﻿1020 
  Dr. 
  R. 
  Pohl 
  and 
  Dr. 
  P. 
  Pringsheim 
  on 
  the 
  

  

  At 
  all 
  events, 
  however, 
  the 
  measurements 
  thus 
  far 
  made 
  

   indicate 
  a 
  relation 
  of: 
  the 
  form 
  

  

  between 
  the 
  initial 
  velocity 
  of 
  the 
  electrons 
  and 
  the 
  frequency 
  

   of 
  the 
  light. 
  In 
  this 
  equation 
  /is 
  a 
  function 
  which 
  increases 
  

   with 
  its 
  argument 
  according 
  to 
  some 
  arbitrary 
  law, 
  and 
  p, 
  

   entirely 
  independently 
  of 
  the 
  exact 
  form 
  of 
  this 
  function, 
  

   gives, 
  as 
  noted 
  above, 
  the 
  least 
  energy 
  which 
  the 
  electron 
  

   can 
  have 
  and 
  escape 
  from 
  the 
  metal. 
  Einstein, 
  in 
  the 
  paper 
  

   already 
  cited, 
  has 
  pointed 
  out 
  that 
  the 
  quantity^ 
  must 
  be 
  

   intimately 
  connected 
  with 
  the 
  contact 
  potential 
  of 
  the 
  par- 
  

   ticular 
  metal 
  under 
  consideration. 
  In 
  fact, 
  Elster 
  and 
  

   Geitel 
  * 
  many 
  years 
  before 
  proved 
  experimentally 
  that 
  all 
  

   electropositive 
  metals 
  were 
  photoelectrically 
  sensitive 
  far 
  

   into 
  the 
  visible 
  spectrum, 
  that 
  is. 
  for 
  these 
  metals 
  the 
  energy 
  

   of 
  separation, 
  p, 
  of 
  the 
  electrons 
  is 
  relativelv 
  small. 
  Later 
  

   the 
  writers 
  succeeded 
  in 
  showing 
  by 
  quantitative 
  measure- 
  

   ments 
  with 
  spectrally 
  resolved 
  light, 
  that 
  the 
  metals 
  with 
  

   respect 
  to 
  the 
  long-wave 
  limits 
  of 
  their 
  photoelectric 
  sensi- 
  

   bility 
  can 
  be 
  arranged 
  in 
  a 
  series 
  which 
  agrees 
  pretty 
  closely 
  

   with 
  the 
  voltaic 
  contact 
  potential 
  series 
  |. 
  We 
  were 
  also 
  

   able 
  to 
  demonstrate 
  that 
  a 
  chemical 
  combination 
  of 
  K 
  with 
  

   more 
  electronegative 
  elements 
  displaces 
  the 
  long- 
  wave 
  limit 
  

   of 
  the 
  normal 
  effect 
  further 
  toward 
  higher 
  frequencies 
  the 
  

   more 
  electronegative 
  the 
  second 
  component 
  is, 
  and 
  that 
  the 
  

   magnitude 
  of 
  this 
  displacement 
  could 
  be 
  regarded 
  as 
  a 
  

   measure 
  of 
  the 
  chemical 
  affinity 
  J. 
  In 
  this 
  connexion 
  it 
  

   is 
  still 
  uncertain 
  whether 
  it 
  is 
  admissible 
  to 
  speak 
  of 
  a 
  

   sudden 
  well-defined 
  cessation 
  of 
  the 
  effect 
  at 
  a 
  perfectly 
  

   definite 
  wave-length, 
  or 
  whether 
  it 
  were 
  not 
  better 
  to 
  speak 
  

   of 
  a 
  gradual 
  asymptotic 
  approach 
  of 
  the 
  curve 
  to 
  the 
  zero 
  

   axis. 
  In 
  this 
  latter 
  case 
  the 
  observed 
  limit 
  would 
  be, 
  to 
  a 
  

   certain 
  extent 
  at 
  least, 
  dependent 
  on 
  the 
  sensibility 
  of 
  the 
  

   measuring 
  instruments. 
  At 
  all 
  events, 
  however, 
  the 
  recent 
  

   investigations 
  by 
  Joffe 
  § 
  on 
  metal 
  particles 
  in 
  suspension 
  

   seem 
  to 
  point 
  to 
  a 
  very 
  rapid 
  decrease 
  in 
  the 
  photoelectric 
  

   sensitiveness 
  at 
  the 
  limit, 
  since 
  by 
  his 
  method 
  the 
  escape 
  of 
  

   each 
  individual 
  electron 
  is 
  registered. 
  The 
  limit 
  of 
  sensi- 
  

   tiveness, 
  X 
  = 
  380 
  fi/.i, 
  for 
  Zn, 
  as 
  given 
  by 
  him, 
  fits 
  in 
  very 
  

  

  * 
  J. 
  Elster 
  & 
  II. 
  Geitel. 
  Wied. 
  Ann. 
  xliii. 
  p. 
  225 
  (1891). 
  

  

  t 
  R. 
  Pohl 
  & 
  P. 
  Pringsheim, 
  Verh. 
  d. 
  D. 
  Phys. 
  Ges. 
  xiii. 
  p. 
  474 
  (1911). 
  

  

  t 
  R. 
  Pohl 
  & 
  P. 
  Prinovsheim, 
  Verh. 
  d. 
  I). 
  Phys. 
  Ges. 
  xii. 
  p. 
  1039 
  

   (1910). 
  The 
  existence 
  of 
  such 
  a 
  relation 
  was 
  later 
  suggested 
  as 
  probable 
  

   by 
  Mr. 
  Hughes, 
  Phil. 
  Mag. 
  [6] 
  xxiv. 
  p. 
  .°>80 
  (1912). 
  

   ' 
  § 
  A. 
  Joffe, 
  Ber. 
  d. 
  Bayr. 
  Akad. 
  Muncheil, 
  1913, 
  p. 
  19. 
  

  

  