﻿of 
  Monochromatic 
  Light 
  hy 
  Mercury 
  Vapour, 
  193 
  

  

  reflexion 
  from 
  an 
  absorbing 
  vapour 
  is 
  the 
  very 
  great 
  density 
  

   necessary 
  before 
  the 
  phenomenon 
  is 
  exhibited. 
  In 
  a 
  gas 
  at 
  

   atmospheric 
  pressure 
  the 
  molecules 
  are 
  so 
  close 
  together 
  that 
  

   there 
  will 
  be 
  about 
  80 
  to 
  the 
  wave-length, 
  i. 
  e. 
  6400 
  in 
  a 
  

   square 
  the 
  sides 
  of 
  which 
  are 
  equal 
  to 
  a 
  light-wave 
  in 
  length. 
  

   This 
  would 
  appear 
  to 
  be 
  more 
  than 
  sufficient 
  for 
  the 
  appli- 
  

   cation 
  of 
  Huygens's 
  principle 
  : 
  experiment 
  shows, 
  however, 
  

   that 
  the 
  reflexion 
  does 
  not 
  occur 
  until 
  we 
  increase 
  this 
  density 
  

   tenfold. 
  I 
  am 
  inclined 
  to 
  think 
  that 
  it 
  is 
  a 
  question 
  of 
  the 
  

   suddenness 
  with 
  which 
  the 
  wave 
  is 
  stopped, 
  rather 
  than 
  of 
  

   packing 
  the 
  resonators 
  close 
  enough 
  together 
  to 
  make 
  the 
  

   application 
  of 
  Huygens^s 
  principle 
  possible. 
  It 
  appears 
  to 
  

   me 
  probable 
  that 
  if 
  the 
  wave-train 
  can 
  penetrate 
  to 
  a 
  depth 
  

   ■of 
  several 
  wave-lengths 
  into 
  the 
  medium, 
  there 
  will 
  be 
  no 
  

   regular 
  reflexion, 
  regardless 
  of 
  the 
  proximity 
  of 
  the 
  resonators. 
  

   An 
  analogous 
  case 
  is 
  that 
  of 
  two 
  media 
  of 
  very 
  different 
  

   refractive 
  indices, 
  between 
  which 
  the 
  transition 
  is 
  gradual 
  

   instead 
  of 
  abrupt. 
  Some 
  authorities 
  hold 
  that 
  reflexion 
  will 
  

   •occur 
  in 
  this 
  case, 
  arguing, 
  if 
  I 
  understand 
  them 
  correctly, 
  

   that 
  we 
  can 
  divide 
  the 
  transition 
  layer 
  into 
  a 
  large 
  number 
  

   of 
  planes, 
  each 
  one 
  of 
  which 
  will 
  reflect 
  a 
  small 
  amount. 
  

   Even 
  if 
  this 
  were 
  the 
  case 
  it 
  appears 
  to 
  me 
  that 
  we 
  should 
  

   have 
  complete 
  destructive 
  interference, 
  for 
  the 
  wave-trains 
  

   reflected 
  from 
  the 
  hypothetical 
  planes 
  would 
  be 
  gradually 
  and 
  

   progressively 
  displaced 
  with 
  reference 
  to 
  each 
  other 
  and 
  give 
  

   us 
  zero 
  for 
  a 
  resultant. 
  The 
  same 
  thing 
  may 
  be 
  considered 
  

   as 
  taking 
  place 
  in 
  the 
  case 
  of 
  the 
  resonators. 
  A 
  rigorous 
  

   theoretical 
  treatment 
  of 
  the 
  resultant 
  effect 
  of 
  the 
  radiations 
  

   from 
  a 
  system 
  of 
  closely 
  packed 
  resonators, 
  excited 
  by 
  plane 
  

   waves, 
  considering 
  several 
  different 
  depths 
  of 
  penetration, 
  is 
  

   much 
  to 
  be 
  desired. 
  At 
  the 
  present 
  time 
  apparatus 
  is 
  

   being 
  constructed 
  for 
  the 
  study 
  of 
  the 
  anomalous 
  dis- 
  

   persion 
  of 
  the 
  vapour. 
  As 
  it 
  is 
  possible 
  to 
  work 
  under 
  

   known 
  conditions 
  of 
  temperature, 
  pressure, 
  and 
  density, 
  the 
  

   results 
  will 
  doubtless 
  be 
  of 
  greater 
  theoretical 
  interest 
  than 
  

   those 
  obtained 
  with 
  sodium 
  vapour. 
  The 
  work 
  on 
  the 
  

   fluorescence 
  and 
  absorption 
  is 
  completed. 
  

  

  In 
  the 
  study 
  of 
  the 
  optical 
  properties 
  of 
  mercury 
  vapour 
  

   I 
  have 
  been 
  aided 
  by 
  a 
  grant 
  from 
  the 
  Humford 
  fund 
  of 
  the 
  

   American 
  Academy. 
  

  

  Phil 
  Mag. 
  S. 
  6. 
  Vol. 
  18. 
  Xo. 
  103. 
  July 
  1909. 
  

  

  