﻿Resonance 
  Spectra 
  of 
  Iodine. 
  245 
  

  

  In 
  discussing 
  the 
  manner 
  in 
  which 
  the 
  groups 
  are 
  formed 
  

   hj 
  multiplex 
  excitation, 
  we 
  must 
  recall 
  that 
  in 
  the 
  case 
  of 
  

   strictlv 
  monochromatic 
  excitation, 
  where 
  a 
  single 
  absorption 
  

   line 
  only 
  is 
  stimulated, 
  we 
  have 
  a 
  series 
  of 
  doublets, 
  the 
  

   shorter 
  wave-length 
  component 
  of 
  the 
  first 
  doublet 
  coinciding 
  

   with 
  the 
  absorption 
  line. 
  

  

  It 
  has 
  been 
  found 
  that 
  the 
  doublets 
  conform 
  very 
  nearly 
  

   to 
  the 
  following 
  formula, 
  in 
  which 
  l/\ 
  represents 
  the 
  

   frequency 
  of 
  the 
  left-hand 
  component 
  of 
  the 
  doublet 
  of 
  

   order 
  m, 
  

  

  ^ 
  = 
  183075- 
  2132m 
  +— 
  !? 
  ^ 
  ) 
  13, 
  

  

  or, 
  putting 
  it 
  in 
  words, 
  that 
  (approximately) 
  the 
  distance 
  

   between 
  the 
  doublets 
  increases 
  by 
  a 
  constant 
  amount 
  as 
  we 
  

   pass 
  from 
  each 
  one 
  to 
  the 
  one 
  of 
  next 
  higher 
  order. 
  The 
  

   degree 
  of 
  accuracy 
  with 
  which 
  this 
  formula 
  is 
  followed 
  will 
  

   be 
  discussed 
  in 
  the 
  communication 
  following 
  this 
  one. 
  The 
  

   circumstance 
  that 
  we 
  have 
  a 
  group 
  of 
  lines 
  formed 
  around 
  

   the 
  (unresolved) 
  absorption 
  lines 
  which 
  are 
  excited 
  by 
  the 
  

   broadened 
  mercury 
  line, 
  furnishes 
  us 
  with 
  the 
  clue 
  as 
  to 
  how 
  

   the 
  groups 
  originate. 
  

  

  These 
  groups 
  originate 
  in 
  the 
  following 
  way 
  : 
  The 
  seven 
  

   absorption 
  lines 
  which 
  are 
  covered 
  by 
  the 
  broadened 
  green 
  

   mercury 
  line 
  are 
  simultaneously 
  excited, 
  and 
  the 
  vapour 
  

   emits 
  these 
  seven 
  wave-lengths 
  withont 
  change. 
  These 
  lines 
  

   we 
  may 
  call 
  the 
  R.R. 
  lines 
  (resonance 
  radiation). 
  Each 
  one 
  

   of 
  these 
  is 
  moreover 
  the 
  first 
  member 
  of 
  a 
  series 
  such 
  as 
  is 
  

   expressed 
  by 
  the 
  formula 
  previously 
  given. 
  The 
  R.R. 
  lines 
  

   are 
  not 
  resolved 
  by 
  the 
  spectrograph 
  employed 
  in 
  photo- 
  

   graphing 
  the 
  resonance 
  spectra 
  and 
  consequently 
  appear 
  

   superposed. 
  But 
  each 
  one 
  is 
  accompanied 
  by 
  one 
  or 
  more 
  

   companion 
  lines, 
  lying 
  to 
  the 
  right 
  or 
  left, 
  and 
  it 
  is 
  these 
  

   companion 
  lines 
  which 
  form 
  the 
  group 
  of 
  order. 
  

  

  The 
  actual 
  width 
  of 
  the 
  group 
  of 
  seven 
  R.R. 
  Hues 
  is 
  

   only 
  about 
  1/30 
  of 
  the 
  width 
  of 
  the 
  group 
  formed 
  by 
  the 
  

   companion 
  lines. 
  

  

  Let 
  ns 
  now 
  see 
  how 
  the 
  groups 
  of 
  higher 
  order 
  are 
  built 
  

   up. 
  Suppose 
  each 
  of 
  the 
  seven 
  R.R. 
  lines 
  to 
  be 
  the 
  first 
  

   member 
  of 
  a 
  series 
  such 
  as 
  was 
  represented 
  by 
  our 
  formula, 
  

   and 
  suppose 
  that 
  for 
  e;ich 
  one 
  we 
  have 
  the 
  same 
  values 
  

   of 
  the 
  constants. 
  Suppose 
  moreover 
  that 
  each 
  member 
  of 
  

   any 
  given 
  series 
  is 
  accompanied 
  by 
  a 
  companion 
  line. 
  In 
  

   this 
  case 
  the 
  group 
  of 
  order 
  will 
  be 
  exactly 
  duplicated 
  at 
  

   intervals 
  along 
  the 
  spectrum. 
  The 
  centre 
  of 
  each 
  group 
  will 
  

   be 
  composed 
  of 
  seven 
  superposed 
  lines 
  (in 
  reality 
  separated 
  

  

  