﻿Resonance 
  Spectra 
  of 
  Iodine. 
  241 
  

  

  photographing 
  the 
  doublets 
  as 
  far 
  as 
  the 
  27th 
  order, 
  with 
  a 
  

   large 
  plane 
  grating 
  and 
  a 
  Cooke 
  lens 
  of 
  one 
  metre 
  focus. 
  

   This 
  permits 
  of 
  wave-length 
  determinations 
  correct 
  to 
  about 
  

   0-1 
  Au. 
  

  

  This 
  marks 
  the 
  end 
  of 
  the 
  resonance 
  spectrum, 
  I 
  believe, 
  

   as 
  the 
  wave-length 
  of 
  the 
  last 
  doublet 
  recorded 
  on 
  the 
  plate 
  

   was 
  7685, 
  and 
  the 
  plates 
  are 
  highly 
  sensitive 
  to 
  well 
  beyond 
  

   8500. 
  Moreover, 
  the 
  absorption 
  spectrum 
  terminates 
  at 
  

   about 
  this 
  point. 
  

  

  Photographs 
  of 
  the 
  doublets 
  are 
  reproduced 
  on 
  Plate 
  VI. 
  

   Fig. 
  e 
  shows 
  the 
  orders 
  0, 
  +1, 
  and 
  +3. 
  The 
  doublet 
  of 
  

   order 
  +2 
  is 
  missing, 
  though 
  a 
  pair 
  of 
  faint 
  lines 
  appear 
  

   nearly 
  in 
  the 
  position 
  in 
  which 
  it 
  should 
  be 
  found. 
  

  

  Fig. 
  b 
  shows 
  the 
  doublets 
  + 
  6 
  to 
  +13 
  inclusive, 
  and 
  fig. 
  c 
  

   + 
  15 
  to 
  +22 
  inclusive; 
  orders 
  9, 
  14, 
  16, 
  19, 
  and 
  21 
  are 
  also 
  

   missing. 
  The 
  variable 
  intensity 
  of 
  the 
  doublets 
  is 
  also 
  to 
  be 
  

   noted. 
  

  

  Comparison 
  Spectrum 
  Neon 
  Short 
  Line. 
  

  

  The 
  law 
  governing 
  the 
  spacing 
  of 
  these 
  doublets 
  will 
  be 
  

   discussed 
  in 
  a 
  separate 
  communication 
  immediately 
  follow- 
  

   ing 
  the 
  present 
  one, 
  in 
  which 
  only 
  the 
  general 
  nature 
  of 
  the 
  

   phenomena 
  will 
  be 
  treated. 
  

  

  Fig. 
  d 
  shows 
  the 
  doublets 
  of 
  order 
  0, 
  +1, 
  and 
  +3 
  taken 
  

   with 
  a 
  large 
  plane 
  grating 
  and 
  an 
  objective 
  of 
  3 
  metres 
  focus 
  

   (exposure 
  15 
  minutes), 
  in 
  superposition 
  with 
  the 
  emission 
  

   band 
  spectrum 
  of 
  iodine 
  electrically 
  excited 
  in 
  a 
  vacuum- 
  

   tube. 
  All 
  of 
  the 
  photographs, 
  with 
  the 
  exception 
  of 
  fig. 
  a, 
  are 
  

   reproduced 
  as 
  negatives. 
  The 
  resolving 
  power 
  employed 
  in 
  

   the 
  case 
  of 
  d 
  was, 
  of 
  course, 
  quite 
  insufficient 
  to 
  completely 
  

   resolve 
  the 
  band 
  spectrum, 
  as 
  can 
  be 
  seen 
  by 
  comparing 
  the 
  

   width 
  of 
  the 
  doublets 
  with 
  the 
  width 
  indicated 
  in 
  fig. 
  a. 
  It 
  

   nevertheless 
  gives 
  an 
  idea 
  of 
  the 
  relation 
  of 
  the 
  doublets 
  to 
  

   the 
  baud 
  absorption 
  spectrum. 
  

  

  If 
  we 
  give 
  a 
  longer 
  exposure, 
  we 
  find 
  that 
  the 
  doublets 
  

   are 
  accompanied 
  by 
  faint 
  companion 
  lines. 
  These 
  appear 
  in 
  

   fig. 
  e, 
  which 
  was 
  exposed 
  for 
  an 
  hour 
  and 
  a 
  quarter. 
  Some 
  

   of 
  these 
  lines 
  are 
  due 
  to 
  the 
  excitation 
  of 
  other 
  iodine 
  absorp- 
  

   tion 
  lines 
  by 
  the 
  satellites 
  of 
  the 
  green 
  mercury 
  line, 
  but 
  

   orhers, 
  I 
  feel 
  sure, 
  result 
  from 
  the 
  stimulation 
  of 
  the 
  absorp- 
  

   tion 
  line 
  covered 
  by 
  the 
  main 
  line. 
  The 
  former 
  come 
  out 
  

   strong 
  when 
  the 
  iodine 
  is 
  excited 
  by 
  the 
  quartz 
  mercury 
  arc, 
  

   in 
  which 
  case 
  the 
  green 
  line 
  can 
  be 
  broadened 
  until 
  it 
  covers 
  

   all 
  of 
  the 
  seven 
  absorption 
  lines 
  between 
  the 
  two 
  arrows 
  in 
  

   fig. 
  a. 
  

  

  If 
  now 
  we 
  give 
  a 
  greatly 
  prolonged 
  exposure, 
  we 
  find 
  that 
  

  

  