CH. iv.] CATHODE RAYS 27 



Such a * something' might be a bullet from a gun, 

 which is quite invisible when looked at sideways, but 

 may produce a flash of flame when it strikes a target, 

 or may do other damage. So it is with these cathode 

 rays : the region of their flight is the dark space ; the 

 boundaries of that space, where the projectiles strike, 

 are illuminated. A substance with phosphorescent 

 power, such as many minerals, or even glass, phos- 

 phoresces brightly ; and the path of the rays can 

 be traced by smearing a sheet of mica with some 

 phosphorescent powder and placing it edgeways 

 along their path. In this way it can be shown 

 that the rays are like particles travelling definitely 

 in straight lines, not colliding against each other, 

 but each shot like bullets from an immense number 

 of parallel guns. Where they strike the sides of 

 the glass they make it phosphoresce; where they 

 strike residual air in the tube, as they do if the 

 exhaustion is not high enough, they make it phos- 

 phoresce also, and give, in fact, the ordinary glow 

 surrounding the dark space. 



These rays possess a considerable amount of 

 energy, as can be shown by concentrating them, by 

 means of a curved saucer-shaped cathode, and 

 bringing them to a focus. The rays can be brought 

 to a focus in consequence qf the fact that they 

 are projected from the cathode initially normal to 

 its surface, though the focus is further from the 

 cathode than the centre of curvature because of 

 something equivalent to mutual repulsion of the 

 rays. A piece of platinum put at that focus will 

 (if the exhaustion is not too high) show evident 

 signs of being red-hot that is to say, will emit 

 light. If the exhaustion proceeds further, less heat 

 is produced, though a phosphorescent light is 



