ACTION OF HEAT AND 'Sv" RAYS 75 



tangent plane. Hence we are in a position 

 to distinguish between the effects produced 

 on phosphorescence by heat on the one hand, 

 and by " N " rays, sound-waves, magnetic and 

 electric fields on the other. 



The following is another case in which the 

 effects are different. Take a rectangular card- 

 board screen, 5 cms. high and 1 2 cms. long, for 

 instance, coated very uniformly with calcium 

 sulphide, and rendered very feebly phosphor- 

 escent. If the temperature of a portion of 

 the screen is raised, this part becomes more 

 luminous than the rest. If, instead of this, 

 we let fall on one half of the screen a pencil 

 of " N " rays, proceeding, for example, from a 

 Nernst lamp, we find no sensible increase in 

 its glow ; but if in front of this half-screen 

 a small opaque object is placed, for instance, 

 a small key or a bit of metal foil, cut off 

 by daylight, this is seen to come out very 

 strongly on the luminous background, while if 

 it is placed on the half not receiving the " N " 

 rays, its outline is vague and indeterminate, 

 and seems even to disappear at times. By 

 shifting slowly the object on the screen, its 



