﻿Resonance 
  Spectra 
  of 
  Iodine 
  under 
  High 
  Dispersion. 
  829 
  

  

  1. 
  Control 
  of 
  the 
  excitation. 
  — 
  The 
  iodine 
  vapour 
  was 
  

   excited 
  by 
  the 
  light 
  of 
  a 
  quartz 
  mercury- 
  vapour 
  lamp 
  

   operating 
  with 
  various 
  resistances 
  so 
  as 
  to 
  show 
  a 
  potential 
  

   drop 
  across 
  the 
  terminals 
  varying 
  from 
  25 
  volts 
  to 
  165 
  volts. 
  

   The 
  tendency 
  of 
  the 
  lamp 
  to 
  go 
  out 
  when 
  running 
  with 
  a 
  

   large 
  resistance 
  in 
  circuit 
  with 
  it 
  was 
  overcome 
  by 
  including 
  

   in 
  the 
  circuit 
  a 
  large 
  amount 
  of 
  self-induction, 
  consisting 
  of 
  

   two 
  coils 
  of 
  very 
  heavily 
  insulated 
  wire 
  with 
  laminated 
  cores, 
  

   for 
  which 
  I 
  am 
  indebted 
  to 
  Mr. 
  Cooper-Hewitt. 
  A 
  voltmeter 
  

   and 
  ammeter 
  w 
  r 
  as 
  used 
  in 
  each 
  case 
  to 
  determine 
  the 
  watt 
  

   consumption 
  of 
  the 
  lamp, 
  and 
  the 
  appearance 
  of 
  the 
  green 
  

   exciting 
  line 
  in 
  coincidence 
  with 
  the 
  absorption 
  spectrum 
  of 
  

   iodine 
  was 
  observed 
  during 
  each 
  exposure, 
  in 
  the 
  fourth 
  

   order 
  spectrum 
  of 
  a 
  6-inch 
  grating 
  with 
  a 
  lens 
  of 
  3 
  metres 
  

   focus. 
  In 
  addition 
  to 
  this, 
  a 
  valuable 
  series 
  of 
  photographs 
  

   of 
  the 
  green 
  line 
  in 
  coincidence 
  with 
  the 
  absorption 
  spectrum 
  

   was 
  made 
  for 
  me 
  by 
  Dr. 
  J. 
  A. 
  Anderson, 
  with 
  the 
  large 
  

   spectrograph 
  of 
  the 
  Mt. 
  Wilson 
  Solar 
  Observatory. 
  This 
  

   instrument 
  has 
  a 
  focal 
  lenoth 
  of 
  75 
  feet 
  and 
  is 
  furnished 
  with 
  

   a 
  Michelson 
  grating. 
  It 
  appears 
  to 
  have 
  about 
  the 
  same 
  re- 
  

   solving 
  power 
  as 
  my 
  East 
  Hampton 
  spectrograph, 
  though 
  i<s 
  

   greater 
  focal 
  length 
  enables 
  it 
  to 
  yield 
  photographs 
  of 
  much 
  

   more 
  satisfactory 
  appearance, 
  the 
  definition 
  in 
  the 
  case 
  of 
  my 
  

   instrument 
  being 
  marred 
  somewhat 
  by 
  the 
  grain 
  of 
  the 
  plate. 
  

  

  The 
  method 
  of 
  controlling 
  the 
  excitation 
  which 
  I 
  adopted 
  

   necessitates 
  the 
  use 
  of 
  two 
  powerful 
  spectrographs, 
  one 
  for 
  

   photographing 
  the 
  resonance 
  spectrum, 
  the 
  other 
  for 
  studying 
  

   the 
  appearance 
  of 
  the 
  green 
  exciting 
  line 
  during 
  the 
  exposure 
  

   and 
  determining 
  which 
  of 
  the 
  iodine 
  absorption 
  lines 
  are 
  

   excited 
  by 
  it. 
  

  

  I 
  have 
  also 
  used 
  a 
  very 
  large 
  and 
  powerful 
  Cooper-Hewitt 
  

   glass 
  lamp, 
  the 
  tube 
  having 
  an 
  internal 
  diameter 
  of 
  5 
  cm., 
  

   and 
  carrying 
  a 
  current 
  which 
  can 
  be 
  varied 
  from 
  three 
  to 
  

  

  Fiff. 
  1. 
  

  

  fifteen 
  amperes. 
  A 
  novel 
  method 
  of 
  illumination 
  was 
  used 
  

   in 
  the 
  case 
  of 
  this 
  lamp, 
  which 
  appears 
  to 
  be 
  the 
  most 
  

   efficient 
  found 
  up 
  to 
  the 
  present 
  time. 
  It 
  is 
  illustrated 
  by 
  

   fig. 
  1. 
  Three 
  enormous 
  cylindrical 
  reflectors 
  were 
  made 
  

  

  