﻿Ultra-violet 
  Absorption 
  ^c. 
  of 
  Sodium 
  Vapour. 
  531 
  

  

  As 
  will 
  be 
  seen 
  from 
  the 
  table, 
  the 
  last 
  22 
  lines, 
  i. 
  e. 
  nearly 
  

   one-half 
  of 
  the 
  whole 
  series 
  thus 
  far 
  observed, 
  fall 
  in 
  a 
  region 
  

   of 
  the 
  spectrum 
  not 
  wider 
  than 
  the 
  distance 
  between 
  the 
  

   D 
  lines 
  ! 
  

  

  Kayser 
  gives 
  on 
  page 
  521 
  the 
  constants 
  of 
  the 
  Balmer 
  

   formula 
  for 
  the 
  sodium 
  series, 
  computed 
  from 
  the 
  seven 
  lines 
  

   known 
  at 
  the 
  time. 
  

  

  These 
  seven 
  lines 
  are 
  well 
  represented 
  by 
  the 
  expression 
  

  

  lO^X-i 
  = 
  41196-34- 
  127040 
  n-2- 
  843841 
  n-\ 
  

  

  Wfth 
  the 
  complete 
  series 
  now 
  at 
  our 
  disposal 
  it 
  will 
  be 
  

   necessary 
  to 
  redetermine 
  the 
  constants. 
  

  

  I 
  have 
  calculated 
  the 
  wave-lengths 
  of 
  the 
  32nd 
  and 
  the 
  

   50th 
  line 
  from 
  the 
  formula, 
  finding 
  2417-5 
  instead 
  of 
  2418'44 
  

   (observed), 
  and 
  2415'2 
  instead 
  of 
  2414*5 
  ; 
  that 
  is, 
  the 
  calcu- 
  

   lated 
  values 
  are 
  separated 
  from 
  the 
  observed 
  by 
  a 
  distance 
  

   equal 
  to 
  the 
  distance 
  between 
  four 
  of 
  the 
  lines 
  at 
  this 
  point 
  

   of 
  the 
  spectrum. 
  

  

  Photographs 
  of 
  the 
  series 
  are 
  reproduced 
  on 
  PL 
  XVI. 
  Fig. 
  1 
  

   is 
  an 
  enlargement 
  showing 
  the 
  lines 
  from 
  7i=12 
  to 
  n 
  = 
  40. 
  

   The 
  last 
  ten 
  members 
  are 
  too 
  close 
  together 
  to 
  appear. 
  

   Fig. 
  2 
  is 
  reproduced 
  natural 
  size, 
  and 
  shows 
  the 
  series 
  from 
  

   ?i 
  = 
  8. 
  The 
  source 
  of 
  light 
  was 
  the 
  cadmium 
  spark, 
  and 
  the 
  

   iron 
  comparison 
  spectrum 
  was 
  taken 
  on 
  the 
  same 
  plate, 
  

   the 
  image 
  of 
  the 
  arc 
  being 
  projected 
  on 
  the 
  slit 
  just 
  below 
  

   the 
  point 
  illuminated 
  by 
  the 
  light 
  of 
  the 
  spark 
  after 
  it 
  had 
  

   traversed 
  the 
  sodiuin 
  vapour. 
  

  

  It 
  is 
  extremely 
  interesting 
  to 
  note 
  that 
  a 
  general 
  absorption 
  

   begins 
  at 
  the 
  head 
  of 
  the 
  series, 
  which 
  extends 
  down 
  to 
  the 
  

   end 
  of 
  the 
  spectrum. 
  In 
  other 
  words, 
  the 
  vapour 
  is 
  much 
  more 
  

   transparent 
  to 
  the 
  light 
  between 
  the 
  lines 
  of 
  the 
  series 
  than 
  

   in 
  the 
  region 
  below 
  the 
  head. 
  This 
  is 
  in 
  accord 
  with 
  the 
  

   recorded 
  observation 
  of 
  a 
  faint 
  continuous 
  emission 
  spectrum 
  

   below 
  the 
  head 
  of 
  the 
  series 
  in 
  the 
  case 
  of 
  solar 
  hydrogen. 
  

   PL 
  XYI. 
  fig. 
  3 
  shows 
  a 
  series 
  of 
  spectra 
  taken 
  with 
  pro- 
  

   gressively 
  increasing 
  vapour 
  density, 
  in 
  which 
  the 
  complete 
  

   Balmer 
  series 
  appears, 
  with 
  the 
  exception 
  of 
  the 
  first 
  member 
  

   (72 
  = 
  3), 
  the 
  D 
  lines, 
  the 
  position 
  of 
  which 
  is 
  however 
  indi- 
  

   cated. 
  As 
  the 
  vapour 
  density 
  increases 
  the 
  higher 
  members 
  

   of 
  the 
  series 
  appear, 
  and 
  the 
  lower 
  members 
  are 
  drowned 
  out 
  

   by 
  channelled 
  spectra 
  which 
  accompany 
  them. 
  

  

  These 
  channelled 
  s})ectra 
  are 
  analogous 
  to 
  the 
  one 
  which 
  

   accompanies 
  the 
  D 
  lines, 
  which 
  is 
  the 
  one 
  previously 
  studied 
  

   with 
  respect 
  to 
  fluorescence 
  and 
  magnetic 
  rotation. 
  They 
  

   differ 
  from 
  it 
  in 
  one 
  respect, 
  however. 
  The 
  channelled 
  

   ijpectrum 
  in 
  the 
  visible 
  region 
  consists 
  of 
  two 
  parts, 
  as 
  is 
  

  

  