﻿168 
  Prof. 
  Q. 
  Majorana 
  on 
  the 
  Second 
  

  

  which 
  will 
  permit 
  us 
  to 
  identify 
  the 
  structure 
  of 
  the 
  luminous 
  

   wave, 
  freed 
  from 
  all 
  external 
  action, 
  in 
  its 
  free 
  propagation 
  

   (or 
  transmission) 
  when 
  the 
  velocity 
  of 
  translation 
  of 
  the 
  

   source 
  can 
  be 
  varied 
  at 
  will. 
  But, 
  apart 
  from 
  the 
  fact 
  that 
  

   we 
  must 
  inevitably 
  experiment 
  under 
  the 
  eventual 
  action 
  of 
  

   our 
  earth 
  *, 
  two 
  serious 
  and 
  almost 
  insurmountable 
  difficulties 
  

   oppose 
  themselves 
  to 
  the 
  realization 
  of 
  such 
  a 
  programme. 
  

   In 
  the 
  first 
  place, 
  it 
  is 
  not 
  easy 
  artificially 
  to 
  endow 
  a 
  

   luminous 
  source 
  with 
  rapid 
  movement 
  t, 
  especially 
  if 
  this 
  

   source 
  (as 
  is 
  necessary 
  in 
  some 
  interference 
  methods) 
  has 
  to 
  

   be 
  very 
  rigorously 
  monochromatic 
  ; 
  moreover, 
  I 
  shall 
  publish 
  

   an 
  account 
  in 
  a 
  forthcoming 
  paper 
  of 
  a 
  disposition 
  of 
  this 
  

   nature 
  with 
  which 
  I 
  am 
  about 
  to 
  experiment. 
  Secondly, 
  in 
  

   order 
  to 
  be 
  able 
  to 
  examine 
  the 
  structure 
  of 
  the 
  light 
  emitted 
  

   by 
  a 
  moving 
  source, 
  with 
  whatever 
  disposition, 
  the 
  light 
  

   itself 
  has 
  to 
  be 
  subjected 
  to 
  reflexions, 
  refractions, 
  <fec, 
  some- 
  

   times 
  fairly 
  numerous 
  ; 
  that 
  is 
  to 
  say, 
  the 
  luminous 
  ray 
  must 
  

   encounter 
  ponderable 
  matter 
  after 
  leaving 
  the 
  source. 
  It 
  

   does 
  not 
  follow, 
  therefore, 
  that 
  even 
  if 
  c 
  in 
  a 
  vacuum 
  varies 
  

   with 
  the 
  particular 
  velocity 
  of 
  the 
  source, 
  this 
  quantity 
  does 
  

   not 
  return 
  to 
  the 
  same 
  fixed 
  value 
  after 
  the 
  said 
  phenomena 
  

   of 
  reflexion, 
  refraction, 
  &c. 
  It 
  will 
  be 
  well, 
  therefore, 
  to 
  

   endeavour 
  to 
  eliminate 
  as 
  far 
  as 
  possible, 
  in 
  an 
  experiment 
  of 
  

   this 
  nature, 
  all 
  causes 
  tending 
  to 
  complicate 
  the 
  phenomenon, 
  

   and 
  in 
  every 
  case 
  to 
  consider 
  its 
  results 
  carefully. 
  

  

  Meanwhile, 
  to 
  begin 
  with 
  a 
  relatively 
  simple 
  experiment, 
  

   we 
  may 
  undertake 
  the 
  study 
  of 
  the 
  wave-length 
  of 
  a 
  ray 
  of 
  

   light 
  reflected 
  by 
  a 
  moving 
  mirror 
  J. 
  This 
  may 
  correspond 
  

   with 
  the 
  experiment 
  already 
  realized, 
  some 
  years 
  ago, 
  by 
  

   Belopolski, 
  and 
  afterwards 
  repeated 
  by 
  Galitzin 
  & 
  Wilip 
  ; 
  

   but 
  if 
  the 
  first 
  of 
  these 
  authors 
  employed 
  prisms 
  for 
  the 
  

   observation 
  of 
  the 
  Doppler 
  effect 
  (and 
  consequently 
  the 
  

   question 
  of 
  the 
  eventual 
  variation 
  of 
  X 
  remained 
  unsolved), 
  

   the 
  other 
  two 
  made 
  use 
  of 
  a 
  diffraction-grating, 
  by 
  which 
  

   the 
  controversy 
  spoken 
  of 
  above 
  arises. 
  It 
  would 
  be 
  better 
  

  

  * 
  I 
  cannot 
  succeed 
  in 
  imagining 
  an 
  interplanetary 
  experiment 
  of 
  the 
  

   nature 
  of 
  that 
  proposed 
  (in 
  jest) 
  bv 
  Rose-Innes 
  ; 
  see 
  Phil. 
  Mag. 
  xxvii. 
  

   p. 
  150 
  (1914). 
  

  

  t 
  I 
  understand 
  by 
  this 
  a 
  velocity 
  higher 
  than 
  some 
  hundreds 
  of 
  metres 
  

   per 
  second; 
  this 
  value 
  may 
  perhaps 
  be 
  attained, 
  but 
  it 
  is 
  difficult 
  to 
  

   conceive 
  a 
  practical 
  disposition 
  for 
  a 
  higher 
  velocity. 
  Naturally 
  I 
  leave 
  

   out 
  of 
  account 
  the 
  employment 
  of 
  canal 
  rays, 
  which 
  do 
  not 
  give 
  simple 
  

   and 
  well-known 
  velocities. 
  

  

  J 
  While 
  this 
  article 
  was 
  in 
  the 
  press, 
  M. 
  Michelson 
  has 
  called 
  my 
  

   attention 
  to 
  his 
  paper 
  on 
  the 
  same 
  subject, 
  published 
  in 
  the 
  'Astro- 
  

   physical 
  Journal,' 
  April 
  1913, 
  the 
  conclusions 
  of 
  which 
  agree 
  with 
  those 
  

   that 
  I 
  am 
  stating. 
  

  

  