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  XXXVI. 
  On 
  Interference 
  Fringes 
  obtained 
  ivith 
  Glass 
  Wedges, 
  

   and 
  their 
  Application 
  to 
  the 
  Examination 
  of 
  Plate 
  Glass. 
  

   By 
  S. 
  R. 
  Milxer, 
  D.Sc* 
  

  

  IF 
  two 
  pieces 
  of 
  ordinary 
  plate-glass 
  of 
  approximately 
  the 
  

   same 
  thickness 
  are 
  placed 
  one 
  on 
  the 
  top 
  of 
  the 
  other, 
  

   and 
  the 
  air-film 
  between 
  them 
  is 
  observed 
  by 
  sodium 
  light, 
  

   the 
  system 
  of 
  Newton's 
  fringes, 
  formed 
  by 
  the 
  interference 
  

   of 
  the 
  light 
  reflected 
  from 
  the 
  two 
  surfaces 
  of 
  the 
  air-film, 
  is 
  

   of 
  course 
  seen. 
  Occasionally, 
  by 
  very 
  close 
  inspection, 
  

   another 
  system 
  of 
  fringes 
  may 
  be 
  seen 
  which 
  is 
  formed 
  by 
  

   the 
  interference 
  of 
  the 
  rays 
  such 
  as 
  ABD, 
  ACD 
  (fig. 
  1), 
  

  

  Fijr. 
  1. 
  

  

  which 
  have 
  been 
  further 
  reflected 
  from 
  the 
  non-adjacent 
  

   faces 
  of 
  the 
  plates 
  so 
  as 
  to 
  meet 
  again 
  at 
  the 
  interface 
  ; 
  

   but 
  these 
  fringes, 
  when 
  observed 
  in 
  the 
  ordinary 
  way, 
  are 
  so 
  

   very 
  faint 
  in 
  comparison 
  with 
  the 
  light 
  reflected 
  direct 
  from 
  

   the 
  interface 
  that 
  they 
  can 
  only 
  very 
  exceptionally 
  be 
  

   distinguished. 
  They 
  will, 
  however, 
  become 
  clearly 
  visible 
  if 
  

   we 
  cut 
  off 
  all 
  the 
  light 
  reflected 
  from 
  the 
  film 
  except 
  that 
  by 
  

   which 
  they 
  themselves 
  are 
  formed. 
  At 
  perpendicular 
  inci- 
  

   dence 
  this 
  would 
  appear 
  to 
  be 
  impossible, 
  but 
  when 
  the 
  

   incident 
  light 
  is 
  inclined 
  to 
  the 
  plates 
  it 
  can 
  be 
  done 
  in 
  a 
  

   very 
  simple 
  way 
  indeed, 
  by 
  merely 
  interposing 
  an 
  obstacle 
  

   having 
  a 
  sharp 
  edge 
  in 
  the 
  path 
  of 
  the 
  incident 
  light. 
  If 
  

   this 
  be 
  done, 
  on 
  looking 
  at 
  the 
  plates 
  by 
  the 
  reflected 
  light 
  a 
  

   series 
  of 
  successive 
  shadow 
  images 
  of 
  the 
  obstacle 
  will 
  be 
  

   seen 
  formed 
  by 
  reflexion 
  at 
  the 
  various 
  surfaces 
  : 
  the 
  first 
  

   It 
  by 
  reflexion 
  at 
  A 
  (fig. 
  2 
  — 
  the 
  refraction 
  in 
  the 
  glass 
  is 
  for 
  

   simplicity 
  omitted 
  in 
  the 
  figure), 
  the 
  second 
  I 
  2 
  by 
  direct 
  

   reflexion 
  from 
  the 
  film 
  BC, 
  and 
  the 
  third 
  I 
  3 
  by 
  the 
  rays 
  

   which 
  have 
  undergone 
  either 
  two 
  reflexions 
  at, 
  or 
  two 
  trans- 
  

   missions 
  through, 
  the 
  film. 
  Where 
  the 
  first 
  and 
  second 
  of 
  

   these 
  images 
  overlap, 
  but 
  not 
  the 
  third, 
  L 
  e.. 
  in 
  the 
  reoion 
  

   FG- 
  of 
  the 
  film, 
  the 
  rays 
  which 
  give 
  rise 
  to 
  the 
  fringe 
  system 
  

   under 
  consideration 
  are 
  the 
  only 
  ones 
  which 
  reach 
  the 
  eye, 
  and 
  

   i 
  i 
  this 
  space 
  the 
  fringes 
  become 
  clearly 
  marked 
  and 
  undiluted 
  

   * 
  Communicated 
  by 
  the 
  Author. 
  

  

  