﻿138 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Anomalous 
  

  

  1*2 
  mm. 
  The 
  angle 
  o£ 
  the 
  prism 
  was 
  130, 
  from 
  whieh 
  data 
  

   we 
  get 
  the 
  following 
  values 
  for 
  the 
  refractive 
  index 
  of 
  the 
  

   vapour 
  for 
  these 
  two 
  wave-lengths 
  (relative 
  to 
  hydrogen): 
  

   ^=1-0005 
  and>= 
  0-9994. 
  

  

  Similar 
  results 
  were 
  obtained 
  with 
  the 
  device 
  shown 
  in 
  

   fig. 
  6, 
  when 
  two 
  elliptical 
  pieces 
  of 
  perforated 
  sheet 
  iron 
  were 
  

   used 
  for 
  moulding 
  the 
  vapour. 
  The 
  images 
  formed 
  in 
  this 
  

   case 
  were 
  blurred 
  by 
  diffraction. 
  

  

  1 
  place 
  very 
  little 
  value 
  however 
  on 
  these 
  figures, 
  for 
  I 
  am 
  

   of 
  the 
  opinion 
  that 
  the 
  effective 
  angle 
  of 
  the 
  sodium 
  prism 
  is 
  

   much 
  less 
  than 
  the 
  angle 
  between 
  the 
  ends 
  of 
  the 
  tubes, 
  it 
  

   seeming 
  probable 
  that 
  the 
  vapour 
  bulges 
  out 
  into 
  the 
  tubes, 
  

   especially 
  near 
  the 
  centre. 
  That 
  this 
  is 
  to 
  a 
  certain 
  extent 
  the 
  

   case 
  is 
  indicated 
  by 
  the 
  fact 
  that 
  the 
  image 
  of 
  the 
  slit 
  is 
  not 
  

   very 
  sharp, 
  though 
  this 
  may 
  well 
  be 
  caused 
  by 
  the 
  varying 
  

   density 
  of 
  the 
  vapour. 
  1 
  have 
  not 
  yet 
  despaired 
  of 
  getting 
  a 
  

   prism 
  bounded 
  by 
  plates 
  of 
  glass, 
  about 
  which 
  there 
  can 
  be 
  

   raised 
  no 
  question, 
  though 
  the 
  problem 
  is 
  a 
  difficult 
  one, 
  and 
  

   observations 
  will 
  have 
  to 
  be 
  made 
  with 
  great 
  rapidity. 
  

  

  The 
  deviations 
  obtained 
  by 
  this 
  method 
  are 
  very 
  much 
  

   less 
  than 
  those 
  obtained 
  with 
  the 
  dispersion-tubes, 
  indicating 
  

   either 
  that 
  the 
  equivalent 
  angle 
  is 
  very 
  large 
  in 
  the 
  latter 
  

   case 
  (it 
  may 
  be 
  nearly 
  180° 
  for 
  all 
  we 
  know) 
  or 
  that 
  the 
  angle 
  

   of 
  the 
  prism 
  formed 
  by 
  the 
  iron 
  tubes 
  was 
  less 
  than 
  the 
  

   estimated 
  value. 
  

  

  If 
  we 
  calculate 
  the 
  refractive 
  indices 
  from 
  the 
  data 
  given 
  

   by 
  the 
  curve 
  traced 
  with 
  the 
  diamond 
  point, 
  assuming 
  the 
  

   equivalent 
  angle 
  of 
  the 
  stratified 
  vapour 
  to 
  be 
  130°, 
  we 
  get 
  

   for 
  the 
  wave-lengths 
  adjacent 
  to 
  the 
  absorption-band 
  the 
  

   values 
  /*= 
  T0024 
  and 
  ^ 
  = 
  0*9969. 
  

  

  The 
  value 
  given 
  by 
  Becquerel 
  (1*0009) 
  is 
  intermediate 
  

   between 
  the 
  values 
  obtained 
  by 
  the 
  two 
  methods, 
  but 
  I 
  feel 
  

   sure 
  that 
  his 
  flame 
  must 
  have 
  been 
  non-homogeneous, 
  and 
  his 
  

   numerical 
  results 
  more 
  or 
  less 
  inaccurate 
  in 
  consequence. 
  

  

  For 
  the 
  exhibition 
  of 
  the 
  actual 
  spectrum 
  produced 
  by 
  a 
  

   prism 
  of 
  sodium 
  vapour, 
  a 
  long 
  dispersion-tube 
  with 
  a 
  battery 
  

   of 
  four 
  or 
  five 
  prisms 
  gives 
  the 
  best 
  results. 
  A 
  small 
  Bunsen 
  

   burner 
  should 
  be 
  used 
  for 
  each 
  of 
  the 
  fragments 
  of 
  sodium, 
  

   which 
  should 
  be 
  at 
  least 
  6 
  or 
  8 
  cm. 
  apart. 
  (This 
  arrangement 
  

   is 
  shown 
  in 
  tig. 
  2, 
  p. 
  132.) 
  

  

  