﻿836 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  Resonance' 
  

  

  sodium 
  I 
  find 
  numerous 
  oases 
  analogous 
  to 
  the 
  ones 
  enu- 
  

   merated, 
  although 
  the 
  groups 
  are 
  not 
  as 
  complicated 
  as 
  in 
  

   the 
  case 
  of 
  iodine 
  vapour. 
  The 
  excitation 
  of 
  sodium 
  vapour 
  

   by 
  the 
  5086 
  line 
  of 
  the 
  cadmium 
  silica 
  lamp 
  gives 
  rise 
  to 
  a 
  

   series 
  of 
  doublets, 
  the 
  components 
  being 
  about 
  6 
  Angstrom 
  

   units 
  apart. 
  One 
  of 
  these 
  is 
  found 
  at 
  the 
  point 
  of 
  excitation 
  

   (5086) 
  ; 
  in 
  other 
  words, 
  we 
  have 
  the 
  R.R. 
  line 
  (5086) 
  witli 
  a 
  

   strong 
  companion 
  6 
  A. 
  U. 
  to 
  the 
  right 
  of 
  it, 
  and 
  the 
  doublet 
  

   thus 
  formed 
  is 
  repeated 
  at 
  regular 
  intervals 
  along 
  the 
  

   spectrum. 
  

  

  We 
  will 
  now 
  take 
  up 
  the 
  study 
  of 
  the 
  groups 
  excited 
  by 
  

   the 
  green 
  mercury 
  line. 
  The 
  problem 
  is 
  to 
  ascertain 
  the 
  

   corresponding 
  lines 
  of 
  the 
  various 
  groups 
  and 
  to 
  determine 
  

   which 
  of 
  the 
  iodine 
  absorption 
  lines 
  are 
  associated 
  with 
  them. 
  

   This 
  was 
  accomplished 
  by 
  operating 
  the 
  quartz 
  arc 
  under 
  

   different 
  conditions, 
  as 
  I 
  have 
  said 
  before. 
  On 
  PI. 
  XV 
  L 
  

   figs. 
  5, 
  6, 
  7, 
  and 
  8, 
  we 
  have 
  photographs 
  of 
  the 
  iodine 
  

   absorption 
  spectrum 
  in 
  coincidence 
  with 
  the 
  green 
  mercury 
  

   line, 
  with 
  voltage 
  drops 
  across 
  the 
  terminals 
  of 
  the 
  exciting 
  

   lamp 
  of 
  45, 
  60, 
  110, 
  and 
  90 
  volts. 
  These 
  were 
  made 
  for 
  me 
  

   by 
  Dr. 
  Anderson 
  at 
  the 
  Mt. 
  Wilson 
  Observatory. 
  I 
  have 
  

   numbered 
  the 
  iodine 
  absorption 
  lines 
  on 
  fij£. 
  5 
  as 
  in 
  my 
  

   previous 
  paper. 
  Another 
  method 
  of 
  studying 
  the 
  relation 
  

   between 
  the 
  exciting 
  line 
  and 
  the 
  absorption 
  lines 
  is 
  to 
  

   photograph 
  the 
  emission 
  line 
  of 
  the 
  quartz 
  arc 
  alone 
  and 
  

   after 
  the 
  light 
  has 
  traversed 
  a 
  bulb 
  of 
  iodine 
  vapour. 
  Fig. 
  4 
  

   shows 
  (upper 
  spectrum) 
  the 
  green 
  line 
  with 
  a 
  voltage 
  drop 
  

   of 
  45, 
  and 
  (lower 
  spectrum) 
  the 
  same 
  line 
  after 
  suffering 
  

   absorption 
  by 
  iodine 
  vapour. 
  This 
  is 
  of 
  interest 
  as 
  showing 
  

   how 
  an 
  exciting 
  line 
  may 
  have 
  its 
  character 
  altered 
  by 
  a 
  

   ray-filter 
  such 
  as 
  bromine. 
  In 
  fig. 
  6 
  the 
  green 
  line 
  below 
  

   the 
  absorption 
  spectrum 
  has 
  been 
  passed 
  through 
  iodine 
  

   vapour. 
  

  

  In 
  the 
  first 
  place 
  we 
  observe 
  (fig. 
  5) 
  that 
  there 
  is 
  a 
  trace 
  

   of 
  reversal 
  even 
  at 
  this 
  low 
  voltage, 
  and 
  that 
  the 
  centre 
  of 
  

   the 
  line 
  falls 
  between 
  absorption 
  lines 
  3 
  and 
  4, 
  lying 
  rather 
  

   nearer 
  the 
  former. 
  During 
  the 
  present 
  summer 
  I 
  have 
  

   photographed 
  the 
  green 
  line 
  of 
  the 
  Cooper-Hewitt 
  glass 
  

   lamp 
  in 
  coincidence 
  with 
  the 
  iodine 
  absorption 
  with 
  my 
  

   42-foot 
  East 
  Hampton 
  spectrograph 
  which 
  has 
  been 
  put 
  

   underground 
  and 
  will 
  be 
  described 
  elsewhere 
  (Astrophysical 
  

   Journal), 
  The 
  line 
  in 
  this 
  case 
  shows 
  no 
  trace 
  of 
  reversal 
  

   and 
  is 
  almost 
  in 
  coincidence 
  with 
  absorption 
  line 
  3. 
  I 
  doubt 
  

   if, 
  in 
  this 
  case, 
  line 
  4 
  receives 
  any 
  excitation. 
  The 
  photo- 
  

   graphs 
  reproduced 
  are 
  but 
  a 
  small 
  part 
  of 
  the 
  total 
  number 
  

   taken. 
  

  

  