﻿82 
  Mr. 
  Nalinimohan 
  Basu 
  on 
  the 
  Diffraction 
  of 
  

  

  cylinder 
  of 
  metal 
  or 
  glass 
  and 
  passes 
  it 
  tangentially 
  at 
  C 
  *. 
  

   The 
  axis 
  of 
  the 
  cylinder 
  is 
  parallel 
  to 
  the 
  slit. 
  A 
  collimating 
  

   lens 
  may, 
  if 
  necessary, 
  be 
  interposed 
  between 
  the 
  slit 
  and 
  

   the 
  cylinder. 
  The 
  fringes 
  bordering 
  the 
  shadow 
  of 
  the 
  edge 
  

   C 
  are 
  observed 
  through 
  the 
  microscope-objective 
  M 
  and 
  the 
  

   micrometer 
  eyepiece 
  E. 
  The 
  latter 
  may 
  be 
  placed 
  at 
  any 
  

   convenient 
  distance 
  from 
  the 
  objective 
  so 
  as 
  to 
  give 
  the 
  

   necessary 
  magnification. 
  The 
  effects 
  are 
  best 
  seen 
  with 
  

   monochromatic 
  light 
  obtained 
  by 
  focussing 
  the 
  spectrum 
  of 
  

   the 
  electric 
  arc 
  on 
  the 
  slit 
  with 
  a 
  small 
  direct-vision 
  prism. 
  

   For 
  photographic 
  work, 
  the 
  eyepiece 
  E 
  is 
  removed 
  and 
  

   replaced 
  by 
  a 
  long 
  light-tight 
  box 
  in 
  front 
  of 
  which 
  the 
  

   objective 
  M 
  is 
  fixed, 
  and 
  at 
  the 
  other 
  end 
  of 
  which 
  the 
  

   photographic 
  plate 
  is 
  exposed. 
  Sufficient 
  illumination 
  for 
  

   photographing 
  the 
  fringes 
  may 
  be 
  secured 
  by 
  using 
  the 
  arc 
  

   and 
  illuminating 
  the 
  slit 
  by 
  the 
  greenish-yellow 
  light 
  trans- 
  

   mitted 
  by 
  a 
  mixture 
  of 
  solutions 
  of 
  copper 
  sulphate 
  and 
  

   potassium 
  bichromate. 
  

  

  5. 
  The 
  phenomena 
  observed 
  depend 
  on 
  the 
  position 
  of 
  the 
  

   focal 
  plane 
  of 
  the 
  objective 
  with 
  reference 
  to 
  the 
  diffracting 
  

   edge 
  of 
  the 
  cylinder, 
  and 
  an 
  interesting 
  sequence 
  of 
  changes 
  

   is 
  observed 
  as 
  the 
  focal 
  plane 
  of 
  the 
  objective 
  is 
  gradually 
  

   moved, 
  towards 
  the 
  light, 
  up 
  to 
  and 
  beyond 
  the 
  edge 
  C 
  (fig. 
  1) 
  

   at 
  which 
  the 
  incident 
  light 
  grazes 
  the 
  cylinder. 
  Some 
  idea 
  

   of 
  these 
  changes 
  will 
  be 
  obtained 
  on 
  a 
  reference 
  to 
  Plate 
  III., 
  

   figs. 
  I. 
  to 
  VIII., 
  in 
  which 
  the 
  fringes 
  photographed 
  with 
  a 
  

   cylinder 
  of 
  radius 
  1*54 
  cm. 
  are 
  reproduced. 
  (A 
  Zeiss 
  

   objective 
  of 
  focal 
  length 
  1*7 
  cm. 
  was 
  used, 
  and 
  the 
  magni- 
  

   fication 
  on 
  the 
  original 
  negative 
  was 
  135 
  diameters.) 
  

  

  6. 
  To 
  interpret 
  the 
  phenomena 
  it 
  is 
  convenient 
  to 
  compare 
  

   them 
  with 
  those 
  obtained 
  by 
  a 
  sharp 
  diffracting 
  edge 
  in 
  the 
  

   same 
  position. 
  Using 
  the 
  cylinder, 
  it 
  is 
  found 
  that 
  when 
  

   the 
  focal 
  plane 
  is 
  between 
  the 
  objective 
  and 
  the 
  cylinder, 
  but 
  

   several 
  centimetres 
  distant 
  from 
  the 
  latter, 
  the 
  fringes 
  are 
  

   practically 
  of 
  the 
  same 
  type 
  as 
  those 
  due 
  to 
  a 
  sharp 
  diffracting 
  

   edge. 
  They 
  are 
  diffuse, 
  few 
  in 
  number 
  (not 
  more 
  than 
  seven 
  

   or 
  eight 
  being 
  visible 
  even 
  in 
  monochromatic 
  light), 
  and 
  the 
  

   first 
  bright 
  band 
  is 
  considerably 
  broader 
  and 
  more 
  luminous 
  

   than 
  the 
  rest. 
  The 
  fringes 
  become 
  narrower 
  (retaining 
  their 
  

   characteristics) 
  as 
  the 
  focal 
  plane 
  is 
  brought 
  nearer 
  the 
  

   cylinder 
  till 
  the 
  distance 
  between 
  the 
  two 
  is 
  about 
  two 
  

   centimetres. 
  At 
  this 
  stage 
  some 
  new 
  features 
  appear 
  ; 
  the 
  

  

  * 
  A 
  glass 
  cylinder 
  may 
  be 
  used 
  without 
  inconvenience 
  as 
  the 
  light 
  

   transmitted 
  through 
  the 
  cylinder 
  is 
  refracted 
  out 
  to 
  one 
  side, 
  and 
  does 
  

   not 
  enter 
  into 
  the 
  field 
  under 
  observation. 
  Very 
  little 
  light 
  is, 
  in 
  fact, 
  

   transmitted 
  through 
  the 
  cylinder 
  at 
  oblique 
  incidences. 
  

  

  