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  XI. 
  On 
  the 
  Asymmetry 
  of 
  the 
  Illumination 
  - 
  Curves 
  in 
  

   Oblique 
  Diffraction. 
  By 
  Sisir 
  Kumar 
  Mitra, 
  M.Sc, 
  

   Sir 
  Rashbehary 
  Ghosh 
  Research 
  Scholar 
  in 
  the 
  University 
  

   of 
  Calcutta 
  *. 
  

  

  [Plate 
  V.] 
  

   Introduction. 
  

  

  IN 
  the 
  Phil. 
  Mag. 
  for 
  May 
  1911, 
  ,C. 
  V. 
  Raman 
  has 
  given 
  

   the 
  results 
  of 
  a 
  photometric 
  study 
  of 
  the 
  unsymmetrical 
  

   difPraction-bands 
  due 
  to 
  an 
  obliquely 
  held 
  rectangular 
  re- 
  

   flecting 
  surface 
  previously 
  observed 
  by 
  himf. 
  The 
  measure- 
  

   ments 
  showed 
  a 
  very 
  marked 
  asymmetry 
  in 
  the 
  distribution 
  

   of 
  intensity 
  in 
  the 
  diffraction 
  pattern, 
  the 
  theoretical 
  expla- 
  

   nation 
  of 
  which 
  is 
  discussed 
  in 
  the 
  papers 
  quoted. 
  The 
  

   following 
  were 
  the 
  principal 
  conclusions 
  arrived 
  at 
  by 
  Raman 
  

   as 
  the 
  result 
  of 
  the 
  quantitative 
  experimental 
  study 
  of 
  the 
  

   case 
  : 
  — 
  

  

  (a) 
  The 
  illumination 
  at 
  the 
  points 
  of 
  minimum 
  intensity 
  

   in 
  the 
  diffraction 
  pattern 
  is 
  zero 
  at 
  all 
  angles 
  of 
  incidence, 
  

   and 
  the 
  positions 
  of 
  the 
  minima 
  are 
  accurately 
  given 
  by 
  the 
  

   formula 
  

  

  S= 
  + 
  7r, 
  +2ir, 
  +37T, 
  &c, 
  

  

  ira 
  

   where 
  8 
  = 
  — 
  — 
  (sin 
  i 
  — 
  sin 
  6) 
  , 
  a 
  being 
  the 
  width 
  of 
  the 
  aperture, 
  

  

  A, 
  

  

  A, 
  the 
  wave-length, 
  and 
  i, 
  6 
  the 
  angles 
  of 
  incidence 
  and 
  

   diffraction 
  respectively 
  ; 
  the 
  fringes 
  are 
  wider 
  on 
  the 
  side 
  

   on 
  which 
  0>i 
  and 
  their 
  number 
  is 
  limited 
  on 
  that 
  side, 
  as 
  6 
  

  

  cannot 
  be 
  greater 
  than 
  ~~. 
  

  

  (b) 
  The 
  formula 
  of 
  the 
  usual 
  type 
  (1 
  = 
  sin 
  2 
  S/S 
  2 
  ) 
  for 
  the 
  

   illumination 
  in 
  the 
  pattern 
  fails 
  to 
  represent 
  the 
  observed 
  

   intensity-curves 
  at 
  oblique 
  incidences 
  except 
  in 
  regard 
  to 
  

   the 
  position 
  of 
  the 
  minima 
  (8 
  — 
  ±ir, 
  + 
  27T, 
  (fee). 
  The 
  inten- 
  

   sities 
  at 
  corresponding 
  points 
  on 
  either 
  side 
  of 
  the 
  central 
  

   fringe 
  for 
  which 
  the 
  values 
  of 
  8 
  are 
  numerically 
  the 
  same 
  

   are 
  not 
  equal. 
  

  

  (c) 
  The 
  observed 
  distribution 
  of 
  intensity 
  was 
  found 
  to 
  fit 
  

   in 
  with 
  the 
  theoretical 
  formula, 
  if 
  the 
  latter 
  is 
  multiplied 
  by 
  

   a 
  factor 
  proportional 
  to 
  the 
  square 
  of 
  the 
  cosine 
  of 
  the 
  

  

  * 
  Communicated 
  by 
  Prof. 
  C. 
  V. 
  Raman. 
  

  

  t 
  C. 
  V. 
  Raman, 
  M.A., 
  " 
  On 
  the 
  Unsymmetrical 
  Diffraction 
  Bands 
  due 
  

   to 
  a 
  Rectangular 
  Aperture/' 
  Phil. 
  Mag. 
  Nov. 
  1906. 
  See 
  also 
  Phil. 
  Mag. 
  

   Jan. 
  1909. 
  

  

  