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  XXXIII. 
  The 
  Velocity 
  of 
  Light. 
  By 
  A. 
  A. 
  Michelson*. 
  

  

  THE 
  fact 
  that 
  the 
  velocity 
  of 
  light 
  is 
  so 
  far 
  beyond 
  the 
  

   conception 
  of 
  the 
  human 
  intellect, 
  coupled 
  with 
  the 
  

   extraordinary 
  accuracy 
  with 
  which 
  it 
  may 
  be 
  measured, 
  makes 
  

   this 
  determination 
  one 
  of 
  the 
  most 
  fascinating 
  problems 
  that 
  

   fall 
  to 
  the 
  lot 
  of 
  the 
  investigator. 
  

  

  The 
  experiment 
  of 
  measuring 
  the 
  velocity 
  of 
  light 
  was 
  

   originally 
  devised 
  by 
  Arago, 
  as 
  a 
  means 
  of 
  furnishing 
  a 
  

   crucial 
  test 
  between 
  the 
  two 
  great 
  rival 
  theories 
  of 
  light. 
  

   History 
  has 
  recorded 
  the 
  triumph 
  of 
  the 
  undulatory 
  theory, 
  

   as 
  the 
  result 
  of 
  the 
  classic 
  experiments 
  of 
  Foucault 
  and 
  

   Fizeau. 
  

  

  It 
  was 
  found 
  in 
  the 
  course 
  of 
  these 
  and 
  subsequent 
  experi- 
  

   ments 
  that 
  the 
  experimental 
  determination 
  of 
  the 
  absolute 
  

   value 
  (V) 
  of 
  the 
  velocity 
  of 
  light 
  was 
  capable 
  of 
  so 
  high 
  a 
  

   degree 
  of 
  accuracy 
  that 
  instead 
  of 
  obtaining 
  V 
  by 
  observations 
  

   of 
  the 
  eclipses 
  of 
  Jupiter's 
  satellites, 
  or 
  the 
  aberration 
  of 
  

   the 
  fixed 
  stars, 
  together 
  with 
  the 
  astronomically 
  calculated 
  

   value 
  of 
  the 
  sun's 
  distance 
  (with 
  results 
  of 
  relatively 
  small 
  

   accuracy) 
  — 
  the 
  process 
  is 
  exactly 
  reversed 
  ; 
  so 
  that 
  the 
  

   experimental 
  value 
  of 
  V, 
  together 
  with 
  observed 
  values 
  of 
  

   the 
  light 
  equation, 
  or 
  the 
  constant 
  of 
  aberration, 
  furnishes 
  

   a 
  far 
  more 
  accurate 
  value 
  of 
  the 
  sun's 
  distance 
  than 
  can 
  be 
  

   obtained 
  by 
  rare 
  and 
  costly 
  expeditions 
  for 
  observing 
  the 
  

   transit 
  of 
  Venus. 
  This 
  appears 
  quite 
  clearly 
  if 
  we 
  contrast 
  

   the 
  numbers 
  given 
  by 
  Professor 
  Harkness 
  f, 
  which 
  give 
  the 
  

   solar 
  parallax 
  as 
  obtained 
  from 
  astronomical 
  data 
  J, 
  

  

  8-78 
  + 
  -05, 
  

  

  with 
  the 
  results 
  given 
  by 
  Professor 
  Todd 
  of 
  the 
  same 
  quantity, 
  

   as 
  determined 
  by 
  the 
  combination 
  of 
  velocity 
  of 
  light 
  with 
  

   the 
  light 
  equation 
  or 
  with 
  the 
  constant 
  of 
  aberration. 
  

  

  In 
  an 
  article 
  on 
  " 
  Solar 
  Parallax 
  from 
  the 
  Velocity 
  of 
  

   Light 
  "§, 
  Professor 
  D. 
  P. 
  Todd 
  gives 
  the 
  following 
  resume 
  

   of 
  the 
  determinations 
  of 
  the 
  solar 
  parallax 
  from 
  L, 
  the 
  light 
  

   equation, 
  and 
  from 
  a, 
  the 
  constant 
  of 
  aberration 
  : 
  — 
  

  

  61 
  The 
  elements 
  of 
  sensible 
  uncertainty 
  considered 
  are 
  : 
  

   " 
  (1) 
  Uncertainty 
  in 
  the 
  determination 
  of 
  terrestrial 
  

   velocity 
  of 
  light. 
  ... 
  I 
  am 
  disposed 
  to 
  think 
  that 
  the 
  limit 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  t 
  The 
  numbers 
  here 
  given 
  are 
  not 
  Professor 
  Harkness's 
  estimates, 
  

   "but 
  only 
  a 
  rather 
  rough 
  guess 
  from 
  the 
  data 
  he 
  presents, 
  which 
  I 
  think 
  

   are 
  nevertheless 
  of 
  about 
  the 
  right 
  order 
  of 
  magnitude. 
  

  

  % 
  Am. 
  Journ. 
  Sci. 
  1881. 
  

  

  § 
  Am. 
  Journ. 
  Sci. 
  vol. 
  xix. 
  p. 
  59. 
  

  

  