﻿and 
  Diffraction- 
  Telescopes. 
  515 
  

  

  .Removing 
  the 
  objective 
  from 
  my 
  5 
  in. 
  Clark, 
  I 
  put 
  a 
  3 
  centim. 
  

   gelatine 
  zone-plate 
  in 
  its 
  place. 
  A 
  low-power 
  eyepiece 
  was 
  

   used, 
  and 
  on 
  viewing 
  terrestrial 
  objects, 
  such 
  as 
  distant 
  

   chimney-tops 
  and 
  trees 
  against 
  the 
  sky, 
  with 
  this 
  combination, 
  

   the 
  result 
  seemed 
  to 
  be 
  very 
  unsatisfactory 
  ; 
  the 
  amplification 
  

   was 
  about 
  60 
  diameters 
  and 
  the 
  outlines 
  were 
  sharp, 
  but 
  there 
  

   was 
  very 
  little 
  difference 
  in 
  light 
  value 
  between 
  the 
  sky 
  and 
  

   dark 
  chimneys. 
  In 
  other 
  words 
  the 
  field 
  was 
  bright. 
  The 
  

   most 
  suitable 
  objects 
  for 
  a 
  telescope 
  of 
  this 
  description 
  are 
  

   small, 
  brightly 
  illuminated 
  ones 
  on 
  a 
  black 
  background; 
  under 
  

   these 
  conditions 
  the 
  general 
  illumination 
  of 
  the 
  field 
  is 
  reduced 
  

   to 
  a 
  minimum. 
  I 
  tried 
  the 
  telescope 
  on 
  the 
  moon, 
  and 
  

   although 
  the 
  image 
  was 
  somewhat 
  dim 
  and 
  hazy, 
  as 
  seen 
  

   through 
  thin 
  clouds, 
  I 
  could 
  distinctly 
  see 
  a 
  number 
  of 
  the 
  

   larger 
  craters. 
  

  

  The 
  zone-plate 
  used 
  in 
  this 
  case 
  had 
  a 
  focal 
  length 
  of 
  about 
  

   five 
  feet; 
  but 
  plates 
  of 
  shorter 
  locus 
  show 
  the 
  principle 
  

   equally 
  well, 
  and 
  make 
  more 
  convenient 
  instruments 
  to 
  

   handle. 
  If 
  we 
  use 
  an 
  ordinary 
  low-power 
  eyepiece 
  in 
  the 
  

   principal 
  focus 
  of 
  the 
  zone-plate, 
  we 
  obtain 
  a 
  highly 
  magnified 
  

   inverted 
  image. 
  It 
  now 
  we 
  push 
  the 
  eyepiece 
  in, 
  the 
  image 
  

   becomes 
  blurred, 
  vanishes, 
  and 
  a 
  second 
  smaller 
  inverted 
  

   image 
  appears 
  ; 
  and 
  on 
  continuing 
  to 
  diminish 
  the 
  distance, 
  

   we 
  get 
  images 
  successively 
  smaller 
  and 
  smaller 
  as 
  the 
  eye- 
  

   piece 
  passes 
  through 
  the 
  different 
  foci 
  of 
  the 
  plate. 
  The 
  best 
  

   object 
  to 
  view 
  is 
  an 
  incandescent 
  electric 
  lamp 
  at 
  a 
  distance 
  

   of 
  six 
  or 
  eight 
  metres. 
  With 
  the 
  Soret 
  form 
  of 
  plate, 
  I 
  have 
  

   never 
  been 
  able 
  to 
  find 
  more 
  than 
  three 
  foci, 
  while 
  the 
  phase- 
  

   reversal 
  plates 
  easily 
  show 
  six. 
  If 
  instead 
  of 
  the 
  glass 
  eye- 
  

   piece 
  we 
  use 
  one 
  of 
  the 
  micro-zoneplates, 
  we 
  obtain 
  both 
  

   erect 
  and 
  inverted 
  images 
  ; 
  the 
  inverted 
  when 
  the 
  eyepiece 
  is 
  

   in 
  the 
  focus, 
  and 
  the 
  erect 
  when 
  it 
  is 
  between 
  the 
  focus 
  and 
  

   the 
  plate 
  — 
  the 
  former 
  image 
  being 
  due 
  to 
  the 
  action 
  of 
  the 
  

   micro-plate 
  as 
  a 
  convex 
  lens, 
  the 
  latter 
  to 
  its 
  action 
  as 
  a 
  con- 
  

   cave 
  lens, 
  as 
  in 
  the 
  Galilean 
  telescope. 
  In 
  spite 
  of 
  the 
  fineness 
  

   of 
  the 
  lines 
  in 
  the 
  micro-plates, 
  I 
  find 
  that 
  they 
  can 
  be 
  made 
  

   to 
  yield 
  very 
  excellent 
  phase-reversal 
  plates 
  by 
  contact 
  print- 
  

   ing, 
  so 
  that 
  by 
  making 
  both 
  objective 
  and 
  eyepiece 
  on 
  the 
  

   new 
  principle, 
  we 
  have 
  a 
  diffraction-telescope 
  wholly 
  without 
  

   lenses 
  or 
  mirrors, 
  which 
  is 
  fairly 
  efficient 
  for 
  viewing 
  small 
  

   brightly-illuminated 
  objects. 
  

  

  Starting 
  with 
  the 
  eyepiece 
  close 
  to 
  the 
  objective 
  and 
  

   moving 
  it 
  slowly 
  back, 
  we 
  get 
  a 
  rapid 
  alternation 
  of 
  erect 
  

   and 
  inverted 
  images, 
  to 
  the 
  number 
  of 
  eight 
  or 
  ten 
  in 
  all. 
  

   As 
  the 
  eyepiece 
  moves 
  the 
  colour 
  of 
  the 
  image 
  changes, 
  the 
  

   foci 
  being 
  differently 
  situated 
  for 
  the 
  different 
  colours. 
  

  

  