﻿134 
  Prof. 
  R. 
  W. 
  Wood 
  on 
  the 
  Anomalous 
  

  

  stage. 
  After 
  the 
  exposure 
  was 
  over, 
  but 
  before 
  the 
  plate 
  

   was 
  removed, 
  a 
  small 
  sodium 
  flame 
  was 
  placed 
  before 
  the 
  

   slit 
  of 
  the 
  spectrometer, 
  and 
  the 
  sodium 
  lines 
  impressed 
  on 
  

   the 
  plate 
  in 
  their 
  proper 
  position, 
  to 
  serve 
  as 
  a 
  reference. 
  

   For 
  securing 
  photographs 
  of 
  the 
  dispersion 
  resulting 
  from 
  

   still 
  denser 
  vapour 
  more 
  light 
  was 
  desirable, 
  and 
  a 
  small 
  

   prism 
  was 
  accordingly 
  substituted 
  for 
  the 
  grating. 
  This 
  prism 
  

   barely 
  resolved 
  the 
  sodium 
  lines 
  in 
  the 
  negatives, 
  but 
  gave 
  

  

  Fig. 
  3. 
  

  

  excellent 
  pictures 
  of 
  the 
  spectrum 
  under 
  strong 
  sodium 
  dis- 
  

   persion. 
  PI. 
  III. 
  figs. 
  10 
  and 
  11 
  were 
  obtained 
  in 
  this 
  way. 
  

   Jn 
  the 
  latter 
  the 
  flutings 
  in 
  the 
  red 
  and 
  green 
  are 
  dis- 
  

   cernible, 
  though 
  not 
  very 
  sharp. 
  

  

  As 
  much 
  of 
  the 
  fine 
  detail 
  is 
  always 
  lost 
  in 
  reproduction 
  

   I 
  have 
  marked 
  with 
  arrows 
  on 
  fig. 
  11 
  the 
  extreme 
  points 
  to 
  

   which 
  the 
  curved 
  branches 
  adjacent 
  to 
  the 
  absorption-band 
  

   can 
  be 
  traced 
  in 
  the 
  original 
  negative. 
  Eye 
  observations 
  

   enable 
  one 
  to 
  trace 
  them 
  much 
  farther, 
  for 
  the 
  tips 
  are 
  

   exceedingly 
  faint, 
  and 
  the 
  light 
  is 
  not 
  very 
  actinic. 
  

  

  In 
  order 
  to 
  secure 
  accurate 
  measurements 
  of 
  the 
  dispersion 
  

   in 
  different 
  parts 
  of 
  the 
  spectrum 
  I 
  have 
  made 
  use 
  of 
  two 
  

   different 
  methods. 
  

  

  The 
  photographs 
  were 
  not 
  very 
  suitable 
  owing 
  to 
  the 
  fact 
  

   that 
  during 
  the 
  exposure 
  the 
  density 
  of 
  the 
  vapour 
  varied 
  some- 
  

   what, 
  producing 
  a 
  slight 
  blurring 
  of 
  the 
  image. 
  Measure- 
  

   ments 
  of 
  the 
  deviation 
  of 
  the 
  continuous 
  spectrum 
  were 
  

   accordingly 
  made 
  with 
  a 
  filar 
  micrometer, 
  an 
  illuminated 
  

   scale 
  giving 
  the 
  wave-lengths 
  at 
  the 
  points 
  where 
  readings 
  

   were 
  taken. 
  Several 
  minutes 
  were 
  necessary, 
  however, 
  to 
  

  

  