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  XXXY. 
  On 
  the 
  Principle 
  of 
  Relativity 
  and 
  the 
  Electromagnetic 
  

   Mass 
  of 
  the 
  Electron. 
  A 
  Reply 
  to 
  Dr. 
  A. 
  H. 
  Bucherer. 
  

   By 
  E. 
  CUNNINGHAM, 
  Lecturer 
  in 
  Applied 
  Mathematics, 
  

   University 
  College, 
  London 
  *. 
  

  

  JX 
  the 
  March 
  number 
  of 
  this 
  Magazine 
  (p. 
  316), 
  Dr. 
  A. 
  H. 
  

   Bucherer 
  objects 
  to 
  the 
  statement 
  made 
  by 
  me 
  in 
  a 
  paper 
  

   published 
  also 
  in 
  this 
  Magazine 
  (Oct. 
  1907) 
  that 
  his 
  Principle 
  

   o£ 
  Relativity 
  f 
  was 
  identical 
  with 
  the 
  Lorentz-Einstein 
  

   principle. 
  Without 
  at 
  all 
  wishing 
  to 
  depreciate 
  the 
  ingenious 
  

   method 
  which 
  Dr. 
  Bucherer 
  has 
  adopted 
  to 
  avoid 
  the 
  diffi- 
  

   culties 
  which 
  cluster 
  round 
  this 
  part 
  of 
  electromagnetic 
  

   theory, 
  I 
  should 
  like 
  to 
  consider 
  his 
  objections 
  and 
  to 
  go 
  more 
  

   fully 
  into 
  the 
  question 
  as 
  to 
  whether 
  the 
  statement 
  which 
  I 
  

   made 
  was 
  correct. 
  

  

  The 
  following 
  paragraph 
  is 
  quoted 
  from 
  my 
  paper 
  : 
  — 
  

   " 
  It 
  is 
  required, 
  among 
  other 
  things, 
  to 
  explain 
  how 
  a 
  light- 
  

   wave 
  travelling 
  outwards 
  in 
  all 
  directions 
  with 
  velocity 
  c 
  

   relative 
  to 
  an 
  observer 
  A, 
  may 
  at 
  the 
  same 
  time 
  be 
  travelling 
  

   outwards 
  in 
  all 
  directions 
  with 
  the 
  same 
  velocity 
  relative 
  to 
  

   an 
  observer 
  B 
  moving 
  relative 
  to 
  A 
  with 
  velocity 
  »." 
  May 
  

   I 
  explain 
  that 
  I 
  did 
  not 
  wish 
  to 
  assert 
  that 
  it 
  was 
  required 
  

   by 
  any 
  known 
  fact 
  of 
  observation, 
  but 
  that 
  I 
  took 
  it 
  

   to 
  be 
  involved 
  in 
  the 
  statement 
  of 
  the 
  principle. 
  I 
  

   may 
  have 
  read 
  into 
  it 
  more 
  than 
  was 
  intended, 
  but 
  if 
  the 
  

   Maxwell 
  equations 
  are 
  assumed 
  to 
  hold 
  when 
  referred, 
  as 
  

   occasion 
  requires, 
  to 
  various 
  frames 
  of 
  reference 
  moving 
  

   relatively 
  to 
  one 
  another, 
  the 
  deduction 
  cannot 
  be 
  escaped 
  

   that 
  the 
  velocity 
  of 
  propagation 
  of 
  a 
  spherical 
  wave 
  will 
  be 
  

   found 
  to 
  be 
  exactly 
  the 
  same, 
  whatever 
  the 
  frame 
  of 
  reference. 
  

   Thus 
  what 
  was 
  proved 
  in 
  my 
  former 
  paper 
  was 
  that 
  if 
  L 
  have 
  

   not 
  read 
  too 
  much 
  into 
  Dr. 
  Bucherer 
  s 
  principle 
  in 
  supposing 
  

   that 
  he 
  assumes 
  the 
  Maxwell 
  equations 
  to 
  hold, 
  whatever 
  

   particular 
  point 
  is 
  considered 
  to 
  be 
  at 
  rest, 
  then 
  that 
  principle 
  

   cannot 
  be 
  applied 
  without 
  taking 
  into 
  consideration 
  a 
  possible 
  

   difference 
  between 
  the 
  space 
  and 
  time 
  measures 
  of 
  two 
  

   observers 
  moving 
  relatively 
  to 
  one 
  another, 
  and 
  that 
  in 
  fact 
  

   this 
  transformation 
  between 
  the 
  space 
  and 
  time 
  measures 
  

   must 
  be 
  that 
  associated 
  with 
  the 
  names 
  of 
  Lorentz 
  and 
  

   Einstein. 
  

  

  Passing 
  to 
  another 
  point 
  raised 
  by 
  Dr. 
  Bucherer, 
  I 
  feel 
  

   myself 
  on 
  firmer 
  ground, 
  inasmuch 
  as 
  I 
  am 
  free 
  from 
  the 
  

   fear 
  that 
  I 
  may 
  still 
  be 
  misconstruing 
  his 
  principle. 
  He 
  asks 
  

  

  * 
  Communicated 
  bv 
  the 
  Author, 
  

   f 
  Phil. 
  Mag. 
  April'1907. 
  

  

  