﻿and 
  D'tf 
  'ruction- 
  Telescopes. 
  513 
  

  

  the 
  transparent 
  plates, 
  I 
  will 
  postpone 
  the 
  discussion 
  of 
  them 
  

   until 
  a 
  little 
  later 
  in 
  the 
  paper. 
  Before 
  taking 
  up 
  the 
  subject 
  

   of 
  the 
  phase-reversal 
  plate 
  I 
  may 
  as 
  well 
  call 
  attention 
  to 
  a 
  

   very 
  pretty 
  method 
  of 
  preparing 
  a 
  zone-plate 
  to 
  illustrate 
  its 
  

   action. 
  

  

  If 
  a 
  profile-face 
  or 
  other 
  design 
  is 
  cut 
  out 
  of 
  black 
  paper 
  

   and 
  pinned 
  on 
  to 
  the 
  original 
  drawing 
  somewhere 
  between 
  

   the 
  centre 
  and 
  the 
  edge, 
  and 
  a 
  photograph 
  of 
  the 
  whole 
  

   about 
  3 
  centim. 
  in 
  diameter 
  be 
  made, 
  on 
  looking 
  at 
  a 
  point 
  of 
  

   light 
  through 
  the 
  plate, 
  the 
  now 
  perfectly 
  transparent 
  profile 
  

   will 
  appear 
  jet-black 
  on 
  a 
  brightly 
  illuminated 
  back-ground 
  

   — 
  that 
  is 
  to 
  say, 
  the 
  space 
  within 
  the 
  design 
  being 
  devoid 
  of 
  

   black 
  lines 
  does 
  not 
  fill 
  up 
  with 
  light. 
  

  

  To 
  prepare 
  a 
  zone-plate 
  in 
  which 
  the 
  light, 
  instead 
  of 
  being 
  

   cut 
  off 
  by 
  the 
  alternate 
  rings, 
  shall 
  suffer 
  a 
  phase-reversal, 
  

   the 
  best 
  method 
  appeared 
  to 
  be 
  to 
  prepare 
  a 
  plate 
  with 
  trans- 
  

   parent 
  zones 
  of 
  a 
  thickness 
  sufficient 
  to 
  retard 
  the 
  light 
  one- 
  

   half 
  wave-length. 
  Sheets 
  of 
  thin 
  plate-glass 
  with 
  the 
  surface 
  

   carefullv 
  cleaned 
  were 
  flowed 
  with 
  a 
  warm 
  solution 
  of 
  gelatine 
  

   (strong 
  enough 
  to 
  just 
  set 
  into 
  a 
  jelly 
  on 
  cooling), 
  and 
  set 
  on 
  

   edge 
  to 
  dry. 
  The 
  films 
  were 
  then 
  sensitized 
  by 
  immersing 
  

   the 
  plates 
  for 
  about 
  five 
  seconds 
  in 
  a 
  weak 
  solution 
  of 
  bichro- 
  

   mate 
  of 
  potassium, 
  and 
  dried 
  in 
  the 
  dark. 
  Properly 
  prepared 
  

   films 
  should 
  exhibit 
  no 
  trace 
  of 
  crystallization, 
  and 
  should 
  

   have 
  a 
  barely 
  perceptible 
  yellow 
  tint. 
  These 
  plates 
  were 
  

   placed 
  in 
  contact 
  with 
  the 
  photographic 
  reduction, 
  and 
  printed 
  

   in 
  direct 
  sunlight; 
  the 
  exposure 
  varying 
  from 
  20 
  seconds 
  to 
  

   1 
  minute 
  according 
  to 
  the 
  density 
  of 
  the 
  negative. 
  They 
  

   were 
  then 
  removed 
  from 
  the 
  frame, 
  washed 
  for 
  a 
  moment 
  in 
  

   cold 
  water, 
  and 
  then 
  immersed 
  in 
  rather 
  warm 
  water, 
  which 
  

   dissolved 
  and 
  washed 
  away 
  the 
  gelatine 
  which 
  had 
  been 
  pro- 
  

   tected 
  by 
  the 
  dark 
  zones, 
  leaving 
  the 
  alternate 
  rings 
  attached 
  

   to 
  the 
  glass. 
  On 
  drying 
  these 
  plates 
  they 
  were 
  perfectly 
  

   transparent, 
  but 
  by 
  holding 
  them 
  in 
  certain 
  lights 
  the 
  rings 
  

   could 
  be 
  seen 
  . 
  The 
  effect 
  produced 
  by 
  them 
  far 
  surpassed 
  my 
  

   expectations. 
  On 
  viewing 
  a 
  brightly 
  luminous 
  point 
  through 
  

   them, 
  they 
  fill 
  up 
  w 
  T 
  ith 
  a 
  blaze 
  of 
  light 
  which 
  compares 
  very 
  

   favourably 
  with 
  a 
  lens. 
  They 
  are 
  not 
  uniformly 
  good, 
  how- 
  

   ever, 
  everything 
  depending 
  on 
  the 
  thickness 
  of 
  the 
  film, 
  and, 
  

   moreover, 
  the 
  exposure 
  must 
  have 
  been 
  exactly 
  right. 
  I 
  find 
  

   that 
  if 
  one 
  of 
  these 
  plates 
  3 
  centim. 
  in 
  diameter 
  be 
  set 
  up 
  at 
  

   a 
  distance 
  of 
  2J 
  metres 
  from 
  an 
  arc-light 
  and 
  a 
  paper 
  screen 
  

   be 
  placed 
  at 
  a 
  suitable 
  distance 
  (3 
  or 
  4 
  metres) 
  behind 
  the 
  

   plate, 
  an 
  exceedingly 
  bright 
  and 
  sharp, 
  somewhat 
  enlarged 
  

   image 
  of 
  the 
  arc 
  is 
  projected 
  ; 
  the 
  surface 
  irregularities 
  of 
  the 
  

   heated 
  carbons 
  can 
  be 
  seen, 
  and 
  the 
  blue 
  flame 
  between 
  them, 
  

  

  