804 Dr. Norman Campbell on 



is greater than 4000 volts. My own experiments (10, 11, 

 12) have shown that there is a similar independence of the 

 velocity o£ the 8 rays on the nature o£ the primary, when 

 the primary rays are a rays or /3 rays excited by X rays. 

 The same observations and those of (4) and (5) also show 

 that the velocity of the 8 rays is independent (or nearly 

 independent) of the metal on which the primary rays act. 



Absolute measurements of the velocity with which the 

 8 rays leave a metal plate have been made by many observers 

 under very different conditions, and have given very various 

 results. Much of the apparent disagreement can be 

 attributed to differences in the form of the apparatus. 

 However, almost all observers agree that the greater part of 

 the 8 rays leave a metal plate with velocities less than 

 10 volts, while some small proportion of them have a velocity 

 as great as 30 volts. The only experiments which cannot 

 be reconciled immediately with this statement are those of 

 (3), which led to the conclusion that the majority of the rays 

 had velocities between 27 and 30 volts ; I think it would be 

 agreed that the method of these experiments is not so reliable 

 as that of the others. Lenard states (2) that the velocity of 

 the secondary rays is not greater than ] 1 volts, but an 

 examination of the observations on which that statement 

 is based shows that they support it only if it is made con- 

 cerning the greater portion of the rays and not concerning 

 all of them. 



4. When the velocity of the primary rays is so small that 

 it is comparable with that of the 8 rays, a change in the 

 nature of the latter is to be expected. The experiments (4) 

 and (5) on the rays liberated by such very slow ft rays have 

 led to the following conclusions : — 



ft rays of which the velocity is less than 11 volts liberate 

 no 8 rays. Electrons may leave the surface of a metal on 

 which such rays fall, but they are to be regarded as a 

 " reflected " primary rather than as a secondary radiation. 



The number of 8 rays liberated by a given number of 

 primary rays is a maximum when the speed of the latter is 

 about 250 volts. 



The velocity of the 8 rays is independent of the metal 

 from which they are liberated and approximately independent 

 of the velocity of the primary, so long as this velocity is not 

 less than 80 volts. It appears, however, from Gehrts' 

 results that there is some variation of the velocities of the 

 8 rays with that of the primary, even when the latter is 

 greater than 80 volts. Table 1. gives, for various values 

 of V x and V 2 , the proportion of 8 rays leaving the plate 



