﻿476 
  A. 
  A. 
  Michelson 
  — 
  Motion 
  of 
  the 
  Earth 
  and 
  the 
  Ether. 
  

  

  ends 
  of 
  the 
  iron 
  pipes; 
  mw, 
  plane 
  glass 
  plates 
  silvered 
  on 
  front 
  

   surface, 
  and 
  provided 
  with 
  adjustments 
  in 
  two 
  planes 
  ; 
  omnb 
  y 
  

   the 
  path 
  of 
  the 
  pencil 
  of 
  light. 
  The 
  apparatus 
  was 
  set 
  up 
  in 
  

   the 
  vertical 
  east 
  and 
  west 
  plane, 
  the 
  light 
  traversing 
  the 
  entire 
  

   circuit 
  of 
  the 
  Ryerson 
  Laboratory, 
  a 
  path 
  about 
  200 
  feet 
  long 
  

   and 
  50 
  feet 
  high. 
  

  

  - 
  

  

  It 
  was 
  found 
  that 
  under 
  ordinary 
  conditions 
  the 
  tempera- 
  

   ture 
  disturbances 
  in 
  this 
  length 
  of 
  air 
  made 
  it 
  impossible 
  to 
  

   measure 
  the 
  position 
  of 
  the 
  fringes 
  ; 
  and 
  the 
  difficulty 
  was 
  only 
  

   slightly 
  remedied 
  by 
  enclosing 
  the 
  whole 
  path 
  of 
  the 
  light 
  in 
  a 
  

   wooden 
  box. 
  By 
  making 
  this 
  enclosure 
  an 
  iron 
  pipe 
  and 
  

   exhausting 
  the 
  air 
  to 
  within 
  a 
  hundredth 
  of 
  an 
  atmosphere, 
  it 
  

   was 
  found 
  possible 
  to 
  measure 
  the 
  position 
  of 
  the 
  central 
  

   bright 
  fringe 
  to 
  within 
  something 
  like 
  a 
  twentieth 
  of 
  the 
  fringe- 
  

   width. 
  

  

  A 
  difficulty 
  is 
  encountered 
  in 
  the 
  selection 
  of 
  a 
  fiducial 
  mark. 
  

   The 
  double 
  image 
  of 
  the 
  source 
  does 
  not 
  remain 
  on 
  the 
  cross 
  

   hairs 
  of 
  the 
  observing 
  telescope 
  for 
  any 
  great 
  length 
  of 
  time, 
  

   notwithstanding 
  the 
  precaution 
  of 
  the 
  double 
  reflections 
  at 
  

   the 
  corners, 
  but 
  by 
  using 
  this 
  double 
  image 
  itself 
  as 
  the 
  fidu- 
  

   cial 
  mark, 
  any 
  possible 
  errors 
  due 
  to 
  daily 
  temperature 
  changes, 
  

   etc., 
  are 
  eliminated. 
  This 
  double 
  image 
  and 
  the 
  interference 
  

   fringes 
  are 
  not 
  in 
  focus 
  at 
  the 
  same 
  time, 
  but 
  by 
  sacrificing 
  a 
  very 
  

  

  