﻿768 
  ProL 
  A. 
  Schuster 
  : 
  What 
  is 
  Liter/erence 
  ? 
  

  

  continuous 
  spectrum 
  from 
  a 
  discontinuous 
  one, 
  and 
  the 
  

   treatment 
  of 
  Stokes 
  becomes 
  therefore 
  to 
  him 
  the 
  more 
  

   natural 
  one. 
  With 
  his 
  usual 
  skill, 
  he 
  devises 
  instructive 
  

   experiments 
  illustrating 
  the 
  intermediate 
  steps 
  ; 
  and 
  those 
  

   who 
  prefer 
  to 
  adhere 
  to 
  the 
  older 
  view 
  of 
  white 
  light 
  may 
  

   find 
  in 
  these 
  experiments 
  sufficient 
  excuse 
  to 
  skip 
  Stokes^s 
  

   rigorous 
  mathematical 
  treatment 
  without 
  too 
  great 
  a 
  strain 
  

   on 
  their 
  conscience. 
  My 
  own 
  investigation, 
  so 
  far 
  as 
  I 
  

   can 
  see, 
  remains 
  untouched, 
  as 
  I 
  dealt 
  with 
  white 
  light 
  

   alone. 
  

  

  Professor 
  Wood 
  now 
  throws 
  doubt 
  on 
  my 
  treatment 
  of 
  the 
  

   subject 
  by 
  an 
  argument 
  which 
  may 
  be 
  summarized 
  as 
  

   follows 
  : 
  — 
  (1) 
  Schuster's 
  explanation 
  of 
  the 
  absence 
  of 
  the 
  

   bands 
  when 
  the 
  plate 
  is 
  introduced 
  on 
  the 
  wrong 
  side 
  is 
  based 
  

   on 
  the 
  principle 
  that 
  there 
  cannot 
  be 
  interference 
  when 
  there 
  

   is 
  no 
  overlapping. 
  (It 
  is 
  admitted 
  that 
  the 
  principle 
  is 
  

   correct.) 
  (2) 
  Interference 
  efifects 
  can 
  be 
  observed 
  with 
  

   homogeneous 
  light 
  even 
  though 
  the 
  plate 
  is 
  introduced 
  on 
  

   the 
  wrong 
  side. 
  (It 
  is 
  admitted 
  that 
  there 
  is 
  overlapping 
  in 
  

   this 
  case.) 
  (3) 
  White 
  light 
  can 
  be 
  built 
  up 
  with 
  homo- 
  

   geneous 
  light 
  ; 
  and 
  if 
  this 
  is 
  done, 
  the 
  plate 
  still 
  being 
  on 
  

   the 
  wrong 
  side, 
  the 
  bands 
  disappear, 
  while 
  interference 
  

   persists^ 
  (4) 
  The 
  absence 
  of 
  interference 
  cannot 
  therefore 
  

   be 
  made 
  the 
  criterion 
  for 
  the 
  disappearance 
  of 
  the 
  bands 
  ; 
  

   and 
  hence 
  " 
  it 
  does 
  not 
  seem 
  as 
  if 
  Schuster's 
  explanation 
  can 
  

   be 
  accepted 
  as 
  the 
  correct 
  one." 
  

  

  The 
  fallacy 
  here 
  lies 
  at 
  the 
  end 
  of 
  the 
  third 
  step 
  where 
  it 
  

   is 
  asserted 
  that 
  interference 
  persists 
  even 
  though 
  the 
  effects 
  

   of 
  interference 
  have 
  disappeared. 
  Matters 
  of 
  definition 
  

   should 
  be 
  kept 
  clear 
  of 
  arguments 
  on 
  the 
  validity 
  of 
  a 
  certain 
  

   reasoning, 
  and 
  if 
  two 
  discussions 
  which 
  both 
  explain 
  observed 
  

   facts 
  satisi'actorily, 
  lead 
  to 
  different 
  results 
  as 
  to 
  whether 
  there 
  

   is 
  " 
  interference 
  '^ 
  or 
  not, 
  the 
  only 
  deduction 
  that 
  can 
  correctly 
  

   be 
  made 
  is, 
  that 
  there 
  is 
  disagreement 
  in 
  the 
  use 
  of 
  the 
  word. 
  

   What 
  indeed 
  is 
  " 
  interference 
  "'' 
  ? 
  Every 
  writer 
  on 
  Optics 
  

   uses 
  the 
  expression, 
  and 
  yet 
  we 
  may 
  search 
  the 
  literature 
  of 
  

   the 
  subject 
  without 
  finding 
  a 
  definition 
  which 
  can 
  be 
  applied 
  

   to 
  test 
  whether 
  in 
  any 
  particular 
  case 
  we 
  can 
  say 
  that 
  there 
  

   has 
  been 
  interference 
  or 
  not. 
  As 
  long 
  as 
  one 
  is 
  satisfied 
  to 
  use 
  

   the 
  term 
  generally 
  and 
  somewhat 
  vaguely 
  to 
  classify 
  a 
  certain 
  

   group 
  of 
  phenomena, 
  no 
  harm 
  is 
  done, 
  and 
  it 
  is 
  sometimes 
  

   useful 
  to 
  retain 
  a 
  word 
  which 
  by 
  common 
  consent 
  has 
  a 
  

   plastic 
  meaning. 
  But 
  if 
  it 
  is 
  to 
  be 
  made 
  the 
  test 
  by 
  which 
  an 
  

   explanatiom 
  is 
  to 
  be 
  judged, 
  then 
  we 
  must 
  have 
  recourse 
  to 
  

   some 
  adequate 
  definition. 
  In 
  Mascart's 
  treatise 
  on 
  Optics 
  it 
  

  

  