﻿240 
  Prof. 
  P. 
  W. 
  Wood 
  on 
  

  

  A 
  portion 
  of 
  this 
  spectrum, 
  in 
  the 
  vicinity 
  of 
  the 
  green 
  mer- 
  

   cury 
  line, 
  reproduced 
  from 
  an 
  earlier 
  paper, 
  is 
  shown 
  by 
  

   fig. 
  a, 
  Plate 
  VI. 
  The 
  entire 
  spectrum, 
  reproduced 
  on 
  the 
  

   same 
  scale, 
  would 
  he 
  about 
  80 
  metres 
  in 
  length. 
  In 
  the 
  

   yellow, 
  orange, 
  and 
  red 
  regions 
  the 
  lines 
  form 
  fluted 
  bands, 
  

   or 
  rather 
  series 
  of 
  overlapping 
  bands. 
  In 
  the 
  green 
  region 
  

   there 
  appears 
  to 
  be 
  so 
  much 
  superposition 
  of 
  bands 
  that 
  all 
  

   appearance 
  of 
  regularity 
  vanishes. 
  A 
  good 
  idea 
  of 
  the 
  

   general 
  appearance 
  of 
  this 
  spectrum 
  in 
  the 
  region 
  5460 
  — 
  

   5700 
  is 
  given 
  by 
  fig. 
  d, 
  Plate 
  VI. 
  This 
  is 
  in 
  reality 
  the 
  

   emission 
  spectrum 
  of 
  iodine 
  in 
  a 
  vacuum-tube, 
  reproduced 
  as 
  

   a 
  negative, 
  and 
  with 
  the 
  dispersion 
  employed 
  could 
  scarcely 
  

   be 
  distinguished 
  from 
  the 
  absorption 
  spectrum, 
  except 
  for 
  

   the 
  strong 
  dark 
  lines, 
  which 
  are 
  iodine 
  emission 
  lines 
  not 
  

   belonging 
  to 
  the 
  band 
  emission. 
  

  

  With 
  this 
  as 
  an 
  introduction, 
  we 
  will 
  now 
  take 
  up 
  the 
  

   remarkable 
  spectrum 
  emitted 
  by 
  the 
  iodine 
  when 
  illuminated 
  

   by 
  the 
  green 
  line 
  of 
  the 
  Cooper-Hewitt 
  lamp. 
  This 
  line 
  

   is 
  shown 
  in 
  coincidence 
  with 
  the 
  absorption 
  spectrum 
  in 
  

   fig. 
  a, 
  Plate 
  VI. 
  As 
  will 
  be 
  seen, 
  the 
  main 
  line 
  falls 
  nearly 
  

   midway 
  between 
  two 
  of 
  the 
  iodine 
  absorption 
  lines. 
  It 
  is 
  

   probable 
  that 
  only 
  the 
  left-hand 
  absorption 
  line 
  is 
  stimulated, 
  

   as 
  the 
  width 
  of 
  the 
  mercury 
  line 
  is 
  not 
  quite 
  sufficient 
  to 
  

   enable 
  it 
  to 
  reach 
  the 
  other. 
  The 
  shortwave-length 
  satellite 
  

   is 
  also 
  in 
  coincidence 
  with 
  an 
  absorption 
  line, 
  but, 
  for 
  the 
  

   present, 
  we 
  shall 
  neglect 
  the 
  effect 
  due 
  to 
  this. 
  The 
  reso- 
  

   nance 
  spectrum 
  excited 
  by 
  the 
  stimulation 
  of 
  this 
  absorption 
  

   line 
  consists 
  of 
  a 
  series 
  of 
  close 
  doublets 
  (doublet 
  separation 
  

   about 
  1*50 
  AIT.) 
  very 
  regularly 
  spaced 
  along 
  the 
  spectrum. 
  

   For 
  convenience 
  we 
  will 
  designate, 
  as 
  before, 
  the 
  one 
  in 
  co- 
  

   incidence 
  with 
  the 
  exciting 
  line 
  as 
  the 
  doublet 
  of 
  order, 
  

   those 
  lying 
  on 
  the 
  long 
  wave-length 
  side 
  as 
  + 
  1, 
  + 
  2, 
  +3, 
  

   &c, 
  orders, 
  and 
  those 
  on 
  the 
  other 
  side 
  as 
  —1, 
  —2, 
  &c. 
  

  

  The 
  doublet 
  of 
  order 
  is 
  indicated 
  on 
  fig. 
  a 
  (Plate 
  VI.) 
  

   immediately 
  above 
  the 
  absorption 
  spectrum. 
  One 
  line 
  

   (5460*74) 
  is 
  in 
  coincidence 
  with 
  the 
  iodine 
  absorption 
  line 
  

   covered 
  by 
  the 
  mercury 
  line, 
  the 
  other 
  (5462*25) 
  lies 
  1*5 
  

   AU. 
  to 
  the 
  right. 
  The 
  former 
  is 
  in 
  reality 
  a 
  re-emission 
  of 
  

   the 
  absorbed 
  energy 
  without 
  change 
  of 
  wave-length 
  (Reso- 
  

   nance 
  radiation), 
  and 
  I 
  have 
  accordingly 
  named 
  this 
  line 
  

   the 
  R.B. 
  line. 
  The 
  other 
  line 
  we 
  may 
  designate 
  the 
  com- 
  

   panion 
  line. 
  On 
  the 
  scale 
  of 
  fig. 
  1 
  the 
  next 
  doublet 
  ( 
  + 
  1 
  

   order) 
  would 
  lie 
  on 
  the 
  right 
  at 
  a 
  distance 
  of 
  nearly 
  two 
  

   metres. 
  

  

  By 
  means 
  of 
  plates 
  sensitized 
  with 
  dicyanine, 
  which 
  were 
  

   prepared 
  for 
  me 
  by 
  Mr. 
  Meggers, 
  I 
  have 
  succeeded 
  in 
  

  

  