﻿Spectra 
  of 
  Iodine 
  under 
  High 
  Dispersion. 
  835 
  

  

  sensibly 
  the 
  same, 
  instead 
  o£ 
  increasing 
  in 
  the 
  proportion 
  

   1, 
  2, 
  3, 
  &c, 
  makes 
  it 
  appear 
  probable 
  that 
  they 
  originate 
  in 
  

   the 
  following 
  way. 
  Each 
  absorption 
  line 
  gives 
  rise 
  to 
  a 
  

   series 
  o£ 
  equidistant 
  lines 
  which 
  are 
  superposed, 
  and 
  which 
  

   form 
  what 
  we 
  might 
  term 
  the 
  cores 
  of 
  the 
  groups. 
  These 
  

   lines 
  are 
  accompanied 
  by 
  companion 
  lines 
  which 
  fall 
  some- 
  

   times 
  to 
  the 
  right 
  and 
  sometimes 
  to 
  the 
  left 
  of 
  the 
  principal 
  

   lines. 
  The 
  position 
  of 
  a 
  companion 
  line 
  depends 
  upon 
  which 
  

   one 
  of 
  the 
  seven 
  absorption 
  lines 
  is 
  excited, 
  but 
  for 
  any 
  given 
  

   absorption 
  line 
  the 
  position 
  of 
  the 
  companion 
  line 
  and 
  its 
  

   distance 
  from 
  the 
  main 
  line 
  (core 
  of 
  the 
  group) 
  is 
  the 
  same, 
  

   both 
  for 
  the 
  R.R. 
  line 
  and 
  the 
  lines 
  of 
  the 
  equidistant 
  series. 
  

   A 
  very 
  careful 
  study 
  of 
  some 
  thirty 
  photographs, 
  taken 
  

   under 
  different 
  conditions, 
  has 
  convinced 
  me 
  that 
  the 
  groups 
  

   arise 
  in 
  the 
  manner 
  outlined. 
  This, 
  of 
  course, 
  holds 
  only 
  for 
  

   groups 
  of 
  identical 
  appearance. 
  

  

  As 
  it 
  will 
  be 
  necessary 
  to 
  discuss 
  some 
  of 
  the 
  other 
  groups 
  

   of 
  lines 
  and 
  compare 
  them 
  with 
  the 
  three 
  enumerated 
  above, 
  

   it 
  will 
  be 
  well 
  to 
  adopt 
  an 
  easy 
  method 
  of 
  identification. 
  As 
  

   they 
  appear 
  to 
  the 
  right 
  and 
  left 
  of 
  the 
  exciting 
  line 
  at 
  

   regular 
  intervals 
  along 
  the 
  spectrum, 
  we 
  may 
  for 
  convenience 
  

   adopt 
  the 
  nomenclature 
  used 
  in 
  the 
  case 
  of 
  grating 
  spectra, 
  

   and 
  speak 
  of 
  them 
  as 
  groups 
  of 
  the 
  first, 
  second, 
  third 
  (&c.) 
  

   order. 
  The 
  R.R. 
  line 
  would 
  be 
  of 
  order 
  0, 
  the 
  groups 
  on 
  

   the 
  short 
  wave-length 
  side 
  —1, 
  — 
  2, 
  &c, 
  and 
  on 
  the 
  long- 
  

   wave-length 
  side 
  1, 
  2, 
  3, 
  4, 
  &c. 
  The 
  characteristic 
  groups 
  

   which 
  have 
  been 
  under 
  discussion, 
  and 
  which 
  are 
  precisely 
  

   similar, 
  are 
  of 
  orders 
  0, 
  1, 
  and 
  3. 
  They 
  are 
  found 
  nowhere 
  

   else, 
  though 
  the 
  fifth 
  order 
  group, 
  which 
  covers 
  the 
  yellow 
  

   mercurv 
  line 
  of 
  longer 
  wave-length, 
  differs 
  from 
  these 
  

   groups 
  only 
  in 
  that 
  the 
  centre 
  is 
  formed 
  of 
  a 
  doublet 
  instead 
  

   of 
  a 
  triplet. 
  This 
  group 
  can 
  be 
  seen 
  only 
  on 
  photographs 
  of 
  

   the 
  resonance 
  spectrum 
  excited 
  by 
  the 
  green 
  line 
  alone, 
  L 
  e., 
  

   with 
  1he 
  yellow 
  light 
  removed 
  from 
  the 
  exciting 
  rays 
  by 
  

   means 
  of 
  a 
  neodymium 
  ray 
  filter. 
  It 
  is 
  well 
  shown 
  on 
  

   fig. 
  ±, 
  PI. 
  XV., 
  the 
  position 
  of 
  the 
  yellow 
  Hg 
  lines 
  being- 
  

   indicated 
  by 
  arrows. 
  

  

  The 
  formation 
  of 
  groups 
  of 
  lines 
  in 
  the 
  manner 
  described 
  

   appears 
  to 
  be 
  quite 
  characteristic, 
  for 
  the 
  same 
  thing 
  occurs 
  

   in 
  the 
  case 
  of 
  the 
  excitation 
  by 
  the 
  two 
  yellow 
  mercury 
  lines, 
  

   as 
  is 
  shown 
  by 
  fig. 
  5, 
  PL 
  XV. 
  In 
  this 
  case 
  the 
  green 
  line 
  has 
  

   been 
  removed 
  from 
  the 
  exciting 
  beam 
  by 
  means 
  of 
  an 
  eosine 
  

   filter. 
  In 
  this 
  case 
  we 
  have 
  the 
  group 
  of 
  order 
  1, 
  and 
  groups 
  

   of 
  order 
  —1 
  and 
  —2, 
  to 
  the 
  right 
  and 
  left 
  of 
  the 
  groups 
  

   which 
  fall 
  in 
  coincidence 
  with 
  the 
  exciting 
  lines 
  (0 
  order). 
  

  

  On 
  looking 
  over 
  my 
  work 
  on 
  the 
  resonance 
  spectra 
  of 
  

  

  3 
  L 
  2 
  

  

  