﻿Dispersion, 
  and 
  Surface- 
  Colour 
  of 
  Selenium. 
  619 
  

  

  seemed 
  worth 
  while 
  to 
  attempt 
  to 
  apply 
  it 
  until 
  a 
  more 
  accu- 
  

   rate 
  curve 
  has 
  been 
  determined. 
  

  

  If 
  we 
  take 
  into 
  account 
  the 
  first 
  member 
  of 
  the 
  series 
  only, 
  

   and 
  apply 
  the 
  results 
  obtained 
  with 
  the 
  prisms 
  to 
  the 
  formula 
  

  

  7L 
  2 
  = 
  b 
  2 
  + 
  

  

  M 
  1 
  

  

  from 
  which 
  the 
  other 
  formula 
  may 
  be 
  derived 
  by 
  letting 
  

   b' 
  2 
  = 
  m 
  + 
  m' 
  and 
  M 
  l 
  = 
  m 
  , 
  X 
  m 
  2 
  , 
  in 
  which 
  b 
  2 
  is 
  the 
  dielectric 
  

   constant, 
  we 
  find 
  b 
  2 
  to 
  be 
  very 
  nearly 
  6*0. 
  This 
  latter 
  form 
  

   of 
  the 
  formula 
  is 
  the 
  one 
  generally 
  used. 
  If 
  we 
  use 
  the 
  

   original 
  formula 
  and 
  determine 
  m 
  and 
  m 
  from 
  the 
  expressions 
  

  

  *«"(V-A»') 
  

  

  Ag 
  A. 
  m 
  

  

  l 
  2 
  

  

  we 
  find 
  m 
  = 
  5 
  and 
  m'=l'02 
  

  

  or 
  b 
  2 
  = 
  m 
  + 
  m' 
  =6*02. 
  

  

  If 
  m 
  in 
  the 
  original 
  formula 
  turns 
  out 
  to 
  be 
  greater 
  than 
  

   unity, 
  it 
  indicates 
  that 
  other 
  absorption-bands 
  must 
  be 
  taken 
  

   into 
  account. 
  The 
  dielectric 
  constant 
  of 
  selenium 
  has 
  been 
  

   measured 
  by 
  Romich 
  and 
  Nowack, 
  the 
  value 
  found 
  being 
  

   10*2. 
  

  

  The 
  dielectric 
  constant 
  of 
  a 
  substance 
  according 
  to 
  the 
  

   electromagnetic 
  dispersion 
  theory 
  is 
  the 
  dielectric 
  constant 
  

   of 
  the 
  aether, 
  plus 
  the 
  dielectric 
  constants 
  of 
  the 
  various 
  ions 
  

   whose 
  action 
  on 
  the 
  aether 
  waves 
  gives 
  rise 
  to 
  the 
  pheno- 
  

   mena 
  of 
  absorption 
  and 
  dispersion. 
  Until 
  the 
  behaviour 
  of 
  

   selenium 
  in 
  the 
  infra-red 
  region 
  has 
  been 
  investigated, 
  it 
  

   will 
  be 
  impossible 
  to 
  say 
  whether 
  the 
  high 
  value 
  of 
  the 
  

   dielectric 
  constant 
  is 
  due 
  wholly, 
  or 
  only 
  in 
  part, 
  to 
  the 
  ions 
  

   whose 
  free 
  periods 
  correspond 
  in 
  frequency 
  to 
  the 
  green, 
  

   blue, 
  and 
  ultra-violet 
  rays. 
  The 
  infra-red 
  work 
  is 
  already 
  

   under 
  way, 
  and 
  until 
  it 
  is 
  completed 
  further 
  discussion 
  of 
  

   the 
  results 
  given 
  in 
  the 
  present 
  paper 
  is 
  impossible*. 
  

  

  The 
  very 
  high 
  values 
  of 
  k 
  in 
  the 
  ultra-violet 
  led 
  me 
  to 
  look 
  

   for 
  traces 
  of 
  selective 
  reflexion 
  in 
  this 
  region. 
  The 
  light 
  of 
  

   an 
  arc-lamp 
  was 
  reflected 
  successively 
  from 
  six 
  surfaces 
  of 
  

   selenium 
  deposited 
  on 
  glass. 
  The 
  image 
  of 
  the 
  crater 
  after 
  

  

  * 
  Since 
  writing* 
  this 
  paper, 
  I 
  have 
  had 
  the 
  opportunity 
  of 
  examining' 
  

   the 
  infra-red 
  transmission 
  up 
  to 
  wave-length 
  23 
  \i 
  with 
  Prof. 
  Menden- 
  

   hall's 
  very 
  sensitive 
  bolometer 
  at 
  the 
  University 
  of 
  Wisconsin. 
  No 
  

   absorption-bands 
  were 
  found 
  up 
  to 
  this 
  point. 
  

  

  