﻿192 
  Prof. 
  E. 
  W. 
  Wood 
  on 
  the 
  Selective 
  Reflexion 
  

  

  It 
  will 
  be 
  necessary 
  to 
  have 
  a 
  cell 
  made 
  of 
  two 
  thick 
  plates 
  

   of 
  fused 
  quartz 
  separated 
  by 
  a 
  distance 
  of 
  say 
  O'Ol 
  mm., 
  and 
  

   capable 
  of 
  standing 
  a 
  pressure 
  of 
  at 
  least 
  ten 
  atmospheres. 
  

   I 
  have 
  ordered 
  the 
  cell, 
  made 
  in 
  prismatic 
  form, 
  with 
  the 
  

   plates 
  in 
  contact 
  at 
  one 
  end, 
  and 
  separated 
  by 
  a 
  distance 
  of 
  

   0*3 
  mm. 
  at 
  the 
  other. 
  They 
  will 
  be 
  fused 
  together 
  along 
  the 
  

   edges, 
  a 
  tube 
  being 
  provided 
  for 
  the 
  introduction 
  of 
  the 
  

   mercury 
  and 
  exhaustion. 
  This 
  cell 
  will 
  also 
  be 
  very 
  useful 
  

   in 
  the 
  further 
  study 
  of 
  the 
  reflecting 
  power 
  of 
  the 
  vapour 
  in 
  

   connexion 
  with 
  the 
  state 
  of 
  polarization 
  of 
  the 
  light. 
  There 
  

   are 
  still 
  many 
  interesting 
  points 
  to 
  investigate^ 
  and 
  I 
  am 
  at 
  

   ihe 
  present 
  time 
  devising 
  methods 
  and 
  apparatus 
  for 
  studying 
  

   the 
  gradual 
  transition 
  from 
  diffuse 
  scattering 
  of 
  the 
  radiation, 
  

   to 
  metallic 
  or 
  specular 
  reflexion. 
  It 
  seems 
  probable 
  that 
  a 
  

   good 
  deal 
  of 
  new 
  information 
  regarding 
  the 
  mechanism 
  of 
  

   reflexion 
  can 
  be 
  obtained 
  in 
  this 
  way. 
  

  

  I 
  have 
  already 
  made 
  a 
  preliminary 
  investigation 
  with 
  a 
  

   polarizing 
  prism. 
  The 
  mercury 
  arc 
  was 
  placed 
  in 
  such 
  a 
  

   position 
  that 
  the 
  reflexion 
  from 
  the 
  inner 
  surface 
  of 
  the 
  bulb 
  

   took 
  place 
  at 
  the 
  polarizing 
  angle. 
  The 
  image 
  from 
  the 
  outer 
  

   surface 
  was 
  hidden 
  by 
  a 
  screen. 
  The 
  narrow 
  line 
  of 
  light 
  

   was 
  then 
  photographed 
  with 
  a 
  quartz 
  lens 
  in 
  combination 
  

   with 
  one 
  half 
  of 
  a 
  quartz 
  Rochon 
  prism. 
  This 
  prism 
  gives 
  

   two 
  superposed 
  spectra 
  of 
  an 
  unpolarized 
  source, 
  one 
  polarized 
  

   vertically, 
  the 
  other 
  horizontally. 
  The 
  light 
  reflected 
  from 
  

   the 
  inner 
  surface 
  of 
  the 
  bulb, 
  being 
  polarized 
  by 
  the 
  reflexion, 
  

   yields 
  only 
  a 
  single 
  spectrum. 
  It 
  was 
  found, 
  however, 
  that 
  

   when 
  the 
  bulb 
  was 
  heated 
  red-hot 
  the 
  mercury 
  line 
  2536 
  

   appeared 
  double, 
  showing 
  that 
  the 
  vapour 
  reflects 
  unpolarized 
  

   light. 
  This 
  is 
  shown 
  in 
  PI. 
  III. 
  fig. 
  7. 
  The 
  upper 
  spectrum 
  

   is 
  of 
  the 
  light 
  reflected 
  from 
  the 
  hot 
  bulb, 
  the 
  lower 
  from 
  the 
  

   cold. 
  In 
  the 
  latter 
  the 
  line 
  to 
  the 
  extreme 
  left 
  is 
  the 
  2536 
  

   line. 
  In 
  the 
  upper 
  spectrum 
  we 
  find 
  this 
  line 
  not 
  only 
  rela- 
  

   tively 
  much 
  stronger, 
  but 
  appearing 
  in 
  duplicate 
  (indicated 
  

   by 
  the 
  arrow). 
  

  

  It 
  must 
  be 
  remembered 
  that 
  the 
  two 
  spectra 
  given 
  by 
  half 
  

   of 
  a 
  Rochon 
  prism 
  with 
  an 
  unpolarized 
  source 
  are 
  superposed, 
  

   the 
  deviations 
  being 
  different, 
  however. 
  A^ 
  will 
  be 
  seen, 
  the 
  

   2536 
  line 
  is 
  the 
  only 
  one 
  which 
  appears 
  twice, 
  that 
  is 
  in 
  the 
  

   two 
  spectra 
  which 
  the 
  prism 
  is 
  capable 
  of 
  forming. 
  The 
  

   metallically 
  reflected 
  light 
  being 
  unpolarized 
  is 
  equally 
  divided 
  

   between 
  the 
  two 
  spectra 
  furnished 
  by 
  the 
  prism. 
  Both 
  lines 
  

   are 
  therefore 
  much 
  brighter 
  than 
  the 
  one 
  in 
  the 
  lower 
  

   spectrum. 
  

  

  One 
  very 
  import 
  nt 
  poiat 
  in 
  connexion 
  with 
  the 
  specular 
  

  

  