﻿24:6 
  

  

  Prof. 
  R. 
  W. 
  Wood 
  on 
  

  

  by 
  the 
  same 
  small 
  intervals 
  as 
  the 
  R.R. 
  lines) 
  each 
  one 
  of 
  

   which 
  is 
  accompanied 
  by 
  a 
  companion 
  line 
  to 
  the 
  right 
  or 
  

   left 
  as 
  the 
  case 
  may 
  be. 
  In 
  an 
  earlier 
  paper 
  I 
  spoke 
  of 
  

   the 
  seven 
  superposed 
  lines 
  as 
  the 
  " 
  core 
  " 
  of 
  the 
  group. 
  As 
  

   a 
  matter 
  of 
  fact, 
  the 
  spacing 
  is 
  not 
  exactly 
  the 
  same 
  for 
  the 
  

   seven 
  series 
  of 
  main 
  lines, 
  consequently, 
  as 
  we 
  ascend 
  to 
  

   higher 
  group 
  orders 
  they 
  begin 
  to 
  separate, 
  even 
  with 
  the 
  

   resolving 
  power 
  employed 
  in 
  photographing 
  the 
  resonance 
  

   spectra. 
  This 
  accounts 
  for 
  the 
  fact 
  that 
  the 
  groups 
  of 
  higher 
  

   order 
  differ 
  in 
  appearance 
  from 
  those 
  of 
  lower. 
  

  

  I 
  have 
  photographed 
  the 
  groups 
  of 
  and 
  + 
  1 
  order 
  with 
  

   the 
  7-inch 
  grating 
  and 
  3-metre 
  objective 
  in 
  the 
  fourth 
  

   order 
  spectrum 
  with 
  an 
  exposure 
  of 
  48 
  hours. 
  The 
  lines 
  

   were 
  very 
  faint 
  but 
  perfectly 
  sharp. 
  The 
  appearance 
  of 
  the 
  

   two 
  groups 
  is 
  shown 
  in 
  fig. 
  3. 
  The 
  resolving 
  power 
  in 
  this 
  

  

  I 
  2 
  

  

  Fig. 
  3. 
  

   3 
  45 
  

  

  7 
  8 
  

  

  order 
  

   group 
  

  

  I 
  st 
  order 
  

   group 
  

  

  case 
  was 
  but 
  little 
  less 
  than 
  that 
  required 
  to 
  separate 
  the 
  

   iodine 
  absorption 
  line?, 
  and 
  we 
  find 
  that 
  the 
  centre 
  of 
  the 
  

   group 
  of 
  order 
  is 
  a 
  narrow 
  band 
  (line 
  JNo. 
  6 
  in 
  the 
  figure) 
  

   made 
  up 
  of 
  five 
  barely 
  resolved 
  lines. 
  It 
  was 
  possible 
  to 
  

   count 
  the 
  lines 
  by 
  holding 
  the 
  plate 
  somewhat 
  foreshortened 
  

   under 
  a 
  magnifying-glass. 
  These 
  are 
  the 
  R.R. 
  lines. 
  The 
  

   other 
  lines 
  which 
  form 
  the 
  group 
  are 
  the 
  companion 
  lines, 
  

   and 
  the 
  fact 
  that 
  there 
  are 
  more 
  of 
  them 
  than 
  R.R. 
  lines 
  

   suggests 
  that 
  probably 
  some 
  of 
  the 
  R.R. 
  lines 
  have 
  two 
  

   companions 
  instead 
  of 
  one. 
  

  

  Passing 
  now 
  to 
  the 
  first 
  order 
  group, 
  we 
  find 
  that 
  the 
  

   main 
  lines 
  which 
  form 
  the 
  core 
  (each 
  one 
  of 
  which 
  belongs 
  

   to 
  a 
  series 
  of 
  which 
  a 
  R.R. 
  line 
  is 
  the 
  first 
  member) 
  are 
  

   more 
  widely 
  separated 
  than 
  in 
  the 
  order 
  group, 
  the 
  spectral 
  

   range 
  having 
  about 
  doubled. 
  This 
  is 
  due 
  to 
  the 
  fact 
  that 
  

   the 
  value 
  of 
  the 
  constant 
  in 
  the 
  second 
  term 
  of 
  our 
  formula 
  

   is 
  not 
  the 
  same 
  for 
  each 
  series. 
  

  

  We 
  will 
  now 
  consider 
  the 
  subject 
  of 
  the 
  companion 
  lines^ 
  

  

  