﻿Experiment 
  relating 
  to 
  the 
  Drift 
  of 
  the 
  JEth 
  

  

  titer. 
  

  

  15 
  

  

  say 
  D'. 
  At 
  the 
  object 
  the 
  two 
  interfering 
  waves 
  have 
  a 
  certain 
  

   phase-difference, 
  but 
  in 
  space 
  their 
  wave-lengths 
  are 
  different, 
  

   and 
  in 
  travelling 
  over 
  the 
  same 
  distance 
  D 
  / 
  their 
  phase- 
  

   differences 
  are 
  changed 
  by 
  the 
  amount 
  D' 
  ( 
  r 
  z- 
  I. 
  

  

  Consequently 
  the 
  optical 
  image 
  of 
  a 
  bright 
  line 
  is 
  not 
  itself 
  

   a 
  bright 
  line 
  ; 
  or, 
  conversely, 
  the 
  bright 
  central 
  band 
  in 
  the 
  

   image 
  is 
  the 
  optical 
  image 
  of 
  a 
  point 
  on 
  the 
  original 
  fringe 
  

   to 
  one 
  side 
  of 
  its 
  central 
  band. 
  This 
  result 
  is 
  important, 
  as 
  

   in 
  certain 
  cases 
  it 
  may, 
  as 
  will 
  be 
  shown 
  later, 
  entirely 
  modify 
  

   the 
  nature 
  of 
  the 
  changes 
  produced 
  as 
  the 
  direction 
  of 
  drift 
  

   of 
  the 
  apparatus 
  alters. 
  Again, 
  superposed 
  on 
  this 
  there 
  will 
  

   be 
  an 
  aberration 
  effect. 
  

  

  So 
  much 
  for 
  the 
  general 
  qualitative 
  theory 
  of 
  the 
  

   phenomenon. 
  It 
  remains 
  now 
  to 
  obtain 
  the 
  quantitative 
  laws. 
  

  

  Reflexion 
  of 
  a 
  Plane 
  Wave 
  at 
  a 
  Moving 
  Mirror. 
  

  

  5. 
  Angle 
  of 
  Reflexion. 
  — 
  In 
  the 
  figure 
  let 
  AB 
  denote 
  a 
  wave- 
  

   front 
  incident 
  at 
  A 
  on 
  the 
  mirror 
  AL. 
  Let 
  V 
  denote 
  the 
  

  

  Fiir. 
  3. 
  

  

  velocity 
  of 
  light, 
  v 
  that 
  of 
  the 
  mirror 
  perpendicular 
  to 
  itself. 
  

   When 
  the 
  mirror 
  has 
  advanced 
  A 
  /1 
  L 
  = 
  vt 
  suppose 
  AB 
  to 
  have 
  

   advanced 
  so 
  as 
  to 
  be 
  incident 
  at 
  A'. 
  During 
  the 
  interval 
  t 
  y 
  

   provided 
  the 
  cether 
  be 
  at 
  rest, 
  the 
  disturbance 
  produced 
  at 
  A 
  

   will 
  have 
  spread. 
  to 
  the 
  spherical 
  surface 
  B', 
  which 
  lies 
  outside 
  

   the 
  new 
  position 
  of 
  the 
  mirror. 
  What 
  happens 
  between 
  the 
  

   old 
  and 
  new 
  positions 
  of 
  the 
  mirror 
  does 
  not 
  concern 
  us. 
  

   Hence, 
  with 
  the 
  usual 
  reasoning, 
  the 
  reflected 
  wave-front 
  

   through 
  A' 
  is 
  A'B' 
  (the 
  fig. 
  explains 
  itself). 
  Let 
  0, 
  0' 
  be 
  

   the 
  angles 
  of 
  incidence 
  and 
  of 
  reflexion, 
  and 
  let 
  a 
  denote 
  the 
  

   angle 
  A/AL. 
  

  

  