CH. V.] 



OF CATHODE RAYS 



43 



value for this ratio, and the cathode ray velocity 

 comes out something quite appropriate to atoms 

 of matter. 



This, however, is a trap. These accidental coin- 

 cidences may retard progress in a most serious 

 manner, for they satisfy the mind and deter people 

 from investigation. It is almost impossible to be 



FIG. 3. Modified Perrin apparatus adopted by J. J. Thomson for, 

 measuring the charge, and at the same time the magnetic deflexion, and 

 sometimes the thermal energy also, of cathode rays. The rays from the 

 cathode, after passing through a perforated anode and proceeding in a 

 straight line, can be deflected by a magnet a measured amount, so as 

 just to enter a hole in an earthed guard-screen D, and then a hollow 

 cavity provided with an electrode E, whereby the aggregate charge 

 conveyed by them is measured. 



completely on guard against them, and they are 

 usually accepted until a more thorough qualitative 

 acquaintance with the subject leads to an instinctive 

 feeling that something is wrong somewhere. 



So it was in this case : the long free path and the 

 penetrating power of the cathode rays kept insisting 

 that the particles were not really atoms of ordinary 

 matter, a truth which both Lenard and Crookes 

 had instinctively grasped, in spite of much criticism 



