﻿520 
  Mr. 
  R. 
  W. 
  Wood 
  on 
  Phase-Reversal 
  Zone-Plates 
  

  

  45° 
  the 
  focus 
  instead 
  of 
  being 
  a 
  point 
  is 
  a 
  line. 
  When 
  the 
  

   screen 
  is 
  near 
  the 
  prism 
  we 
  get 
  a 
  very 
  bright 
  vertical 
  line, 
  

   which 
  fades 
  away 
  as 
  the 
  screen 
  is 
  moved 
  off, 
  being 
  replaced 
  

   with 
  a 
  horizontal 
  line. 
  This 
  is 
  exactly 
  what 
  we 
  get 
  when 
  

   we 
  hold 
  a 
  transmission 
  plate 
  at 
  an 
  angle 
  with 
  the 
  screen, 
  and 
  

   the 
  greater 
  the 
  obliquity 
  the 
  shorter 
  the 
  focus, 
  the 
  increasing 
  

   obliquity 
  decreasing 
  the 
  grating 
  space 
  on 
  the 
  sides 
  of 
  the 
  plate. 
  

  

  To 
  get 
  a 
  point-focus 
  from 
  a 
  reflecting 
  plate 
  on 
  a 
  prism 
  

   the 
  zones 
  must 
  be 
  elliptical, 
  the 
  major 
  axes 
  being 
  double 
  the 
  

   minor, 
  the 
  whole 
  being 
  so 
  placed 
  that 
  its 
  projection 
  on 
  each 
  

   of 
  the 
  other 
  two 
  faces 
  of 
  the 
  prism 
  is 
  a 
  circle. 
  To 
  make 
  an 
  

   elliptical 
  zone-plate 
  we 
  have 
  only 
  to 
  photograph 
  the 
  original 
  

   drawing 
  in 
  an 
  oblique 
  position. 
  A 
  slight 
  error 
  due 
  to 
  per- 
  

   spective 
  will 
  of 
  course 
  be 
  introduced, 
  but 
  it 
  is 
  insufficient 
  to 
  

   materially 
  affect 
  the 
  results. 
  

  

  From 
  a 
  negative 
  made 
  from 
  the 
  large 
  drawing 
  set 
  at 
  an 
  

   angle 
  of 
  45° 
  I 
  prepared 
  a 
  silver-on-glass 
  elliptical 
  plate, 
  and 
  

   cemented 
  it 
  onto 
  the 
  prism 
  with 
  balsam. 
  This 
  combination 
  

   fulfilled 
  every 
  expectation, 
  and 
  yielded 
  a 
  very 
  brilliant 
  and 
  

   sharp 
  image. 
  It 
  furnished, 
  moreover, 
  a 
  very 
  nice 
  means 
  of 
  

   comparing 
  the 
  effect 
  of 
  an 
  ordinary 
  plate 
  with 
  a 
  phase- 
  

   reversal 
  one 
  ; 
  for 
  by 
  turning 
  it 
  round 
  a 
  focal 
  image 
  was 
  

   formed 
  by 
  the 
  reflexion 
  from 
  the 
  outer 
  surface 
  of 
  the 
  silver 
  

   zones 
  alone, 
  which 
  was 
  many 
  times 
  fainter 
  than 
  that 
  produced 
  

   by 
  the 
  internal 
  reflexion. 
  In 
  making 
  this 
  comparison 
  I 
  noted 
  

   the 
  curious 
  circumstance 
  that 
  the 
  focus 
  from 
  internal 
  reflexion 
  

   is 
  longer 
  than 
  that 
  from 
  external 
  reflexion, 
  but 
  can 
  as 
  yet 
  offer 
  

   no 
  explanation 
  of 
  the 
  fact. 
  

  

  Fig. 
  2. 
  

  

  POSITION 
  OF 
  

  

  PRISM 
  FOR 
  

  

  PHASE 
  -REVERSAL 
  

  

  REFLECTION 
  

  

  ttT 
  

  

  u 
  I 
  ! 
  

  

  POSITION 
  OF 
  

  

  11 
  i 
  

   PRISM 
  

  

  REFLECTION 
  

  

  FOR 
  

  

  U 
  i 
  

  

  WITHOUT 
  PHASE 
  

  

  REVERSAL 
  

  

  ill! 
  

  

  m 
  

  

  In 
  order 
  to 
  give 
  some 
  idea 
  of 
  the 
  relative 
  intensities 
  of 
  the 
  

   focal 
  images 
  produced 
  by 
  the 
  plate 
  with 
  and 
  without 
  a 
  phase- 
  

   reversal, 
  I 
  photographed 
  an 
  incandescent 
  lamp-filament, 
  first 
  

   from 
  the 
  outside 
  surface 
  of 
  the 
  zones 
  and 
  then 
  from 
  the 
  

   inside, 
  by 
  the 
  arrangement 
  shown 
  in 
  fig. 
  2. 
  The 
  time 
  of 
  

  

  