﻿Postulate 
  of 
  the 
  Theory 
  of 
  Relativity. 
  167 
  

  

  light. 
  In 
  any 
  case 
  it 
  should 
  be 
  remarked 
  that 
  astronomical 
  

   observations 
  of 
  the 
  Doppler 
  effect 
  are 
  not 
  always 
  made 
  with 
  

   an 
  a 
  priori 
  knowledge 
  of 
  the 
  relative 
  velocities 
  of 
  source 
  and 
  

   observer. 
  In 
  the 
  case 
  of 
  the 
  solar 
  limb 
  it 
  is 
  necessary, 
  

   moreover, 
  to 
  be 
  cautious 
  in 
  establishing 
  a 
  relation 
  between 
  

   the 
  measure 
  of 
  displacement 
  of 
  the 
  lines 
  and 
  the 
  velocity 
  of 
  

   the 
  limb 
  established 
  by 
  observation 
  of 
  the 
  solar 
  spots 
  ; 
  in 
  

   fact, 
  the 
  light 
  from 
  the 
  limb 
  may 
  be 
  strongly 
  refracted 
  by 
  

   the 
  perispherical 
  incandescent 
  gases, 
  and 
  consequently 
  the 
  

   value 
  of 
  the 
  Doppler 
  effect 
  may 
  vary 
  considerably 
  *. 
  So 
  

   far 
  as 
  terrestrial 
  observations 
  are 
  concerned, 
  and 
  those 
  on 
  

   the 
  canal 
  rays 
  (Stark, 
  Paschen), 
  they 
  give 
  measures 
  of 
  the 
  

   phenomenon 
  of 
  only 
  small 
  precision, 
  and 
  it 
  is 
  impossible 
  to 
  

   foresee 
  by 
  another 
  method 
  the 
  exact 
  velocity 
  of 
  the 
  luminous 
  

   particles 
  : 
  finally, 
  observations 
  made 
  with 
  moving 
  mirrors 
  

   bear 
  no 
  relation 
  to 
  those 
  with 
  moving 
  sources, 
  and 
  these 
  may 
  

   produce 
  different 
  consequences 
  f. 
  

  

  From 
  all 
  this 
  we 
  may 
  conclude 
  that 
  up 
  to 
  the 
  present 
  

   time 
  we 
  do 
  not 
  possess 
  any 
  quite 
  certain 
  evidence 
  of 
  the 
  

   immutability 
  of 
  c 
  with 
  variable 
  velocity 
  of 
  the 
  source 
  if,, 
  be 
  

   it 
  understood, 
  we 
  are 
  not 
  willing 
  to 
  admit 
  as 
  conclusive 
  the 
  

   simple 
  electromagnetic 
  theory 
  or 
  that 
  of 
  bodies 
  in 
  motion 
  

   according 
  to 
  Lorentz 
  or 
  else 
  Einstein's 
  theory 
  of 
  relativity. 
  

   This 
  conclusion 
  is 
  confirmed 
  by 
  the 
  study 
  of 
  the 
  works 
  of 
  

   the 
  chief 
  supporters 
  of 
  the 
  last 
  theory, 
  and, 
  implicitly, 
  of 
  

   the 
  second 
  postulate. 
  In 
  these 
  works 
  we 
  frequently 
  find 
  

   expressed 
  the 
  desire 
  to 
  discover 
  further 
  facts 
  which 
  will 
  

   definitely 
  confirm 
  the 
  said 
  theory 
  : 
  this 
  desire 
  corresponds 
  

   with 
  the 
  crisis 
  of 
  the 
  latter 
  years 
  of 
  the 
  said 
  theory. 
  

  

  But 
  on 
  the 
  other 
  hand, 
  as 
  Levi-Civita 
  observes, 
  after 
  the 
  

   latest 
  researches 
  of 
  Einstein, 
  which 
  collect 
  in 
  an 
  admirably 
  

   comprehensive 
  synthesis 
  all 
  the 
  physical 
  phenomena 
  (gravi- 
  

   tation 
  included), 
  it 
  is 
  difficult 
  to 
  avoid 
  the 
  impression 
  that 
  

   we 
  are, 
  as 
  regards 
  the 
  theory 
  of 
  relativity, 
  face 
  to 
  face 
  with 
  

   some 
  definite 
  acquisition. 
  But, 
  while 
  taking 
  account 
  of 
  this, 
  

   it 
  is 
  not 
  expedient 
  to 
  neglect 
  any 
  attempt 
  at 
  a 
  definite 
  con- 
  

   firmation, 
  from 
  an 
  experimental 
  point 
  of 
  view, 
  of 
  a 
  theory 
  

   which 
  has 
  subverted 
  to 
  so 
  large 
  an 
  extent 
  our 
  simplest 
  

   physical 
  notions. 
  This 
  confirmation 
  may 
  follow 
  from 
  a 
  

   precise 
  study 
  of 
  the 
  velocity 
  of 
  propagation 
  of 
  light 
  emitted 
  

   from 
  a 
  moving 
  source, 
  or, 
  which 
  is 
  equivalent, 
  of 
  the 
  wave- 
  

   length 
  of 
  this 
  light. 
  

  

  To 
  realize 
  this 
  study 
  we 
  must 
  devise 
  an 
  arrangement 
  

  

  * 
  Michelson, 
  Astrophys. 
  Journ. 
  xiii. 
  p. 
  192 
  (1901) 
  ; 
  Harnack, 
  Ann. 
  d. 
  

   Thys. 
  xlvi. 
  p. 
  558 
  (1915). 
  

  

  t 
  See 
  the 
  theory 
  proposed 
  by 
  Ritz, 
  (Euvres, 
  pp. 
  321, 
  371, 
  444. 
  

  

  