﻿Dispersion, 
  arid 
  Surface- 
  Colour 
  of 
  Selenium, 
  G09 
  

  

  well 
  except 
  at 
  the 
  edge 
  of 
  the 
  absorption-band, 
  where 
  the 
  

   chance 
  of 
  error 
  is 
  much 
  greater 
  owing 
  to 
  the 
  broadening 
  

   of 
  the 
  image 
  of 
  the 
  spectrometer-slit 
  by 
  diffraction, 
  the 
  light 
  

   only 
  passing 
  through 
  a 
  very 
  narrow 
  strip 
  of 
  the 
  prism 
  

   bordering 
  the 
  refracting 
  edge. 
  

  

  The 
  spectrometer 
  was 
  illuminated 
  with 
  monochromatic 
  

   light 
  obtained 
  by 
  prismatic 
  analysis, 
  the 
  wave-length 
  being 
  

   determined 
  for 
  each 
  observation 
  with 
  a 
  glass 
  grating 
  of 
  14,000 
  

   lines. 
  I 
  found 
  it 
  quite 
  impossible 
  to 
  construct 
  prisms 
  of 
  small 
  

   enough 
  angle 
  to 
  make 
  determinations 
  below 
  wave-length 
  61 
  

   possible. 
  With 
  one 
  very 
  acute 
  prism 
  I 
  fancied 
  that 
  I 
  detected 
  

   evidences 
  of 
  a 
  decrease 
  of 
  refractive 
  index 
  after 
  passing 
  this 
  

   point, 
  and 
  accordingly 
  determined 
  to 
  follow 
  the 
  curve 
  further 
  

   down 
  into 
  the 
  spectrum 
  by 
  means 
  of 
  the 
  interferometer, 
  

   in 
  the 
  same 
  manner 
  as 
  has 
  already 
  been 
  done 
  in 
  the 
  case 
  

   of 
  cyanine. 
  

  

  Beautifully 
  uniform 
  films 
  of 
  selenium 
  were 
  obtained 
  on 
  

   plates 
  of 
  plane 
  parallel 
  glass, 
  by 
  means 
  of 
  a 
  flat 
  selenium 
  

   cathode 
  in 
  a 
  high 
  vacuum. 
  This 
  cathode 
  was 
  made 
  by 
  

   rubbing 
  a 
  stick 
  of 
  selenium 
  over 
  a 
  hot 
  aluminium 
  plate, 
  the 
  

   device 
  being 
  that 
  used 
  by 
  Longden 
  in 
  obtaining 
  films 
  showing 
  

   Newton's 
  rings. 
  

  

  Displacements 
  of 
  the 
  interferometer 
  fringes 
  by 
  the 
  films 
  

   were 
  obtained 
  in 
  the 
  manner 
  described 
  in 
  previous 
  papers 
  on 
  

   the 
  dispersion 
  of 
  cyanine 
  and 
  carbon 
  ; 
  a 
  film 
  covering 
  one 
  halt 
  

   of 
  each 
  of 
  the 
  two 
  interferometer 
  paths 
  giving 
  double 
  the 
  

   displacement 
  of 
  a 
  single 
  film 
  without 
  any 
  increased 
  loss 
  of 
  

   light. 
  The 
  displaced 
  fringes 
  were 
  photographed 
  by 
  mono- 
  

   chromatic 
  light 
  of 
  known 
  wave-length, 
  the 
  photographs 
  being 
  

   subsequently 
  measured 
  with 
  a 
  filar 
  micrometer. 
  

  

  In 
  the 
  photographic 
  work 
  some 
  trouble 
  was 
  had 
  with 
  

   superfluous 
  light 
  which, 
  being 
  superposed 
  on 
  the 
  image 
  of 
  

   the 
  fringe 
  system, 
  lessened 
  the 
  contrast 
  between 
  the 
  maxima 
  

   and 
  minima. 
  Some 
  of 
  this 
  light 
  comes 
  from 
  the 
  glass 
  

   surfaces 
  and 
  some 
  from 
  the 
  selenium 
  films. 
  Mr. 
  Plund 
  

   finally 
  got 
  rid 
  of 
  this 
  light 
  by 
  the 
  ingenious 
  device 
  of 
  placing 
  

   a 
  convex 
  lens 
  between 
  the 
  interferometer 
  and 
  the 
  photo- 
  

   graphic 
  plate, 
  which 
  brought 
  the 
  light 
  to 
  two 
  point-foci 
  less 
  

   than 
  a 
  millimetre 
  apart. 
  A 
  screen 
  with 
  a 
  pin-hole 
  held 
  back 
  

   the 
  superfluous 
  light, 
  which 
  was 
  collected 
  at 
  one 
  focus, 
  

   allowing 
  the 
  light 
  which 
  formed 
  the 
  fringes 
  to 
  pass. 
  It 
  was 
  

   also 
  found 
  that 
  better 
  results 
  were 
  obtained 
  with 
  unsilvered 
  

   interferometer-plates. 
  

  

  When 
  working 
  in 
  the 
  red 
  and 
  orange 
  comparatively 
  thick 
  

   films 
  could 
  be 
  used, 
  but 
  in 
  the 
  blue 
  and 
  violet 
  only 
  exceed- 
  

   ingly 
  thin 
  ones 
  allowed 
  any 
  light 
  to 
  pass 
  through. 
  

  

  