﻿86 
  

  

  Mr. 
  Nalinimohan 
  Basu 
  on 
  the 
  Diffraction 
  of 
  

   Table 
  I. 
  

   a 
  = 
  l-54 
  cm. 
  X 
  = 
  6562 
  x 
  10" 
  8 
  cm. 
  

  

  n. 
  

  

  Calculated 
  width 
  

   of 
  Band. 
  

  

  Observed 
  width 
  

   of 
  Band. 
  

  

  1. 
  

  

  •001775 
  cm. 
  

  

  •00174 
  cm. 
  

  

  2. 
  

  

  •001019 
  „ 
  

  

  •00102 
  „ 
  

  

  3. 
  

  

  •000875 
  „ 
  

  

  •00086 
  „ 
  

  

  4. 
  

  

  •000781 
  „ 
  

  

  •00076 
  „ 
  

  

  5. 
  

  

  •000717 
  „ 
  

  

  •00069 
  „ 
  

  

  6. 
  

  

  1 
  

  

  •000671 
  „ 
  

  

  •00068 
  „ 
  

  

  The 
  discrepancies 
  are 
  within 
  the 
  limits 
  of 
  experimental 
  

   error. 
  When 
  making 
  these 
  measurements, 
  the 
  focal 
  plane 
  

   was, 
  in 
  the 
  first 
  instance, 
  set 
  in 
  approximate 
  coincidence 
  with 
  

   the 
  edge 
  of 
  the 
  cylinder 
  by 
  noting 
  the 
  stage 
  at 
  which 
  a 
  further 
  

   movement 
  of 
  the 
  focal 
  plane 
  towards 
  the 
  light 
  results 
  in 
  a 
  

   movement 
  of 
  the 
  fringes 
  into 
  the 
  region 
  of 
  the 
  shadow. 
  There 
  

   was, 
  however, 
  a 
  slight 
  uncertainty 
  in 
  regard 
  to 
  this 
  adjust- 
  

   ment, 
  and 
  the 
  best 
  position 
  of 
  the 
  focal 
  plane 
  was 
  finally 
  

   ascertained 
  by 
  actual 
  trial. 
  

  

  10. 
  The 
  ratio 
  between 
  the 
  maxima 
  and 
  minima 
  of 
  illumi- 
  

   nation 
  in 
  the 
  fringes 
  at 
  the 
  edge 
  may 
  readily 
  be 
  calculated* 
  

   Dividing 
  up 
  the 
  pencil 
  of 
  rays 
  incident 
  on 
  the 
  cylinder 
  into 
  

   elements 
  of 
  width 
  asin6d0 
  or 
  a0dO 
  approximately, 
  the 
  

   width 
  of 
  the 
  corresponding 
  elements 
  of 
  the 
  reflected 
  pencil 
  

   in 
  the 
  plane 
  of 
  the 
  edge 
  is 
  da, 
  that 
  is, 
  'daO 
  dd. 
  The 
  amplitude 
  

   of 
  the 
  disturbance 
  at 
  any 
  point 
  in 
  this 
  plane 
  due 
  to 
  the 
  

   reflected 
  rays 
  is 
  thus 
  only 
  l/\/3 
  of 
  that 
  due 
  to 
  the 
  direct 
  

   rays, 
  multiplied 
  by 
  the 
  reflecting 
  power 
  of 
  the 
  surface. 
  If 
  

   the 
  reflecting 
  power 
  be 
  unity 
  (as 
  is 
  practically 
  the 
  case 
  at 
  

   such 
  oblique 
  incidences), 
  the 
  ratio 
  of 
  the 
  intensities 
  of 
  the 
  

   maxima 
  and 
  minima 
  is 
  (1 
  + 
  1/ 
  \/Z) 
  2 
  /(l 
  — 
  1/ 
  v'S) 
  2 
  , 
  that 
  is 
  

   approximately 
  14: 
  1. 
  The 
  dark 
  bands 
  are 
  thus 
  nearly, 
  but 
  

   not 
  quite, 
  perfectly 
  black. 
  

  

  Theory 
  of 
  the 
  Fringes 
  at 
  the 
  edge 
  of 
  the 
  shadow. 
  

  

  11. 
  If 
  the 
  fringes 
  be 
  observed 
  in 
  a 
  plane 
  (such 
  as 
  C'P' 
  

   in 
  fig. 
  2) 
  which 
  is 
  farther 
  from 
  the 
  source 
  of 
  light 
  than 
  the 
  

   edge 
  of 
  the 
  cylinder, 
  the 
  diffraction 
  and 
  mutual 
  interference 
  

   of 
  the 
  direct 
  and 
  the 
  reflected 
  rays 
  have 
  both 
  to 
  be 
  taken 
  

   into 
  account. 
  Since 
  the 
  reflected 
  rays 
  form 
  a 
  divergent 
  

  

  