Properties of Thin Films of Platinum. 131 



that as the thickness of nickel increases up to about 8 wave- 

 lengths, the incident photo-current increases. It attains its 

 maximum value when all the light is absorbed. 



Another estimate of the maximum depth for platinum at 

 which Light can produce a photoelectric effect can be obtained 

 from measurements of the emergent current. We find by 

 extrapolation in fig. 2 that the emergent current would be 

 zero at about 5 x 10 -6 cm. This means that the light can 

 produce electrons only to a depth less than 5xl0 -6 cm. 

 This estimate of the maximum depth to which light can 

 produce electrons is much smaller than that of Ladenburg, 

 who found it to be 8 wave-lengths, or of the order of 

 10 ~ 4 cm. This discrepancy may be due to the fact that the 

 estimates are given for different metals. 



13. As to the causes of the dissymmetrical effect it is very 

 difficult to suggest a satisfactory explanation. As more elec- 

 trons emerge in the direction of the light, and as they have 

 a larger velocity in this direction, there must be some in- 

 fluence at work in the actual process of causing electrons to 

 leave the molecules, which tends to make them emerge more 

 readily in the direction of the light. If this were not the 

 •case, and if electrons emerge from molecules equally in all 

 directions, then we should be left with the task of explaining 

 the dissymmetry by the pressure of light on the electrons 

 after they have emerged from the molecules. Such a con- 

 sideration does not appear to account for the magnitude of 

 the dissymmetry. 



It may be asked why the emergent velocity diminishes as 

 the thickness of the film increases. All the films used were 

 semitransparent. Hence some light gets right through the 

 films, and as these films absorb all wave-lengths uniformly * 

 some ultra-violet light also gets right through. If the 

 general view is correct that the velocity of photoelectrons 

 is independent of the intensity of the light used, then we 

 ought not to expect the emergent velocity to diminish so 

 lon|>- as some light goes right through the films. As the 

 emergent velocity does diminish we may account for it by 

 some assumption as : — 



(a) Perhaps light must have a certain minimum intensity 



to produce electrons ; 



(b) Possibly the velocity of the electrons is a function of 



the intensity of (he light ; 



* Kobiuson, Phil. Mag. April 1912, p. 549. 

 K2 



