﻿of 
  Monochromatic 
  Light 
  hy 
  Mercury 
  Vapour. 
  191 
  

  

  farther 
  than 
  can 
  be 
  indicated 
  in 
  the 
  figure), 
  the 
  third 
  iron 
  

   line 
  being 
  still 
  transmitted, 
  however. 
  Below 
  this 
  is 
  the 
  

   reflexion 
  from 
  Hg 
  vapour 
  at 
  about 
  ten 
  atmospheres, 
  the 
  

   third 
  iron 
  line 
  being 
  strongly 
  reflected. 
  In 
  the 
  lower 
  line 
  

  

  Fi-. 
  2. 
  

  

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  1 
  1 
  

  

  

  

  

  

  

  

  

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  1 
  

  

  

  

  - 
  

  

  XI 
  

  

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  -. 
  

  

  ^ 
  

  

  - 
  

  

  - 
  

  

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  X 
  

  

  

  - 
  

  

  - 
  

  

  - 
  

  

  - 
  

  

  ~A 
  

  

  -^ 
  

  

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  1 
  

  

  - 
  

  

  1 
  

  

  

  II 
  li 
  

  

  u 
  

  

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  I 
  

  

  I 
  II 
  

  

  /r^^. 
  lines 
  

  

  /Ids. 
  ly 
  /^ 
  l/apor 
  

   • 
  denser 
  - 
  

  

  /fe/l. 
  6y 
  /^. 
  y^por 
  (fO/ltm.) 
  

  

  ■■ 
  a'enser 
  ■■ 
  (^5 
  /^t7?t) 
  

  

  f 
  /^osi^iOTi 
  of 
  //^. 
  Une 
  cpz 
  //g^./^rc. 
  

  

  we 
  have 
  the 
  reflexion 
  bj 
  vapour 
  at 
  25 
  atmospheres, 
  the 
  

   third 
  and 
  fourth 
  (double) 
  iron 
  lines 
  appearing 
  strongly 
  

   reflected. 
  This 
  fignre 
  is 
  a 
  diagram, 
  of 
  course. 
  Photographs 
  

   are 
  reproduced 
  in 
  figs. 
  3, 
  4, 
  5, 
  & 
  6 
  (PI. 
  III.). 
  Fig. 
  3 
  a 
  is 
  enlarged 
  

   Irom 
  fig. 
  5, 
  and 
  was 
  made 
  with 
  a 
  concave 
  grating 
  of 
  two 
  

   metres 
  radius. 
  The 
  powerful 
  reflexion 
  of 
  the 
  third 
  and 
  fourth 
  

   iron 
  lines 
  (indicated 
  by 
  arrows) 
  is 
  well 
  shown. 
  Fig. 
  3 
  h 
  

   shows 
  the 
  absorption 
  of 
  the 
  fourth 
  iron 
  line 
  by 
  the 
  vapour 
  at 
  

   small 
  density, 
  while 
  fig. 
  3 
  c 
  shows 
  that 
  the 
  reflexion 
  of 
  the 
  

   third 
  line 
  is 
  much 
  stronger 
  than 
  that 
  of 
  the 
  fourth 
  by 
  vapour 
  

   at 
  about 
  ten 
  atmospheres. 
  In 
  fig. 
  4 
  we 
  have 
  the 
  same 
  thing 
  

   shown 
  by 
  the 
  quartz 
  spectrograph, 
  the 
  upper 
  spectrum 
  

   reflected 
  from 
  rare, 
  the 
  lower 
  from 
  dense 
  mercury 
  vapour 
  ; 
  

   third 
  and 
  fourth 
  lines 
  indicated 
  by 
  arrows. 
  Fig. 
  5 
  was 
  made 
  

   with 
  the 
  grating, 
  and 
  shows 
  a 
  wider 
  range 
  of 
  the 
  spectrum 
  

   than 
  the 
  enlargement 
  ; 
  the 
  powerful 
  selective 
  reflexion 
  of 
  

   the 
  two 
  iron 
  lines 
  is 
  beautifully 
  shown. 
  Fig. 
  4 
  was 
  enlaro-ed 
  

   from 
  fig. 
  6 
  (quartz 
  spectrograph). 
  

  

  It 
  is 
  useless 
  to 
  speculate 
  about 
  the 
  cause 
  of 
  this 
  surprising 
  

   apparent 
  shift 
  of 
  the 
  reflexion 
  band 
  until 
  the 
  absorption 
  of 
  a 
  

   very 
  thin 
  layer 
  of 
  the 
  vapour 
  at 
  very 
  great 
  density 
  has 
  been 
  

   studied, 
  for 
  the 
  absorption 
  band 
  may 
  be 
  shifted 
  towards 
  the 
  

   short 
  wave-length 
  region 
  as 
  the 
  density 
  increases, 
  though 
  

   this 
  IS 
  precisely 
  the 
  opposite 
  to 
  what 
  we 
  should 
  expect. 
  I 
  

   have 
  attempted 
  to 
  study 
  this 
  with 
  small-bore 
  tubes, 
  but 
  the 
  

   thickness 
  of 
  the 
  layer 
  was 
  too 
  great. 
  

  

  