﻿Resonance 
  Spectra 
  of 
  Iodine. 
  239 
  

  

  advantageous 
  if 
  an 
  image 
  is 
  to 
  be 
  projected 
  on 
  the 
  slit 
  of 
  a 
  

   spectroscope. 
  

  

  This 
  is 
  the 
  arrangement 
  which 
  I 
  have 
  used 
  in 
  all 
  of 
  the 
  

   recent 
  work, 
  and 
  besides 
  having 
  a 
  very 
  high 
  efficiency, 
  it 
  is 
  

   easy 
  to 
  construct. 
  

  

  The 
  fluorescence 
  of 
  the 
  iodine 
  is 
  so 
  intense 
  that 
  the 
  

   doublets 
  excited 
  by 
  the 
  green 
  mercury 
  line 
  can 
  be 
  seen 
  widely 
  

   separated 
  in 
  the 
  first 
  order 
  spectrum 
  of 
  a 
  large 
  grating 
  

   with 
  a 
  telescope 
  of 
  three 
  metres 
  focus. 
  In 
  a 
  small 
  prism 
  

   spectroscope 
  the 
  complete 
  resonance 
  spectrum 
  is 
  extremely 
  

   brilliant. 
  

  

  The 
  excitation 
  of 
  the 
  iodine 
  vapour 
  results 
  from 
  the 
  cir- 
  

   cumstance 
  that 
  the 
  green 
  mercury 
  line 
  and 
  the 
  two 
  yellow 
  

   lines 
  coincide 
  with 
  absorption 
  lines 
  of 
  the 
  iodine, 
  of 
  which, 
  

   as 
  I 
  have 
  previously 
  shown, 
  there 
  are 
  between 
  forty 
  and 
  fifty 
  

   thousand 
  in 
  the 
  visible 
  spectrum. 
  

  

  We 
  will 
  consider, 
  first, 
  the 
  resonance 
  spectrum 
  excited 
  by 
  

   the 
  green 
  Hg 
  line. 
  To 
  obtain 
  this 
  pure, 
  it 
  is 
  necessary 
  to 
  

   screen 
  off 
  the 
  light 
  of 
  the 
  two 
  yellow 
  lines. 
  This 
  can 
  be 
  

   done 
  with 
  a 
  solution 
  of 
  neodymium 
  chloride, 
  or 
  the 
  double 
  

   salt 
  of 
  neodymium 
  and 
  ammonium, 
  and 
  as 
  the 
  use 
  of 
  a 
  fluid 
  

   screen 
  is 
  impractical 
  in 
  the 
  case 
  of 
  the 
  method 
  of 
  illumina- 
  

   tion 
  just 
  described, 
  it 
  is 
  necessary 
  in 
  this 
  case 
  to 
  illuminate 
  

   the 
  tube 
  with 
  a 
  large 
  condensing 
  lens. 
  As 
  a 
  matter 
  of 
  fact, 
  

   however, 
  the 
  yellow 
  lines 
  are 
  comparatively 
  feeble 
  in 
  the 
  

   case 
  of 
  the 
  Cooper-Hewitt 
  glass 
  lamp, 
  and 
  the 
  doublets 
  

   excited 
  by 
  the 
  green 
  line 
  are 
  so 
  intense 
  that, 
  in 
  the 
  greater 
  

   part 
  of 
  the 
  work, 
  no 
  screen 
  has 
  been 
  employed. 
  

  

  The 
  more 
  recent 
  investigations 
  have 
  brought 
  some 
  ex- 
  

   tremely 
  interesting 
  phenomena 
  to 
  light, 
  especially 
  with 
  

   respect 
  to 
  the 
  transfer 
  of 
  energy 
  from 
  the 
  doublet 
  series 
  to 
  

   the 
  band 
  spectra, 
  as 
  a 
  result 
  of 
  the 
  admixture 
  of 
  helium 
  or 
  

   other 
  rare 
  gases 
  with 
  the 
  iodine. 
  

  

  On 
  account 
  of 
  the 
  complexity 
  of 
  the 
  subject, 
  it 
  will 
  be 
  

   necessary 
  to 
  touch 
  briefly 
  on 
  some 
  of 
  the 
  relations 
  which 
  

   have 
  been 
  discussed 
  in 
  the 
  earlier 
  papers. 
  

  

  The 
  band 
  absorption 
  spectrum 
  of 
  iodine 
  covers 
  the 
  spectrum 
  

   range 
  comprised 
  between 
  wave-lengths 
  5100 
  and 
  7700. 
  It 
  

   is 
  made 
  up 
  of 
  exceedingly 
  fine 
  lines 
  averaging 
  20 
  to 
  the 
  

   Angstrom 
  unit 
  in 
  the 
  green 
  and 
  yellow 
  regions, 
  or 
  some 
  

   50,000 
  in 
  all 
  making 
  the 
  estimate 
  on 
  the 
  above 
  average. 
  It 
  

   is 
  covered 
  on 
  the 
  short 
  wave-length 
  side 
  by 
  a 
  band 
  of 
  con- 
  

   tinuous 
  absorption 
  in 
  the 
  blue-green 
  region, 
  which 
  makes 
  

   the 
  exact 
  determination 
  of 
  its 
  end 
  impossible. 
  In 
  the 
  red 
  it 
  

   has 
  been 
  followed 
  by 
  means 
  of 
  dicyanine 
  plates 
  sensitive 
  to 
  

   X 
  = 
  9000, 
  and 
  its 
  termination 
  discovered 
  at 
  about 
  \ 
  = 
  7700. 
  

  

  S2 
  

  

  