﻿Light 
  by 
  Cylinders 
  of 
  Large 
  Radius. 
  83 
  

  

  contrast 
  between 
  the 
  minima 
  and 
  maxima 
  of 
  illumination 
  

   becomes 
  greater 
  than 
  in 
  the 
  fringes 
  o£ 
  the 
  usual 
  Fresnel 
  type, 
  

   and 
  the 
  number 
  that 
  can 
  be 
  seen 
  and 
  counted 
  in 
  mono- 
  

   chromatic 
  light 
  increases 
  considerably. 
  These 
  features 
  

   become 
  more 
  and 
  more 
  marked 
  as 
  the 
  focal 
  plane 
  approaches 
  

   the 
  cylinder, 
  and 
  the 
  dark 
  bands 
  then 
  become 
  almost 
  per- 
  

   fectly 
  black. 
  The 
  difference 
  between 
  the 
  intensity 
  of 
  the 
  

   first 
  maximum 
  and 
  of 
  those 
  following 
  it 
  also 
  becomes 
  less 
  

   conspicuous. 
  Figs. 
  L, 
  II., 
  and 
  III. 
  in 
  the 
  Plate 
  represent 
  

   these 
  stages. 
  A 
  considerable 
  brightening-up 
  of 
  the 
  whole 
  

   field 
  is 
  also 
  noticed 
  as 
  the 
  focal 
  plane 
  approaches 
  the 
  cylinder, 
  

   but 
  this 
  is 
  not 
  shown 
  in 
  the 
  photographs, 
  as 
  the 
  exposures 
  

   obtained 
  with 
  the 
  light 
  of 
  the 
  arc 
  were 
  very 
  variable. 
  When 
  

   the 
  focal 
  plane 
  is 
  within 
  a 
  millimetre 
  or 
  two 
  of 
  the 
  edge 
  at 
  

   which 
  the 
  incident 
  light 
  grazes 
  the 
  cylinder, 
  a 
  change 
  in 
  the 
  

   law 
  of 
  spacing 
  of 
  the 
  fringes 
  also 
  becomes 
  evident, 
  the 
  widths 
  

   of 
  the 
  successive 
  bright 
  bands 
  decreasing 
  less 
  rapidly 
  than 
  in 
  

   the 
  fringes 
  of 
  the 
  Fresnel 
  type. 
  Fig. 
  IV. 
  in 
  the 
  plate 
  illus- 
  

   trates 
  this 
  feature, 
  which 
  is 
  most 
  marked 
  when 
  the 
  focal 
  plane 
  

   coincides 
  with 
  the 
  edge 
  of 
  the 
  cylinder. 
  At 
  this 
  stage, 
  of 
  

   course, 
  the 
  fringes 
  due 
  to 
  a 
  sharp 
  diffracting 
  edge 
  would 
  

   vanish 
  altogether. 
  

  

  7. 
  When 
  the 
  focal 
  plane 
  is 
  gradually 
  moved 
  further 
  in, 
  so 
  

   that 
  it 
  lies 
  between 
  the 
  cylinder 
  and 
  the 
  source 
  of 
  light, 
  some 
  

   very 
  interesting 
  effects 
  are 
  observed. 
  The 
  fringes 
  contract 
  

   a 
  little, 
  and 
  the 
  first 
  band, 
  instead 
  of 
  remaining 
  in 
  the 
  fixed 
  

   position 
  defined 
  by 
  the 
  geometrical 
  edge, 
  moves 
  into 
  the 
  

   region 
  of 
  the 
  shadow, 
  and 
  is 
  followed 
  by 
  a 
  new 
  system 
  

   of 
  fringes, 
  characterized 
  by 
  intensely 
  dark 
  minima, 
  that 
  

   appears 
  to 
  emerge 
  from 
  the 
  field 
  occupied 
  by 
  the 
  fringes 
  

   seen 
  in 
  the 
  previous 
  stages. 
  (See 
  figs. 
  V. 
  and 
  VI.) 
  The 
  

   first 
  band 
  of 
  this 
  new 
  system 
  is 
  considerably 
  more 
  brilliant 
  

   than 
  those 
  that 
  follow 
  it. 
  It 
  is 
  evident 
  on 
  careful 
  inspection 
  

   that 
  the 
  fringes 
  that 
  move 
  into 
  the 
  shadow 
  form 
  an 
  inde- 
  

   pendent 
  system. 
  For 
  it 
  is 
  found 
  that 
  the 
  part 
  of 
  the 
  field 
  

   from 
  which 
  the 
  new 
  system 
  has 
  separated 
  out 
  appears 
  greatly 
  

   reduced 
  in 
  intensity 
  in 
  comparison 
  with 
  the 
  part 
  on 
  which 
  it 
  

   is 
  still 
  superimposed. 
  When 
  the 
  separation 
  of 
  the 
  field 
  into 
  

   two 
  parts 
  is 
  complete, 
  a 
  few 
  diffraction-fringes 
  of 
  the 
  usual 
  

   Fresnel 
  type 
  are 
  observed 
  at 
  the 
  geometrical 
  edge 
  of 
  the 
  

   shadow 
  of 
  the 
  cylinder. 
  (See 
  figs. 
  VII. 
  and 
  VIII. 
  in 
  the 
  

   Plate, 
  in 
  which 
  this 
  position 
  is 
  indicated 
  by 
  an 
  arrow.) 
  

  

  8. 
  A 
  comparison 
  of 
  the 
  effects 
  described 
  in 
  the 
  preceding 
  

   paragraph 
  and 
  of 
  those 
  obtained 
  with 
  a 
  sharp 
  diffracting 
  

   edge 
  in 
  the 
  same 
  position, 
  furnishes 
  the 
  clue 
  to 
  the 
  correct 
  

   explanation 
  of 
  the 
  phenomena 
  observed 
  and 
  dealt 
  with 
  in 
  the 
  

  

  G 
  2 
  

  

  