﻿518 
  Mr. 
  R. 
  W. 
  Wood 
  on 
  Phase-Reversal 
  Zone-Plates 
  

  

  respect, 
  namely, 
  that 
  with 
  films 
  of 
  certain 
  thickness 
  the 
  

   central 
  direct 
  image 
  disappeared 
  entirely 
  when 
  monochro- 
  

   matic 
  light 
  was 
  used, 
  and 
  passed 
  through 
  a 
  succession 
  of 
  

   colours 
  when 
  white 
  light 
  was 
  used. 
  By 
  making 
  the 
  films 
  

   slightly 
  wedge-shaped 
  and 
  ruling 
  the 
  lines 
  at 
  right 
  angles 
  to 
  

   the 
  thin 
  edge, 
  he 
  obtained 
  a 
  grating 
  in 
  which 
  the 
  thickness 
  

   increased 
  continually 
  towards 
  the 
  bottom. 
  On 
  looking 
  at 
  an 
  

   illuminated 
  slit 
  through 
  the 
  top 
  of 
  this 
  grating 
  where 
  the 
  

   thickness 
  of 
  the 
  film 
  was 
  zero, 
  the 
  central 
  image 
  alone 
  was 
  

   observed. 
  On 
  moving 
  the 
  grating 
  up, 
  the 
  film 
  began 
  to 
  have 
  

   an 
  appreciable 
  thickness, 
  and 
  side 
  or 
  diffracted 
  images 
  appeared 
  

   on 
  either 
  side 
  of 
  the 
  central 
  one. 
  These 
  increased 
  in 
  bright- 
  

   ness 
  with 
  increasing 
  thickness 
  of 
  the 
  film, 
  while 
  the 
  central 
  

   image 
  became 
  fainter, 
  and 
  finally 
  disappeared 
  when 
  the 
  thick- 
  

   ness 
  was 
  such 
  as 
  to 
  give 
  to 
  the 
  light 
  passing 
  through 
  it 
  a 
  

   retardation 
  of 
  one 
  half 
  wave-length 
  on 
  the 
  light 
  passing 
  by 
  it. 
  

   We 
  thus 
  have 
  a 
  transparent 
  plate 
  which 
  will 
  not 
  allow 
  a 
  ray 
  

   to 
  pass 
  directly 
  through 
  it, 
  or, 
  in 
  other 
  words, 
  a 
  transparent 
  

   grating 
  which 
  gives 
  no 
  central 
  image. 
  I 
  find 
  that 
  very 
  

   satisfactory 
  lamina 
  gratings 
  can 
  be 
  prepared 
  in 
  the 
  same 
  way 
  

   as 
  the 
  zone-plates 
  ; 
  by 
  drawing 
  black 
  lines 
  of 
  uniform 
  width 
  

   and 
  with 
  uniform 
  spaces 
  between 
  them 
  on 
  white 
  paper, 
  

   reducing 
  them 
  by 
  photography, 
  and 
  printing 
  on 
  the 
  sensitized 
  

   gelatine. 
  The 
  same 
  effects 
  can 
  be 
  observed 
  by 
  viewing 
  an 
  

   illuminated 
  slit 
  through 
  the 
  outer 
  edge 
  of 
  the 
  lamina 
  zone- 
  

   plate 
  held 
  close 
  to 
  the 
  eye. 
  

  

  Quincke 
  found 
  that 
  if 
  a 
  portion 
  of 
  the 
  hypothenuse 
  surface 
  

   of 
  a 
  right-angle 
  prism 
  be 
  silvered, 
  a 
  ray 
  reflected 
  from 
  the 
  

   metal 
  suffers 
  a 
  phase 
  displacement 
  relative 
  to 
  a 
  ray 
  which 
  is 
  

   totally 
  reflected 
  from 
  the 
  boundary 
  between 
  glass 
  and 
  air. 
  

   Whether 
  or 
  not 
  this 
  change 
  is 
  a 
  complete 
  phase- 
  reversal 
  or 
  

   not 
  depends 
  on 
  a 
  number 
  of 
  conditions. 
  If 
  polarized 
  light 
  

   is 
  used, 
  the 
  change 
  of 
  phase 
  depends 
  on 
  whether 
  the 
  plane 
  

   of 
  polarization 
  is 
  parallel 
  or 
  perpendicular 
  to 
  the 
  plane 
  of 
  inci- 
  

   dence, 
  on 
  the 
  angle 
  of 
  incidence, 
  and 
  on 
  the 
  thickness 
  of 
  the 
  

   film. 
  It 
  appeared 
  probable 
  that 
  a 
  silver 
  zone-plate, 
  formed 
  on 
  

   the 
  surface 
  of 
  a 
  reflecting 
  prism, 
  would 
  give 
  excellent 
  results 
  

   under 
  certain 
  conditions. 
  Such 
  a 
  plate 
  could 
  be 
  ruled 
  on 
  a 
  

   slowly 
  revolving 
  prism, 
  but 
  the 
  process 
  of 
  advancing 
  the 
  knife- 
  

   point 
  over 
  the 
  right 
  distances 
  would 
  be 
  tedious 
  in 
  the 
  extreme, 
  

   and 
  I 
  accordingly 
  hunted 
  about 
  for 
  a 
  photomechanical 
  process. 
  

   After 
  a 
  little 
  experimenting 
  I 
  devised 
  a 
  method 
  which 
  yielded 
  

   beautiful 
  results. 
  Plate-glass, 
  carefully 
  cleaned 
  and 
  silvered 
  

   by 
  the 
  chemical 
  method, 
  is 
  coated 
  with 
  an 
  exceedingly 
  thin 
  

   film 
  of 
  gelatine 
  : 
  the 
  solution 
  should 
  be 
  just 
  too 
  weak 
  to 
  set 
  

   into 
  a 
  jelly 
  on 
  cooling, 
  merely 
  thickening 
  a 
  little. 
  It 
  should 
  

  

  