﻿Dispersion 
  of 
  Sodium 
  Vapour. 
  137 
  

  

  might 
  be 
  possible 
  to 
  do 
  away 
  with 
  glass 
  surfaces 
  entirely, 
  

   moulding 
  the 
  vapour 
  into 
  the 
  required 
  prismatic 
  form 
  by 
  the 
  

   proximity 
  of 
  cold 
  bodies. 
  Experiments 
  along 
  these 
  lines 
  

   were 
  partially 
  successful. 
  Two 
  pieces 
  of 
  thick-walled 
  iron 
  

   tubing, 
  the 
  ends 
  of 
  which 
  had 
  been 
  cut 
  off 
  at 
  an 
  angle, 
  were 
  

   introduced 
  into 
  a 
  glass 
  tube, 
  and 
  the 
  sodium 
  placed 
  in 
  the 
  

   clear 
  space 
  between 
  the 
  bevelled 
  ends, 
  as 
  is 
  shown 
  in 
  fig. 
  5. 
  

   The 
  ends 
  of 
  the 
  tube 
  were 
  closed 
  in 
  the 
  usual 
  manner 
  and 
  the 
  

  

  whole 
  mounted 
  between 
  the 
  collimator 
  and 
  telescope, 
  in 
  such 
  

   a 
  position 
  that 
  the 
  prism 
  formed 
  by 
  the 
  sloping 
  ends 
  of 
  the 
  

   iron 
  tubes 
  stood 
  with 
  its 
  refracting 
  edge 
  vertical. 
  This 
  was 
  

   necessary, 
  for 
  in 
  any 
  other 
  position 
  the 
  refraction 
  due 
  to 
  the 
  

   non-homogeneity 
  of 
  the 
  vapour 
  would 
  have 
  made 
  itself 
  felt. 
  

   With 
  a 
  vertical 
  slit 
  and 
  a 
  vertical 
  prism 
  no 
  lateral 
  deviation 
  

   could 
  result 
  from 
  this 
  cause. 
  It 
  was 
  hoped 
  that 
  the 
  sodium 
  

   vapour 
  formed 
  by 
  sudden 
  and 
  rapid 
  heating 
  of 
  the 
  glass 
  tube 
  

   would 
  refuse 
  to 
  enter 
  the 
  colder 
  iron 
  tube, 
  and 
  that 
  a 
  pris- 
  

   matic 
  form 
  would 
  result 
  from 
  the 
  bevelled 
  ends. 
  The 
  slit 
  of 
  

   the 
  spectroscope 
  was 
  illuminated 
  with 
  monochromatic 
  light 
  

   obtained 
  by 
  prismatic 
  analysis, 
  the 
  wave-length 
  of 
  which 
  

   could 
  be 
  changed 
  by 
  turning 
  the 
  prism, 
  which 
  was 
  furnished 
  

   with 
  a 
  mirror 
  in 
  the 
  manner 
  described 
  by 
  Wadsworth. 
  

  

  On 
  heating 
  the 
  tube 
  the 
  black 
  vapour 
  was 
  seen 
  to 
  spread 
  

   out 
  and 
  fill 
  the 
  prismatic 
  clear 
  space 
  between 
  the 
  iron 
  tubes, 
  

   while 
  the 
  lateral 
  deviation 
  of 
  the 
  image 
  of 
  the 
  slit, 
  as 
  observed 
  

   in 
  the 
  telescope, 
  indicated 
  that, 
  to 
  a 
  certain 
  extent 
  at 
  least, 
  

   the 
  desired 
  prismatic 
  form 
  had 
  been 
  secured. 
  In 
  the 
  extreme 
  

   red 
  the 
  deviation 
  was 
  very 
  slight, 
  but 
  as 
  the 
  spectrum 
  was 
  

   advanced 
  across 
  the 
  slit 
  by 
  slowly 
  turning 
  the 
  prism 
  the 
  image 
  

   in 
  the 
  telescope 
  moved 
  off 
  to 
  one 
  side, 
  the 
  deviation 
  in 
  this 
  

   direction 
  reaching 
  its 
  maximum 
  value 
  just 
  before 
  the 
  wave- 
  

   length 
  reached 
  that 
  of 
  the 
  sodium 
  lines. 
  At 
  this 
  point 
  the 
  

   image 
  jumped 
  abruptly 
  to 
  the 
  other 
  side 
  just 
  as 
  we 
  should 
  

   expect 
  it 
  to 
  do 
  on 
  crossing 
  the 
  D 
  lines 
  in 
  the 
  spectrum, 
  and 
  

   from 
  now 
  on 
  the 
  image 
  slowly 
  crawled 
  back 
  to 
  its 
  undeviated 
  

   position. 
  The 
  focal 
  length 
  of 
  the 
  telescope 
  was 
  460 
  mm., 
  

   and 
  the 
  maximum 
  deviation 
  of 
  the 
  rays 
  adjacent 
  to 
  the 
  

   D 
  lines 
  on 
  the 
  red 
  side, 
  as 
  measured 
  by 
  an 
  eyepiece 
  filar 
  

   micrometer, 
  was 
  but 
  1 
  mm., 
  while 
  the 
  deviation 
  in 
  the 
  opposite 
  

   direction 
  of 
  the 
  rays 
  on 
  the 
  other 
  side 
  of 
  the 
  D 
  lines 
  was 
  

  

  