﻿Selective 
  Reflexion 
  of 
  Monochromatic 
  Light. 
  187 
  

  

  No 
  objection 
  can 
  be 
  made 
  to 
  these 
  results 
  on 
  the 
  ground 
  

   o£ 
  varying 
  intensity 
  of 
  sound, 
  as 
  the 
  measurements 
  o£ 
  each 
  

   of 
  the 
  parts, 
  left, 
  middle, 
  and 
  right, 
  are 
  for 
  the 
  same 
  set 
  o£ 
  

   ripples, 
  and 
  thus 
  no 
  doubt 
  remains 
  that 
  the 
  distance 
  between 
  

   the 
  ripples 
  diminishes 
  in 
  going 
  from 
  the 
  antinode 
  to 
  the 
  

   node. 
  

  

  My 
  thanks 
  are 
  due 
  to 
  Prof. 
  Greh. 
  Rat 
  Riecke 
  for 
  his 
  kind 
  

   advice 
  and 
  encouragement 
  during 
  the 
  progress 
  of 
  this 
  work, 
  

   which 
  was 
  carried 
  out 
  in 
  the 
  Physical 
  Laboratory 
  of 
  the 
  

   University 
  of 
  Gottingen. 
  

  

  The 
  Physical 
  Laboratory, 
  ^ 
  

  

  University 
  of 
  Gotti 
  no-en, 
  \ 
  / 
  

  

  April 
  2-2, 
  1909. 
  " 
  \ 
  ' 
  

  

  XX. 
  The 
  SeUctive 
  Reflexion 
  of 
  Monocliromatic 
  Light 
  hi/ 
  Mercury 
  

   Vapour. 
  By 
  R. 
  W. 
  WooD^ 
  Professor 
  of 
  Experimental 
  

   Physics, 
  Johns 
  Hopkins 
  University* 
  . 
  

  

  [Plate 
  III.] 
  

  

  THE 
  present 
  paper 
  forms 
  the 
  first 
  of 
  a 
  series 
  now 
  in 
  pre- 
  

   paration 
  upon 
  the 
  optical 
  properties 
  of 
  mercury 
  vapour, 
  

   which 
  has 
  yielded 
  results 
  quite 
  as 
  interesting 
  as 
  those 
  which 
  

   have 
  been 
  obtained 
  with 
  the 
  vapour 
  of 
  metallic 
  sodium^ 
  though 
  

   of 
  a 
  somewhat 
  different 
  nature. 
  

  

  The 
  vapour 
  of 
  mercury 
  has 
  a 
  strong 
  and 
  narrow 
  absorption 
  

   line 
  in 
  the 
  ultra-violet 
  at 
  w^ave-length 
  2536* 
  7, 
  not 
  unlike 
  one 
  

   of 
  the 
  D 
  lines 
  of 
  sodium 
  in 
  character. 
  I 
  have 
  already 
  

   published 
  some 
  observations 
  of 
  this 
  very 
  remarkable 
  asym- 
  

   metrical 
  band 
  in 
  the 
  Astrophysical 
  Journal 
  t- 
  

  

  There 
  are 
  a 
  number 
  of 
  other 
  absorption 
  lines 
  and 
  bands, 
  

   but 
  it 
  is 
  with 
  the 
  one 
  above 
  mentioned 
  that 
  we 
  are 
  chiefly 
  

   concerned 
  in 
  the 
  present 
  paper. 
  There 
  is 
  a 
  fainter 
  line 
  at 
  

   2539*3, 
  which 
  fuses 
  with 
  the 
  stronger 
  line 
  as 
  soon 
  as 
  the 
  

   vapour 
  acquires 
  any 
  considerable 
  density. 
  

  

  Planck's 
  theory 
  of 
  absorption 
  is 
  based 
  upon 
  the 
  supposition 
  

   that 
  the 
  energy, 
  taken 
  from 
  the 
  oncoming 
  waves, 
  is 
  re-emitted 
  

   laterally 
  by 
  the 
  resonators. 
  Though 
  it 
  is 
  my 
  opinion 
  that 
  

   this 
  re-emission 
  only 
  occurs 
  in 
  exceptional 
  cases, 
  we 
  do 
  find 
  

   it 
  in 
  some 
  instances. 
  As 
  I 
  have 
  shown 
  in 
  a 
  previous 
  paper, 
  

   sodium 
  vapour, 
  when 
  illuminated 
  with 
  sodium 
  light, 
  emits 
  

   the 
  absorbed 
  wave-lengths 
  without 
  change. 
  The 
  emission 
  is 
  

   diffuse, 
  however, 
  that 
  is, 
  it 
  is 
  scattered 
  in 
  all 
  directions. 
  

   Radiation 
  of 
  this 
  nature 
  I 
  have 
  called 
  resonance 
  radiation, 
  to 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  t 
  " 
  Chanjre 
  in 
  the 
  appearance 
  and 
  apparent 
  position 
  of 
  an 
  absorption 
  

   band 
  caused 
  by 
  the 
  presence 
  of 
  an 
  inert 
  gas." 
  

  

  