126 



Dr. J. Robinson on the Photoelectric 



(b) Although the velocities are only given for one film, the 



dissymmetry is not so marked as for the currents. 



(c) The maximum value of the ratio y currents for the 



arc is now smaller than in the preceding experi- 

 ments, and now they agree more nearly with the 

 value given by Stuhlmann. As the apparatus was 

 not altered at all, we must conclude that the films 

 are modified by allowing the spark radiations to fall 

 on them, and in such a way as to diminish the ratio 



Y as measured with the arc as source of light. 



9. Influence of the angle of incidence and the orientation 

 of the plane of polarization of the light on the 

 dissymmetry. 



In the experiments described up to the present only normal 

 incidence of light was used. If the apparatus described is 

 used for oblique incidence, then more attention ought to be 

 directed to the influence of reflected light. Because of this, 

 only measurements of the photo-currents were made, and no 

 attention paid to the velocities. The walls of the vessel were 

 blackened and kept at a potential of + 60 volts. To polarize 

 the ultra-violet light, Prof. W. M. Hicks kindly placed a 

 calcite polarizer at my disposal. 



The actual magnitudes of the currents were found to 

 increase slightly as the angle of incidence was increased up to 

 45°, whether total light or light polarized parallel to or per- 

 pendicular to the plane of incidence was used. On the 

 other hand, it was not found possible to establish an influence 

 of the plane of polarization on the dissymmetry, as Table Y. 

 shows. The variations are small and irregular, so that they 

 are most probably due to experimental errors. 



Table V. 



Thickness 



of 



Film. 



Angle 



of 



Incidence. 



Emergent 

 Incident 



Currents. 











E|| 



E_L 



<10" 7 cm. ... 



0° 



1-14 



1-15 





32° 



112 



1-145 



>10~ 7 cm, ... 



0° 



0-85 



0-83 





32° 



0-84 



0-85 



