﻿188 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Selective 
  Reflexion 
  

  

  distinojuish 
  it 
  from 
  fluorescence, 
  in 
  which 
  case 
  there 
  is 
  a 
  

   change 
  of 
  wave-length. 
  

  

  I 
  predicted 
  some 
  years 
  ago 
  that 
  if 
  the 
  molecular 
  resonators 
  

   were 
  packed 
  closely 
  enough 
  together^ 
  the 
  secondary 
  wavelets 
  

   which 
  they 
  emit, 
  having 
  a 
  definite 
  phase 
  relation, 
  would 
  

   unite 
  into 
  a 
  single 
  wave, 
  and 
  the 
  scattered 
  light 
  w^ould 
  dis- 
  

   appear, 
  regular 
  reflexion 
  of 
  the 
  absorbed 
  light 
  taking 
  its 
  

   place. 
  Repeated 
  efforts 
  to 
  discover 
  the 
  phenomenon 
  with 
  

   sodium 
  vapour 
  yielded 
  negative 
  results, 
  since 
  it 
  was 
  impossible 
  

   to 
  obtain 
  the 
  vapour 
  at 
  great 
  density 
  with 
  a 
  sharply 
  defined 
  

   surface, 
  on 
  account 
  of 
  its 
  corrosive 
  action 
  upon 
  the 
  trans- 
  

   parent 
  walls 
  of 
  the 
  containing 
  vessel. 
  

  

  In 
  the 
  course 
  of 
  a 
  long 
  series 
  of 
  experiments, 
  which 
  I 
  have 
  

   been 
  making 
  upon 
  the 
  fluorescence, 
  dispersion, 
  magnetic 
  

   rotation,, 
  &c. 
  of 
  mercury 
  vapour, 
  it 
  occurred 
  to 
  me 
  that 
  this 
  

   substance 
  was 
  well 
  suited 
  to 
  the 
  study 
  of 
  the 
  phenomenon, 
  if 
  

   it 
  existed, 
  for 
  it 
  is 
  not 
  difficult 
  to 
  obtain 
  the 
  vapour 
  at 
  a 
  

   pressure 
  of 
  20 
  or 
  30 
  atmospheres, 
  in 
  bulbs 
  of 
  fused 
  quartz. 
  

  

  To 
  separate 
  the 
  images 
  formed 
  by 
  reflexion 
  from 
  the 
  inner 
  

   and 
  outer 
  surfaces 
  of 
  the 
  bulb, 
  the 
  necks 
  were 
  made 
  of 
  very 
  

   thick-walled 
  tubing, 
  so 
  that 
  the 
  walls 
  of 
  the 
  bulb 
  were 
  

  

  Fig. 
  1. 
  

  

  prismatic 
  as 
  show^n 
  in 
  fig. 
  1. 
  These 
  bulbs 
  were 
  made 
  

   especially 
  for 
  the 
  work 
  by 
  Heraeus 
  of 
  Hanau, 
  and 
  were 
  found 
  

   most 
  satisfactory. 
  

  

  A 
  good-sized 
  globule 
  of 
  mercury 
  was 
  placed 
  in 
  a 
  bulb 
  

   which 
  was 
  then 
  highly 
  exhausted 
  and 
  sealed. 
  

  

  It 
  seemed 
  best 
  to 
  begin 
  by 
  using 
  b'ght 
  of 
  exactly 
  the 
  right 
  

   frequency, 
  that 
  is 
  of 
  a 
  wave-length 
  identical 
  with 
  that 
  of 
  the 
  

   absorption 
  line. 
  The 
  light 
  from 
  the 
  mercury 
  arc 
  in 
  a 
  quartz 
  

   tube 
  shows 
  a 
  strong 
  line 
  of 
  exactly 
  the 
  right 
  frequency. 
  

   The 
  bulb 
  was 
  mounted 
  in 
  a 
  small 
  tube 
  of 
  thin 
  steel 
  provided 
  

   with 
  an 
  oval 
  aperture 
  in 
  the 
  side 
  : 
  the 
  ends 
  were 
  closed 
  with 
  

   disks 
  of 
  asbestos 
  board, 
  one 
  of 
  which 
  supported 
  the 
  neck 
  of 
  

   the 
  bulb; 
  as 
  shown 
  in 
  fig. 
  1. 
  The 
  steel 
  tube 
  was 
  heated 
  by 
  

   two 
  Bunsen 
  burners, 
  usually 
  to 
  a 
  full 
  red 
  heat, 
  and 
  the 
  

   mercury 
  arc 
  placed 
  as 
  close 
  as 
  possible 
  to 
  the 
  aperture 
  and 
  a 
  

   little 
  to 
  one 
  side, 
  so 
  that 
  its 
  image 
  appeared 
  reflected 
  in 
  the 
  

   tapering 
  neck 
  of 
  the 
  bulb, 
  as 
  shown 
  in 
  the 
  figure. 
  The 
  flame 
  

  

  