﻿Dispersion 
  of 
  Sddiiim 
  Vapour. 
  129 
  

  

  spectrum, 
  and 
  the 
  molecular 
  condition 
  is 
  in 
  all 
  probability 
  

   vastly 
  more 
  complicated 
  than 
  in 
  the 
  gaseous 
  state. 
  

  

  In 
  liquid 
  oxygen 
  and 
  in 
  the 
  salts 
  of 
  erbium 
  and 
  didymium 
  

   we 
  have 
  the 
  only 
  examples, 
  so 
  far 
  as 
  I 
  know, 
  of 
  substances 
  

   other 
  than 
  gases 
  which 
  exhibit 
  narrow 
  absorption-bands, 
  and 
  

   in 
  these 
  cases 
  the 
  absorption 
  is 
  scarcely 
  strong 
  enough 
  to 
  

   influence 
  the 
  dispersion 
  to 
  any 
  marked 
  degree. 
  A 
  careful 
  

   study 
  of 
  the 
  dispersion 
  of 
  some 
  substance 
  with 
  absorption- 
  

   bands 
  as 
  narrow 
  and 
  strong 
  as 
  those 
  of 
  sodium 
  vapoui^ 
  would 
  

   be 
  of 
  great 
  value 
  in 
  aiding 
  us 
  to 
  pass 
  judgment 
  on 
  the 
  

   various 
  modifications 
  of 
  Sellmeier's 
  original 
  theory. 
  

  

  The 
  anomalous 
  dispersion 
  of 
  sodium 
  vapour 
  in 
  the 
  

   immediate 
  vicinity 
  of 
  the 
  D 
  lines 
  was 
  first 
  observed 
  by 
  

   Kundt, 
  and 
  has 
  since 
  been 
  studied 
  by 
  Becquerel 
  and 
  Julius, 
  

   but 
  none 
  of 
  these 
  investigators 
  has 
  traced 
  the 
  effect 
  over 
  a 
  

   range 
  of 
  the 
  spectrum 
  greater 
  than 
  two 
  or 
  three 
  times 
  the 
  

   distance 
  between 
  the 
  lines. 
  

  

  Jn 
  a 
  previous 
  paper* 
  I 
  have 
  shown 
  that 
  a 
  spectrum 
  

   closely 
  resembling 
  the 
  bright-line 
  spectrum 
  of 
  incandescent 
  

   sodium 
  can 
  be 
  produced 
  under 
  certain 
  conditions 
  by 
  the 
  

   anomalous 
  refraction 
  of 
  white 
  light 
  in 
  a 
  non-homoo;eneous 
  

   atmosphere 
  of 
  sodium 
  vapour, 
  in 
  precisely 
  the 
  manner 
  in 
  

   which 
  Julius 
  very 
  ingeniously 
  suggests 
  that 
  it 
  may 
  occur 
  

   in 
  the 
  reversing 
  layer 
  of 
  the 
  sun, 
  giving 
  rise 
  to 
  the 
  pheno- 
  

   menon 
  of 
  the 
  " 
  flash 
  spectrum." 
  In 
  view 
  of 
  the 
  seeming 
  

   importance 
  of 
  the 
  subject 
  in 
  its 
  bearing 
  on 
  the 
  subject 
  of 
  

   the 
  dynamics 
  of 
  dispersion, 
  and 
  its 
  possible 
  connexion 
  with 
  

   solar 
  physics, 
  I 
  have 
  undertaken 
  to 
  determine 
  whether 
  

   sufficiently 
  accurate 
  quantitative 
  data 
  can 
  be 
  obtained, 
  to 
  

   make 
  a 
  test 
  of 
  the 
  dispersion 
  formula, 
  as 
  applied 
  to 
  sodium 
  

   vapour, 
  possible. 
  

  

  Though 
  the 
  absolute 
  values 
  of 
  the 
  refractive 
  indices 
  which 
  

   have 
  been 
  found 
  are 
  not 
  as 
  satisfactory 
  as 
  is 
  desirable, 
  very 
  

   interesting 
  relative 
  values 
  have 
  been 
  obtained, 
  and, 
  what 
  is 
  

   of 
  especial 
  interest, 
  the 
  dispersion 
  has 
  been 
  traced 
  and 
  

   measured 
  throughout 
  the 
  entire 
  range 
  of 
  the 
  visible 
  spectrum, 
  

   the 
  refractive 
  index 
  for 
  all 
  waves 
  of 
  higher 
  frequency 
  than 
  

   D 
  2 
  being 
  less 
  than 
  one, 
  and 
  for 
  all 
  waves 
  of 
  lower 
  frequency 
  

   than 
  D 
  l 
  greater 
  than 
  one. 
  In 
  other 
  words, 
  I 
  have 
  been 
  

   able 
  to 
  form 
  a 
  complete 
  anomalous 
  spectrum 
  by 
  means 
  of 
  

   sodium 
  vapour, 
  in 
  which 
  all 
  of 
  the 
  colours 
  between 
  the 
  

   extreme 
  red 
  and 
  violet 
  are 
  present, 
  with 
  the 
  exception 
  of 
  a 
  

   range 
  of 
  perhaps 
  twenty 
  Angstrom 
  units 
  at 
  the 
  D 
  lines. 
  

  

  Previous 
  investigations 
  have 
  been 
  made 
  exclusively 
  with 
  

  

  * 
  Phil. 
  Mag. 
  May 
  1901. 
  

   Phil 
  Mac,. 
  S. 
  0. 
  Vol. 
  3. 
  No. 
  13. 
  Jan. 
  1902. 
  K 
  

  

  