806 Dr. Norman Campbell on 



smallest value obtained by v. Baeyer and Gehrts for this 

 velocity is 25 volts; by no modification of the apparatus was 

 it found possible to obtain a smaller value. Similar evidence 

 would be obtained if positively charged particles were found 

 leaving the plate on which the primary rays fell; such 

 particles were found to be present when the velocity of the 

 primary rays exceeded 30 volts, but never when it was smaller 

 than that value. (It is almost impossible to tell whether the 

 ionization giving rise to these positive particles takes place 

 at the surface of the metal plate or at a layer of gas adhering 

 to it, or in the trace of gas left when the vessel is evacuated 

 to the highest possible degree.) 



Thus no evidence could be obtained which threw doubt on 

 the conclusion, based on other experiments, that primary 

 rays of a velocity less than 11 volts do not cause ionization 

 or liberate 8 rays. 



(2) If primary rays of any speed greater than 11 volt-s- 

 can liberate 8 rays and the 8 rays produced by fast primary 

 rays have speeds as great as 30 volts, and if these speeds 

 are independent of that of the primary, it is possible that 

 primary rays with speeds between 11 and 30 volts might 

 liberate 8 rays faster than themselves. Can any evidence be 

 found of such rays ? 



Several times in the course of the investigation it was- 

 thought that the presence of 8 rays faster than the primary 

 had been detected, but the evidence always disappeared on 

 further investigation. (See § 7.) It appears to me quite 

 certain that the 8 or reflected rays never have a speed greater - 

 than that of the primary. 



(3) Can any evidence be found of reflected electrons when-; 

 the speed of the primary is considerably greater than that 

 required to produce 8 rays ? 



A search was made for 8 rays having a speed between 

 30 volts and that of the primary rays when the speed of the 

 latter exceeded 40 volts. The presence of the positively 

 charged particles made it very difficult to be sure that there 

 were no such rays, but no evidence whatever of their 

 existence could be detected*. The observations which have 

 been made on the scattering of very fast rays suggest that 

 there must be some electrons returned from the plate with 

 velocities near to that of the primary, but the proportion of 

 such rays scattered must be very small compared with that 



* It may be noted that, if there are no such rays, the explanation 

 given on p. 533 of (11) of a curious phenomenon noted by Pound and 

 myself is inadequate; the phenomenon is sufficiently explained by 

 Bumstead's secondary ,3 radiation excited by a rays. 



