﻿Dispersion 
  of 
  Sodium 
  Vapour. 
  133 
  

  

  later 
  on, 
  the 
  breadth 
  of 
  the 
  absorption-band 
  is 
  sometimes 
  

   twenty 
  times 
  the 
  width 
  of 
  the 
  spectrum 
  comprised 
  within 
  

   the 
  D 
  lines. 
  

  

  The 
  oppositely-curved 
  branches 
  adjacent 
  to 
  the 
  region 
  of 
  

   absorption 
  grow 
  out 
  rapidly 
  as 
  the 
  tube 
  grows 
  hotter, 
  the 
  

   ends 
  finally 
  passing 
  out 
  of 
  the 
  field 
  of 
  the 
  instrument. 
  A 
  

   beautiful 
  fluted 
  absorption 
  appears 
  in 
  the 
  red 
  and 
  the 
  greenish- 
  

   blue, 
  which 
  finally 
  blots 
  out 
  a 
  region 
  in 
  the 
  blue 
  almost 
  

   entirely. 
  Meanwhile 
  the 
  curvature 
  of 
  the 
  spectrum 
  increases 
  

   in 
  a 
  most 
  remarkable 
  manner, 
  and 
  the 
  entire 
  red 
  end 
  is 
  

   lifted 
  high 
  above 
  the 
  green-blue 
  end. 
  As 
  the 
  density 
  of 
  the 
  

   vapour 
  increases 
  the 
  red 
  gradually 
  fades 
  away, 
  leaving 
  only 
  

   the 
  yellow 
  and 
  green 
  and 
  the 
  remote 
  blue 
  and 
  violet, 
  the 
  

   curvature 
  increasing 
  all 
  the 
  while. 
  The 
  fluted 
  or 
  channelled 
  

   spectrum 
  was 
  described 
  by 
  Roscoe 
  and 
  Schuster 
  about 
  

   twenty-five 
  years 
  ago, 
  but 
  so 
  far 
  as 
  I 
  know 
  no 
  work 
  has 
  

   been 
  done 
  on 
  it 
  since. 
  I 
  have 
  recently 
  secured 
  excellent 
  

   photographs 
  of 
  it 
  with 
  a 
  Rowland 
  concave 
  grating, 
  from 
  the 
  

   extreme 
  red 
  to 
  the 
  violet, 
  and 
  find 
  that 
  it 
  is 
  much 
  more 
  

   extensive 
  than 
  has 
  been 
  supposed, 
  for 
  the 
  flutings 
  run 
  right 
  

   up 
  to 
  the 
  absorption 
  - 
  band 
  at 
  the 
  JD 
  lines 
  on 
  both 
  sides, 
  

   though 
  they 
  are 
  very 
  faint 
  on 
  the 
  side 
  of 
  shorter 
  wave- 
  

   lengths. 
  This 
  spectrum 
  will 
  be 
  described 
  in 
  a 
  subsequent 
  

   paper. 
  

  

  Very 
  satisfactory 
  photographs 
  of 
  the 
  dispersed 
  grating- 
  

   spectrum 
  were 
  secured, 
  some 
  of 
  which 
  are 
  reproduced. 
  It 
  

   was 
  found 
  impossible 
  to 
  maintain 
  a 
  sufficiently 
  uniform 
  

   density, 
  at 
  the 
  low 
  temperature, 
  for 
  a 
  sufficient 
  length 
  of 
  

   time 
  to 
  enable 
  a 
  negative 
  to 
  be 
  secured 
  showing 
  the 
  

   appearance 
  before 
  the 
  light 
  between 
  the 
  D 
  lines 
  vanished. 
  

   I 
  therefore 
  went 
  back 
  to 
  the 
  old 
  plan 
  of 
  using 
  a 
  prismatic 
  

   flame. 
  After 
  some 
  experimenting 
  it 
  was 
  found 
  that 
  the 
  

   most 
  satisfactory 
  flame 
  was 
  secured 
  by 
  passing 
  hydrogen 
  

   through 
  a 
  tube 
  containing 
  metallic 
  sodium 
  and 
  strongly 
  

   heated, 
  and 
  burning 
  the 
  gas 
  at 
  a 
  flat 
  jet 
  piece 
  made 
  of 
  

   platinum 
  foil. 
  An 
  exceedingly 
  dense 
  and 
  very 
  uniform 
  

   sodium 
  flame 
  is 
  obtained 
  in 
  this 
  way, 
  which 
  can 
  be 
  main- 
  

   tained 
  almost 
  indefinitely. 
  The 
  arrangement 
  of 
  the 
  lamp 
  is 
  

   shown 
  in 
  fig. 
  3 
  (p. 
  134), 
  the 
  diagram 
  requiring 
  no 
  description. 
  

  

  With 
  this 
  flame 
  as 
  a 
  dispersing 
  piece 
  the 
  photographs 
  

   shown 
  in 
  Plate 
  III. 
  fig. 
  8 
  were 
  obtained. 
  In 
  comparison 
  with 
  

   some 
  of 
  the 
  other 
  photographs 
  this 
  example 
  is 
  very 
  poor, 
  and 
  

   I 
  have 
  included 
  it 
  merely 
  to 
  make 
  the 
  set 
  complete. 
  An 
  

   exposure 
  of 
  about 
  ten 
  minutes 
  was 
  necessary, 
  with 
  an 
  

   " 
  Erythro 
  " 
  plate. 
  Fig. 
  9 
  was 
  taken 
  with 
  one 
  of 
  the 
  

   dispersion-tubes, 
  and 
  shows 
  what 
  may 
  be 
  called 
  the 
  second 
  

  

  