106 Prof. A. LI. Hughes on the Velocities of 



which they are emitted? If the selective electrons were 

 slower than the normal electrons, but had identical direction 

 distributions, then the smaller potentials would suffice to 

 gather the same fraction of the total number of electrons 

 emitted from the alloy into the electrode E. On the other 

 hand, if the velocity distributions in both effects were the 

 same, but if the concentration of electrons along directions 

 near the normal were greater in the selective effect than in the 

 normal effect, then again it would require smaller potentials 

 to gather in a given fraction of the selective electrons than of 

 the normal electrons. Hence we cannot decide from these 

 experiments whether the curves imply a difference between 

 the velocity distributions in the two cases, or in the direction 

 distributions. To find out exactly the origin of the difference 

 between the curves in fig. 4 would require a more elaborate 

 experiment. It will probably be necessary to investigate 

 both the direction distributions of the electrons and also 

 their velocity distributions along each direction. The 

 maximum emission velocity could be measured by noting 

 the least potential applied to the alloy which would keep the 

 electrode E from receiving any electrons. Owing to the 

 arrangement of the electrode E relative to the illuminated 

 alloy, the apparatus was very insensitive for the test. A 

 difference in the maximum energies of '15 volt could have 

 been detected ; no difference could be observed in these 

 experiments. Richardson and Compton, Kadesch, and 

 Millikan, using unpolarized light, found that the maximum 

 emission energy of the photo-electrons was a linear function 

 of the frequency. Thus there is no indirect evidence from 

 these results of any departure in the selective effect from the 

 law obeyed by the normal effect. 



Experiments such as those made by Pohl and Pringsheim 

 on the numbers of electrons liberated from the alkali metals 

 by polarized light point to a fundamental difference between 

 the two effects. The appearance of the curve for the selective 

 effect suggests strongly that we have to deal with some kind 

 of a resonance effect, and that each alkali metal contains 

 electronic systems whose frequency is characteristic of the 

 metal. These systems become unstable the more readily, 

 the closer the frequency of the incident light to this character- 

 istic frequency. The electrons associated with the normal 

 effect are not related to any system possessing a frequency 

 characteristic of the metal. Thus, investigations of the 

 number of electrons emitted in the selective and in the normal 

 effects suggest different origins for the two effects. But 

 there is no evidence from the various experiments on the 



