116 



Dr. J. Robinson on the Photoelectric 



film was altered, i. e. for thin films the emergent velocities 

 and currents were larger than the incident velocities and 

 currents, and vice versa for thick films. It was also found 

 that the dissymmetry for the velocities was not so marked as 

 for the currents. 



2. The present investigation was undertaken to get more 

 knowledge of these effects. The points investigated fall 

 under the four heads : — 



(a) There is a certain thickness of film for which the ratio 



E . . . E 



Y currents = unity, and also a thickness for which -p 



velocities = unity. It was desired to find whether 

 for the same source of light these two thicknesses of 

 film are the same. 



(b) The variation of the actual magnitude of the photo- 



current (incident and emergent) was investigated 

 for different thicknesses of film. 



(c) Whether the dissymmetry is a function of the actual 



magnitude of the velocity of the emitted electrons. 



(d) Whether the angle of incidence and the orientation of 



the plane of polarization of the light influences the 

 dissymmetry. 



3. Apparatus. 



The apparatus used was similar to that described in the 

 forementioned paper. Modifications were introduced to 

 make still more sure that the effect of reflected light was 

 reduced to a minimum, and that the film was uniform. 



Fig. 1. 



- ^rnrn-jy 



{ir^j: 



M 



These improvements consisted in making the tube KK 

 (fig. 1) wider (7 cm. diameter), the inlet for the light 

 narrower, and the platinum electrode G larger. As before 



