﻿Dispersion 
  of 
  Sodium 
  Vapour. 
  135 
  

  

  get 
  anything 
  like 
  a 
  satisfactory 
  set 
  of 
  readings, 
  and 
  changes 
  

   occurred 
  even 
  during 
  that 
  short 
  period, 
  as 
  was 
  at 
  once 
  

   apparent 
  on 
  repeating 
  observations. 
  I 
  accordingly 
  adopted 
  

   the 
  following 
  method. 
  

  

  The 
  telescope 
  of 
  the 
  spectrometer 
  was 
  removed 
  and 
  a 
  

   telescope-objective 
  of 
  nearly 
  two 
  metres 
  focus 
  put 
  in 
  its 
  place. 
  

   In 
  the 
  focal 
  plane 
  of 
  this 
  lens 
  a 
  plate 
  of 
  glass 
  was 
  firmly 
  

   mounted, 
  on 
  which 
  a 
  horizontal 
  diamond 
  scratch 
  had 
  been 
  

   ruled. 
  This 
  diamond 
  scratch 
  was 
  brought 
  accurately 
  into 
  

   coincidence 
  with 
  the 
  narrow 
  continuous-spectrum 
  image 
  in 
  

   its 
  undeviated 
  position. 
  An 
  eyepiece 
  mounted 
  behind 
  the 
  

   plate 
  enabled 
  the 
  spectrum 
  to 
  be 
  observed. 
  When 
  the 
  dis- 
  

   persion-tube 
  was 
  in 
  good 
  condition, 
  and 
  the 
  deviated 
  branches 
  

   of 
  the 
  spectrum 
  appeared 
  sharp 
  and 
  steady, 
  their 
  positions 
  on 
  

   the 
  plate 
  were 
  marked 
  with 
  a 
  writing 
  diamond, 
  the 
  line 
  being 
  

   drawn 
  along 
  the 
  middle 
  of 
  the 
  spectrum, 
  which 
  was 
  only 
  

   about 
  a 
  millimetre 
  wide 
  on 
  the 
  plate. 
  The 
  dispersion-tube 
  

   was 
  then 
  removed 
  and 
  various 
  metals 
  fed 
  to 
  the 
  arc, 
  the 
  

   bright 
  lines 
  being 
  marked 
  on 
  the 
  plate 
  with 
  the 
  diamond. 
  

   In 
  this 
  way 
  a 
  scale 
  of 
  wave-lengths 
  was 
  secured. 
  For 
  the 
  

   determination 
  of 
  refractive 
  indices 
  we 
  require, 
  however, 
  the 
  

   angular 
  deviation 
  of 
  the 
  different 
  rays. 
  This 
  can 
  be 
  

   determined 
  if 
  we 
  know 
  the 
  focal 
  length 
  of 
  the 
  lens 
  forming 
  

   the 
  image 
  of 
  the 
  spectrum 
  on 
  the 
  slit 
  of 
  the 
  spectrometer, 
  

   and 
  the 
  actual 
  deviations 
  measured 
  in 
  millimetres 
  o£ 
  the 
  

   images 
  of 
  the 
  first 
  slit 
  formed 
  by 
  light 
  of 
  various 
  wave-length 
  

   at 
  this 
  point. 
  To 
  secure 
  a 
  record 
  of 
  this 
  a 
  glass 
  plate, 
  ruled 
  

   vuth 
  half-millimetre 
  lines, 
  was 
  placed 
  over 
  the 
  slit 
  of 
  the 
  

   spectrometer, 
  and 
  the 
  position 
  of 
  the 
  lines 
  on 
  the 
  glass 
  plate 
  

   in 
  the 
  focus 
  of 
  the 
  large 
  lens 
  recorded 
  with 
  the 
  writing- 
  

   diamond. 
  This 
  scale 
  enabled 
  the 
  recorded 
  deviations 
  to 
  be 
  

   reduced 
  to 
  the 
  actual 
  deviations 
  as 
  they 
  existed 
  on 
  the 
  slit 
  

   of 
  the 
  instrument. 
  

  

  Fig. 
  4. 
  

  

  Line 
  of 
  zero 
  

   deviation 
  

  

  jm 
  = 
  I 
  67 
  6666 
  64 
  65 
  62 
  6' 
  60 
  

  

  D 
  Lines. 
  

  

  Wave-Length 
  scabe. 
  

  

  53 
  57 
  56 
  55 
  54 
  53 
  52 
  51 
  5049 
  46 
  47 
  4645 
  44 
  43 
  42 
  41 
  40 
  39 
  3d 
  37 
  

  

  -Half 
  miLLi 
  metre 
  

   marks. 
  

  

  The 
  dispersion 
  curve 
  obtained 
  in 
  this 
  manner, 
  with 
  the 
  

   half-millimetre 
  marks, 
  is 
  reproduced 
  on 
  the 
  same 
  scale 
  as 
  

   the 
  original 
  in 
  fig. 
  4, 
  the 
  prismatic 
  spectrum 
  having 
  been 
  

  

  