﻿512 
  Mr. 
  R. 
  W. 
  Wood 
  on 
  Phase-Reversal 
  Zone-Plates 
  

  

  The 
  preliminary 
  drawing 
  which 
  I 
  made 
  contained 
  115 
  dark 
  

   rinos, 
  and 
  was 
  drawn 
  on 
  a 
  scale 
  three 
  times 
  as 
  large 
  as 
  that 
  used 
  

   by 
  Soret. 
  In 
  order 
  to 
  avoid 
  the 
  errors 
  due 
  to 
  shrinkage 
  and 
  

   expansion 
  of 
  the 
  paper 
  during 
  the 
  progress 
  of 
  the 
  work, 
  the 
  

   following 
  method 
  was 
  adopted. 
  A 
  sheet 
  of 
  heavy 
  smooth 
  

   surface 
  drawing-paper, 
  considerably 
  over 
  a 
  metre 
  square, 
  was 
  

   thoroughly 
  dampened 
  and 
  the 
  edges 
  glued 
  down 
  to 
  a 
  heavy 
  

   drawing-board. 
  The 
  paper 
  was 
  kept 
  damp 
  until 
  the 
  edges 
  

   had 
  thoroughly 
  dried 
  to 
  prevent 
  the 
  shrinkage 
  from 
  tearing 
  

   them 
  loose. 
  On 
  drying, 
  the 
  paper 
  was 
  in 
  close 
  contact 
  with 
  

   the 
  board 
  and 
  as 
  tight 
  as 
  a 
  drumhead. 
  A 
  copper 
  tack 
  with 
  a 
  

   small 
  conical 
  dent 
  in 
  its 
  head 
  was 
  driven 
  into 
  the 
  centre 
  of 
  

   the 
  board 
  to 
  serve 
  as 
  a 
  centre 
  for 
  the 
  beam-compass 
  used 
  in 
  

   drawing 
  the 
  circles. 
  The 
  pen 
  of 
  the 
  compass 
  was 
  accurately 
  

   set 
  for 
  each 
  circle 
  by 
  means 
  of 
  a 
  Brown 
  and 
  Sharp 
  steel 
  

   metre-bar, 
  care 
  being 
  taken 
  to 
  set 
  by 
  the 
  inner 
  and 
  outer 
  edge 
  

   of 
  the 
  pen 
  alternately, 
  in 
  order 
  that 
  no 
  error 
  due 
  to 
  the 
  width 
  

   of 
  the 
  line 
  should 
  be 
  present. 
  Two 
  hundred 
  and 
  thirty 
  circles 
  

   were 
  drawn 
  in 
  this 
  manner, 
  using 
  the 
  greatest 
  care 
  possible 
  

   to 
  ensure 
  accuracy. 
  The 
  spaces 
  between 
  the 
  alternate 
  rings 
  

   were 
  then 
  blackened 
  by 
  means 
  of 
  the 
  beam-compass 
  and 
  a 
  

   broad 
  pen. 
  In 
  spite 
  of 
  every 
  precaution 
  errors 
  crept 
  in 
  which 
  

   manifest 
  themselves 
  as 
  slight 
  shadings 
  at 
  certain 
  places 
  ; 
  their 
  

   effect, 
  however, 
  is 
  small. 
  

  

  The 
  labour 
  involved 
  in 
  the 
  preparation 
  of 
  such 
  a 
  drawing 
  

   was 
  so 
  great 
  that 
  it 
  has 
  seemed 
  worth 
  while 
  to 
  publish 
  with 
  

   this 
  paper 
  a 
  reduced 
  photographic 
  copy, 
  from 
  which 
  other 
  

   still 
  more 
  reduced 
  copies 
  can 
  be 
  prepared 
  on 
  glass 
  by 
  anyone. 
  

   I 
  have 
  made 
  photographic 
  negatives 
  of 
  this 
  drawing 
  on 
  

   lantern-slide 
  plates 
  of 
  various 
  sizes, 
  and 
  find 
  that 
  they 
  act 
  

   very 
  satisfactorily. 
  These 
  copies 
  were 
  made 
  with 
  a 
  very 
  fine 
  

   Zeiss 
  lens, 
  and 
  had 
  focal 
  lengths 
  varying 
  from 
  half 
  a 
  metre 
  

   to 
  ten 
  metres. 
  Smaller 
  plates 
  could 
  not 
  be 
  made 
  with 
  this 
  

   lens, 
  since 
  the 
  limit 
  in 
  the 
  defining 
  power 
  is 
  about 
  '02 
  millim. 
  

   In 
  order 
  to 
  produce 
  plates 
  of 
  very 
  short 
  focus 
  I 
  made 
  some 
  

   copies 
  with 
  a 
  Beck 
  microscope-objective 
  of 
  about 
  2 
  eentim. 
  

   focus. 
  The 
  focal 
  lengths 
  of 
  these 
  plates 
  varied 
  from 
  3 
  to 
  

   10 
  eentim., 
  and 
  they 
  were 
  found 
  to 
  act 
  admirably 
  as 
  eyepieces. 
  

   The 
  central 
  circle 
  of 
  one 
  of 
  these 
  micro-plates 
  had 
  a 
  radius 
  of 
  

  

  /.1KQV2 
  

  

  •158 
  millim.; 
  its 
  focus 
  for 
  yellow 
  light 
  is 
  therefore 
  .r 
  )()i) 
  n 
  , 
  

  

  or 
  4*1 
  centime 
  In 
  spite 
  of 
  its 
  small 
  size, 
  the 
  rings 
  were 
  sharp 
  

   out 
  to 
  the 
  edge, 
  where 
  their 
  width, 
  measured 
  on 
  a 
  small 
  

   dividing-engine, 
  was 
  found 
  to 
  be 
  only 
  '005 
  millim. 
  

  

  A 
  number 
  of 
  experiments 
  were 
  tried 
  with 
  these 
  plates 
  ; 
  

   but 
  as 
  thev 
  can 
  be 
  shown 
  to 
  a 
  much 
  better 
  advantage 
  with 
  

  

  