﻿Necessity 
  for 
  Postulating 
  an 
  JEther, 
  125 
  

  

  the 
  aether 
  as 
  a 
  disturbing 
  cause 
  in 
  terrestrial 
  experiments, 
  

   and 
  so 
  establish 
  such 
  a 
  basis, 
  but 
  without 
  success. 
  It 
  is 
  still 
  

   open 
  to 
  any 
  one 
  to 
  go 
  a 
  step 
  further 
  than 
  Mill 
  and 
  to 
  say 
  

   that 
  in 
  his 
  opinion 
  light 
  is 
  an 
  action 
  of 
  one 
  body 
  upon 
  

   another, 
  such 
  action 
  taking 
  place 
  at 
  a 
  distance 
  not 
  only 
  in 
  

   space 
  but 
  in 
  time. 
  

  

  When 
  we 
  leave 
  the 
  earth 
  and 
  turn 
  to 
  observation 
  of 
  the 
  

   stars, 
  however, 
  we 
  find 
  that 
  there 
  is 
  a 
  possibility 
  of 
  ex- 
  

   periencing 
  the 
  aether 
  as 
  a 
  cause. 
  It 
  is 
  a 
  consequence 
  of 
  the 
  

   undulatory 
  theory 
  that 
  aberration 
  depends 
  on 
  the 
  motion 
  of 
  

   the 
  earth 
  relative 
  to 
  the 
  free 
  aether, 
  and 
  is 
  entirely 
  unaffected 
  

   by 
  the 
  motion 
  of 
  the 
  star. 
  This, 
  if 
  true, 
  furnishes 
  our 
  logical 
  

   basis. 
  For 
  suppose 
  the 
  earth 
  and 
  the 
  star 
  to 
  be 
  fixed, 
  relative 
  

   to 
  one 
  another, 
  but 
  in 
  uniform 
  motion 
  relative 
  to 
  the 
  aether. 
  

   There 
  will 
  be 
  aberration, 
  the 
  star 
  will 
  appear 
  to 
  the 
  observer 
  

   to 
  occupy 
  a 
  different 
  position 
  from 
  that 
  which 
  it 
  would 
  

   occupy 
  if 
  there 
  were 
  no 
  motion 
  relative 
  to 
  the 
  aether, 
  the 
  

   difference 
  depending 
  on 
  the 
  amount 
  of 
  such 
  motion. 
  To 
  one 
  

   who 
  had 
  not 
  thought 
  of 
  postulating 
  an 
  aether 
  but 
  merely 
  

   concerned 
  himself 
  with 
  classifying 
  phenomena 
  and 
  expressing 
  

   their 
  laws 
  mathematically, 
  this 
  new 
  phenomenon 
  would 
  

   appear 
  wholly 
  arbitrary 
  and 
  inexplicable. 
  He 
  would 
  in 
  fact 
  

   have 
  to 
  invent 
  a 
  new 
  cause 
  to 
  explain 
  it, 
  and 
  that 
  cause 
  would 
  

   in 
  its 
  essence 
  be 
  our 
  aether. 
  Jn 
  a 
  way, 
  it 
  may 
  be 
  said 
  that 
  

   the 
  aether 
  would 
  become 
  manifest 
  to 
  his 
  senses 
  as 
  a 
  something- 
  

   other 
  than 
  matter 
  having 
  parts 
  and 
  position. 
  We 
  should 
  

   apprehend 
  the 
  existence 
  of 
  an 
  atmosphere 
  in 
  exactly 
  the 
  same 
  

   way, 
  and 
  should 
  ascribe 
  the 
  same 
  properties 
  to 
  it, 
  if 
  we 
  had 
  

   to 
  depend 
  only 
  on 
  the 
  sense 
  of 
  hearing 
  for 
  perceiving 
  it. 
  

   The 
  fact 
  that 
  the 
  noise 
  of 
  a 
  rail 
  way- 
  whistle 
  does 
  not 
  always 
  

   appear 
  to 
  proceed 
  from 
  the 
  whistle, 
  though 
  the 
  whistle 
  has 
  

   no 
  motion 
  relative 
  to 
  the 
  hearer, 
  furnishes 
  a 
  logical 
  necessity 
  

   for 
  an 
  atmosphere. 
  

  

  The 
  only 
  question 
  that 
  remains 
  is 
  this 
  : 
  Is 
  there 
  any 
  direct 
  

   evidence 
  that 
  the 
  motion 
  of 
  the 
  luminous 
  body 
  produces 
  no 
  

   effect 
  on 
  aberration 
  ? 
  For 
  of 
  course 
  it 
  will 
  not 
  do 
  to 
  take 
  it 
  

   merely 
  as 
  a 
  consequence 
  of 
  the 
  undulatory 
  theory. 
  In 
  the 
  

   ordinary 
  observations 
  of 
  aberration 
  there 
  is 
  no 
  such 
  evidence. 
  

   It 
  is 
  quite 
  consistent 
  with 
  them 
  to 
  suppose 
  that 
  the 
  motion 
  of 
  

   the 
  star 
  which 
  is 
  uniform 
  produces 
  the 
  same 
  effect 
  on 
  

   aberration 
  as 
  that 
  of 
  the 
  earth. 
  We 
  can 
  in 
  fact 
  only 
  observe 
  

   the 
  changes 
  in 
  aberration, 
  such 
  as 
  those 
  which 
  depend 
  on 
  the 
  

   periodic 
  motion 
  of 
  the 
  earth, 
  and 
  not 
  its 
  absolute 
  amount. 
  

   But 
  there 
  are 
  certain 
  stars 
  — 
  spectroscopic 
  binaries 
  — 
  which, 
  

   apparently 
  single 
  as 
  seen 
  in 
  a 
  telescope, 
  are 
  inferred 
  to 
  

   consist 
  of 
  two 
  components 
  revolving 
  one 
  about 
  the 
  other 
  

  

  