﻿Experiment 
  relating 
  to 
  the 
  Drift 
  of 
  the 
  JEtlier. 
  31 
  

  

  Also 
  to 
  this 
  order 
  ^ 
  = 
  C, 
  or 
  the 
  direction 
  of.' 
  the 
  uiaximals 
  is 
  

   constant. 
  

  

  15. 
  It 
  remains 
  to 
  determine 
  the 
  magnitude 
  of 
  the 
  effect 
  

   referred 
  to 
  in 
  § 
  4. 
  By 
  means 
  of 
  the 
  lenses 
  of 
  the 
  eye, 
  aided 
  

   or 
  not 
  by 
  an 
  optical 
  instrument, 
  an 
  image 
  is 
  thrown 
  on 
  the 
  

   retina 
  of 
  points 
  on 
  a 
  certain 
  plane 
  on 
  which 
  the 
  instrument 
  is 
  

   focussed. 
  Of 
  any 
  such 
  point 
  P 
  an 
  image 
  is 
  formed 
  at 
  Q 
  (say) 
  

   on 
  the 
  retina. 
  Through 
  P 
  two 
  rays 
  (belonging 
  to 
  the 
  two 
  

   systems 
  of 
  waves) 
  pass, 
  which 
  by 
  the 
  optical 
  apparatus 
  are 
  

   again 
  brought 
  to 
  pass 
  through 
  Q, 
  having 
  passed 
  over 
  the 
  

   same 
  optical 
  distance 
  D', 
  say. 
  The 
  waves 
  travel 
  with 
  the 
  

   same 
  velocity 
  but 
  have 
  different 
  wave-lengths. 
  Suppose 
  at 
  

   P 
  the 
  phase-difference 
  is 
  II, 
  i. 
  e. 
  the 
  A 
  2 
  phase 
  greater 
  by 
  II 
  

   than 
  the 
  \ 
  phase. 
  Then 
  in 
  the 
  wave 
  diagram 
  the 
  two 
  paths 
  

   (D') 
  are 
  occupied 
  by 
  waves 
  of 
  different 
  lengths 
  \ 
  x 
  , 
  A 
  2 
  , 
  and 
  

  

  D' 
  D' 
  

  

  therefore 
  at 
  Q 
  the 
  phase-difference 
  of 
  \% 
  over 
  \ 
  x 
  is 
  II 
  + 
  — 
  — 
  — 
  , 
  

  

  or 
  there 
  is 
  an 
  increased 
  difference 
  of 
  phase 
  of 
  amount 
  

  

  1 
  1 
  \ 
  

  

  D' 
  ( 
  - 
  r-\. 
  Consequently 
  the 
  resulting 
  intensity 
  at 
  Q 
  will 
  

  

  not 
  be 
  the 
  same 
  as 
  that 
  at 
  P 
  — 
  the 
  maxima 
  and 
  minima 
  on 
  the 
  

   retina 
  are 
  not 
  the 
  images 
  of 
  the 
  maxima 
  and 
  minima 
  of 
  the 
  

   object 
  viewed. 
  Now 
  the 
  \ 
  2 
  waves 
  are 
  ahead 
  of 
  the 
  Xi 
  on 
  

   the 
  right 
  hand 
  of 
  the 
  maximals. 
  Hence 
  in 
  order 
  that 
  Q 
  may 
  

   be 
  the 
  central 
  bright 
  band 
  it 
  must 
  be 
  the 
  optical 
  image 
  of 
  a 
  

   point 
  on 
  the 
  left 
  of 
  the 
  centre 
  of 
  the 
  original 
  fringe, 
  for 
  at 
  Q 
  

   the 
  X2 
  waves 
  lead 
  still 
  more 
  than 
  at 
  P, 
  and 
  the 
  distance 
  from 
  

  

  it 
  must 
  be 
  IV 
  (z- 
  — 
  r- 
  J 
  bands. 
  

   Now 
  by 
  (8) 
  

  

  Xi-X 
  2 
  _ 
  U(V*-^)P.Q 
  

   ~ 
  A 
  D^D,* 
  

  

  X,X 
  2 
  _ 
  (V'-«* 
  /(V»-tfl(T»--ii*) 
  

  

  A 
  2 
  " 
  ~WW 
  

   . 
  n 
  , 
  Ai 
  - 
  xi 
  _ 
  & 
  UPQ 
  

  

  _ 
  D 
  f 
  (V 
  cos 
  6- 
  u) 
  U 
  cos 
  a 
  . 
  Q 
  

   X 
  V 
  4 
  

  

  D' 
  Q, 
  

  

  = 
  - 
  f 
  cos 
  a 
  ; 
  

  

  