Effect caused by Incident and Emergent Light. 857 



This method avoids any difficulties which might be caused by 

 fluctuations in the illumination. 



The absorption of the thinnest films became a negligible 



quantity because the ratio — L , when plotted against the 



2 



thickness of the films, approached a limiting value as the 



thickness was indefinitely diminished. Hence the limiting 



B 



value corresponds to the case in which the value ~ arises 



2 



from a true difference in the ionization produced by emergent 



and incident light of equal intensity. 



Data. 



The metals investigated for the difference in the photo- 

 electric effect caused by the incident and the emergent light 

 include magnesium, copper, zinc, silver, tin, platinum *, lead, 

 and iron. Ln addition selenium, arsenic, and carbon gave 

 unsatisfactory results. Selenium could not be sputtered in 

 vacuo owing to its low melting-point. The cathode became 

 distorted, in consequence of heating by the current, before any 

 satisfactory films could be deposited. Carbon gave very 

 satisfactory films of uniform texture, but its photo-electric 

 activities are so small that the results became worthless 

 owing to the fact that the magnitude of the results was never 

 greater than the experimental error. 



The above metals were chemically pure and were supplied 

 by Kahlbaum, Berlin, except silver which was commercial 

 " Fine Silver/' and was supplied by a local firm. 



It was desirable wherever possible to test such metals as 

 exhibited the most diverse chemical and physical properties. 

 Here one is limited by the possibility of sputtering, rapidity 

 of oxidation, lack of photo-electric response and rapidity of 

 fatigue. j> -g 



The values of the ratios — l and -~ obtained for a series of 



films of different thicknesses and for the metals cited above, 

 are given in the following tables. The third column headed 



^ gives the ratio of ~ to ~ or the ratio of the emergent 



B2 A. A 



photo-electric effect to the incident effect at the same film. 



B . 



The last column gives the values of log -~, which is pro- 

 portional to the optical thickness of the film plus that of the 

 quartz plate used, as was shown in the first paper on this 



subject. 



* For data on platinum sec pt. i. loc. cit. 



