Electrons emitted in Photo- Electric Effects. 107 



velocities of the electrons for the view that the two effects 

 are fundamentally different. If then, for a moment, we 

 assume that the two effects are ultimately identical, we must 

 look for some intermediate effect which can account for the 

 great difference in the number of electrons emitted. If, 

 over a certain range of wave-lengths, the light polarized in 

 the Ell plane were absorbed with unusual rapidity in the 

 surface layers, then we should expect an unusually large 

 emission of electrons, for they would have a greater oppor- 

 tunity of emerging than if they bad been released in the 

 deeper layers. The optical properties of the alkali metals in 

 the region of the selective effect are unknown, and therefore 

 there is no evidence to justify this conjecture. We cannot 

 say that, in the case of those metals whose optical properties 

 have been investigated in some detail, there is any evidence 

 to support such a view as advanced above, but then these 

 metals exhibit only the normal photo-electric effect. 



Summary. 



Although the number of the photo-electrons emitted in 

 the selective photo-electric effect of the sodium-potassium 

 alloy used in these experiments is twenty-six times as great 

 as the number emitted in the normal photo-electric effect, 

 yet the way in which the two sets of electrons are emitted 

 differs only to a small extent. This small difference may 

 arise either from a somewhat smaller average velocity of the 

 photo-electrons emitted in the selective effect, or else from a 

 greater proportion of the selective photo-electrons being 

 emitted in directions closer to the normal to the surface. 

 These results on the velocities of the photo-electrons in the 

 selective and normal effects are hardly to be expected, if one 

 concludes that the great difference in the magnitude of the 

 two effects, and also their relation to the frequency of the 

 light used, indicates a fundamental difference between the 

 two effects. 



This investigation was carried out in the Palmer Physical 

 Laboratory of Princeton University. I wish to express my 

 best thanks to Professor Magie for permission to work in the 

 laboratory and for placing every facility at my disposal. 



The Rice Institute, 

 Houston, Texas, 

 Oct. 28, 1915. 



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