﻿Dispersion, 
  and 
  Surface- 
  Colour 
  of 
  Selenium. 
  613 
  

  

  films 
  being 
  indicated 
  by 
  circles, 
  wbile 
  points 
  determined 
  

   with 
  wedge-shaped 
  films 
  are 
  denoted 
  by 
  crosses. 
  The 
  

   turning 
  point 
  of 
  the 
  curve 
  is 
  at 
  wave-length 
  "0305, 
  where 
  the 
  

   refractive 
  index 
  reaches 
  the 
  value 
  3-13, 
  which, 
  so 
  far 
  as 
  I 
  

   know, 
  is 
  the 
  highest 
  value 
  ever 
  found 
  for 
  any 
  substance. 
  

  

  Fig. 
  4. 
  

  

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  30 
  32 
  34 
  36 
  38 
  40 
  42 
  44 
  46 
  48 
  50 
  52 
  54 
  56 
  58 
  60 
  62 
  64 
  66 
  68 
  70 
  72 
  74 
  76 
  

  

  The 
  continued 
  rise 
  of 
  the 
  dispersion- 
  curve 
  after 
  entrance 
  

   into 
  the 
  region 
  of 
  the 
  absorption-band 
  is 
  worthy 
  of 
  notice, 
  

   for 
  usually 
  the 
  curve 
  falls 
  abruptly 
  as 
  soon 
  as 
  the 
  band 
  is 
  

   entered. 
  As 
  the 
  matter 
  is 
  of 
  considerable 
  theoretical 
  im- 
  

   portance 
  it 
  seemed 
  best 
  to 
  check, 
  if 
  possible, 
  the 
  data 
  furnished 
  

   by 
  the 
  interferometer 
  by 
  means 
  of 
  very 
  acute 
  prisms. 
  

   Examination 
  of 
  the 
  wedge-shaped 
  plate 
  by 
  means 
  of 
  sodium 
  

   light, 
  showed 
  that 
  a 
  strip 
  about 
  8 
  mm. 
  wide 
  was 
  suitable 
  for 
  

   the 
  purpose, 
  the 
  interference-fringes 
  being 
  straight, 
  parallel, 
  

   and 
  equidistant. 
  The 
  angle 
  was 
  found 
  Irom 
  the 
  fringes 
  to 
  

   be 
  very 
  nearly 
  22 
  seconds. 
  

  

  The 
  deviations 
  were 
  observed 
  by 
  means 
  of 
  a 
  large 
  spec- 
  

   trometer, 
  which 
  I 
  have 
  recently 
  set 
  up 
  for 
  quantitative 
  

   measurements 
  of 
  the 
  dispersion 
  of 
  sodium 
  vapour. 
  The 
  slit 
  

   is 
  illuminated 
  by 
  moans 
  of 
  the 
  monochromatic 
  illuminator 
  

   made 
  by 
  Fuess. 
  The 
  telescope 
  has 
  a 
  focal 
  length 
  of 
  1*5 
  

   metres, 
  the 
  eyepiece 
  being 
  furnished 
  with 
  a 
  filar 
  micrometer. 
  

   The 
  graduated 
  screw 
  of 
  the 
  monochromatic 
  illuminator, 
  by 
  

   means 
  of 
  which 
  the 
  wave-length 
  of 
  the 
  light 
  on 
  the 
  slit 
  is 
  

   changed, 
  is 
  operated 
  from 
  the 
  eye-end 
  of 
  the 
  telescope 
  by 
  

   means 
  of 
  a 
  long 
  steel 
  rod, 
  while 
  the 
  graduations 
  are 
  read 
  by 
  

   means 
  of 
  the 
  finder 
  of 
  the 
  telescope, 
  which 
  points 
  towards 
  a 
  

   mirror 
  in 
  which 
  a 
  portion 
  of 
  the 
  illuminator 
  is 
  seen 
  reflected. 
  

  

  This 
  arrangement 
  j 
  s 
  extremelv 
  convenient, 
  for 
  the 
  observer 
  

  

  