118 Dr. J. Robinson on the Photoelectric 



and the steady deflexion read. On certain occasions this high 

 resistance was dispensed with, and the leak measured directly. 

 As source of light a quartz mercury lamp was used. For 

 some experiments a spark was also used. 



4. Corrections j ? or Velocity Measurements. 



Some attention has recently been given to accurate 

 measurements of the photoelectric velocities. Hughes * has 

 shown that by distilling metals in vacuum, velocities can be 

 measured accurately, and quite free from the errors intro- 

 duced by surface films. For the purposes of the present 

 investigation, the method for obtaining films, that of 

 sputtering, is preferable to the distillation method, for it is 

 easier to estimate the thickness of sputtered films, and 

 possibly they may be of more uniform thickness over the 

 whole of the quartz plate than if they had been distilled. 

 An objection might be raised to the method of sputtering 

 because the electric discharge may cause some kind of 

 polarization of the film. Hughes t refers to such an 

 influence of the discharge on thick electrodes, where it is 

 possible to increase the maximum potential attained under 

 exposure to light by using the electrode as a cathode in a 

 discharge for a short time. 



In the present experiments consistent values for the 

 maximum potential were obtained for any one film. The 

 velocities generally varied from film to film, but with very 

 few exceptions they ranged from 2*3 to 2' 9 volts. These do 

 not differ much from the velocities given by Hughes for 

 most of the metals that he distilled, and as he proved that 

 the distillation method gave films free from gas layers, we 

 are justified in concluding that the present method achieves 

 the same result. 



It is not in the scope of the present work to investigate 

 the actual distribution of velocities, although the apparatus 

 was designed in such a way as to make this possible if 

 necessary. A disturbing factor in such investigations, and 

 as was suggested by V. Baeyer f also in velocity measure- 

 ments, is the reflexion of electrons from the walls of the 

 vessel. If this has to be taken into account, we must know 

 how the emission of photoelectrons depends on the angle of 

 emission. Hughes § came to the conclusion that the 

 emission is the same for all angles, but this is not in 

 harmony with some measurements made by the writer on 



* Phil. Trans. A, vol. 212. p. 205 (1912). 

 t Proc. Camb. Phil. Soc. xvi. p. 107 (1911). 

 I Verh. d. deutsch. Phys. Ges. x. p. 96 (1908). 

 § Loc. cit. 



