﻿Emission 
  of 
  Polarized 
  Light 
  by 
  Fluorescent 
  Gases. 
  185 
  

  

  sources 
  of 
  polarization, 
  such 
  as 
  fog 
  or 
  mist 
  in 
  the 
  tube. 
  

   Exhaustion 
  with 
  a 
  Gaede 
  mercurial 
  pump 
  while 
  the 
  tube 
  was 
  

   heated 
  was 
  sufficient 
  to 
  completely 
  banish 
  all 
  trace 
  of 
  fog. 
  

  

  The 
  complete 
  absence 
  of 
  fog 
  can 
  be 
  told 
  by 
  illuminating 
  

   the 
  vapour 
  with 
  an 
  intense 
  beam 
  of 
  light 
  embracing 
  the 
  

   spectrum 
  region 
  comprised 
  between 
  wave-lengths 
  5400 
  and 
  

   5700, 
  obtained 
  by 
  prismatic 
  dispersion. 
  Such 
  a 
  beam 
  is 
  

   incapable 
  of 
  exciting 
  any 
  fluorescence, 
  and 
  if 
  fog 
  is 
  absent 
  

   the 
  cone 
  of 
  light 
  is 
  absolutely 
  invisible 
  when 
  viewed 
  through 
  

   the 
  lateral 
  tube. 
  The 
  Savart 
  fringes 
  were 
  very 
  distinct, 
  and 
  tlie 
  

   percentage 
  of 
  polarization 
  was 
  determined 
  by 
  compensating 
  

   it 
  with 
  a 
  pair 
  of 
  glass 
  plates 
  which 
  could 
  be 
  turned 
  about 
  on 
  

   a 
  vertical 
  axis 
  furnished 
  with 
  a 
  graduated 
  circle. 
  This 
  method 
  

   gives 
  bv 
  far 
  the 
  most 
  accurate 
  results 
  when 
  we 
  are 
  dealing 
  

   with 
  small 
  percentages 
  of 
  polarization. 
  Cornu's 
  method, 
  

   which 
  depends 
  upon 
  the 
  unequal 
  brilliancy 
  of 
  two 
  images 
  

   seen 
  through 
  a 
  Rochon 
  or 
  other 
  double-image 
  prism, 
  cannot 
  

   be 
  used 
  when 
  there 
  is 
  less 
  than 
  twenty-five 
  or 
  thirty 
  per 
  cent, 
  of 
  

   polarized 
  light 
  present, 
  for 
  with 
  less 
  than 
  this 
  amount 
  it 
  is 
  

   difficult 
  or 
  impossible 
  to 
  detect 
  any 
  difference 
  in 
  the 
  brilliancy 
  

   of 
  the 
  two 
  images. 
  This 
  is 
  true 
  at 
  all 
  events 
  for 
  the 
  green 
  

   fluorescent 
  light 
  of 
  sodium 
  vapour. 
  The 
  first 
  measurements 
  

   were 
  made 
  with 
  the 
  exciting 
  light 
  polarized 
  (electric 
  vector) 
  

   vertically. 
  

  

  It 
  was 
  found 
  that 
  the 
  polarization 
  was 
  strongest 
  when 
  the 
  

   temperature 
  of 
  the 
  tube 
  was 
  comparatively 
  low, 
  i. 
  e. 
  when 
  

   the 
  fluorescence 
  first 
  appeared. 
  At 
  a 
  higher 
  temperature, 
  

   with 
  very 
  bright 
  fluorescence, 
  compensation 
  was 
  secured 
  

   with 
  a 
  single 
  plate 
  at 
  an 
  angle 
  of 
  59°, 
  or 
  with 
  two 
  plates 
  at 
  

   an 
  angle 
  of 
  -47°. 
  Taking 
  the 
  refractive 
  index 
  of 
  the 
  glass 
  as 
  

   1*52, 
  and 
  making 
  use 
  of 
  the 
  formula 
  which 
  expresses 
  the 
  

   ratio 
  of 
  the 
  amplitudes 
  of 
  the 
  components 
  of 
  the 
  vibration 
  in 
  

   and 
  perpendicular 
  to 
  the 
  plane 
  of 
  incidence 
  

  

  dI 
  = 
  e; 
  C052 
  ^-^* 
  

  

  'p 
  XJp 
  

  

  we 
  find 
  the 
  percentage 
  of 
  polarization 
  to 
  be 
  20. 
  

  

  At 
  the 
  lowest 
  temperature 
  consistent 
  with 
  a 
  fairly 
  bright 
  

   fluorescence 
  compensation 
  was 
  secured 
  with 
  two 
  plates 
  at 
  an 
  

   angle 
  of 
  52°. 
  This 
  gives 
  us 
  30 
  per 
  cent, 
  of 
  polarized 
  light. 
  

   The 
  change 
  in 
  the 
  intensity 
  of 
  the 
  fluorescent 
  spot 
  when 
  

   viewed 
  through 
  a 
  slowly 
  revolving 
  nicol 
  was 
  easily 
  observed 
  

   in 
  this 
  case, 
  and 
  a 
  separate 
  determination 
  was 
  made 
  by 
  Cornu's 
  

   method, 
  which 
  gave 
  28 
  per 
  cent, 
  of 
  polarized 
  light. 
  

  

  * 
  'Drude's 
  Optics/ 
  page 
  265, 
  1st 
  edition. 
  

  

  