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  XCIII. 
  The 
  Apparent 
  Dispersion 
  of 
  Light 
  in 
  Space, 
  and 
  the 
  

   Minuteness 
  of 
  Structure 
  of 
  the 
  y^ther. 
  By 
  C. 
  V; 
  BuRTON, 
  

  

  D.Sc* 
  

  

  1. 
  "DECENT 
  observations 
  by 
  Nordmannf 
  and 
  by 
  TikhofFf 
  

   JI\j 
  have 
  been 
  held 
  by 
  these 
  authors 
  to 
  establish 
  a 
  

   dispersion 
  o£ 
  light 
  in 
  interstellar 
  space 
  : 
  the 
  velocity 
  of 
  pro- 
  

   pagation 
  appearing 
  to 
  be 
  a 
  direct 
  function 
  of 
  the 
  wave- 
  

   length. 
  On 
  the 
  other 
  hand, 
  the 
  reality 
  of 
  the 
  phenomenon 
  

   has 
  been 
  denied 
  by 
  Lebedew§, 
  who 
  attributes 
  the 
  effects 
  

   recorded 
  to 
  peculiarities 
  of 
  the 
  star-systems 
  observed. 
  It 
  is- 
  

   not 
  the 
  purpose 
  of 
  this 
  note 
  to 
  discuss 
  questions 
  of 
  astro- 
  

   physics 
  on 
  which 
  the 
  views 
  of 
  specialists 
  are 
  so 
  widely 
  

   different, 
  but 
  rather 
  to 
  direct 
  attention 
  to 
  a 
  theoretical 
  point 
  

   regarding 
  the 
  possible 
  scale 
  of 
  structure 
  of 
  the 
  aether, 
  and 
  

   the 
  relation 
  which 
  might 
  be 
  expected 
  to 
  obtain 
  between 
  that 
  

   scale 
  of 
  structure 
  and 
  differences 
  of 
  velocity 
  corresponding 
  

   to 
  radiations 
  of 
  given 
  wave-length. 
  

  

  2. 
  Amongst 
  the 
  considerations 
  advanced 
  by 
  Lebedew, 
  there 
  

   is 
  one 
  concerned 
  with 
  the 
  inevitable 
  connexion 
  between 
  dis- 
  

   persion 
  and 
  absorption 
  of 
  light 
  in 
  material 
  media. 
  He 
  points 
  

   out 
  that 
  any 
  known 
  substance 
  (hydrogen 
  at 
  low 
  pressure, 
  for 
  

   example), 
  if 
  present 
  in 
  sufficient 
  amount 
  to 
  give 
  rise 
  to 
  the 
  

   dispersion 
  which 
  has 
  been 
  inferred, 
  would 
  absorb 
  so 
  much 
  

   light 
  that 
  the 
  sun 
  and 
  the 
  stars 
  would 
  be 
  invisible 
  to 
  us. 
  

   This 
  appears 
  to 
  be 
  undeniable, 
  and 
  accordingly 
  it 
  would 
  

   seem 
  that 
  any 
  definitely 
  proved 
  dispersion 
  of 
  light 
  in 
  inter- 
  

   stellar 
  space 
  would 
  have 
  to 
  be 
  attributed 
  to 
  properties 
  of 
  the 
  

   sether 
  itself. 
  

  

  3. 
  In 
  any 
  homogeneous 
  isotropic 
  || 
  elastic 
  medium, 
  which 
  

   can 
  be 
  adequately 
  treated 
  as 
  a 
  continuum— 
  that 
  is 
  to 
  say, 
  in 
  

   which 
  the 
  specific 
  properties 
  of 
  the 
  medium 
  in 
  bulk 
  exist 
  

   unimpaired 
  in 
  any 
  volume 
  thereof, 
  however 
  small 
  — 
  any 
  dis- 
  

  

  * 
  Communicated 
  by 
  tlie 
  Author. 
  

  

  t 
  C. 
  Nordmann, 
  Comptes 
  Rendus, 
  cxlvi. 
  pp. 
  266 
  & 
  383 
  (1908) 
  ; 
  

   cxlvii. 
  pp. 
  24 
  & 
  620 
  (1908). 
  

  

  X 
  G. 
  A. 
  Tikhoff, 
  Comptes 
  RenduSj 
  cxlvi. 
  p. 
  570 
  (1908) 
  ; 
  cxlvii. 
  p. 
  170- 
  

   (1908). 
  

  

  § 
  P. 
  Lebedew, 
  Astrophys. 
  Journ. 
  xxix. 
  no. 
  2, 
  p. 
  101 
  (March 
  1909). 
  

  

  II 
  Though 
  isotropy 
  is 
  not 
  a 
  necessary 
  condition 
  for 
  ensuring 
  that 
  

   (in 
  any 
  given 
  direction) 
  the 
  velocity 
  of 
  propagation 
  is 
  independent 
  of 
  the 
  

   wave-leng-th, 
  the 
  statement 
  in 
  the 
  text 
  is 
  limited 
  to 
  isotropic 
  media 
  for 
  

   the 
  sake 
  of 
  simplicity 
  of 
  expression. 
  The 
  term 
  elastic 
  is 
  used 
  in 
  its 
  most 
  

   extended 
  sense, 
  denoting 
  merely 
  that 
  it 
  is 
  possible 
  to 
  increase 
  the 
  potential 
  

   energy 
  of 
  the 
  medium 
  by 
  doing 
  work 
  upon 
  it, 
  and 
  that 
  the 
  medium 
  is 
  

   capable 
  of 
  doing 
  an 
  equivalent 
  amount 
  of 
  work 
  in 
  reverting 
  to 
  its 
  normal 
  

   state. 
  

  

  