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  Llll. 
  Phase-Reversal 
  Zone-Plates, 
  and 
  Diffraction- 
  Telescopes, 
  

  

  By 
  R 
  W. 
  Wood*. 
  

  

  [Plate 
  XXIV.] 
  

  

  IN 
  a 
  paper 
  published 
  in 
  PoggendorfFs 
  Annalen 
  (1875) 
  

   Soret 
  showed 
  that 
  if 
  we 
  describe 
  a 
  number 
  of 
  small 
  

   concentric 
  circles 
  on 
  a 
  glass 
  plate, 
  with 
  radii 
  proportional 
  to 
  

   the 
  square 
  roots 
  of 
  the 
  natural 
  numbers, 
  and 
  blacken 
  the 
  

   spaces 
  between 
  the 
  alternate 
  rings, 
  the 
  plate 
  will 
  have 
  the 
  

   property 
  of 
  bringing 
  parallel 
  rays 
  of 
  light 
  to 
  a 
  focus, 
  like 
  a 
  

   condensing-lens. 
  The 
  dark 
  rings 
  check 
  the 
  disturbances 
  on 
  

   the 
  alternate 
  Huygens's 
  zones 
  on 
  the 
  wave-front, 
  which 
  by 
  

   destructive 
  interference 
  with 
  the 
  disturbances 
  from 
  the 
  other 
  

   zones 
  ordinarily 
  produce 
  darkness 
  at 
  points 
  within 
  the 
  geo- 
  

   metrical 
  shadow. 
  

  

  Soret's 
  method 
  was 
  to 
  draw 
  circles 
  of 
  suitable 
  size 
  on 
  a 
  

   sheet 
  of 
  paper, 
  blackening 
  the 
  alternate 
  rings, 
  and 
  make 
  a 
  

   reduced 
  photographic 
  copy 
  of 
  the 
  whole. 
  He 
  showed 
  that 
  

   such 
  a 
  plate 
  formed 
  real 
  images 
  of 
  luminous 
  objects, 
  and 
  

   could 
  be 
  used 
  as 
  the 
  objective 
  of 
  a 
  telescope 
  or 
  as 
  the 
  eyepiece. 
  

   He 
  also 
  showed 
  that 
  in 
  addition 
  to 
  acting 
  as 
  a 
  condensing-lens, 
  

   the 
  zone-plate 
  acted 
  as 
  a 
  concave 
  or 
  dispersing-lens. 
  More- 
  

   over, 
  he 
  pointed 
  out 
  that 
  the 
  plate 
  has 
  multiple 
  foci 
  at 
  distances 
  

  

  c? 
  a? 
  d^ 
  

  

  —5 
  ^— 
  ? 
  pt-, 
  where 
  a 
  is 
  the 
  radius 
  of 
  the 
  central 
  circle. 
  

  

  A- 
  OA. 
  DA, 
  

  

  Since 
  the 
  appearance 
  of 
  Soret's 
  paper 
  very 
  little 
  seems 
  to 
  

   have 
  been 
  done 
  with 
  the 
  subject, 
  though 
  it 
  is 
  alluded 
  to 
  briefly 
  

   in 
  the 
  textbooks, 
  and 
  it 
  has 
  been 
  pointed 
  out 
  by 
  Lord 
  Rayleigh 
  

   that 
  if 
  it 
  were 
  possible 
  to 
  provide 
  that 
  the 
  light 
  stopped 
  by 
  

   alternate 
  zones 
  were 
  replaced 
  by 
  a 
  phase-reversal, 
  a 
  fourfold 
  

   effect 
  would 
  be 
  produced. 
  After 
  some 
  experimenting 
  I 
  have 
  

   succeeded 
  in 
  producing 
  such 
  a 
  zone-plate, 
  perfectly 
  trans- 
  

   parent 
  over 
  its 
  entire 
  extent, 
  which 
  yields 
  an 
  image 
  at 
  least 
  six 
  

   or 
  eight 
  times 
  as 
  bright 
  as 
  those 
  produced 
  with 
  the 
  old 
  form 
  

   of 
  plate. 
  This 
  increase 
  in 
  intensity 
  is 
  greater 
  than 
  one 
  would 
  

   expect, 
  which 
  is 
  probably 
  accounted 
  for 
  by 
  the 
  lack 
  of 
  per- 
  

   fection 
  of 
  the 
  black 
  zone-plates, 
  the 
  black 
  rings 
  being 
  partly 
  

   transparent 
  and 
  the 
  clear 
  ones 
  not 
  wholly 
  so. 
  Using 
  one 
  of 
  

   the 
  new 
  plates 
  as 
  the 
  objective 
  of 
  a 
  telescope 
  in 
  connexion 
  

   with 
  a 
  low-power 
  eyepiece, 
  I 
  have 
  distinctly 
  seen 
  the 
  lunar 
  

   craters, 
  and 
  have 
  constructed 
  telescopes 
  in 
  which 
  both 
  objective 
  

   and 
  eyepiece 
  were 
  zone-plates. 
  

  

  The 
  largest 
  plate 
  made 
  by 
  Soret 
  contained 
  98 
  dark 
  circles, 
  

   and 
  since 
  the 
  scale 
  on 
  which 
  it 
  was 
  drawn 
  was 
  rather 
  small, 
  

   the 
  outer 
  zones 
  could 
  not 
  have 
  been 
  very 
  accurately 
  placed. 
  

   * 
  Communicated 
  by 
  the 
  Author. 
  

  

  