﻿136 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Anomalous 
  

  

  converted 
  into 
  a 
  normal 
  spectrum. 
  It 
  will 
  be 
  seen 
  thai 
  

   for 
  all 
  wave-lengths 
  shorter 
  than 
  those 
  of 
  the 
  D 
  lines, 
  the 
  

   refractive 
  index 
  is 
  less 
  than 
  one, 
  while 
  for 
  the 
  rest 
  of 
  the 
  spec- 
  

   trum 
  it 
  is 
  greater 
  than 
  one. 
  This 
  is 
  the 
  only 
  case 
  that 
  I 
  

   know 
  of 
  in 
  which 
  we 
  have 
  a 
  medium 
  beautifully 
  transparent 
  

   even 
  in 
  considerable 
  thicknesses, 
  in 
  which 
  light 
  travels 
  faster 
  

   than 
  in 
  a 
  vacuum. 
  

  

  To 
  determine 
  the 
  refractive 
  indices 
  we 
  require 
  the 
  angle 
  

   of 
  the 
  sodium 
  prism, 
  and 
  here 
  we 
  encounter 
  a 
  grave 
  difficulty, 
  

   for 
  it 
  appears 
  to 
  be 
  quite 
  impossible 
  to 
  determine 
  this 
  directly. 
  

   As 
  I 
  have 
  said 
  before, 
  we 
  are 
  dealing 
  with 
  the 
  optical 
  

   equivalent 
  of 
  a 
  prism, 
  namely, 
  a 
  non-homogeneous 
  medium, 
  

   in 
  which 
  the 
  vertical 
  wave-front 
  is 
  retarded 
  or 
  accelerated 
  in 
  

   a 
  progressively 
  increasing 
  amount 
  as 
  we 
  pass 
  from 
  its 
  upper 
  

   to 
  its 
  lower 
  edge. 
  The 
  equivalent 
  prism 
  appears 
  from 
  ex- 
  

   periment 
  to 
  be 
  bounded 
  by 
  concave 
  instead 
  of 
  plane 
  surfaces; 
  

   in 
  other 
  words, 
  the 
  effective 
  angle 
  is 
  greater 
  near 
  the 
  bottom 
  

   of 
  the 
  tube 
  than 
  at 
  the 
  top. 
  This 
  can 
  be 
  shown 
  by 
  screening 
  

   off 
  different 
  levels. 
  

  

  The 
  lower 
  part 
  of 
  the 
  tube 
  acting 
  alone 
  gives 
  a 
  much 
  

   greater 
  deviation 
  than 
  the 
  upper. 
  In 
  practice 
  I 
  have 
  found 
  

   it 
  best 
  to 
  screen 
  off 
  the 
  upper 
  and 
  lower 
  portions, 
  utilizing 
  

   only 
  the 
  light 
  which 
  passes 
  in 
  the 
  median 
  plane, 
  where 
  the 
  

   change 
  in 
  density 
  appears 
  to 
  be 
  quite 
  uniform, 
  this 
  method 
  

   having 
  been 
  used 
  in 
  preparing 
  the 
  dispersion 
  curve 
  alluded 
  

   to 
  above. 
  The 
  effective 
  angle 
  lies 
  probably 
  somewhere 
  

   between 
  90° 
  and 
  130°, 
  but 
  even 
  this 
  estimate 
  is 
  mere 
  guess- 
  

   work. 
  

  

  If 
  a 
  single 
  observation 
  could 
  be 
  made 
  with 
  a 
  vapour 
  prism 
  

   of 
  known 
  angle, 
  in 
  which 
  we 
  could 
  be 
  sure 
  that 
  the 
  density 
  

   was 
  uniform, 
  quantitative 
  values 
  could 
  be 
  assigned 
  to 
  deter- 
  

   minations 
  made 
  with 
  the 
  dispersion-tubes. 
  I 
  have 
  accordingly 
  

   tried 
  in 
  every 
  way 
  possible 
  to 
  obtain 
  a 
  prism 
  fulfilling 
  the 
  

   required 
  conditions. 
  These 
  endeavours 
  have 
  thus 
  far 
  met 
  

   with 
  only 
  partial 
  success, 
  but 
  an 
  account 
  of 
  the 
  devices 
  tried 
  

   may 
  be 
  of 
  value 
  to 
  others 
  working 
  along 
  similar 
  lines. 
  

  

  Glass 
  and 
  mica 
  are 
  so 
  quickly 
  attacked 
  by 
  the 
  sodium 
  

   vapour, 
  that 
  the 
  use 
  of 
  these 
  substances 
  for 
  prism-faces 
  seems 
  

   to 
  be 
  out 
  of 
  the 
  question. 
  Added 
  to 
  this 
  there 
  is 
  the 
  

   difficulty 
  of 
  making 
  gas-tight 
  joints 
  between 
  plate-glass 
  and 
  

   metal 
  which 
  will 
  stand 
  a 
  temperature 
  but 
  little 
  below 
  a 
  red 
  

   heat. 
  Repeated 
  failures 
  to 
  secure 
  prisms 
  in 
  this 
  way 
  com- 
  

   pelled 
  me 
  to 
  seek 
  some 
  other 
  method 
  of 
  giving 
  the 
  vapour 
  

   the 
  required 
  form. 
  I 
  had 
  observed 
  that 
  in 
  glass 
  tubes 
  held 
  

   before 
  sodium 
  flames 
  the 
  black 
  vapour 
  retreated 
  before 
  the 
  

   approach 
  of 
  a 
  cold 
  obstacle. 
  This 
  suggested 
  to 
  me 
  that 
  it 
  

  

  