﻿Second 
  Postulate 
  of 
  the 
  Theory 
  of 
  Relativity. 
  163 
  

  

  movements 
  testify 
  to 
  the 
  presence 
  of 
  the 
  oil. 
  Perhaps 
  this 
  

   was 
  the 
  reason 
  why 
  in 
  my 
  experiments 
  of 
  1890 
  I 
  found 
  

   the 
  approximate, 
  rather 
  than 
  the 
  absolute, 
  stoppage 
  of 
  the 
  

   movements 
  to 
  give 
  the 
  sharpest 
  results. 
  The 
  absolute 
  

   stoppage, 
  dependent 
  upon 
  the 
  presence 
  of 
  impurity, 
  might 
  

   well 
  be 
  less 
  defined. 
  

  

  If, 
  after 
  the 
  deposition 
  of 
  a 
  drop 
  of 
  purified 
  oil, 
  the 
  

   surface 
  be 
  again 
  dusted 
  over 
  with 
  sulphur 
  or 
  talc 
  and 
  then 
  

   touched 
  with 
  a 
  very 
  small 
  quantity 
  of 
  the 
  original 
  oil, 
  

   the 
  dust 
  is 
  driven 
  away 
  a 
  second 
  time 
  and 
  camphor- 
  

   movements 
  cease. 
  

  

  The 
  manner 
  in 
  which 
  impurity 
  operates 
  in 
  these 
  phenomena 
  

   merits 
  close 
  attention. 
  It 
  seems 
  pretty 
  clear 
  that 
  from 
  pure 
  

   oil 
  water 
  will 
  only 
  take 
  a 
  layer 
  one 
  molecule 
  thick. 
  But 
  

   when 
  oleic 
  acid 
  is 
  available, 
  a 
  further 
  drop 
  of 
  tension 
  ensues. 
  

   The 
  question 
  arises 
  how 
  does 
  this 
  oleic 
  acid 
  distribute 
  

   itself? 
  Is 
  it 
  in 
  substitution 
  for 
  the 
  molecules 
  of 
  oil, 
  or 
  

   an 
  addition 
  to 
  them 
  constituting 
  a 
  second 
  layer 
  ? 
  The 
  

   latter 
  seems 
  the 
  more 
  probable. 
  Again, 
  how 
  does 
  the 
  

   impurity 
  act 
  when 
  it 
  leads 
  the 
  general 
  mass 
  into 
  the 
  un- 
  

   stable 
  flattened-out 
  form 
  ? 
  In 
  considering 
  such 
  questions 
  

   Laplace's 
  theory 
  is 
  of 
  little 
  service, 
  its 
  fundamental 
  postulate 
  

   of 
  forces 
  operating 
  over 
  distances 
  large 
  in 
  comparison 
  with 
  

   molecular 
  dimensions 
  being 
  plainly 
  violated. 
  

  

  Terling 
  Place, 
  Witham, 
  

   Dec. 
  31, 
  1917. 
  

  

  XVIII. 
  On 
  the 
  Second 
  Postulate 
  of 
  the 
  Theory 
  of 
  Relativity 
  : 
  

   Experimental 
  Demonstration 
  of 
  the 
  Constancy 
  of 
  Velo- 
  

   city 
  of 
  the 
  Light 
  reflected 
  from 
  a 
  Moving 
  Mirror. 
  By 
  

   Q. 
  Majokana, 
  Professor 
  of 
  Physics 
  at 
  the 
  Polytechnic 
  

   School 
  of 
  Turin*. 
  

  

  THE 
  Theory 
  of 
  Kelativity 
  is 
  based 
  upon 
  two 
  well-known 
  

   fundamental 
  postulates. 
  The 
  first 
  affirms 
  the 
  impos- 
  

   sibility 
  of 
  discovering 
  the 
  movement 
  of 
  a 
  system 
  without 
  

   referring 
  this 
  system 
  to 
  other 
  systems 
  ; 
  that 
  is 
  to 
  say, 
  it 
  

   denies 
  the 
  physical 
  reality 
  of 
  absolute 
  motion. 
  The 
  second 
  

   postulate 
  affirms 
  that 
  the 
  velocity 
  c 
  of 
  propagation 
  of 
  light 
  

   in 
  vacuo 
  is 
  a 
  universal 
  constant. 
  Both 
  these 
  postulates 
  are 
  

   generalizations 
  of 
  facts 
  or 
  principles 
  already 
  admitted 
  by 
  

   physicists. 
  

  

  In 
  fact, 
  we 
  may 
  regard 
  the 
  first 
  as 
  the 
  extension 
  to 
  optical 
  

   or 
  electrical 
  phenomena 
  of 
  a 
  classical 
  principle 
  of 
  mechanics,. 
  

   * 
  Communicated 
  by 
  the 
  Author. 
  

  

  