﻿80 
  Mr. 
  Nalinimohan 
  Basu 
  on 
  the 
  Diffraction 
  of 
  

  

  cylinders 
  of 
  various 
  radii 
  (the 
  finer 
  ones 
  being 
  screened 
  on 
  

   one 
  side 
  so 
  as 
  to 
  confine 
  diffraction 
  to 
  the 
  other 
  side 
  only), 
  

   and 
  observing 
  the 
  fringes 
  formed 
  within 
  a 
  few 
  millimetres 
  of 
  

   the 
  diffracting 
  edge 
  through 
  a 
  microscope, 
  found 
  that 
  they 
  

   appeared 
  brighter 
  and 
  sharper 
  with 
  every 
  increase 
  in 
  the 
  

   radius 
  of 
  the 
  cylinder. 
  The 
  fringes 
  obtained 
  with 
  a 
  smooth 
  

   rod 
  of 
  one 
  or 
  two 
  centimetres 
  radius 
  differed 
  very 
  markedly 
  

   from 
  those 
  formed 
  by 
  a 
  sharp 
  edge 
  or 
  by 
  a 
  cylinder 
  of 
  small 
  

   radius. 
  They 
  were 
  brighter, 
  more 
  numerous, 
  showed 
  greater 
  

   contrast 
  between 
  the 
  maxima 
  and 
  minima 
  of 
  illumination, 
  

   and 
  their 
  spacing 
  was 
  different 
  from 
  that 
  given 
  by 
  the 
  usual 
  

   Fresnel 
  formulas. 
  Brush 
  also 
  observed 
  that 
  when 
  the 
  radius 
  

   of 
  the 
  cylinder 
  was 
  a 
  millimetre 
  or 
  more, 
  the 
  fringes 
  did 
  not 
  

   vanish 
  when 
  the 
  focal 
  plane 
  of 
  the 
  microscope 
  was 
  put 
  

   forward 
  so 
  as 
  to 
  coincide 
  with 
  the 
  edge 
  of 
  the 
  cylinder. 
  

   Sharp 
  narrow 
  fringes 
  were 
  observed 
  with 
  the 
  focal 
  plane 
  in 
  

   this 
  position, 
  becoming 
  broader 
  and 
  more 
  numerous 
  as 
  the 
  

   radius 
  of 
  the 
  cylinder 
  was 
  increased. 
  

  

  2. 
  To 
  account 
  for 
  these 
  phenomena 
  Brush 
  has 
  suggested 
  

   an 
  explanation, 
  the 
  nature 
  of 
  which 
  is 
  indicated 
  by 
  the 
  title 
  

   of 
  his 
  paper. 
  The 
  diffraction-pattern 
  formed 
  by 
  the 
  cylinder 
  

   is, 
  according 
  to 
  Brush, 
  the 
  result 
  of 
  the 
  superposition 
  of 
  a 
  

   number 
  of 
  diffraction-patterns 
  which 
  are 
  almost, 
  but 
  not 
  

   quite, 
  in 
  register. 
  He 
  regards 
  the 
  cylindrical 
  diffracting 
  

   surface 
  as 
  consisting 
  of 
  a 
  great 
  many 
  parallel 
  elements, 
  each 
  

   of 
  which 
  acts 
  as 
  a 
  diffracting 
  edge 
  and 
  produces 
  its 
  own 
  

   fringe-pattern, 
  which 
  is 
  superposed 
  on 
  those 
  of 
  the 
  other 
  

   elements. 
  Brush 
  has 
  made 
  no 
  attempt 
  to 
  arrive, 
  mathe- 
  

   matically 
  or 
  empirically, 
  at 
  any 
  quantitative 
  laws 
  of 
  the 
  

   phenomena 
  described 
  in 
  his 
  paper. 
  A 
  careful 
  examination 
  

   of 
  the 
  subject 
  shows 
  that 
  the 
  view 
  put 
  forward 
  by 
  him 
  

   presents 
  serious 
  difficulties, 
  and 
  is 
  open 
  to 
  objection. 
  One 
  

   of 
  the 
  defects 
  of 
  the 
  treatment 
  suggested 
  by 
  Brush 
  is 
  that 
  

   it 
  entirely 
  ignores 
  the 
  part 
  played 
  by 
  the 
  light 
  regularly 
  

   reflected 
  from 
  the 
  surface 
  of 
  the 
  obstacle 
  at 
  oblique 
  or 
  nearly 
  

   grazing 
  incidences. 
  I 
  propose 
  in 
  the 
  present 
  paper 
  (a) 
  to 
  

   describe 
  the 
  observed 
  effects 
  in 
  some 
  detail, 
  drawing 
  attention 
  

   to 
  some 
  interesting 
  features 
  overlooked 
  by 
  Brush 
  ; 
  (6) 
  to 
  show 
  

   that 
  they 
  can 
  be 
  interpreted 
  in 
  a 
  manner 
  entirely 
  different 
  

   from 
  that 
  suggested 
  by 
  him 
  ; 
  and 
  (c) 
  to 
  give 
  a 
  mathematical 
  

   theory 
  together 
  with 
  the 
  results 
  of 
  a 
  quantitative 
  expe- 
  

   rimental 
  test. 
  

  

  3. 
  Reference 
  should 
  be 
  made 
  here 
  to 
  the 
  problem 
  of 
  the 
  

   diffraction 
  of 
  plane 
  electromagnetic 
  waves 
  by 
  a 
  cylinder 
  

   with 
  its 
  axis 
  parallel 
  to 
  the 
  incident 
  waves. 
  The 
  solution 
  of 
  

   this 
  problem 
  for 
  a 
  perfectly 
  conducting 
  cylinder 
  has 
  been 
  

  

  