﻿Postulate 
  of 
  the 
  Theory 
  of 
  Relativity. 
  171 
  

  

  second. 
  The 
  number 
  of 
  revolutions 
  of 
  the 
  wheel 
  was 
  deter- 
  

   mined 
  acoustically 
  in 
  each 
  experiment. 
  The 
  mirrors, 
  at 
  

   equal 
  intervals 
  on 
  the 
  periphery 
  of 
  the 
  wheel, 
  are 
  inclined 
  

   to 
  the 
  radius 
  from 
  R 
  passing 
  through 
  the 
  centre 
  of 
  each 
  of 
  

   them 
  at 
  an 
  angle 
  a. 
  = 
  29°. 
  They 
  are 
  fixed 
  solidly 
  to 
  R 
  by 
  

   screw 
  movements 
  capable 
  of 
  permitting 
  a 
  rigorous 
  adjust- 
  

   ment. 
  The 
  support 
  for 
  the 
  bearings 
  of 
  the 
  axle 
  of 
  R 
  carries 
  

   also 
  fixed 
  mirrors 
  F, 
  vertical 
  like 
  M, 
  of 
  which 
  the 
  number 
  

   in 
  the 
  figure 
  is 
  three 
  ; 
  but 
  this 
  number 
  may, 
  at 
  will, 
  be 
  

   reduced, 
  or 
  increased 
  up 
  to 
  nine. 
  The 
  position 
  of 
  the 
  F's 
  

   and 
  M's 
  is 
  such 
  that 
  a 
  parallel 
  beam 
  of 
  light 
  L, 
  after 
  a 
  

   certain 
  number 
  of 
  reflexions 
  from 
  the 
  F's 
  and 
  M.'s 
  (seven 
  in 
  

   the 
  figure), 
  may 
  be 
  received 
  at 
  L/ 
  when 
  R 
  has 
  determinate 
  

   angular 
  positions. 
  Naturally 
  the 
  intensity 
  of 
  L' 
  is 
  much 
  

   weaker 
  than 
  that 
  of 
  L, 
  and 
  this 
  enfeeblement 
  is 
  much 
  more 
  

   marked 
  if 
  R 
  is 
  in 
  rotation, 
  because 
  in 
  this 
  case 
  the 
  light 
  

   arrives 
  at 
  L' 
  only 
  during 
  certain 
  very 
  short 
  instants 
  (ten 
  

   times 
  per 
  revolution) 
  . 
  I 
  have 
  observed 
  in 
  practice, 
  however, 
  

   that 
  the 
  four 
  moving 
  and 
  three 
  fixed 
  reflexions 
  of 
  the 
  figure 
  

   allow 
  of 
  experimenting 
  with 
  light 
  sufficiently 
  intense 
  at 
  L 
  ; 
  

   even 
  if 
  R 
  is 
  in 
  motion 
  : 
  that 
  is 
  to 
  say, 
  that 
  direct 
  observation 
  

   (without 
  photography) 
  suffices 
  to 
  establish 
  the 
  luminous 
  

   phenomenon 
  of 
  which 
  we 
  have 
  spoken 
  above. 
  

  

  To 
  study 
  the 
  value 
  of 
  X 
  the 
  light 
  L/ 
  was 
  examined 
  with 
  

   the 
  well-known 
  interferometer 
  of 
  Michelson, 
  shown 
  diagram- 
  

   matically 
  in 
  the 
  figure. 
  It 
  is 
  known 
  that 
  if 
  the 
  distances 
  

   S^ 
  and 
  S 
  2 
  8 
  3 
  are 
  exactly 
  equal 
  fringes 
  are 
  observed 
  with 
  

   the 
  telescope 
  C 
  even 
  if 
  the 
  light 
  is 
  not 
  monochromatic 
  ; 
  

   these 
  fringes 
  then 
  have 
  the 
  coloration 
  of 
  Newton's 
  rings. 
  

   As 
  soon 
  as 
  a 
  difference 
  of 
  path 
  occurs 
  (even 
  if 
  only 
  of 
  a 
  few 
  

   microns) 
  observation 
  with 
  white 
  light 
  is 
  no 
  longer 
  possible. 
  

   Monochromatic 
  light 
  must 
  then 
  be 
  used, 
  and 
  the 
  order 
  of 
  the 
  

   interference 
  fringes 
  increases 
  with 
  this 
  difference. 
  Their 
  

   visibility 
  is 
  greater, 
  the 
  simpler 
  the 
  luminous 
  vibrations. 
  

   From 
  the 
  researches 
  of 
  Michelson 
  * 
  it 
  is 
  known 
  that 
  from 
  

   this 
  point 
  of 
  view 
  the 
  line 
  that 
  gives 
  the 
  greatest 
  visibility 
  

   of 
  the 
  fringes 
  with 
  the 
  greatest 
  difference 
  in 
  path 
  is 
  the 
  

   green 
  one 
  of 
  mercury 
  (A, 
  = 
  54 
  6yu,/z). 
  In 
  this 
  case 
  numberless 
  

   circular 
  fringes 
  are 
  visible 
  even 
  for 
  a 
  difference 
  of 
  path 
  

   I 
  = 
  2(8x83 
  — 
  8383) 
  = 
  40 
  cm. 
  I 
  have 
  therefore 
  employed 
  as 
  

   source 
  L 
  a 
  mercury 
  arc 
  in 
  vacuo 
  the 
  light 
  of 
  which 
  is 
  con- 
  

   veniently 
  filtered 
  by 
  solutions 
  of 
  chromate 
  of 
  potassium 
  

   and 
  chloride 
  of 
  nickel 
  to 
  absorb 
  the 
  violet 
  and 
  yellow 
  rays 
  ; 
  

  

  * 
  Travaux 
  et 
  Memoires, 
  Bur. 
  Lit. 
  de 
  poids 
  et 
  mesures, 
  xi. 
  p. 
  146 
  

   (1895). 
  

  

  