﻿130 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Anomalous 
  

  

  sodium 
  flames, 
  which 
  have 
  been 
  made 
  to 
  assume 
  a 
  prismatic 
  

   form. 
  In 
  view 
  of 
  the 
  great 
  uncertainty 
  regarding- 
  the 
  prism 
  

   angle, 
  density 
  of 
  the 
  vapour, 
  and 
  its 
  molecular 
  condition, 
  

   that 
  is 
  whether 
  dissociated 
  or 
  not, 
  it 
  seemed 
  advisable 
  to 
  

   employ 
  non-luminous 
  vapour, 
  obtained 
  by 
  heating 
  metallic 
  

   sodium 
  in 
  some 
  inert 
  gas 
  or 
  in 
  vacuo. 
  

  

  The 
  first 
  experiments 
  were 
  made 
  with 
  a 
  large 
  prism 
  of 
  

   cast 
  iron, 
  furnished 
  with 
  windows 
  of 
  mica 
  or 
  thin 
  plate 
  glass, 
  

   in 
  which 
  the 
  metal 
  was 
  heated 
  in 
  an 
  atmosphere 
  of 
  hydrogen. 
  

   Very 
  beautiful 
  results 
  were 
  at 
  once 
  obtained, 
  but 
  certain 
  

   peculiarities 
  of 
  the 
  vapour's 
  action 
  showed 
  that 
  the 
  refraction 
  

   was 
  due 
  chiefly 
  to 
  the 
  action 
  of 
  a 
  non 
  -homogeneous 
  medium, 
  

   the 
  planes 
  of 
  constant 
  density 
  being 
  horizontal. 
  Great 
  

   trouble 
  was 
  had 
  with 
  the 
  windows, 
  which 
  soon 
  became 
  

   covered 
  with 
  a 
  white 
  deposit, 
  which 
  cut 
  off 
  most 
  of 
  the 
  light. 
  

   It 
  being 
  apparent, 
  however, 
  that 
  the 
  oblique 
  faces 
  played 
  but 
  

   a 
  very 
  small 
  part, 
  the 
  effect 
  being 
  due 
  almost 
  wholly 
  to 
  the 
  

   variable 
  density 
  of 
  the 
  vapour, 
  it 
  seemed 
  best 
  to 
  make 
  the 
  

   most 
  of 
  this 
  circumstance, 
  and 
  dispense 
  with 
  the 
  trouble 
  

   entirely 
  by 
  removing 
  the 
  glass 
  plates 
  to 
  such 
  a 
  distance 
  

   from 
  the 
  heated 
  vapour 
  that 
  no 
  deposit 
  took 
  place. 
  The 
  

   arrangement 
  finally 
  adopted 
  was 
  simply 
  a 
  tube 
  of 
  glass 
  about 
  

   30 
  cm. 
  long, 
  provided 
  with 
  plate-glass 
  ends 
  cemented 
  on 
  with 
  

   sealing-wax. 
  Hydrogen, 
  dried 
  by 
  passage 
  over 
  calcium 
  

   chloride, 
  was 
  conducted 
  into 
  and 
  out 
  of 
  the 
  tube 
  by 
  means 
  of 
  

   two 
  fine 
  glass 
  tubes 
  arranged 
  as 
  shown 
  in 
  fig. 
  1. 
  

  

  Fig. 
  1. 
  

  

  4U,< 
  

  

  'f'rfr.,. 
  

  

  w 
  

  

  B 
  

  

  The 
  diameter 
  of 
  the 
  tubes 
  should 
  not 
  be 
  over 
  2 
  mm., 
  and 
  

   they 
  should 
  lie 
  close 
  against 
  the 
  sides 
  of 
  the 
  large 
  tube 
  in 
  

   order 
  not 
  to 
  cut 
  off 
  any 
  of 
  the 
  light. 
  The 
  most 
  suitable 
  

   diameter 
  for 
  the 
  large 
  tube 
  is 
  2 
  cm. 
  As 
  the 
  experiments 
  to 
  

   be 
  described 
  in 
  this 
  paper 
  will, 
  I 
  feel 
  sure, 
  repay 
  any 
  one 
  

   who 
  takes 
  the 
  trouble 
  to 
  repeat 
  them, 
  and 
  will 
  be 
  extremely 
  

   useful 
  in 
  illustrating 
  the 
  subject 
  of 
  anomalous 
  dispersion 
  to 
  

   small 
  classes 
  of 
  advanced 
  students, 
  I 
  shall 
  describe 
  in 
  some 
  

  

  