﻿560 
  Mr. 
  K. 
  T. 
  Compton 
  and 
  Prof. 
  0. 
  W. 
  Richardson 
  on 
  

  

  Therefore 
  we 
  may 
  neglect 
  all 
  of 
  the 
  resistance 
  except 
  R, 
  

   and 
  write 
  

  

  1=5 
  

  

  R' 
  

  

  The 
  drop 
  across 
  S 
  was 
  

  

  d 
  - 
  is 
  - 
  n 
  -r' 
  

  

  where 
  K 
  is 
  a 
  constant. 
  When 
  the 
  resistance 
  R 
  was 
  ad- 
  

   justed 
  so 
  that 
  no 
  current 
  passed 
  through 
  the 
  galvanometer, 
  

   obviously 
  

  

  K_ 
  

  

  where 
  e 
  is 
  the 
  electromotive 
  force 
  generated 
  by 
  the 
  thermo- 
  

   pile. 
  Thus 
  the 
  electromotive 
  force 
  was 
  inversely 
  propor- 
  

   tional 
  to 
  the 
  resistance 
  R. 
  But 
  the 
  electromotive 
  force 
  is 
  

   proportional 
  to 
  the 
  rise 
  in 
  temperature, 
  which 
  in 
  turn 
  is 
  

   proportional 
  to 
  the 
  energy 
  of 
  the 
  incident 
  light. 
  Therefore 
  

   the 
  reciprocal 
  of 
  the 
  resistance 
  required 
  for 
  compensation 
  

   was 
  a 
  measure 
  of 
  the 
  energy 
  of 
  the 
  light 
  incident 
  on 
  the 
  strip. 
  

  

  By 
  this 
  method 
  the 
  galvanometer 
  was 
  a 
  detector 
  only, 
  and 
  

   the 
  measurements 
  were 
  independent 
  of 
  any 
  changes 
  in 
  its 
  

   -sensitiveness. 
  For 
  measurements 
  in 
  the 
  extreme 
  ultra-violet 
  

   between 
  wave-lengths 
  200 
  fifju 
  and 
  250 
  ji/j, 
  the 
  sensitiveness 
  

   of 
  the 
  galvanometer 
  was 
  about 
  5 
  x 
  10~ 
  n 
  ampere 
  per 
  division. 
  

   In 
  the 
  rest 
  of 
  the 
  spectrum 
  it 
  was 
  unnecessary 
  to 
  maintain 
  

   such 
  high 
  sensitiveness. 
  

  

  The 
  calibration 
  of 
  the 
  spectrum 
  was 
  repeated 
  at 
  intervals 
  

   during 
  the 
  course 
  of 
  the 
  investigation. 
  It 
  was 
  found 
  that, 
  

   for 
  a 
  given 
  number 
  of 
  amperes 
  running 
  through 
  the 
  arc- 
  

   lamp, 
  the 
  energy 
  distribution 
  in 
  the 
  spectrum 
  remained 
  

   practically 
  constant 
  during 
  the 
  period 
  of 
  the 
  three 
  months' 
  

   investigation. 
  The 
  slight 
  variations 
  that 
  occurred 
  were 
  

   allowed 
  for. 
  

  

  There 
  is 
  one 
  peculiarity 
  of 
  the 
  apparatus 
  which 
  should 
  be 
  

   mentioned 
  before 
  taking 
  up 
  the 
  experimental 
  results. 
  The 
  

   collimator 
  slit, 
  the 
  metal 
  strip, 
  and 
  the 
  slit 
  in 
  front 
  of 
  the 
  

   thermopile 
  were 
  each 
  T 
  ] 
  ^ 
  inch 
  wide. 
  This 
  large 
  width 
  re- 
  

   sulted 
  in 
  considerable 
  overlapping 
  and 
  impurity 
  in 
  the 
  

   visible 
  part 
  of 
  the 
  spectrum 
  from 
  wave-lengths 
  400 
  /jl/jl 
  to 
  

   700 
  fjbfjb. 
  But 
  out 
  in 
  the 
  ultra-violet 
  the 
  dispersive 
  power 
  of 
  

   the 
  instrument 
  was 
  about 
  twenty 
  times 
  as 
  great 
  as 
  in 
  the 
  

   visible 
  spectrum, 
  so 
  that 
  in 
  this 
  region, 
  which 
  was 
  the 
  most 
  

   important, 
  the 
  error 
  due 
  to 
  the 
  finite 
  sizes 
  of 
  the 
  slits 
  was 
  

   negligible. 
  The 
  reason 
  for 
  making 
  the 
  slits 
  so 
  wide 
  was 
  

   that 
  in 
  the 
  visible 
  spectrum 
  the 
  photoelectric 
  currents 
  were 
  

  

  