422 



Dr. J. Robinson on the Photoelectric 



vessel thus acts nearly as a black body, and very little light 

 which enters at B can escape again. Thus it is all absorbed 

 somewhere in the vessel. is a platinum film on quartz 

 arranged so that a parallel beam of light entering at B falls 

 on it. Let I be the intensity of light entering at B, and 



Fiar. 1. 



hence falling on C. A certain proportion of this, al , is 

 absorbed by the film, /3I is reflected, and yl is transmitted, 

 where a 4-/8 + 7 = 1. The amount of light (/3 + 7)I is 

 absorbed by the vessel. The amount of light al is that 

 which produces the photo current in the film. We need to 

 find i l9 the photo current from the film, and al , the light 

 absorbed by the film. The photo current ii can be measured 

 by charging the walls of the vessel to a potential to drive 

 the electrons away from the film. 



Suppose that I is kept constant, and with a mercury- 

 vapour lamp it can be kept practically constant. Then 

 a~l — (/3 + ry) 5 where (J3 + y)I is the amount of light ab- 

 sorbed by the walls of the vessel . These walls remain constant 

 throughout a series of experiments, and if the photo current 

 from the walls is measured by applying a potential to drive 

 'electrons to the film, this current is proportional to the 

 amount of light absorbed by the walls, and hence to (/3 -f- «y ) . 

 Thus a can be obtained in terms of the photo current of the 

 walls of the vessel, a itself cannot he absolutely determined 

 in each case unless some means is found for measuring the 

 intensity of the light, or unless another set of experiments 

 is made, so that I can be eliminated from the equations. 



