﻿242 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  

  

  a 
  band 
  spectrum 
  also 
  appears. 
  Fig. 
  / 
  is 
  a 
  20-hour 
  exposure 
  

   for 
  the 
  same 
  region 
  of 
  the 
  spectrum. 
  The 
  doublets 
  have 
  

   fused 
  to 
  a 
  wide 
  band, 
  owing 
  to 
  over 
  exposure. 
  The 
  com- 
  

   panion 
  lines, 
  above 
  referred 
  to, 
  come 
  out 
  strong, 
  and 
  in 
  addi- 
  

   tion 
  there 
  is 
  a 
  fluted 
  band 
  to 
  the 
  right 
  of 
  the 
  doublets 
  of 
  

   order 
  4- 
  1 
  and 
  + 
  3. 
  

  

  It 
  will 
  be 
  observed 
  that 
  these 
  doublets 
  lie 
  just 
  within 
  the 
  

   heads 
  of 
  the 
  fluted 
  bands, 
  a 
  circumstance 
  which 
  is 
  better 
  

   shown 
  by 
  fig. 
  7i, 
  in 
  which 
  the 
  heads 
  of 
  the 
  bands 
  are 
  indi- 
  

   cated 
  by 
  arrows. 
  In 
  the 
  case 
  of 
  fig. 
  g 
  the 
  iodine 
  tube, 
  

   instead 
  of 
  being 
  highly 
  exhausted, 
  contained 
  xenon 
  at 
  a 
  

   pressure 
  of 
  1*5 
  mm. 
  As 
  is 
  apparent, 
  the 
  effect 
  of 
  the 
  xenon 
  

   is 
  to 
  reduce 
  tremendously 
  the 
  intensity 
  of 
  the 
  doublets, 
  and 
  

   bring 
  out 
  strongly 
  a 
  number 
  of 
  fluted 
  bands 
  between 
  the 
  

   doublets, 
  of 
  which 
  scarcely 
  a 
  trace 
  can 
  be 
  seen 
  in 
  the 
  case 
  of 
  

   iodine 
  in 
  vacuo. 
  In 
  the 
  case 
  of 
  fig. 
  h, 
  we 
  have 
  the 
  ioHine 
  in 
  

   helium 
  at 
  4 
  mm. 
  The 
  doublets 
  are 
  still 
  further 
  reduced 
  in 
  

   intensity, 
  the 
  bands 
  are 
  stronger, 
  and 
  a 
  new 
  band 
  appears 
  at 
  

   the 
  centre, 
  no 
  trace 
  of 
  which 
  can 
  be 
  seen 
  in 
  fig. 
  g. 
  The 
  heads 
  

   of 
  the 
  bands 
  are 
  not 
  resolved, 
  though 
  on 
  the 
  original 
  plate 
  a 
  

   number 
  of 
  the 
  component 
  lines 
  can 
  be 
  seen 
  to 
  the 
  left 
  of 
  the 
  

   doublets. 
  The 
  doublet 
  of 
  the 
  second 
  order, 
  which 
  is 
  missing, 
  

   would 
  fall 
  at 
  a 
  considerable 
  distance 
  from 
  the 
  head 
  of 
  the 
  

   band. 
  There 
  is 
  in 
  fact 
  a 
  group 
  of 
  lines 
  at 
  this 
  point 
  in 
  

   fig. 
  /, 
  but 
  it 
  is 
  my 
  opinion 
  that 
  they 
  result 
  from 
  excitation 
  

   of 
  the 
  vapour 
  by 
  some 
  of 
  the 
  satellite 
  lines 
  ; 
  at 
  all 
  events, 
  

   none 
  of 
  them 
  fits 
  into 
  the 
  series 
  of 
  doublets 
  excited 
  by 
  the 
  

   main 
  line. 
  

  

  If 
  we 
  compare 
  fig. 
  h 
  with 
  fig. 
  d, 
  we 
  see 
  at 
  once 
  that 
  the 
  

   band 
  spectrum 
  emitted 
  by 
  iodine 
  in 
  helium 
  with 
  monochro- 
  

   matic 
  excitation 
  is 
  much 
  simpler 
  than 
  the 
  complete 
  band 
  

   spectrum. 
  For 
  example, 
  there 
  is 
  in 
  fig. 
  d 
  a 
  strong 
  band-head 
  

   at 
  A, 
  of 
  which 
  no 
  trace 
  appears 
  in 
  fig. 
  h. 
  Moreover, 
  fewer 
  

   of 
  the 
  bands 
  appear 
  in 
  the 
  case 
  of 
  iodine 
  in 
  vacuo 
  than 
  in 
  the 
  

   case 
  of 
  iodine 
  in 
  helium. 
  

  

  If 
  the 
  excitation 
  is 
  by 
  the 
  quartz 
  mercury 
  arc 
  the 
  bands 
  

   become 
  more 
  complicated, 
  and 
  in 
  place 
  of 
  the 
  doublets 
  we 
  

   have 
  groups 
  of 
  lines, 
  which 
  will 
  be 
  discussed 
  more 
  in 
  detail 
  

   presently. 
  

  

  Relation 
  between 
  the 
  Doublets 
  and 
  the 
  Band 
  Spectrum. 
  

  

  The 
  absorption 
  spectrum 
  of 
  iodine 
  is 
  made 
  up 
  of 
  more 
  or 
  

   less 
  regular 
  fluted 
  bands, 
  resolvable 
  under 
  high 
  dispersion 
  

   into 
  fine 
  lines. 
  The 
  heads 
  of 
  these 
  bands 
  lie 
  towards 
  the 
  

   region 
  of 
  shorter 
  wave-lengths, 
  and 
  there 
  is 
  considerable 
  

  

  