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  LXXXI. 
  An 
  Extension 
  of 
  the 
  principal 
  Series 
  of 
  the 
  Sodium 
  

   Spectrum. 
  By 
  R. 
  W. 
  Wood, 
  Professor 
  of 
  Experimental 
  

   Physics, 
  Johns 
  Hopkins 
  University 
  *. 
  

  

  [Plate 
  XXVIII.] 
  

  

  THE 
  largest 
  number 
  o£ 
  lines 
  in 
  a 
  Balraer 
  series 
  which 
  has 
  

   ever 
  been 
  observed 
  in 
  the 
  laboratory 
  is 
  thirteen, 
  which 
  

   were 
  found 
  in 
  the 
  hydrogen 
  spectrum 
  by 
  Cornu 
  and 
  by 
  

   Ames. 
  In 
  the 
  spectrum 
  o£ 
  the 
  Chromosphere 
  twenty-nine 
  

   hydrogen 
  lines 
  of 
  the 
  series 
  have 
  been 
  recorded, 
  the 
  last 
  line 
  

   (w=31 
  in 
  the 
  formula) 
  being 
  within 
  only 
  fifteen 
  Angstrom 
  

   units 
  o£ 
  the 
  theoretical 
  end 
  of 
  the 
  series. 
  In 
  the 
  case 
  o£ 
  

   sodium 
  vapour 
  seven 
  lines 
  of: 
  the 
  principal 
  series 
  have 
  been 
  

   discovered 
  up 
  to 
  the 
  present 
  time. 
  In 
  studying 
  the 
  ultra- 
  

   violet 
  absorption 
  spectrum 
  of 
  the 
  vapour 
  I 
  have 
  already 
  raised 
  

   the 
  number 
  to 
  thirty, 
  though 
  the 
  last 
  six 
  lines 
  are 
  so 
  close 
  

   together, 
  with 
  the 
  dispersion 
  at 
  my 
  command, 
  that 
  it 
  is 
  

   impossible 
  to 
  determine 
  their 
  wave-lengths. 
  This 
  makes 
  by 
  

   far 
  the 
  most 
  complete 
  Balmer 
  series 
  ever 
  observed 
  in 
  the 
  

   laboratory, 
  and 
  places 
  the 
  sodium 
  spectrum 
  on 
  an 
  equal 
  

   footing 
  with 
  the 
  hydrogen 
  spectrum 
  of 
  the 
  chromosphere. 
  

   In 
  my 
  opinion 
  a 
  further 
  extension 
  of 
  the 
  series 
  is 
  merely 
  

   a 
  question 
  of 
  vapour-density 
  and 
  dispersive 
  power. 
  No 
  

   lines 
  other 
  than 
  those 
  of 
  the 
  principal 
  series 
  appear 
  in 
  the 
  

   absorption 
  spectrum 
  (disregarding 
  channelled 
  spectra), 
  con- 
  

   sequently 
  we 
  are 
  not 
  obliged 
  to 
  hunt 
  about 
  in 
  a 
  confusion 
  of 
  

   lines 
  for 
  the 
  ones 
  which 
  belong 
  to 
  the 
  series, 
  as 
  is 
  the 
  case 
  

   with 
  the 
  spectrum 
  of 
  hydrogen 
  obtained 
  with 
  vacuum-tubes. 
  

  

  The 
  vapour 
  is 
  obtained 
  by 
  heating 
  the 
  metal 
  in 
  a 
  steel 
  tube 
  

   about 
  a 
  metre 
  long 
  to 
  a 
  dull 
  red 
  heat. 
  It 
  is 
  almost 
  opaque 
  

   to 
  all 
  visible 
  radiations, 
  only 
  the 
  extreme 
  violet 
  being 
  trans- 
  

   mitted. 
  It 
  is 
  best 
  to 
  work 
  with 
  a 
  pressure 
  of 
  about 
  15 
  cms. 
  

   of 
  nitrogen 
  in 
  the 
  tube, 
  as 
  in 
  a 
  highly 
  exhausted 
  tube 
  it 
  is 
  

   difficult 
  to 
  get 
  the 
  requisite 
  density. 
  It 
  is 
  unnecessary 
  to 
  fill 
  

   the 
  tube 
  with 
  nitrogen, 
  for 
  the 
  sodium 
  vapour 
  soon 
  removes 
  

   the 
  oxygen, 
  the 
  smoke 
  settling 
  in 
  a 
  few 
  minutes. 
  

  

  The 
  source 
  of 
  light 
  was 
  the 
  cadmium 
  spark 
  furnished 
  by 
  

   a 
  large 
  induction-coil 
  with 
  condenser. 
  This 
  gives 
  a 
  fairly 
  

   bright 
  continuous 
  background, 
  and 
  has 
  no 
  bright 
  lines 
  which 
  

   coincide 
  with 
  any 
  of 
  the 
  sodium 
  lines. 
  

  

  The 
  spectrum 
  was 
  photographed 
  with 
  a 
  small 
  quartz 
  

   spectrograph 
  made 
  by 
  Fuess, 
  which, 
  though 
  having 
  a 
  focal 
  

   length 
  of 
  but 
  12 
  cms., 
  gives 
  in 
  the 
  region 
  under 
  investigation 
  

   a 
  dispersion 
  about 
  equal 
  to 
  that 
  of 
  a 
  concave 
  grating 
  of 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

   Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  16. 
  No, 
  96. 
  Dec. 
  1908. 
  3 
  Q 
  

  

  