﻿Spectra 
  of 
  Iodine 
  under 
  High 
  Dispersion. 
  837 
  

  

  The 
  groups 
  excited 
  by 
  the 
  5461 
  line 
  are 
  recorded 
  in 
  fig. 
  2 
  

   (p. 
  834) 
  diagrammatically, 
  the 
  distances 
  between 
  the 
  groups 
  

   being 
  reduced 
  somewhat 
  to 
  save 
  space. 
  The 
  spectrum, 
  

   excited 
  by 
  the 
  lamp 
  operating 
  at. 
  140 
  volts, 
  is 
  identical 
  with 
  

   the 
  photograph 
  reproduced 
  on 
  PI. 
  XY. 
  fig. 
  4. 
  The 
  exact 
  

   wave-lengths 
  of 
  the 
  lines 
  will 
  be 
  given 
  in 
  a 
  table 
  presently. 
  

  

  1 
  have 
  numbered 
  the 
  lines 
  in 
  the 
  characteristic 
  groups 
  for 
  

   the 
  purpose 
  of 
  identification, 
  and 
  the 
  chart 
  shows 
  the 
  groups 
  

   as 
  they 
  appear 
  in 
  the 
  photographs 
  made 
  in 
  the 
  first 
  order 
  

   spectrum 
  of 
  the 
  large 
  grating. 
  The 
  " 
  intensities 
  " 
  are 
  

   represented 
  by 
  the 
  lengths 
  of 
  the 
  lines. 
  Their 
  appearance 
  

   under 
  the 
  higher 
  resolving 
  power 
  of 
  the 
  fourth 
  order 
  will 
  be 
  

   discussed 
  later 
  on. 
  

  

  We 
  will 
  begin 
  by 
  comparing 
  the 
  resonance 
  spectrum 
  

   excited 
  by 
  the 
  Cooper-Hewitt 
  lamp, 
  and 
  the 
  quartz 
  lamp 
  

   operating 
  at 
  35 
  volts, 
  which 
  we 
  should 
  expect 
  to 
  be 
  

   identical, 
  for 
  the 
  vapour 
  density 
  in 
  the 
  lamps 
  is 
  about 
  the 
  

   same. 
  This 
  is 
  far 
  from 
  being 
  the 
  case, 
  however. 
  The 
  first 
  

   order 
  group 
  in 
  the 
  case 
  of 
  the 
  C.-H. 
  lamp 
  is 
  made 
  up 
  of 
  the 
  

   "core 
  w 
  line 
  5525 
  with 
  a 
  fainter 
  companion 
  to 
  the 
  right, 
  and 
  

   a 
  still 
  fainter 
  one 
  far 
  off 
  to 
  the 
  left, 
  which 
  I 
  have 
  identified 
  

   with 
  line 
  1 
  of 
  the 
  complete 
  group 
  as 
  it 
  appears 
  in 
  the 
  

   spectrum 
  excited 
  by 
  the 
  lamp 
  at 
  140 
  volts. 
  In 
  the 
  case 
  of 
  

   the 
  quartz 
  lamp 
  at 
  35 
  volts 
  the 
  group 
  consists 
  of 
  two 
  lines 
  of 
  

   nearly 
  equal 
  intensity, 
  with 
  no 
  trace 
  of 
  line 
  1. 
  This 
  puzzled 
  

   me 
  for 
  a 
  long 
  time, 
  but 
  1 
  believe 
  that 
  I 
  have 
  found 
  the 
  

   explanation. 
  In 
  the 
  case 
  of 
  excitation 
  by 
  the 
  C.-H. 
  lamp, 
  

   the 
  light 
  is 
  emitted 
  by 
  a 
  long 
  column 
  of 
  iodine 
  vapour, 
  and 
  

   is, 
  in 
  consequence, 
  subjected 
  to 
  the 
  absorbing 
  power 
  of 
  the 
  

   vapour. 
  Now 
  control 
  experiments 
  showed 
  that 
  the 
  right- 
  

   hand 
  member 
  of 
  this 
  doublet 
  was 
  much 
  more 
  strongly 
  ab- 
  

   sorbed 
  by 
  iodine 
  vapour 
  than 
  the 
  left-hand 
  member. 
  The 
  

   experiment 
  was 
  made 
  in 
  the 
  following 
  way. 
  An 
  iodine 
  tube, 
  

   half 
  a 
  metre 
  long, 
  was 
  mounted 
  in 
  front 
  of 
  the 
  slit 
  of 
  the 
  

   spectroscope 
  with 
  its 
  axis 
  parallel 
  to 
  the 
  axis 
  of 
  the 
  collimator 
  

   in 
  the 
  usual 
  manner. 
  The 
  image 
  of 
  the 
  quartz 
  mercury 
  arc 
  

   was 
  focussed 
  first 
  at 
  the 
  end 
  of 
  the 
  tube 
  nearest 
  the 
  slit, 
  and 
  

   then 
  at 
  the 
  remote 
  end. 
  In 
  the 
  latter 
  case 
  the 
  light 
  from 
  the 
  

   luminous 
  vapour 
  was 
  obliged 
  to 
  traverse 
  a 
  considerable 
  

   thickness 
  of 
  iodine 
  vapour 
  of 
  the 
  same 
  density 
  as 
  the 
  radiating 
  

   vapour. 
  The 
  two 
  photographs 
  are 
  reproduced 
  on 
  PL 
  XV. 
  fig. 
  tf, 
  

   the 
  lower 
  spectrum 
  being 
  made 
  with 
  iodine 
  absorption, 
  the 
  

   upper 
  without. 
  All 
  other 
  conditions 
  were 
  the 
  same, 
  yet 
  we 
  

   find 
  that 
  only 
  the 
  left-hand 
  member 
  of 
  the 
  strong 
  middle 
  

   group 
  of 
  lines 
  in 
  the 
  first 
  order 
  group 
  remains, 
  while 
  the 
  

  

  