424 



Dr. J. Robinson on the Photoelectric 



These results are shown graphically in fig. 2. 



100 200 200 



Time of Depceit of Film in Seconds 



Curve A. — Photo current for various thicknesses of film. 

 Curve B. — Photo current from the walls of the vessel. 



It is seen that the current from the film varies with 

 the thickness in the way already demonstrated * ; i. e., it 

 increases gradually with the thickness at first, then it 

 suddenly increases rapidly, after which it falls again. The 

 current from the walls of the vessel diminishes uniformly, 

 and it is proportional to the light absorbed by the vessel. 

 Hence (/3 + 7)I diminishes uniformly as the thickness of 

 film increases, and hence al c the light absorbed by the film 

 increases uniformly with the thickness. Producing the 

 curve for the current from the vessel backwards, we find 

 the current that would have been obtained had no light been 

 absorbed by the film. This value is 57 in the units employed, 

 and this gives an arbitrary measure of the intensity of light 

 I on entering the vessel. Curve A (fig. 3) shows how the 

 light absorbed by the film varies with the thickness. It 

 was found by subtracting the ordinates of the curve B (fig. 2) 

 from 57. 



It is at once obvious that the large photo current at the 

 thickness 10" 7 cm. is not produced by an abnormal absorption 

 of light. 



In Table II. values for the current from the film and light 

 * Robinson, loc. Git. 



