﻿770 
  Prof. 
  A. 
  Schuster 
  : 
  What 
  is 
  Interference? 
  

  

  white 
  ah 
  initio 
  and 
  the 
  other 
  white 
  through 
  the 
  overlapping 
  

   of 
  homogeneous 
  waves. 
  Though 
  the 
  distinction 
  would 
  only 
  

   be 
  a 
  verbal 
  one, 
  it 
  is 
  obviously 
  inconvenient. 
  If 
  we 
  adopt 
  

   the 
  above 
  definition 
  of 
  interference 
  the 
  matter 
  becomes 
  

   definite. 
  We 
  should 
  have 
  to 
  interpret 
  the 
  result 
  of 
  expe- 
  

   riment 
  by 
  saying 
  that 
  interference 
  has 
  disappeared 
  during 
  

   the 
  process 
  of 
  forming 
  white 
  light. 
  In 
  efi^ect 
  interference 
  is 
  

   not 
  like 
  a 
  substance 
  which, 
  if 
  present 
  in 
  the 
  several 
  con- 
  

   stituents 
  of 
  a 
  mixture, 
  must 
  also 
  be 
  present 
  in 
  the 
  mixture 
  

   itself. 
  It 
  may 
  be 
  destroyed 
  as 
  well 
  as 
  ])roduced 
  by 
  over- 
  

   lapping. 
  If 
  this 
  be 
  accepted 
  difficulties 
  such 
  as 
  those 
  raised 
  

   by 
  Professor 
  Wood 
  will 
  cease 
  to 
  trouble 
  us. 
  

  

  The 
  case 
  of 
  Talbot's 
  bands 
  with 
  the 
  retarding 
  plate 
  intro- 
  

   duced 
  on 
  the 
  wrong 
  side 
  is 
  best 
  represented 
  by 
  a 
  grating- 
  

   ruled 
  in 
  two 
  parts 
  which 
  are 
  separated 
  by 
  an 
  opaque 
  portion. 
  

   With 
  white 
  light 
  we 
  should 
  then 
  have 
  at 
  each 
  point 
  of 
  the 
  

   spectrum 
  a 
  succession 
  of 
  impulses 
  followed 
  first 
  by 
  a 
  quiet 
  

   interval 
  and 
  then 
  by 
  another 
  succession 
  of 
  impulses. 
  There 
  

   cannot 
  be 
  a 
  question 
  of 
  interference 
  here 
  ; 
  but 
  if 
  we 
  apply 
  a 
  

   second 
  grating 
  to 
  investigate 
  the 
  spectrum 
  formed 
  by 
  the 
  

   first, 
  then 
  if 
  the 
  resolving 
  power 
  be 
  sufficient, 
  we 
  can 
  make 
  

   the 
  second 
  group 
  of 
  impulses 
  overlap 
  the 
  first 
  and 
  interference 
  

   would 
  result. 
  For 
  this 
  reason 
  Professor 
  Wood's 
  second 
  

   spectroscope 
  complicates 
  the 
  phenomenon 
  by 
  introducing 
  

   the 
  conditions 
  necessary 
  for 
  interference 
  and 
  the 
  observations 
  

   made 
  with 
  it 
  have 
  no 
  bearing 
  on 
  the 
  question 
  whether 
  there 
  

   has 
  been 
  interference 
  in 
  the 
  original 
  spectrum 
  or 
  not. 
  These 
  

   questions 
  were 
  fully 
  considered 
  and 
  settled 
  during 
  the 
  dis- 
  

   cussion 
  on 
  the 
  nature 
  of 
  white 
  light 
  which 
  it 
  is 
  not 
  necessary 
  

   to 
  revive 
  now. 
  The 
  manner 
  in 
  which 
  Professor 
  Wood 
  tries 
  

   to 
  prove 
  that 
  there 
  is 
  interference 
  with 
  the 
  plate 
  in 
  Talbot's 
  

   experiment 
  on 
  the 
  wrong 
  side, 
  is 
  identical 
  with 
  that 
  by 
  which 
  

   the 
  regularity 
  of 
  vibration 
  in 
  white 
  light 
  used 
  to 
  be 
  thought 
  

   capable 
  of 
  being 
  proved 
  by 
  means 
  of 
  experiments 
  with 
  spec- 
  

   troscopes, 
  which 
  themselves 
  manufactured 
  the 
  regularity 
  

   they 
  were 
  supposed 
  to 
  disclose. 
  

  

  1 
  have 
  entered 
  into 
  these 
  elementary 
  matters 
  because 
  I 
  

   think 
  that 
  under 
  present 
  circumstances 
  a 
  more 
  scientific 
  use 
  

   of 
  the 
  word 
  interference 
  would 
  help 
  students 
  over 
  difficulties. 
  

   At 
  any 
  rate, 
  the 
  point 
  at 
  issue 
  between 
  Professor 
  Wood 
  and 
  

   myself 
  can 
  only 
  resolve 
  itself 
  into 
  a 
  question 
  of 
  the 
  proper 
  

   use 
  of 
  words, 
  so 
  long 
  as 
  he 
  cannot 
  point 
  to 
  some 
  experi- 
  

   mental 
  and 
  observable 
  distinction 
  between 
  light 
  which 
  is 
  

   white 
  ab 
  initio 
  and 
  light 
  which 
  is 
  made 
  white 
  by 
  mixing- 
  

   homogeneous 
  waves. 
  

  

  