﻿and 
  Diffraction- 
  Telescopes. 
  517 
  

  

  seen 
  given 
  anywhere, 
  probably 
  because 
  of 
  its 
  obvious 
  nature, 
  

   is 
  the 
  following: 
  — 
  We 
  may 
  regard 
  the 
  zone-plate 
  as 
  a 
  circular 
  

   grating 
  in 
  which 
  the 
  grating 
  space 
  becomes 
  less 
  and 
  less 
  as 
  

   we 
  proceed 
  outward 
  from 
  the 
  centre, 
  consequently 
  the 
  bending 
  

   or 
  deviation 
  of 
  the 
  diffracted 
  ray 
  from 
  the 
  normal 
  becomes 
  

   greater 
  as 
  we 
  near 
  the 
  edge, 
  and 
  the 
  change 
  in 
  the 
  grating 
  

   space 
  is 
  such 
  as 
  to 
  bring 
  all 
  the 
  deviated 
  rays 
  of 
  a 
  single 
  

   colour 
  together 
  in 
  a 
  point. 
  The 
  principal 
  real 
  focus 
  of 
  the 
  

   plate 
  corresponds 
  to 
  the 
  superimposed 
  spectra 
  of 
  the 
  first 
  

   order 
  ; 
  the 
  second 
  order 
  spectra 
  being 
  bent 
  more 
  come 
  

   together 
  at 
  a 
  point 
  nearer 
  the 
  plate, 
  forming 
  a 
  second 
  focus, 
  

   and 
  so 
  on 
  — 
  the 
  different 
  foci 
  corresponding 
  to 
  spectra 
  of 
  

   different 
  orders. 
  There 
  are 
  also 
  spectra 
  of 
  the 
  first 
  order 
  

   bent 
  outwards, 
  or 
  away 
  from 
  the 
  centre, 
  and 
  these 
  rays 
  pro- 
  

   jected 
  backwards 
  behind 
  the 
  plate 
  will 
  meet, 
  forming 
  a 
  

   virtual 
  focus 
  in 
  a 
  position 
  corresponding 
  to 
  that 
  of 
  the 
  real 
  

   focus 
  on 
  the 
  opposite 
  side 
  of 
  the 
  plate 
  ; 
  the 
  second 
  order 
  

   spectra 
  are 
  bent 
  out 
  more, 
  consequently 
  the 
  virtual 
  focus 
  of 
  

   these 
  rays 
  is 
  nearer 
  the 
  plate, 
  and 
  we 
  thus 
  see, 
  that 
  for 
  every 
  

   real 
  focus 
  on 
  the 
  one 
  side 
  of 
  the 
  plate, 
  there 
  is 
  a 
  corresponding 
  

   virtual 
  focus 
  on 
  the 
  other. 
  

  

  Plates 
  of 
  very 
  long 
  focus 
  are 
  also 
  useful 
  for 
  demonstration. 
  

   I 
  have 
  one 
  7 
  centim. 
  in 
  diameter, 
  with 
  a 
  focal 
  length 
  of 
  

   about 
  twelve 
  metres. 
  This 
  plate 
  will 
  project 
  a 
  very 
  good 
  

   image 
  of 
  the 
  sun, 
  about 
  11 
  centim. 
  in 
  diameter, 
  on 
  a 
  screen 
  

   placed 
  at 
  the 
  focus. 
  The 
  effect 
  is 
  especially 
  fine 
  when 
  the 
  

   sun 
  is 
  behind 
  the 
  leafless 
  branches 
  of 
  distant 
  trees, 
  for 
  then 
  

   each 
  twig 
  stands 
  out 
  sharp 
  and 
  distinct 
  on 
  the 
  image. 
  When 
  

   this 
  plate 
  is 
  set 
  up 
  at 
  a 
  distance 
  of 
  3 
  metres 
  from 
  an 
  arc-light 
  

   and 
  the 
  eye 
  is 
  brought 
  into 
  one 
  of 
  the 
  nearer 
  foci, 
  3 
  or 
  4 
  

   metres 
  behind 
  the 
  plate, 
  the 
  effect 
  is 
  very 
  fine 
  ; 
  the 
  whole 
  

   area 
  filling 
  up 
  with 
  a 
  dazzling 
  white 
  light 
  of 
  almost 
  insup- 
  

   portable 
  brilliancy. 
  

  

  The 
  diffraction 
  phenomena 
  produced 
  by 
  openings 
  in 
  thin 
  

   transparent 
  films 
  were 
  studied 
  by 
  Quincke 
  and 
  published 
  in 
  

   Poggendorff's 
  Annalen, 
  1867. 
  Quincke, 
  moreover, 
  prepared 
  

   gratings 
  with 
  narrow 
  strips 
  of 
  a 
  thin 
  transparent 
  lamina 
  

   instead 
  of 
  the 
  usual 
  dark 
  spaces 
  : 
  these, 
  by 
  retarding 
  the 
  

   light, 
  threw 
  the 
  w 
  r 
  aves 
  out 
  of 
  phase 
  with 
  those 
  which 
  passed 
  

   between 
  them. 
  These 
  gratings 
  he 
  prepared 
  by 
  ruling 
  the 
  

   silvered 
  surface 
  of 
  a 
  piece 
  of 
  glass 
  on 
  a 
  dividing-engine, 
  

   forming 
  an 
  ordinary 
  grating 
  in 
  which 
  the 
  dark 
  lines 
  were 
  

   narrow 
  strips 
  of 
  silver 
  equal 
  in 
  width 
  to 
  the 
  clear 
  spaces. 
  By 
  

   covering 
  the 
  plate 
  with 
  iodine 
  the 
  silver 
  was 
  changed 
  into 
  

   transparent 
  iodide. 
  He 
  found 
  that 
  these 
  transparent, 
  gratings 
  

   gave 
  spectra 
  very 
  similar 
  to 
  ordinary 
  gratings, 
  save 
  in 
  one 
  

   Phil. 
  Mag. 
  S. 
  5. 
  Vol. 
  45. 
  No. 
  277. 
  June 
  18y8. 
  2 
  N 
  

  

  