﻿516 
  Mr. 
  R. 
  W. 
  Wood 
  on 
  Pliase-Reversal 
  Zone-Plates 
  

  

  I 
  have 
  tried 
  making 
  landscape-photographs 
  

  

  using 
  one 
  of 
  

  

  these 
  short-focus 
  retardation-plates 
  instead 
  of 
  a 
  lens. 
  The 
  

   results 
  far 
  surpassed 
  my 
  expectations. 
  There 
  are 
  good 
  

   qualities 
  in 
  the 
  zone-plate 
  picture 
  that 
  almost 
  offset 
  the 
  

   deficiencies. 
  Very 
  soft 
  and 
  artistic 
  effects 
  can 
  be 
  produced. 
  

   Where 
  there 
  is 
  a 
  mass 
  of 
  twigs 
  against 
  the 
  sky 
  the 
  zone-plate 
  

   rubs 
  in 
  a 
  little 
  extra 
  shade 
  for 
  a 
  background 
  instead 
  of 
  cutting- 
  

   each 
  stem 
  sharp 
  and 
  black 
  against 
  the 
  white 
  sky. 
  The 
  

   shadows 
  are 
  massed 
  to 
  a 
  great 
  extent, 
  and 
  yet 
  there 
  is 
  defi- 
  

   nition 
  enough 
  to 
  prevent 
  the 
  picture 
  from 
  appearing 
  out 
  of 
  

   focus. 
  The 
  landscape 
  reproduced 
  (fig. 
  1) 
  was 
  made 
  with 
  a 
  plate 
  

   of 
  4 
  millim. 
  aperture 
  and 
  14 
  centim. 
  focus, 
  with 
  an 
  exposure 
  of 
  

   half 
  a 
  second, 
  which 
  is 
  certainly 
  a 
  great 
  improvement 
  on 
  pin- 
  

   hole 
  photography. 
  The 
  picture 
  has 
  lost 
  much 
  in 
  reproduction. 
  

   The 
  actinic 
  focus 
  of 
  the 
  plate 
  must 
  be 
  found 
  and 
  the 
  image 
  

   on 
  the 
  ground-glass 
  is 
  never 
  sharp, 
  owing 
  to 
  the 
  chromatic 
  

   aberration. 
  I 
  have 
  used 
  a 
  blue 
  glass 
  to 
  advantage 
  in 
  finding 
  

   the 
  focus 
  ; 
  and 
  a 
  low 
  sun 
  in 
  thin 
  clouds 
  makes 
  an 
  excellent 
  

   object 
  to 
  focus 
  upon. 
  

  

  Ifig.l. 
  

  

  A 
  very 
  simple 
  elementary 
  way 
  of 
  explaining 
  the 
  multiple 
  

   foci, 
  both 
  real 
  and 
  virtual, 
  of 
  the 
  zone-plate 
  which 
  I 
  have 
  not 
  

  

  