684: Dr. A. LI. Huohes on the Velocities with which 



tend to make li too small. I think, however, that the 

 deviations are larger than the possible errors of experiment, 

 and moreover, I found that there was a small but quite 

 regular variation in h' with the position of the metal in the 

 periodic table. 



The reconciliation between the experimental and theoretical 

 results is perhaps to be sought in another direction. In the 

 above experiments the maximum velocities of photo-electrons 

 emitted from the same side of the plate as the incident light 

 were investigated. Robinson * has lately studied the velo- 

 cities of photo-electrons from both sides of a thin transparent 

 film of platinum placed perpendicularly to the light. He 

 finds that the velocities are greater on the emergent side than 

 on the incident side. In the figure, curve A s;ives the ratio 



I-30-' 



Ratuo 



IZO- 

 HO'"- 



too- 



of the numbers of photo- electrons emitted on either side, and 

 curve B gives the ratio of the corresponding maximum 

 velocities measured in volts. One may surmise that, perhaps, 

 all the electrons are emitted originally from the molecules 

 more or less in the direction of the light, and that subsequent 

 scattering by collisions leads to the emission in all directions. 

 On producing curve B backwards, we find it cuts the ordinate 

 through the origin at 1*12 in such a way as to indicate that 

 this is the limiting ratio of the emergent to the incident 

 energies, for a film of zero thickness. We may regard this 

 as indicating that the energy lost by a photo-electron in 

 swinging round from its original direction of emission is at 

 least 11 per cent. Now Richardson and Oompton found li 

 for platinum to be 5*85 X 10~ 27 by a direct method and 5*60 

 by an indirect method, which they considered to give rather 

 more accurate results. Increasing these values by 12 per 

 cent, we get 6"55 x 10~ 27 and 6*27 x 10 27 , which are in good 

 agreement with Planck's constant, G'55 x 10~ 27 erg sec. It 



* Kobinson, Phil. Mag. xxv. p. 115 (1913). 



