﻿of 
  Monochromatic 
  Light 
  hy 
  Mercury 
  Vapour, 
  189 
  

  

  of 
  the 
  burner 
  must 
  play 
  over 
  the 
  aperture 
  to 
  prevent 
  con- 
  

   densation 
  of 
  mercury 
  drops 
  at 
  the 
  point 
  where 
  reflexion 
  

   occurs. 
  

  

  By 
  properly 
  choosing 
  the 
  direction 
  in 
  which 
  the 
  bulb 
  was 
  

   viewed^ 
  the 
  reflexion 
  from 
  the 
  outer 
  surface 
  disappeared, 
  and 
  

   the 
  slit 
  of 
  a 
  small 
  quartz 
  spectrograph 
  was 
  directed 
  towards 
  

   the 
  bright 
  image 
  of 
  the 
  arc 
  reflected 
  from 
  the 
  inner 
  surface 
  

   of 
  the 
  wall. 
  

  

  A 
  number 
  of 
  photographs 
  of 
  the 
  spectrum 
  of 
  the 
  reflected 
  

   light 
  were 
  taken, 
  the 
  first 
  with 
  the 
  bulb 
  cold, 
  the 
  succeeding 
  

   ones 
  at 
  gradually 
  increasing 
  temperatures. 
  It 
  was 
  found 
  

   that 
  the 
  relative 
  intensity 
  of 
  the 
  2536 
  "7 
  line 
  in 
  the 
  spectrum 
  

   of 
  the 
  reflected 
  light 
  increased 
  rapidly 
  as 
  the 
  temperature 
  of 
  

   the 
  bulb 
  increased. 
  A 
  series 
  of 
  these 
  photographs 
  is 
  repro- 
  

   duced 
  on 
  PI. 
  III. 
  fig. 
  1. 
  The 
  time 
  of 
  exposure 
  was 
  less 
  in 
  

   the 
  case 
  of 
  the 
  red-hot 
  bulb, 
  in 
  spite 
  of 
  which 
  the 
  line 
  2536 
  

   is 
  very 
  strong 
  while 
  the 
  other 
  lines 
  do 
  not 
  appear 
  at 
  all. 
  

  

  Previous 
  work 
  having 
  shown, 
  however, 
  that 
  mercury 
  

   vapour 
  emits 
  scattered 
  resonance 
  radiation, 
  when 
  stimulated 
  

   by 
  the 
  light 
  of 
  the 
  2536 
  line, 
  it 
  was 
  next 
  necessary 
  to 
  prove 
  

   that 
  it 
  was 
  not 
  a 
  diffuse 
  radiation 
  which 
  had 
  caused 
  the 
  

   increased 
  intensity 
  of 
  the 
  line. 
  A 
  very 
  nice 
  method 
  was 
  

   found 
  of 
  proving 
  this. 
  If 
  our 
  eyes 
  were 
  sensitive 
  to 
  the 
  

   ultra-violet 
  light, 
  we 
  should 
  see, 
  in 
  the 
  case 
  of 
  the 
  diffuse 
  

   emission, 
  the 
  entire 
  surface 
  of 
  the 
  bulb 
  glowing 
  with 
  light, 
  

   while 
  if 
  the 
  radiation 
  was 
  regularly 
  (i. 
  e. 
  specularly) 
  reflected 
  

   by 
  the 
  vapour, 
  we 
  should 
  see 
  merely 
  the 
  small 
  image 
  of 
  the 
  

   arc 
  increase 
  in 
  brilliancy. 
  

  

  The 
  steel 
  tube 
  was 
  mounted 
  vertically 
  in 
  such 
  a 
  position 
  

   that 
  the 
  two 
  images 
  due 
  to 
  reflexion 
  from 
  the 
  inner 
  and 
  

   outer 
  surfaces 
  of 
  the 
  front 
  wall 
  of 
  the 
  bulb 
  appeared 
  one 
  

   above 
  the 
  other. 
  Inasmuch 
  as 
  the 
  mercury 
  arc 
  consisted 
  of 
  

   a 
  narrow 
  vertical 
  column 
  of 
  light, 
  these 
  images 
  appeared 
  as 
  

   narrow 
  vertical 
  lines 
  of 
  light, 
  and 
  could 
  be 
  used 
  in 
  place 
  of 
  

   the 
  slit 
  of 
  the 
  spectrograph. 
  The 
  arc 
  in 
  this 
  case 
  was 
  placed 
  

   at 
  a 
  distance 
  of 
  about 
  a 
  metre 
  from 
  the 
  bulb. 
  

  

  The 
  slit 
  tube 
  of 
  the 
  spectrograph 
  was 
  removed, 
  and 
  the 
  

   instrument 
  placed 
  in 
  such 
  a 
  position 
  that 
  the 
  two 
  reflected 
  

   images 
  occupied 
  the 
  position 
  previously 
  occupied 
  by 
  the 
  slit. 
  

   On 
  exposing 
  a 
  plate 
  we 
  obtain 
  two 
  spectra 
  one 
  above 
  the 
  

   other, 
  the 
  one 
  that 
  of 
  the 
  light 
  reflected 
  from 
  the 
  outer 
  

   surface, 
  the 
  other 
  that 
  of 
  the 
  light 
  reflected 
  from 
  the 
  inner 
  

   surface. 
  

  

  Two 
  photographs 
  were 
  taken, 
  one 
  with 
  the 
  bulb 
  cold, 
  the 
  

   other 
  with 
  the 
  bulb 
  red-hot. 
  The 
  latter 
  showed 
  that 
  the 
  

   ima<ie 
  of 
  the 
  arc 
  as 
  seen 
  in 
  the 
  2536 
  liofht 
  had 
  been 
  increased 
  

  

  