﻿the 
  Eclielon 
  Spectroscope. 
  391 
  

  

  with 
  the 
  difference 
  in 
  phase 
  between 
  the 
  primary 
  and 
  the 
  

   secondary 
  light 
  produced 
  by 
  the 
  two 
  reflexions 
  o£ 
  the 
  latter. 
  

   Apart 
  from 
  these 
  phase 
  changes 
  at 
  reflexion, 
  the 
  primary 
  and 
  

   secondary 
  light 
  would 
  be 
  in 
  phase 
  at 
  the 
  points 
  of 
  intersection 
  

   of 
  their 
  maxima, 
  as 
  the 
  retardations 
  in 
  both 
  systems 
  are 
  

   whole 
  numbers 
  of 
  wave-lengths 
  in 
  the 
  direction 
  in 
  which 
  the 
  

   maxima 
  are 
  formed. 
  There 
  are 
  two 
  cases 
  to 
  be 
  considered 
  

   for 
  the 
  secondary 
  light, 
  according 
  as 
  it 
  has 
  undergone 
  both 
  

   reflexions 
  at 
  interfaces 
  or 
  one 
  reflexion 
  at 
  an 
  interface 
  and 
  

   the 
  other 
  at 
  an 
  external 
  surface. 
  In 
  the 
  former 
  case, 
  as 
  the 
  

   air-films 
  are 
  very 
  thin, 
  it 
  is 
  possible 
  that 
  a 
  change 
  of 
  phase 
  

  

  of 
  ^ 
  may 
  be 
  introduced 
  at 
  each 
  reflexion 
  which 
  would 
  give 
  

  

  the 
  best 
  phase 
  conditions 
  for 
  interference 
  between 
  the 
  maxima. 
  

   The 
  latter 
  case 
  may 
  account 
  for 
  the 
  second 
  set 
  of 
  bands 
  

   shifted 
  relatively 
  to 
  the 
  first. 
  

  

  Part 
  III. 
  — 
  Structure 
  of 
  the 
  G-reen 
  Mercury 
  Line. 
  

  

  (5461.) 
  

  

  Description 
  of 
  Spectrum 
  given 
  hy 
  an 
  Arons 
  Lamp^ 
  and 
  

  

  Comparison 
  with 
  the 
  Results 
  obtained 
  hy 
  other 
  Observers. 
  

  

  This 
  spectrum 
  consists 
  of 
  a 
  bright 
  principal 
  line, 
  which 
  is 
  

   a 
  close 
  double, 
  with 
  six 
  companion 
  lines, 
  three 
  on 
  either 
  side. 
  

   A 
  photograph 
  of 
  the 
  primary 
  echelon 
  spectrum 
  is 
  reproduced 
  

   in 
  Plate 
  XII. 
  together 
  with 
  a 
  diagram 
  of 
  the 
  spectrum. 
  The 
  

   photograph 
  shows, 
  in 
  addition 
  to 
  the 
  genuine 
  components, 
  

   a 
  number 
  of 
  faint 
  lines 
  which 
  have 
  their 
  origin 
  in 
  the 
  

   echelon. 
  The 
  genuine 
  components 
  are 
  numbered 
  from 
  

   1 
  to 
  8, 
  and 
  the 
  false 
  lines 
  are 
  marked 
  1 
  a, 
  1 
  6, 
  3 
  a, 
  &c. 
  The 
  

   lines 
  1 
  a, 
  lb, 
  1 
  c, 
  &c., 
  mark 
  the 
  positions 
  of 
  the 
  secondary 
  

   diffraction 
  maxima 
  on 
  the 
  longer 
  wave-length 
  side 
  of 
  the 
  

   principal 
  diffraction 
  maximum 
  1, 
  and 
  the 
  lines 
  3 
  a, 
  5 
  a, 
  t5 
  a, 
  

   7 
  a, 
  and 
  8 
  a 
  represent 
  the 
  first 
  secondary 
  maxima 
  on 
  the 
  

   longer 
  wave-length 
  side 
  of 
  the 
  lines 
  3, 
  5, 
  6, 
  7, 
  and 
  8. 
  When 
  

   the 
  aperture 
  of 
  the 
  echelon 
  is 
  reduced 
  by 
  covering 
  the 
  first 
  

   ten 
  step-faces 
  at 
  the 
  smaller 
  end, 
  the 
  faint 
  lines 
  move 
  away 
  

   from 
  their 
  parents 
  into 
  the 
  new 
  positions 
  of 
  the 
  secondary 
  

   maxima 
  corresponding 
  to 
  the 
  reduced 
  number 
  of 
  apertures. 
  

  

  The 
  numbers 
  in 
  Table 
  II. 
  and 
  Table 
  III. 
  give 
  the 
  distances 
  

   of 
  the 
  various 
  components 
  from 
  the 
  component 
  of 
  shortest 
  

   wave-length. 
  I 
  adopted 
  this 
  method 
  of 
  measuring 
  the 
  

   positions 
  because 
  the 
  bright 
  central 
  line 
  appeared 
  to 
  be 
  the 
  

   most 
  variable 
  component, 
  while 
  th^ 
  component 
  of 
  shortest 
  

   wave-length 
  is 
  a 
  good 
  reference 
  line. 
  The 
  results 
  of 
  the 
  

   other 
  observers, 
  given 
  in 
  the 
  Tables, 
  are 
  not 
  convenient 
  for 
  

  

  2 
  D2 
  

  

  