﻿186 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Emission 
  of 
  

  

  If 
  the 
  exciting 
  beam 
  was 
  polarized 
  with 
  its 
  electric 
  vector 
  

   horizontal, 
  no 
  trace 
  of 
  the 
  Savart 
  fringes 
  could 
  be 
  detected, 
  

   which 
  was 
  to 
  be 
  expected 
  as 
  everything 
  was 
  then 
  symmetrical 
  

   about 
  the 
  line 
  of 
  sight. 
  

  

  If 
  the 
  exciting 
  light 
  was 
  unpolarized 
  to 
  start 
  with 
  the 
  

   fluorescent 
  light 
  was 
  found 
  to 
  be 
  polarized, 
  but 
  not 
  to 
  the 
  

   same 
  extent, 
  the 
  percentage 
  in 
  each 
  case 
  being 
  one 
  half 
  of 
  its 
  

   former 
  value, 
  which 
  was 
  also 
  to 
  be 
  expected. 
  The 
  question 
  

   at 
  once 
  arose 
  as 
  to 
  why 
  we 
  have 
  only 
  a 
  partial 
  polarization 
  of 
  

   the 
  fluorescent 
  light 
  when 
  we 
  start 
  with 
  plane-polarized 
  light. 
  

  

  As 
  I 
  have 
  already 
  shown 
  (Phil. 
  Mag., 
  May 
  1908) 
  excitation 
  

   of 
  the 
  vapour 
  with 
  monochromatic 
  light 
  causes 
  it 
  to 
  emit 
  this 
  

   same 
  wave-length 
  and 
  a 
  large 
  number 
  of 
  other 
  wave-lengths, 
  

   the 
  spectrum 
  exhibiting 
  in 
  the 
  most 
  typical 
  case 
  a 
  number 
  of 
  

   very 
  sharp 
  lines 
  spaced 
  at 
  nearly 
  equal 
  intervals 
  along 
  a 
  

   normal 
  spectrum. 
  It 
  at 
  once 
  occurred 
  to 
  me 
  to 
  see 
  whether 
  

   the 
  polarization 
  was 
  confined 
  to 
  the 
  line 
  corresponding 
  to 
  the 
  

   exciting 
  line, 
  for 
  it 
  seemed 
  not 
  impossible 
  that 
  the 
  electron 
  

   vibrating 
  in 
  synchronism 
  with 
  the 
  exciting 
  light 
  might 
  emit 
  

   light 
  which 
  was 
  completely 
  plane-polarized, 
  whereas 
  the 
  other 
  

   disturbed 
  electrons 
  might 
  emit 
  unpolarized 
  light. 
  The 
  vapour 
  

   was 
  accordingly 
  excited 
  w 
  r 
  ith 
  the 
  light 
  of 
  the 
  cadmium 
  arc, 
  

   and 
  the 
  spectrum 
  examined 
  with 
  a 
  small 
  spectroscope 
  furnished 
  

   with 
  a 
  nicol 
  prism. 
  It 
  was 
  found, 
  however, 
  that 
  the 
  polariza- 
  

   tion 
  was 
  quite 
  as 
  strong 
  in 
  all 
  parts 
  of 
  the 
  spectrum 
  as 
  it 
  was 
  

   at 
  the 
  lines 
  coinciding 
  in 
  position 
  with 
  the 
  exciting 
  lines. 
  As 
  

   a 
  check 
  on 
  this 
  observation 
  the 
  following 
  experiment 
  was 
  

   then 
  tried. 
  The 
  light 
  from 
  the 
  arc 
  was 
  passed 
  through 
  a 
  

   filter 
  which 
  removed 
  everything 
  above 
  wave-length 
  5000, 
  i. 
  e* 
  

   which 
  transmitted 
  a 
  deep 
  blue 
  light. 
  In 
  front 
  of 
  the 
  Savart 
  

   polariscope 
  I 
  placed 
  a 
  dense 
  screen 
  of 
  aurantia 
  which 
  cut 
  off 
  

   everything 
  below 
  5000. 
  This 
  removed 
  all 
  the 
  wave-lengths 
  

   from 
  the 
  fluorescent 
  spectrum 
  which 
  were 
  to 
  be 
  found 
  in 
  the 
  

   exciting 
  light, 
  transmitting, 
  however, 
  the 
  upper 
  end 
  of 
  the 
  

   fluorescent 
  spectrum. 
  This 
  light 
  was 
  found 
  to 
  be 
  strongly 
  

   polarized, 
  the 
  fringes 
  appearing 
  quite 
  as 
  distinct 
  as 
  in 
  the 
  

   ab><>nce 
  of 
  the 
  screens. 
  It 
  will 
  be 
  observed 
  that 
  this 
  experi- 
  

   ment 
  completely 
  eliminates 
  nil 
  possible 
  sources 
  of 
  error, 
  such 
  

   as 
  polarization 
  produced 
  byfog 
  or 
  by 
  reflexion 
  from 
  the 
  back 
  

   wall 
  of 
  the 
  tube. 
  

  

  The 
  I) 
  lines 
  appear 
  in 
  the 
  fluoresced 
  spectrum 
  stimulated 
  

   by 
  white 
  light, 
  and 
  the 
  spectroscope 
  showed 
  that 
  they 
  were 
  

   polarized 
  to 
  about 
  the 
  Bame 
  degree 
  as 
  the 
  rest 
  of 
  the 
  spectrum. 
  

   A- 
  1 
  have 
  shown, 
  the 
  D 
  lines 
  can 
  also 
  be 
  caused 
  to 
  appear 
  by 
  

   stimulation 
  with 
  blue 
  light, 
  but 
  in 
  this 
  case 
  they 
  are 
  too 
  faint 
  

   to 
  make 
  a 
  study 
  of 
  their 
  polarization 
  possible. 
  

  

  

  