﻿514 
  Mr. 
  R. 
  W. 
  Wood 
  on 
  Phase-Reversal 
  Zone-Plates 
  

  

  the 
  small 
  incandescent 
  particles 
  that 
  are 
  thrown 
  oft' 
  being 
  also 
  

   clearly 
  visible. 
  

  

  If 
  the 
  screen 
  is 
  near 
  the 
  plate, 
  the 
  image 
  of 
  the 
  carbons 
  is 
  

   orange- 
  red; 
  and 
  as 
  it 
  is 
  moved 
  back 
  the 
  colour 
  changes 
  to 
  

   yellow, 
  green, 
  and 
  blue, 
  the 
  foci 
  for 
  different 
  colours 
  being 
  

   far 
  apart. 
  The 
  best 
  definition 
  appears 
  to 
  be 
  in 
  the 
  yellow 
  

   and 
  green. 
  On 
  setting 
  the 
  plates 
  up 
  at 
  a 
  greater 
  distance 
  

   from 
  the 
  arc 
  and 
  placing 
  a 
  sheet 
  of 
  paper 
  in 
  the 
  focus, 
  

   a 
  large 
  part 
  of 
  the 
  light 
  incident 
  on 
  the 
  plate 
  is 
  brought 
  to 
  

   a 
  very 
  small 
  sharp 
  focus 
  surrounded 
  by 
  a 
  dark 
  area, 
  corre- 
  

   sponding 
  to 
  the 
  geometrical 
  projection 
  of 
  that 
  part 
  of 
  the 
  

   plate 
  which 
  is 
  effective. 
  With 
  some 
  plates 
  this 
  dark 
  area 
  

   is 
  very 
  small, 
  and 
  the 
  central 
  bright 
  spot 
  correspondingly 
  

   feeble, 
  showing 
  that 
  what 
  little 
  action 
  there 
  is 
  comes 
  from 
  a 
  

   few 
  rings 
  in 
  the 
  centre. 
  This 
  forms 
  a 
  very 
  good 
  method 
  of 
  

   testing 
  the 
  plates 
  ; 
  and 
  in 
  general 
  I 
  find 
  that 
  about 
  one 
  out 
  of 
  

   four 
  can 
  be 
  considered 
  first 
  class. 
  With 
  more 
  care 
  in 
  the 
  

   preparation 
  of 
  the 
  films 
  greater 
  uniformity 
  in 
  the 
  results 
  

   could 
  doubtless 
  be 
  secured. 
  

  

  In 
  making 
  these 
  plates 
  it 
  is 
  necessary 
  to 
  use 
  mirror-glass 
  

   with 
  a 
  ground 
  surface 
  if 
  good 
  definition 
  is 
  to 
  be 
  obtained. 
  I 
  

   find 
  the 
  German 
  mirror-glass 
  which 
  comes 
  with 
  a 
  deposit 
  of 
  

   metallic 
  silver 
  backed 
  by 
  varnish 
  gives 
  very 
  good 
  results. 
  

   The 
  varnish 
  can 
  be 
  removed 
  by 
  alcohol, 
  the 
  silver 
  with 
  

   nitric 
  acid, 
  and 
  the 
  surface 
  cleaned. 
  It 
  is 
  best 
  to 
  prepare 
  a 
  

   large 
  number 
  of 
  plates 
  and 
  select 
  the 
  best, 
  either 
  by 
  throwing 
  

   an 
  image 
  of 
  an 
  arc-light 
  with 
  them 
  on 
  a 
  screen, 
  or 
  by 
  the 
  

   following 
  method 
  which 
  is 
  perhaps 
  better. 
  

  

  A 
  small 
  hole 
  a 
  millimetre 
  or 
  so 
  in 
  diameter 
  is 
  punched 
  in 
  a 
  

   black 
  card 
  or 
  a 
  metal 
  plate 
  and 
  set 
  up 
  in 
  front 
  of 
  a 
  bright 
  

   sodium 
  flame 
  ; 
  the 
  zone-plate 
  is 
  mounted 
  at 
  a 
  distance 
  of 
  

   three 
  metrgs 
  from 
  this, 
  and 
  the 
  eye 
  brought 
  into 
  the 
  focus. 
  Li' 
  

   the 
  plate 
  is 
  good, 
  it 
  lights 
  up 
  brightly 
  and 
  uniformly. 
  Most 
  

   plates, 
  however, 
  show 
  irregularity 
  in 
  the 
  illumination, 
  and 
  

   may 
  even 
  have 
  spots 
  that 
  appear 
  quite 
  devoid 
  of 
  light. 
  These 
  

   dark 
  spots 
  may 
  be 
  caused 
  by 
  the 
  film 
  being 
  so 
  thin 
  as 
  to 
  give 
  

   no 
  appreciable 
  retardation, 
  or 
  so 
  thick 
  as 
  to 
  give 
  a 
  retardation 
  

   of 
  a 
  w 
  T 
  hole 
  wave 
  or 
  a 
  number 
  of 
  whole 
  waves. 
  Plates 
  that 
  do 
  

   not 
  come 
  up 
  to 
  this 
  test 
  often 
  give 
  very 
  good 
  images, 
  how- 
  

   ever, 
  and 
  need 
  not 
  be 
  rejected. 
  Those 
  which 
  light 
  up 
  feebly 
  

   and 
  but 
  a 
  short 
  distance 
  from 
  the 
  centre 
  should 
  be 
  thrown 
  

   away. 
  

  

  Although 
  a 
  large 
  percentage 
  of 
  the 
  light 
  is 
  wasted 
  by 
  a 
  

   zone-plate, 
  the 
  focal 
  images 
  produced 
  by 
  these 
  plates 
  were 
  so 
  

   sharp 
  and 
  brilliant, 
  that 
  I 
  determined 
  to 
  see 
  just 
  what 
  results 
  

   could 
  be 
  obtained 
  with 
  a 
  telescope 
  constructed 
  with 
  one. 
  

  

  