﻿244 
  Prof. 
  It. 
  W. 
  Wood 
  on 
  

  

  that 
  all 
  of 
  the 
  strong 
  doublets 
  are 
  located 
  near 
  the 
  heads 
  of 
  

   the 
  bands, 
  and 
  the 
  missing 
  ones 
  near 
  the 
  tails. 
  

  

  The 
  doublet 
  of 
  the 
  sixth 
  order 
  is 
  very 
  strong, 
  and 
  it 
  lies 
  

   just 
  within 
  the 
  head 
  of 
  a 
  strong 
  band 
  shown 
  by 
  electrical 
  

   excitation, 
  and 
  the 
  same 
  thing 
  appears 
  to 
  be 
  true 
  of 
  the 
  

   eighth 
  and 
  tenth 
  order 
  doublets. 
  The 
  interesting 
  point, 
  

   however, 
  is 
  that 
  a 
  simple 
  system 
  of 
  fluted 
  bands, 
  spaced 
  

   apparently 
  according 
  to 
  a 
  law 
  similar 
  to 
  that 
  which 
  governs 
  

   the 
  spacing 
  of 
  the 
  doublets, 
  is 
  excited 
  by 
  the 
  stimulation 
  of 
  a 
  

   single 
  absorption 
  line. 
  

  

  Multiplex 
  Excitation. 
  

  

  If, 
  instead 
  of 
  the 
  glass 
  Cooper-Hewitt 
  lamp, 
  we 
  employ 
  a 
  

   quartz 
  mercury 
  arc 
  (Westinghouse, 
  Cooper-Hewitt) 
  for 
  the 
  

   excitation 
  of 
  the 
  iodine 
  vapour, 
  we 
  find 
  complicated 
  groups 
  

   of 
  lines 
  in 
  place 
  of 
  the 
  simple 
  doublets. 
  This 
  is 
  clue 
  to 
  the 
  

   fact 
  that 
  the 
  green 
  mercury 
  line 
  has 
  broadened 
  to 
  such 
  a 
  

   degree 
  that 
  it 
  covers 
  a 
  number 
  of 
  the 
  iodine 
  absorption 
  lines. 
  

   This 
  we 
  may 
  call 
  multiplex 
  excitation. 
  

  

  The 
  first 
  point 
  of 
  interest 
  which 
  we 
  should 
  note 
  is 
  that 
  the 
  

   intensity 
  distribution 
  among 
  the 
  groups 
  is 
  practically 
  the 
  

   same 
  as 
  for 
  the 
  doublets, 
  i. 
  e., 
  groups 
  of 
  strong 
  lines 
  are 
  

   built 
  up 
  around 
  the 
  strong 
  doublets, 
  weak 
  groups 
  around 
  

   weak 
  doublets, 
  and 
  only 
  a 
  few 
  very 
  faint 
  lines 
  at 
  the 
  points 
  

   where 
  the 
  doublets 
  are 
  missing. 
  This 
  means 
  that 
  the 
  

   dynamics 
  of 
  the 
  vibrating 
  system 
  excited 
  is 
  verj 
  r 
  much 
  the 
  

   same 
  in 
  the 
  case 
  of 
  the 
  several 
  absorption 
  lines 
  covered 
  by 
  

   the 
  broadened 
  mercury 
  line. 
  

  

  The 
  complexity 
  of 
  the 
  groups 
  depends 
  upon 
  the 
  width 
  of 
  

   the 
  green 
  line 
  which 
  increases 
  with 
  the 
  potential 
  drop 
  across 
  

   the 
  terminals 
  of 
  the 
  quartz 
  arc, 
  as 
  has 
  been 
  shown 
  in 
  previous 
  

   communications. 
  

  

  If 
  sufficient 
  resistance 
  is 
  put 
  in 
  circuit 
  with 
  the 
  arc 
  to 
  

   keep 
  the 
  potential 
  down 
  to 
  35 
  volts, 
  the 
  iodine 
  emits 
  the 
  

   doublets 
  only, 
  fig.,;, 
  Plate 
  VII. 
  With 
  the 
  potential 
  at 
  60 
  volts 
  

   we 
  have 
  two 
  new 
  lines 
  to 
  the 
  left 
  of 
  the 
  doublets, 
  as 
  shown 
  

   by 
  fig. 
  k, 
  Plate 
  VII., 
  while 
  with 
  a 
  potential 
  difference 
  of 
  

   110 
  volts 
  we 
  have 
  the 
  complicated 
  groups 
  shown 
  by 
  figs. 
  I 
  

   and 
  m, 
  the 
  latter 
  showing 
  the 
  group 
  of 
  —1 
  order. 
  These 
  

   groups 
  are 
  so 
  similar 
  in 
  appearance 
  that, 
  until 
  very 
  recently, 
  

   I 
  have 
  considered 
  that 
  the 
  lines 
  corresponded 
  to 
  each 
  other, 
  

   that 
  is 
  to 
  say, 
  that 
  the 
  fourth 
  line 
  from 
  the 
  left 
  in 
  each 
  group 
  

   was 
  excited 
  by 
  the 
  same 
  absorption 
  line. 
  I 
  now 
  feel 
  certain, 
  

   however, 
  that 
  we 
  must 
  be 
  a 
  little 
  careful 
  about 
  accepting 
  

   this 
  conclusion, 
  for 
  reasons 
  which 
  will 
  appear 
  presently. 
  

  

  