﻿S32 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  Resonance 
  

  

  being, 
  however, 
  the 
  only 
  ones 
  involved 
  in 
  the 
  excitation. 
  

   Immediately 
  below 
  (PI. 
  XV. 
  fig. 
  2) 
  is 
  the 
  spectrum 
  excited 
  

   bv 
  the 
  light 
  o£ 
  the 
  quartz 
  lamp 
  operating 
  with 
  a 
  potential 
  drop 
  

   of 
  140 
  volts 
  across 
  the 
  terminals, 
  in 
  which 
  case 
  we 
  have 
  the 
  

   mercury 
  lines 
  so 
  broadened 
  that 
  they 
  cover 
  a 
  number 
  of 
  

   iodine 
  absorption 
  lines. 
  The 
  superposed 
  spectrum 
  o£ 
  

   short 
  lines 
  is 
  that 
  of 
  neon. 
  For 
  the 
  purpose 
  of 
  comparison 
  

   I 
  have 
  mounted 
  the 
  spectrum 
  obtained 
  with 
  the 
  Cooper- 
  

   Hewitt 
  excitation 
  immediately 
  below, 
  and 
  in 
  coincidence 
  

   with 
  it. 
  Both 
  of 
  these 
  photegraphs 
  were 
  made 
  with 
  a 
  large 
  

   plane 
  grating 
  in 
  the 
  first 
  order 
  spectrum 
  and 
  are 
  therefore 
  

   very 
  nearly 
  normal. 
  

  

  The 
  most 
  interesting 
  part 
  of 
  the 
  spectrum, 
  and 
  the 
  one 
  to 
  

   which 
  the 
  most 
  study 
  has 
  been 
  given, 
  is 
  the 
  region 
  between 
  

   the 
  green 
  and 
  the 
  yellow 
  mercury 
  lines. 
  In 
  this 
  region 
  

   ordinary 
  isochromatic 
  plates 
  (Cramer) 
  are 
  extremely 
  sensitive, 
  

   fully 
  eight 
  times 
  as 
  sensitive 
  as 
  spectrum 
  plates, 
  consequently 
  

   the 
  time 
  of 
  exposure 
  is 
  proportionally 
  reduced. 
  Some 
  of 
  the 
  

   plates 
  were 
  given 
  an 
  exposure 
  of 
  only 
  ten 
  or 
  fifteen 
  minutes, 
  

   conssquently 
  many 
  different 
  observations 
  could 
  be 
  made 
  in 
  a 
  

   single 
  day. 
  On 
  the 
  other 
  hand, 
  when 
  using 
  spectrum 
  plates 
  

   and 
  the 
  bromine 
  filter 
  the 
  exposures 
  were 
  often 
  of 
  five 
  days 
  s 
  

   duration, 
  the 
  quartz 
  lamp 
  running 
  day 
  and 
  night 
  without 
  

   attention. 
  

  

  We 
  will 
  begin 
  with 
  a 
  discussion 
  of 
  the 
  spectra 
  excited 
  

   by 
  the 
  green 
  line 
  with 
  the 
  lamp 
  running 
  under 
  varying 
  

   conditions. 
  It 
  will 
  be 
  necessary 
  first 
  to 
  give 
  a 
  general 
  

   statement 
  of 
  the 
  phenomena 
  and 
  adopt 
  a 
  nomenclature, 
  as 
  

   the 
  subject 
  is 
  a 
  little 
  involved. 
  In 
  the 
  first 
  place 
  it 
  is 
  of 
  the 
  

   utmost 
  importance 
  to 
  ascertain 
  what 
  occurs 
  in 
  the 
  immediate 
  

   vicinity 
  of 
  the 
  exciting 
  line. 
  As 
  I 
  have 
  shown 
  in 
  previous 
  

   papers, 
  the 
  vapour 
  always 
  emits 
  a 
  line 
  the 
  wave-length 
  of 
  

   which 
  is 
  identical 
  with 
  that 
  of 
  the 
  exciting 
  light. 
  This 
  line 
  

   we 
  will 
  term 
  for 
  brevity 
  the 
  R.R. 
  line 
  (resonance 
  radiation). 
  

   If 
  we 
  carefully 
  screen 
  from 
  the 
  slit 
  of 
  the 
  spectroscope 
  all 
  

   light 
  except 
  that 
  which 
  comes 
  from 
  the 
  iodine 
  vapour, 
  which 
  

   we 
  can 
  do 
  by 
  means 
  of 
  a 
  small 
  diaphragm 
  placed 
  between 
  

   the 
  slit 
  and 
  the 
  rounded 
  end 
  of 
  the 
  iodine 
  tube, 
  we 
  find 
  that 
  

   the 
  R.R. 
  line 
  is 
  accompanied 
  by 
  companion 
  lines 
  which 
  form 
  

   a 
  group 
  very 
  similar 
  in 
  appearance 
  to 
  the 
  other 
  groups 
  which 
  

   appear 
  at 
  intervals 
  along 
  the 
  spectrum. 
  Now 
  the 
  actual 
  

   width 
  of 
  the 
  group 
  of 
  seven 
  absorption 
  lines 
  which 
  are 
  

   covered 
  by 
  the 
  green 
  line 
  of 
  mercury 
  is 
  only 
  about 
  1/30 
  of 
  

   the 
  width 
  of 
  the 
  group 
  of 
  lines 
  accompanying 
  the 
  R.R. 
  

   line. 
  

  

  On 
  Plate 
  XVI. 
  fig. 
  1 
  we 
  have 
  a 
  photograph 
  of 
  the 
  iodine 
  

  

  