﻿Spectra 
  of 
  Iodine 
  under 
  High 
  Dispersion. 
  831 
  

  

  excitation 
  of 
  the 
  fluorescence 
  in 
  the 
  long 
  tube 
  by 
  the 
  Cooper- 
  

   Hewitt 
  lamp, 
  the 
  light 
  from 
  the 
  more 
  remote 
  portions 
  of 
  the 
  

   tube 
  is 
  obliged 
  to 
  traverse 
  a 
  considerable 
  thickness 
  of 
  iodine 
  

   vapour, 
  and 
  the 
  intensity 
  distribution 
  amongst 
  the 
  lines 
  of 
  

   the 
  various 
  groups 
  may 
  be 
  modified 
  by 
  absorption. 
  It 
  was 
  

   found, 
  as 
  a 
  matter 
  of 
  fact, 
  that 
  this 
  was 
  the 
  case, 
  which 
  at 
  

   once 
  explained 
  why 
  the 
  resonance 
  spectrum 
  excited 
  by 
  the 
  

   Cooper-Hewitt 
  lamp 
  appeared 
  to 
  be 
  quite 
  different 
  from 
  that 
  

   excited 
  by 
  a 
  quartz 
  lamp 
  running 
  with 
  a 
  large 
  resistance 
  

   in 
  circuit 
  with 
  it. 
  

  

  In 
  the 
  case 
  of 
  excitation 
  by 
  the 
  quartz 
  lamp, 
  the 
  image 
  of 
  

   which 
  was 
  focussed 
  along 
  the 
  axis 
  of 
  the 
  iodine 
  tube 
  by 
  

   means 
  of 
  a 
  large 
  condeuser, 
  the 
  emitted 
  light 
  had 
  but 
  a 
  short 
  

   path 
  to 
  traverse 
  iu 
  iodine 
  vapour 
  ; 
  consequently 
  absorption 
  

   played 
  a 
  lesser 
  role, 
  but 
  to 
  still 
  further 
  remove 
  its 
  influence 
  

   a 
  portion 
  of 
  the 
  tube 
  was 
  cooled 
  with 
  ice 
  and 
  salt, 
  which 
  

   reduced 
  the 
  density 
  of 
  the 
  iodine 
  vapour 
  about 
  tenfold. 
  

   A 
  new 
  phenomenon 
  was 
  at 
  once 
  discovered, 
  namely, 
  that 
  

   under 
  these 
  conditions 
  the 
  band 
  spectrum, 
  which, 
  as 
  I 
  have 
  

   shown, 
  is 
  strongly 
  developed 
  if 
  the 
  iodine 
  is 
  in 
  helium 
  at 
  a 
  

   pressure 
  of 
  a 
  few 
  millimetres, 
  is 
  quite 
  strongly 
  developed 
  

   even 
  when 
  the 
  iodine 
  is 
  in 
  a 
  high 
  vacuum. 
  This 
  matter 
  will 
  

   be 
  more 
  fully 
  discussed 
  later 
  on. 
  

  

  3. 
  Photography 
  of 
  the 
  Spectra. 
  — 
  The 
  spectra 
  excited 
  by 
  

   the 
  green 
  and 
  the 
  two 
  yellow 
  lines 
  of 
  mercury 
  have 
  been 
  

   photographed 
  both 
  together 
  and 
  separately 
  in 
  the 
  first 
  order 
  

   spectrum 
  of 
  the 
  large 
  grating, 
  by 
  the 
  use 
  of 
  ray 
  filters 
  of 
  

   neodymium 
  or 
  cosine 
  interposed 
  between 
  the 
  mercury 
  lamp 
  

   and 
  the 
  iodine 
  tube, 
  and 
  the 
  more 
  interesting 
  groups 
  of 
  

   resonance 
  lines 
  have 
  been 
  taken 
  in 
  the 
  fourth 
  order 
  spectrum 
  

   of 
  the 
  same 
  grating, 
  which 
  showed 
  that 
  they 
  were 
  more 
  

   complicated 
  than 
  w 
  T 
  as 
  at 
  first 
  supposed, 
  and 
  explained 
  the 
  

   peculiar 
  behaviour 
  of 
  some 
  of 
  the 
  lines 
  when 
  the 
  exciting- 
  

   lamp 
  was 
  ..operated 
  under 
  different 
  conditions. 
  

  

  It 
  is 
  quite 
  impossible 
  to 
  give 
  reproductions 
  of 
  the 
  photo- 
  

   graphs 
  which 
  bring 
  out 
  all 
  of 
  the 
  interesting 
  points 
  owing 
  to 
  

   the 
  enormous 
  difference 
  in 
  the 
  intensities 
  of 
  the 
  lines 
  forming 
  

   the 
  groups. 
  I 
  have 
  given, 
  however, 
  on 
  Plate 
  XV. 
  fig. 
  1 
  a 
  

   reproduction 
  of 
  the 
  complete 
  resonance 
  spectrum 
  excited 
  by 
  

   the 
  large 
  Cooper-Hewitt 
  lamp, 
  with 
  the 
  wave-length 
  of 
  each 
  

   line 
  recorded 
  on 
  a 
  strip 
  below. 
  It 
  appears 
  to 
  me 
  that 
  this 
  

   is 
  preferable 
  to 
  a 
  table 
  of 
  wave-lengths, 
  for 
  the 
  reason 
  that 
  

   there 
  is 
  no 
  question 
  in 
  this 
  case 
  about 
  establishing 
  the 
  

   identity 
  of 
  any 
  given 
  line. 
  This 
  spectrum 
  is 
  emitted 
  by 
  

   the 
  iodine 
  vapour 
  when 
  it 
  is 
  illuminated 
  by 
  the 
  total 
  radiation 
  

   of 
  the 
  mercury 
  lamp, 
  the 
  green 
  and 
  the 
  two 
  yellow 
  lines 
  

  

  