﻿142 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Anomalous 
  

  

  of 
  the 
  D 
  lines, 
  but 
  I 
  do 
  not 
  feel 
  absolutely 
  sure 
  of 
  it. 
  It 
  will 
  

   be 
  interesting 
  to 
  see 
  whether 
  the 
  fluorescence 
  persists 
  for 
  an 
  

   appreciable 
  time 
  after 
  the 
  light 
  is 
  cut 
  off, 
  which 
  can 
  be 
  easily 
  

   determined 
  with 
  an 
  especially 
  designed 
  phosphoroscope, 
  a 
  

   matter 
  which 
  I 
  expect 
  to 
  take 
  up 
  next. 
  

  

  A 
  quantitative 
  study 
  of 
  the 
  absorption 
  of 
  the 
  vapour 
  is 
  

   extremely 
  desirable, 
  though 
  the 
  experimental 
  difficulties 
  will 
  

   be 
  very 
  great. 
  As 
  I 
  have 
  already 
  said, 
  the 
  width 
  of 
  the 
  band 
  

   at 
  the 
  D 
  lines 
  is 
  often 
  from 
  ten 
  to 
  fifteen 
  times 
  the 
  distance 
  

   between 
  the 
  lines. 
  Julius 
  has, 
  however, 
  called 
  attention 
  to 
  

   the 
  fact 
  that 
  we 
  must 
  be 
  on 
  our 
  guard 
  against 
  attributing 
  the 
  

   absence 
  of 
  light 
  in 
  the 
  spectrum 
  to 
  absorption, 
  when 
  the 
  

   conditions 
  are 
  such 
  that 
  the 
  rays 
  may 
  have 
  been 
  merely 
  

   turned 
  to 
  one 
  side 
  by 
  refraction. 
  This 
  he 
  believes, 
  if 
  I 
  read 
  

   his 
  paper 
  rightly, 
  is 
  the 
  true 
  explanation 
  of 
  the 
  widening 
  of 
  

   the 
  D 
  lines 
  in 
  the 
  absorption-spectrum. 
  To 
  eliminate 
  the 
  

   possibility 
  of 
  this 
  lateral 
  deviation 
  it 
  is 
  necessary, 
  if 
  we 
  are 
  

   working 
  with 
  a 
  non-homogeneous 
  medium, 
  to 
  arrange 
  matters 
  

   so 
  that 
  the 
  rays 
  of 
  light 
  are 
  perpendicular 
  to 
  the 
  surfaces 
  of 
  

   equal 
  refractive 
  index, 
  instead 
  of 
  parallel, 
  as 
  is 
  the 
  case 
  in 
  the 
  

   dispersion-tubes. 
  One 
  olmous 
  method 
  of 
  getting 
  round 
  the 
  

   difficulty 
  would 
  be 
  to 
  vaporize 
  the 
  sodium 
  in 
  a 
  vacuum, 
  thus 
  

   doing 
  away 
  with 
  the 
  variations 
  in 
  the 
  density, 
  but 
  this 
  

   necessitates 
  contact 
  between 
  the 
  corrosive 
  vapour 
  and 
  the 
  

   glass. 
  A 
  better 
  plan 
  appeared 
  to 
  be 
  to 
  send 
  the 
  light 
  through 
  

   the 
  stratified 
  vapour 
  in 
  such 
  a 
  direction 
  that 
  no 
  lateral 
  

   deviation 
  could 
  result, 
  a 
  matter 
  of 
  some 
  difficulty 
  until 
  the 
  

   expedient 
  occurred 
  to 
  me 
  of 
  using 
  the 
  surface 
  of 
  the 
  molten 
  

   metal 
  as 
  a 
  reflector, 
  the 
  rays 
  thus 
  twice 
  traversing 
  the 
  non- 
  

   homogeneous 
  medium 
  in 
  a 
  direction 
  perpendicular 
  to 
  the 
  

   equi-indicial 
  surfaces. 
  

  

  A 
  photograph 
  of 
  the 
  absorption-spectrum 
  of 
  the 
  vapour 
  in 
  

   the 
  red 
  and 
  yellow 
  region, 
  obtained 
  with 
  a 
  10-foot 
  concave 
  

   grating, 
  is 
  reproduced 
  on 
  Plate 
  IV. 
  fig. 
  12. 
  The 
  D 
  lines 
  were 
  

   photographed 
  just 
  below 
  the 
  spectrum 
  for 
  comparison. 
  The 
  

   flutings 
  in 
  the 
  red 
  are 
  well 
  shown 
  in 
  this 
  picture, 
  which 
  is 
  

   but 
  one 
  of 
  a 
  large 
  number 
  that 
  have 
  been 
  taken. 
  Measure- 
  

   ments 
  of 
  wave-length 
  have 
  not 
  yet 
  been 
  made, 
  as 
  I 
  feel 
  

   confident 
  of 
  getting 
  better 
  plates 
  in 
  the 
  near 
  future. 
  The 
  fine 
  

   dark 
  lines 
  can 
  be 
  found 
  on 
  both 
  sides 
  of 
  the 
  heavy 
  band 
  at 
  

   the 
  D 
  lines, 
  and 
  I 
  have 
  found 
  that 
  they 
  are 
  present 
  throughout 
  

   the 
  entire 
  spectrum. 
  An 
  enlargement 
  of 
  a 
  portion 
  of 
  fig. 
  12 
  

   is 
  shown 
  in 
  fig. 
  13 
  (PI. 
  IV.) 
  which 
  gives 
  a 
  better 
  idea 
  of 
  the 
  

   appearance 
  of 
  the 
  fluted 
  bands. 
  

  

  On 
  increasing 
  the 
  density 
  of 
  the 
  vapour 
  the 
  red 
  end 
  of 
  the 
  

   spectrum 
  finally 
  disappears, 
  and 
  by 
  employing 
  a 
  very 
  powerful 
  

  

  