﻿190 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Selective 
  Reflexion 
  

  

  tremendously 
  in 
  brilliancy 
  by 
  the 
  presence 
  of 
  the 
  mercury 
  

   vapour. 
  The 
  two 
  photographs 
  are 
  reproduced 
  on 
  PL 
  III. 
  

   fia. 
  2, 
  the 
  spectrum 
  image 
  formed 
  by 
  the 
  light 
  of 
  wave- 
  

   length 
  2536 
  being 
  indicated. 
  The 
  light 
  reflected 
  from 
  the 
  

   inner 
  surface 
  is 
  not 
  as 
  intense 
  as 
  that 
  reflected 
  from 
  the 
  outer, 
  

   consequently 
  one 
  of 
  the 
  spectra 
  is 
  weaker 
  than 
  the 
  other 
  in 
  

   each 
  case. 
  The 
  2536 
  line 
  is, 
  however, 
  many 
  times 
  brighter 
  

   in 
  the 
  spectrum 
  of 
  the 
  image 
  formed 
  by 
  the 
  inner 
  surface 
  of 
  

   the 
  bulb 
  when 
  hot. 
  

  

  This 
  experiment 
  shows 
  that 
  the 
  mercury 
  vapour 
  reflects 
  

   light 
  of 
  this 
  particular 
  wave-length 
  in 
  much 
  the 
  same 
  way 
  as 
  

   would 
  a 
  coating 
  of 
  silver 
  on 
  the 
  inside 
  of 
  the 
  bulb. 
  

  

  Experiments 
  were 
  next 
  undertaken 
  to 
  ascertain 
  how 
  nearly 
  

   the 
  frequency 
  of 
  the 
  light 
  must 
  agree 
  with 
  that 
  of 
  the 
  

   absorption 
  band 
  in 
  order 
  that 
  metallic 
  reflexion 
  should 
  take 
  

   place. 
  

  

  It 
  was 
  found 
  that 
  the 
  spectrum 
  of 
  the 
  iron 
  arc 
  showed 
  a 
  

   group 
  of 
  closely 
  packed 
  lines 
  exactly 
  in 
  the 
  region 
  required, 
  

   and 
  it 
  was 
  accordingly 
  put 
  in 
  place 
  of 
  the 
  mercury 
  arc. 
  

   The 
  slit 
  was 
  replaced 
  on 
  the 
  quartz 
  spectrograph, 
  and 
  photo- 
  

   graphs 
  of 
  the 
  reflected 
  image 
  of 
  the 
  arc 
  were 
  taken 
  with 
  the 
  

   bulb 
  cold 
  and 
  heated 
  to 
  different 
  temperatures. 
  A 
  very 
  

   remarkable 
  discovery 
  was 
  at 
  once 
  made, 
  for 
  it 
  turned 
  out 
  

   that 
  the 
  iron 
  line 
  which 
  was 
  metallically 
  reflected 
  (2535*67) 
  

   Avas 
  about 
  one 
  Angstrom 
  unit 
  on 
  the 
  short 
  wave-length 
  side 
  

   of 
  the 
  absorption 
  line. 
  As 
  the 
  temperature 
  and 
  vapour 
  

   density 
  increased 
  a 
  second 
  iron 
  line 
  was 
  strongly 
  reflected, 
  

   this 
  one 
  coinciding 
  almost 
  exactly 
  with 
  the 
  absorption 
  line. 
  

   It 
  is 
  in 
  reality 
  a 
  double 
  line, 
  with 
  wave-lengths 
  2536'90 
  and 
  

   2537-21. 
  

  

  To 
  make 
  absolutely 
  sure 
  that 
  no 
  error 
  had 
  occurred, 
  I 
  

   photographed 
  the 
  spectrum 
  of 
  the 
  iron 
  arc, 
  passing 
  the 
  light 
  

   through 
  mercury 
  vapour 
  at 
  different 
  densities. 
  

  

  The 
  iron 
  line 
  which 
  first 
  disappeared 
  was 
  the 
  double 
  one, 
  

   which 
  was 
  not 
  reflected 
  until 
  the 
  mercary 
  vapour 
  was 
  at 
  its 
  

   greatest 
  density. 
  The 
  line 
  which 
  was 
  metallically 
  reflected 
  

   by 
  the 
  vapour 
  at 
  a 
  lesser 
  density 
  was 
  not 
  absorbed 
  by 
  the 
  

   vapour 
  even 
  when 
  its 
  density 
  was 
  so 
  great 
  that 
  four 
  or 
  five 
  

   lines 
  on 
  the 
  long 
  wave-length 
  side 
  of 
  the 
  line 
  first 
  absorbed, 
  

   were 
  completely 
  blotted 
  out. 
  This 
  can 
  be 
  better 
  understood 
  

   by 
  reference 
  to 
  fig. 
  2 
  (p. 
  191). 
  In 
  the 
  upper 
  line 
  we 
  have 
  

   the 
  group 
  of 
  iron 
  lines 
  in 
  question. 
  The 
  next 
  line 
  shows 
  the 
  

   absorption 
  by 
  Hg 
  vapour 
  when 
  its 
  density 
  is 
  small, 
  the 
  fourth 
  

   iron 
  line 
  (double, 
  mean 
  X 
  2537) 
  having 
  disappeared. 
  Just 
  

   below 
  is 
  the 
  absor[)tion 
  by 
  dense 
  vapour, 
  the 
  absorption 
  having 
  

   extended 
  towards 
  the 
  visible 
  region 
  of 
  the 
  spectrum 
  (much 
  

  

  