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  LXX. 
  Resonance 
  Spectra 
  of 
  Iodine 
  under 
  High 
  Dispersion. 
  

   By 
  K. 
  "W. 
  Wood, 
  Professor 
  of 
  Experimental 
  Physics, 
  

   Johns 
  Hopkins 
  University, 
  and 
  Adams 
  Research 
  Fellow 
  of 
  

   Columbia 
  University 
  *. 
  

  

  [Plates 
  XV. 
  & 
  XVI.] 
  

  

  IN 
  my 
  last 
  communication 
  on 
  the 
  resonance 
  spectra 
  of 
  

   iodine 
  (Phil. 
  Mag. 
  Oct. 
  1912, 
  p. 
  673) 
  I 
  showed 
  that 
  the 
  

   green 
  line 
  of 
  the 
  mercury-vapour 
  lamp 
  had 
  a 
  width 
  sufficient 
  

   to 
  enable 
  it 
  to 
  cover 
  from 
  two 
  to 
  seven 
  of 
  the 
  fine 
  absorption 
  

   lines 
  of 
  iodine. 
  In 
  the 
  case 
  of 
  the 
  Cooper-Hewitt 
  glass 
  

   lamp 
  the 
  green 
  Hg 
  line 
  was 
  so 
  narrow 
  that 
  only 
  two 
  

   absorption 
  lines 
  were 
  excited, 
  while 
  in 
  the 
  case 
  of 
  the 
  quartz 
  

   lamp, 
  running 
  at 
  a 
  high 
  temperature, 
  the 
  line 
  was 
  so 
  

   broadened 
  that 
  seven 
  absorption 
  lines 
  were 
  covered. 
  In 
  the 
  

   latter 
  case 
  the 
  resonance 
  spectrum 
  consisted 
  of 
  groups 
  of 
  

   lines 
  occurring 
  at 
  regular 
  intervals 
  along 
  the 
  spectrum, 
  and 
  

   to 
  this 
  type 
  of 
  stimulation 
  I 
  gave 
  the 
  name 
  " 
  multiplex 
  

   excitation/ 
  - 
  ' 
  I 
  predicted 
  at 
  this 
  time 
  that 
  the 
  appearances 
  

   of 
  the 
  groups 
  of 
  lines 
  could 
  be 
  profoundly 
  modified 
  by 
  

   running 
  the 
  quartz 
  lamp 
  under 
  different 
  conditions 
  as 
  to 
  

   temperature 
  and 
  voltage 
  drop 
  across 
  the 
  terminals, 
  as 
  in 
  this 
  

   way 
  the 
  width 
  of 
  the 
  exciting 
  line 
  could 
  be 
  varied, 
  and 
  the 
  

   number 
  of 
  absorption 
  lines 
  excited 
  easily 
  controlled. 
  The 
  

   adoption 
  of 
  this 
  plan 
  has 
  thrown 
  much 
  new 
  light 
  upon 
  

   the 
  genesis 
  of 
  resonance 
  spectra, 
  and 
  further 
  improvements 
  

   in 
  the 
  methods 
  of 
  utilizing 
  the 
  exciting 
  light 
  have 
  made 
  it 
  

   possible 
  to 
  photograph 
  the 
  resonance 
  spectra 
  in 
  the 
  fourth 
  

   order 
  spectrum 
  of* 
  the 
  large 
  plane 
  grating 
  which 
  I 
  use 
  in 
  the 
  

   42-foot 
  spectrograph, 
  though 
  with 
  a 
  lens 
  of 
  shorter 
  focus 
  (2 
  

   metres) 
  and 
  with 
  an 
  exposure 
  of 
  only 
  24 
  hours. 
  When 
  it 
  is 
  

   remembered 
  that 
  in 
  the 
  first 
  study 
  of 
  the 
  iodine 
  fluorescence 
  

   an 
  exposure 
  of 
  24 
  hours 
  was 
  necessary 
  to 
  secure 
  a 
  satis- 
  

   factory 
  photograph 
  with 
  a 
  Hilger 
  one-prism 
  spectrograph, 
  the 
  

   immense 
  gain 
  in 
  efficiency 
  is 
  apparent. 
  I 
  can 
  now 
  secure 
  

   good 
  photographs 
  of 
  the 
  resonance 
  spectrum 
  in 
  the 
  first 
  

   order 
  spectrum 
  of 
  the 
  grating 
  in 
  30 
  minutes. 
  

  

  The 
  work 
  of 
  the 
  past 
  year 
  has 
  furnished 
  some 
  very 
  

   definite 
  problems 
  in 
  molecular 
  mechanics 
  for 
  the 
  theoretical 
  

   physicists 
  to 
  solve, 
  and 
  I 
  feel 
  that, 
  for 
  the 
  first 
  time, 
  I 
  am 
  

   now 
  able 
  to 
  furnish 
  some 
  very 
  exact 
  knowledge 
  of 
  the 
  nature 
  

   of 
  these 
  remarkable 
  spectra, 
  which 
  it 
  is 
  most 
  important 
  to 
  

   account 
  for 
  by 
  theory. 
  

  

  The 
  methods 
  which 
  I 
  have 
  adopted 
  in 
  the 
  present 
  work 
  

   may 
  be 
  outlined 
  briefly 
  as 
  follows 
  : 
  — 
  

  

  * 
  Communicated 
  bv 
  the 
  Author. 
  

  

  