Effect caused by Incident and Emergent Light. 339 



platinum. This was now placed in one arm of a Wheatstone's 

 bridge and its resistance measured. Using a value of the 

 specific resistance for thin platinum films equal to 5 X 10" 5 ohm 

 as found by Patterson *, a value for the thickness was found 

 comparable to *3 x 10" 6 cm. Hence the thinnest films used 

 were evidently of order 1 -7 cm. in thickness. Similar values 

 were found by Patterson for the thinnest films measured 

 by him. 



Experiments on other metals are now in progress which 

 Mill determine the variation of this forward effect with the 

 atomic weight of the element. 



Conclusion. 



The above experiments show that when beams of ultra- 

 violet light of equal intensity are compared, the ionization 

 they produce is greater on the emergent than on the incident 

 side of a thin platinum film. For a film so thin that the 

 absorption of the light in it is negligible, the ratio of the 

 ionization on the emergent to that on the incident side is as 

 1*17 is to unity. There is thus an increase of 17 per cent. 

 in favour of the emergent side of the film. 



An effect of this kind would obviously be expected on any 

 corpuscular theory of light. It can also be explained on an 

 undulatory theory by a process of the nature of light-pressure, 

 which tends to push the electrons forward in the direction 

 in which the light is propagated. In its ordinary electro- 

 magnetic form, however, the undulatory theory does not 

 appear to give rise to effects large enough to explain the 

 phenomena observed. The difficulty appears to be similar 

 to that which arises when Ihe ordinary theory attempts to 

 explain why the ultra-violet light is capable of causing the 

 expulsion of the electrons, with their observed properties, 

 under any circumstances. 



This investigation was suggested by Professor 0. W. 

 Richardson, and I wish to thank him here for valuable 

 suggestions and advice throughout the course of the work. 



Palmer Physical Laboratory, 

 Princeton University. 



* J. Patterson, Phil. Mag. iv. p. 663 (1902). 



