﻿Prof. 
  A. 
  Schuster 
  : 
  What 
  is 
  Interference 
  ? 
  769 
  

  

  is 
  stated 
  that 
  the 
  principle 
  of 
  interference 
  is 
  identical 
  with 
  

   the 
  principle 
  of 
  superposition. 
  This 
  is 
  clear 
  and 
  definite, 
  

   but 
  it 
  would 
  force 
  us 
  to 
  abandon 
  the 
  word 
  altogether, 
  because 
  

   every 
  optical 
  phenomenon 
  would 
  become 
  a 
  phenomenon 
  of 
  

   interference, 
  and 
  we 
  should 
  have 
  to 
  admit 
  that 
  rays 
  from 
  

   independent 
  sources 
  of 
  light 
  could 
  and 
  always 
  would 
  interfere 
  

   with 
  each 
  other. 
  In 
  order 
  to 
  retain 
  a 
  meaning 
  wdiich 
  shall 
  

   be 
  definite 
  and 
  concordant 
  with 
  the 
  present 
  usage, 
  I 
  have 
  

   introduced 
  a 
  definition 
  into 
  the 
  second 
  edition 
  of 
  my 
  ' 
  Optics,' 
  

   which 
  I 
  think 
  specifies 
  what 
  I 
  believe 
  to 
  represent 
  the 
  

   consensus 
  of 
  opinion 
  on 
  the 
  subject. 
  

  

  If 
  the 
  observed 
  illumination 
  of 
  a 
  surface 
  by 
  two 
  or 
  more 
  

   pencils 
  of 
  light 
  is 
  not 
  equal 
  to 
  the 
  sum 
  of 
  the 
  illuminations 
  of 
  

   the 
  separate 
  pencils, 
  we 
  sai/ 
  that 
  the 
  pencils 
  have 
  interfered 
  with 
  

   each 
  other 
  and 
  class 
  the 
  pheiiomenon 
  as 
  one 
  of 
  inteiference. 
  

  

  Stress 
  should 
  be 
  laid 
  on 
  the 
  adjective 
  " 
  observed." 
  It 
  

   means 
  that 
  the 
  efiect 
  must 
  be 
  observable 
  in 
  the 
  actual 
  case 
  

   considered 
  and 
  not 
  only 
  be 
  capable 
  of 
  being 
  made 
  visible 
  by 
  

   some 
  additional 
  appliance. 
  Adopting 
  this 
  definition, 
  the 
  only 
  

   way 
  in 
  which 
  it 
  can 
  be 
  decided 
  experimentally 
  whether 
  inter- 
  

   ference 
  takes 
  place 
  when 
  the 
  plate 
  in 
  Talbot^s 
  experiment 
  is 
  

   introduced 
  on 
  the 
  wrong 
  side, 
  would 
  be 
  to 
  observe 
  each 
  part 
  

   of 
  the 
  aperture 
  separately, 
  and 
  see 
  whether 
  the 
  combined 
  

   efiect 
  is 
  equal 
  to 
  the 
  sum 
  of 
  the 
  separate 
  effects. 
  There 
  is 
  no 
  

   indication 
  in 
  Professor 
  Wood's 
  paper 
  that 
  he 
  has 
  found 
  any 
  

   changes 
  of 
  illumination 
  which 
  would 
  indicate 
  that 
  there 
  is 
  

   interference 
  when 
  the 
  bands 
  are 
  not 
  visible. 
  

  

  Professor 
  Wood^s 
  difficulty 
  can 
  be 
  brought 
  to 
  an 
  issue 
  by 
  

   means 
  of 
  a 
  much 
  simpler 
  case 
  than 
  the 
  one 
  treated 
  by 
  him. 
  

   Consider 
  the 
  formation 
  of 
  a 
  spectrum 
  by 
  a 
  grating. 
  I 
  assume 
  

   it 
  to 
  be 
  generallj^ 
  known 
  how 
  an 
  " 
  impulse 
  " 
  of 
  white 
  light 
  is 
  

   converted 
  into 
  a 
  succession 
  of 
  impulses 
  by 
  the 
  grating, 
  and 
  

   how 
  at 
  each 
  point 
  of 
  the 
  spectrum 
  more 
  or 
  less 
  homogeneous 
  

   light 
  is 
  formed 
  by 
  such 
  a 
  succession. 
  Shall 
  we 
  say 
  in 
  this 
  

   case 
  that 
  a 
  grating 
  acts 
  by 
  interference 
  ? 
  My 
  answer 
  is 
  " 
  No,^^ 
  

   if 
  the 
  impulses 
  are 
  sufficiently 
  short 
  not 
  to 
  overlap. 
  If 
  instead 
  

   of 
  white 
  light 
  we 
  use 
  homogeneous 
  light, 
  there 
  is 
  undoubtedly 
  

   interference 
  because 
  the 
  illumination 
  of 
  the 
  whole 
  grating 
  is 
  

   not 
  equal 
  to 
  the 
  sum 
  of 
  the 
  illuminations 
  due 
  to 
  each 
  line 
  

   separately. 
  Let 
  now 
  white 
  light 
  be 
  formed 
  by 
  a 
  large 
  

   number 
  of 
  homogeneous 
  waves 
  spread 
  all 
  over 
  the 
  spectrum. 
  

   If 
  we 
  argue 
  that 
  the 
  interference 
  of 
  the 
  homogeneous 
  waves 
  

   is 
  preserved 
  when 
  they 
  are 
  sufficiently 
  crowded 
  together 
  to 
  

   form 
  a 
  continuous 
  spectrum, 
  we 
  should 
  have 
  to 
  draw 
  a 
  

   distinction 
  between 
  two 
  kinds 
  of 
  white 
  light, 
  one 
  of 
  which 
  is 
  

  

  