﻿Prof. 
  A. 
  Schuster 
  : 
  What 
  is 
  Interference 
  ? 
  767 
  

  

  Our 
  pile 
  of 
  plates 
  forms 
  an 
  echelon 
  grating, 
  and 
  the 
  dif- 
  

   fraction 
  maxima 
  are 
  much 
  narrower, 
  consequently 
  the 
  light 
  

   of 
  the 
  continuous 
  spectrum 
  is 
  squeezed 
  into 
  regions 
  ^Yhich 
  

   are 
  narrow 
  in 
  comparison 
  to 
  the 
  width 
  of 
  the 
  dark 
  regions. 
  

  

  The 
  mica 
  echelon, 
  which 
  I 
  described 
  in 
  a 
  previous 
  paper 
  

   (Phil. 
  Mag. 
  vol. 
  i. 
  1901, 
  p. 
  627), 
  answers 
  admirably 
  for 
  the 
  

   production 
  of 
  these 
  fringes, 
  a 
  photograph 
  of 
  which 
  is 
  repro- 
  

   duced 
  on 
  PI. 
  XXIII. 
  fig. 
  2. 
  

  

  Fainter 
  secondary 
  maxima 
  appear 
  in 
  the 
  dark 
  regions, 
  

   their 
  number, 
  position, 
  and 
  the 
  distribation 
  of 
  the 
  intensity 
  

   among 
  them 
  depending 
  upon 
  the 
  angle 
  at 
  which 
  the 
  echelon 
  

   is 
  set. 
  We 
  find 
  very 
  unsymmetrical 
  distribution 
  of 
  intensity 
  

   when 
  the 
  echelon 
  is 
  quite 
  oblique, 
  a 
  circumstance 
  which 
  is 
  

   of 
  interest 
  in 
  connexion 
  with 
  the 
  recent 
  work 
  of 
  Raman 
  on 
  

   the 
  diffraction 
  by 
  oblique 
  apertures 
  (Phil. 
  Mag. 
  Nov. 
  1906). 
  

  

  LXXIX. 
  What 
  is 
  Interference 
  ? 
  A 
  Rejoinder 
  to 
  Professor 
  

   Wood. 
  By 
  Arthuk 
  Schuster, 
  F.R.S* 
  \ 
  ]'y^'' 
  

  

  THROUGH 
  the 
  courtesy 
  of 
  Professor 
  Wood 
  I 
  am 
  enabled 
  

   to 
  add 
  a 
  few 
  words 
  of 
  comment 
  to 
  the 
  foregoing 
  paper. 
  

   The 
  question 
  at 
  issue 
  is 
  the 
  explanation 
  of 
  the 
  observed 
  fact 
  

   that 
  when 
  a 
  spectrum 
  is 
  viewed 
  with 
  the 
  pupil 
  partly 
  covered 
  

   with 
  a 
  thin 
  transparent 
  plate, 
  it 
  is 
  found 
  to 
  be 
  crossed 
  by 
  

   dark 
  bands 
  only 
  when 
  the 
  plate 
  is 
  introduced 
  on 
  that 
  side 
  on 
  

   which 
  the 
  violet 
  end 
  appears. 
  In 
  the 
  complete 
  theory 
  of 
  

   these 
  bands 
  which 
  has 
  been 
  given 
  by 
  Airy 
  and 
  Stokes, 
  the 
  

   characteristic 
  feature, 
  that 
  the 
  bands 
  do 
  not 
  appear 
  when 
  

   the 
  plate 
  is 
  introduced 
  from 
  the 
  red 
  end, 
  requires 
  a 
  compli- 
  

   cated 
  mathematical 
  analysis, 
  which 
  I 
  have 
  shown 
  to 
  become 
  

   unnecessary 
  when 
  white 
  light 
  is 
  treated 
  as 
  a 
  collection 
  of 
  

   impulses. 
  It 
  is 
  then 
  at 
  once 
  seen 
  to 
  be 
  obvious 
  that 
  in 
  order 
  

   to 
  bring 
  two 
  series 
  of 
  impulses 
  to 
  interference, 
  we 
  must 
  

   retard 
  the 
  front 
  one 
  or 
  accelerate 
  the 
  one 
  that 
  is 
  in 
  the 
  rear. 
  

   The 
  explanation, 
  which 
  I 
  have 
  based 
  on 
  this 
  principle, 
  throws 
  

   a 
  new 
  light 
  on 
  the 
  phenomenon, 
  but 
  was 
  not 
  intended 
  to 
  

   invalidate 
  the 
  correctness 
  of 
  the 
  older 
  theory. 
  The 
  two 
  

   representations 
  of 
  white 
  light 
  (by 
  homogeneous 
  waves 
  or 
  by 
  

   impulses) 
  are 
  not 
  mutually 
  exclusive; 
  they 
  represent 
  two 
  

   points 
  of 
  view, 
  and 
  we 
  may 
  adopt 
  either 
  one 
  or 
  the 
  other 
  in 
  

   difi'erent 
  problems 
  according 
  to 
  convenience. 
  

  

  Professor 
  Wood 
  in 
  his 
  experimental 
  investigation 
  of 
  

   Talbot's 
  bands 
  a[)proaches 
  the 
  subject 
  by 
  building 
  up 
  his 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  