﻿the 
  Photoelectric 
  Effect. 
  

  

  555 
  

  

  of 
  incidence. 
  Therefore 
  we 
  may 
  use 
  their 
  data 
  on 
  the 
  wave- 
  

   length 
  at 
  which 
  the 
  photoelectric 
  emission 
  is 
  a 
  maximum 
  for 
  

   various 
  metals, 
  and 
  compare 
  these 
  results 
  with 
  the 
  require- 
  

   ments 
  of 
  the 
  theory 
  expressed 
  by 
  equation 
  (6), 
  or 
  rather, 
  by 
  

   equation 
  (7). 
  This 
  comparison 
  is 
  shown 
  in 
  Table 
  II. 
  The 
  

   calculated 
  values 
  of 
  X 
  max 
  . 
  are 
  obtained 
  from 
  equation 
  (7), 
  

   taking 
  iv 
  Q 
  for 
  each 
  metal 
  as 
  given 
  by 
  equation 
  (2), 
  taking- 
  

   values 
  of 
  the 
  contact 
  difference 
  of 
  potential 
  with 
  respect 
  

   to 
  platinum 
  from 
  standard 
  tables, 
  and 
  taking 
  for 
  platinum 
  

   M7 
  =4*7 
  volts. 
  The 
  observed 
  values 
  of 
  A 
  max< 
  are 
  taken 
  directly 
  

   from 
  Pohl 
  and 
  Pringsheim's 
  papers. 
  The 
  blank 
  spaces 
  re- 
  

   present 
  cases 
  in 
  which 
  the 
  data 
  are 
  not 
  available 
  and 
  are 
  

   put 
  in 
  because 
  they 
  may 
  be 
  of 
  interest 
  later. 
  

  

  Table 
  II. 
  

  

  Metal. 
  

  

  Xmax. 
  calculated. 
  Amax. 
  observed. 
  

  

  Rb 
  

  

  475 
  mi 
  

   404 
  (ip 
  440 
  

   367 
  335 
  

   360 
  

   280 
  

   280 
  

   260 
  250 
  

   240 
  247-5 
  

   223 
  

   200 
  

   184 
  

   180 
  

  

  K 
  

  

  Na 
  

  

  Ca 
  

  

  Ba 
  

  

  Li 
  

  

  Mo- 
  

  

  Al 
  

  

  Zn 
  

  

  Sn 
  

  

  Ou 
  

  

  Pt 
  

  

  

  Wherever 
  comparison 
  is 
  possible 
  the 
  agreement 
  with 
  the 
  

   theory 
  is 
  quite 
  good. 
  Pohl 
  and 
  Pringsheim 
  did 
  not 
  deter- 
  

   mine 
  points 
  close 
  together 
  on 
  their 
  curves, 
  and 
  the 
  exact 
  

   positions 
  of 
  the 
  maxima 
  are 
  doubtful 
  within 
  limits 
  large 
  

   enough 
  to 
  account 
  for 
  most 
  of 
  the 
  discrepancies 
  in 
  the 
  

   table. 
  

  

  The 
  most 
  obvious 
  discrepancy 
  between 
  the 
  theoretical 
  

   equation 
  and 
  the 
  curves 
  of 
  Pohl 
  and 
  Pringsheim 
  is 
  that 
  the 
  

   sensitiveness 
  in 
  the 
  region 
  of 
  very 
  short 
  wave-lengths 
  is 
  

   increasing 
  as 
  the 
  wave-length 
  decreases, 
  while 
  the 
  theory 
  

   requires 
  that 
  it 
  should 
  finally 
  fall 
  to 
  zero. 
  The 
  further 
  ex- 
  

   perimental 
  test 
  of 
  the 
  theory, 
  as 
  expressed 
  by 
  equation 
  (6), 
  

   which 
  we 
  shall 
  now 
  proceed 
  to 
  describe, 
  differs 
  from 
  the 
  

   work 
  of 
  Pohl 
  and 
  Pringsheim 
  principally 
  in 
  the 
  fact 
  that 
  we 
  

   secure 
  approximate 
  isotropic 
  monochromatic 
  radiation 
  falling 
  

   on 
  the 
  strip, 
  and 
  that 
  we 
  extend 
  the 
  measurements 
  farther 
  

   out 
  into 
  the 
  ultra-violet 
  than 
  has 
  hitherto 
  been 
  done. 
  

  

  