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  950 
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  LXXXIV. 
  Telescopic 
  Vision. 
  

   Bij 
  G. 
  Johnstone 
  Stoney, 
  M.A., 
  Sc.D., 
  F.R.S* 
  

  

  Part 
  III. 
  — 
  How 
  the 
  Image 
  is 
  formed, 
  and 
  of 
  what 
  

  

  PARTIALS 
  IT 
  CONSISTS. 
  

  

  [Plate 
  XIL] 
  

   Explanation 
  of 
  terms 
  : 
  — 
  

  

  1. 
  u 
  f 
  W, 
  undulation 
  of 
  flat 
  waves, 
  like 
  the 
  light 
  of 
  wave-length 
  X 
  

  

  received 
  from 
  a 
  star. 
  

   3- 
  a 
  B 
  W. 
  undulation 
  of 
  spherical 
  waves, 
  like 
  the 
  light 
  of 
  wave- 
  

   length 
  X 
  received 
  from 
  a 
  luminous 
  point. 
  

  

  3. 
  Stellades 
  and 
  Punctades 
  — 
  see 
  §§62 
  and 
  69. 
  Observation 
  — 
  A 
  

  

  u 
  f 
  W 
  is 
  a 
  special 
  kind 
  of 
  usW; 
  and 
  a 
  stellade 
  is 
  a 
  special 
  

   kind 
  of 
  punctade. 
  

  

  4. 
  ( 
  Somplete 
  image 
  and 
  its 
  partials 
  — 
  see 
  §§ 
  56 
  and 
  80. 
  

  

  5. 
  Spile 
  and 
  anti-spile 
  — 
  see 
  Chapter 
  :>, 
  in 
  the 
  November 
  Phil. 
  Mag. 
  

  

  p. 
  796. 
  

  

  CHAPTER 
  5. 
  

  

  Outline 
  of 
  the 
  Argument. 
  

  

  54, 
  V\ 
  T 
  I 
  j 
  have 
  now 
  to 
  enter 
  upon 
  the 
  most 
  important 
  

   t 
  t 
  part 
  of 
  our 
  undertaking, 
  in 
  which 
  we 
  are 
  to 
  

   find 
  out 
  win/ 
  it 
  is 
  that 
  when 
  a 
  large 
  and 
  a 
  small 
  object 
  

   exactly 
  >imilar 
  to 
  one 
  another 
  are 
  examined 
  with 
  the 
  same 
  

   telescope, 
  the 
  large 
  one 
  will 
  be 
  seen 
  satisfactorily, 
  while 
  the 
  

   small 
  one, 
  though 
  of 
  precisely 
  the 
  same 
  shape, 
  will 
  if 
  small 
  

   enongh 
  appear 
  when 
  viewed 
  through 
  the 
  telescope 
  to 
  be 
  

   transformed 
  into 
  something 
  unlike 
  itself. 
  In 
  the 
  last 
  chapter 
  

   we 
  have 
  met 
  with 
  specimens 
  of 
  such 
  distorted 
  appearances 
  

   and 
  we 
  have 
  now 
  to 
  learn 
  the 
  cause 
  of 
  these 
  imperfections, 
  

   and 
  of 
  other 
  imperfections 
  due 
  to 
  similar 
  causes. 
  

  

  Hitherto 
  in 
  examining 
  these 
  objects 
  (see 
  Chap. 
  4) 
  we 
  

   have 
  employed 
  diffused 
  light 
  as 
  the 
  light 
  to 
  illuminate 
  

   object 
  z\ 
  in 
  order 
  that 
  by 
  doing 
  so 
  we 
  may 
  reproduce 
  in 
  

   our 
  experimental 
  apparatus 
  the 
  same 
  optical 
  conditions 
  

   as 
  those 
  under 
  which 
  natural 
  objects 
  are 
  seen 
  through 
  

   telescopes, 
  see 
  § 
  80. 
  But 
  this 
  mode 
  of 
  illumination 
  will 
  no 
  

   longer 
  suffice, 
  because 
  so 
  long 
  as 
  we 
  allow 
  the 
  light 
  incident 
  

   upon 
  z' 
  to 
  be 
  diffused 
  light 
  the 
  whole 
  phenomenon 
  of 
  the 
  

   formation 
  of 
  its 
  image 
  in 
  the 
  pro-telescope 
  is 
  too 
  complicated 
  

   for 
  investigation. 
  It 
  will 
  be 
  shown 
  that 
  what 
  then 
  happens 
  

   is 
  due 
  to 
  the 
  simultaneous 
  operation 
  of 
  a 
  vast 
  number 
  of 
  

   optical 
  causes 
  which 
  produce 
  different 
  effects, 
  and 
  which 
  in 
  

   producing 
  these 
  effects 
  act 
  independently 
  of 
  one 
  another. 
  

   What, 
  accordingly, 
  we 
  have 
  to 
  do 
  is 
  to 
  separate 
  these 
  causes, 
  

   and 
  study 
  what 
  each 
  does 
  and 
  why 
  it 
  does 
  it 
  ; 
  and 
  then, 
  

   * 
  Communicated 
  by 
  the 
  Author. 
  

  

  