﻿telescopic 
  Vision. 
  339 
  

  

  images 
  in 
  telescopes 
  depends. 
  By 
  persistent 
  interference, 
  

   is 
  meant 
  an 
  interference 
  which 
  lasts 
  for 
  a 
  time 
  sufficiently 
  

   long 
  for 
  the 
  human 
  eye 
  to 
  take 
  note 
  of 
  it, 
  that 
  is 
  which 
  lasts 
  

   for 
  a 
  vast 
  seon 
  of 
  time 
  if 
  compared 
  with 
  the 
  durations 
  we 
  

   have 
  to 
  consider 
  when 
  treating 
  of 
  the 
  molecular 
  events 
  

   which 
  determine 
  the 
  great 
  bulk 
  of 
  the 
  interference 
  effects 
  

   that 
  occur 
  in 
  light. 
  

  

  The 
  reason 
  of 
  this 
  persistence 
  is 
  that 
  the 
  fluctuations 
  that 
  

   occur 
  in 
  a 
  complex 
  u 
  f 
  W 
  are 
  functions 
  of 
  the 
  time 
  only, 
  

   so 
  that 
  although 
  consecutive 
  waves 
  will 
  in 
  some 
  parts 
  of 
  the 
  

   undulation 
  differ, 
  yet 
  every 
  wave 
  will 
  be 
  absolutely 
  the 
  same 
  

   throughout 
  all 
  parts 
  of 
  its 
  own 
  extent 
  ; 
  and 
  this 
  is 
  the 
  con- 
  

   dition 
  which 
  renders 
  a 
  lasting 
  interference 
  effect 
  possible. 
  

  

  Thus 
  when 
  a 
  wave 
  of 
  the 
  complex 
  u 
  f 
  W 
  which 
  issues 
  

   from 
  the 
  pro-star 
  s' 
  reaches 
  the 
  object 
  z\ 
  which 
  is 
  usually 
  a 
  

   small 
  opening 
  of 
  some 
  definite 
  shape 
  in 
  the 
  sheet 
  of 
  copper 
  

   Z', 
  a 
  part 
  of 
  this 
  wave 
  and 
  its 
  successors 
  are 
  allowed 
  to 
  

   advance 
  through 
  the 
  hole, 
  and 
  are 
  scattered 
  into 
  different 
  

   forward 
  directions 
  in 
  a 
  way 
  which 
  will 
  be 
  explained 
  in 
  the 
  

   next 
  Chapter. 
  These 
  scattered 
  portions 
  or 
  some 
  of 
  them 
  will 
  

   enter 
  the 
  pro-telescope 
  and 
  by 
  its 
  objective 
  be 
  made 
  to 
  con- 
  

   verge 
  upon 
  a 
  small 
  space 
  at 
  x\ 
  and 
  it 
  is 
  in 
  consequence 
  oj 
  

   their 
  being 
  capable 
  of 
  interfering 
  ivith 
  one 
  another 
  that 
  they 
  

   will 
  produce 
  there 
  an 
  image 
  of 
  the 
  opening 
  at 
  zf. 
  

  

  This 
  u 
  f 
  W 
  will 
  produce 
  at 
  x' 
  an 
  image 
  of 
  some 
  kind 
  of 
  

   object 
  z', 
  and 
  this 
  image 
  is 
  one 
  of 
  the 
  partials 
  of 
  the 
  com- 
  

   plete 
  image 
  which 
  will 
  be 
  formed 
  when 
  diffused 
  light 
  instead 
  

   of 
  only 
  one 
  u 
  f 
  W 
  is 
  allowed 
  to 
  operate 
  as 
  the 
  light 
  incident 
  

   upon 
  z 
  f 
  . 
  

  

  It 
  is 
  well 
  here 
  to 
  observe 
  that 
  no 
  two 
  of 
  the 
  u 
  f 
  W's 
  need 
  

   be 
  so 
  related 
  to 
  one 
  another 
  that 
  the 
  light 
  of 
  the 
  one 
  can 
  

   produce 
  visible 
  interference 
  effects 
  with 
  the 
  light 
  of 
  the 
  

   other 
  : 
  and 
  accordingly 
  the 
  image 
  we 
  see 
  in 
  the 
  pro-telescope 
  

   when 
  we 
  employ 
  diffused 
  light, 
  consists 
  of 
  our 
  seeing, 
  as 
  it 
  

   were, 
  superposed 
  upon 
  one 
  another, 
  the 
  images 
  formed 
  inde- 
  

   pendently 
  of 
  one 
  another 
  byufW's 
  into 
  which 
  the 
  diffused 
  

   light 
  can 
  be 
  resolved 
  Thus 
  the 
  partials, 
  as 
  they 
  exist 
  on 
  

   disk 
  x, 
  are 
  light 
  superposed 
  upon 
  light 
  under 
  the 
  condition 
  

   that 
  they 
  can 
  produce 
  no 
  interference 
  effect 
  one 
  with 
  another. 
  

   They 
  accordingly 
  furnish 
  at 
  each 
  point 
  of 
  disk 
  x 
  an 
  illumi- 
  

   nation 
  which 
  is 
  the 
  simple 
  sum 
  of 
  the 
  intensities 
  of 
  them 
  

   separately 
  at 
  that 
  point. 
  It 
  is 
  in 
  this 
  way 
  that 
  a 
  telescopic 
  

   image 
  is 
  formed. 
  

  

  In 
  the 
  second 
  part 
  of 
  this 
  paper 
  it 
  is 
  proposed 
  to 
  go 
  fully 
  

   into 
  this 
  subject, 
  and 
  into 
  others 
  allied 
  with 
  it. 
  

  

  [To 
  be 
  continued.] 
  

  

  