out. The variations in glow are especi- 

 ally easy to catch near the contours of the 

 luminous patch formed by the phosphorescent 

 body on the dark background ; when the " N " 

 rays are intercepted, these contours lose their 

 sharpness, regaining the same when the screen 

 is removed. However, these variations in glow 

 do not appear to be instantaneous. Interposing 



