﻿Telescopic 
  Vision. 
  321 
  

  

  with 
  which 
  we 
  are 
  concerned 
  is 
  an 
  enormously 
  long 
  cone 
  of 
  

   light 
  with 
  its 
  base 
  upon 
  Mars 
  and 
  its 
  vertex 
  at 
  p, 
  and 
  consists 
  

   of 
  that 
  portion 
  of 
  the 
  whole 
  spherical 
  undulation 
  converging 
  

   towards 
  p, 
  which 
  happens 
  to 
  lie 
  within 
  the 
  geometrical 
  cone 
  

   we 
  have 
  indicated. 
  After 
  passing 
  p, 
  this 
  portion 
  o£ 
  the 
  

   undulation 
  becomes 
  a 
  small 
  diverging 
  cone 
  which 
  enters 
  the 
  

   objective, 
  and 
  (unless 
  the 
  axis 
  of 
  the 
  cone 
  is 
  already 
  on 
  the 
  

   optic 
  axis) 
  the 
  little 
  cone 
  in 
  passing 
  through 
  the 
  objective 
  is 
  

   bent 
  so 
  that 
  the 
  light 
  advances 
  towards 
  a 
  certain 
  tiny 
  patch 
  

   where 
  we 
  shall 
  find 
  an 
  image 
  of 
  Mars, 
  if 
  a 
  suitable 
  screen, 
  

   which 
  we 
  may 
  call 
  screen 
  X, 
  be 
  set 
  up 
  perpendicularly 
  to 
  

   the 
  optic 
  axis 
  at 
  the 
  principal 
  focus 
  of 
  the 
  objective. 
  Upon 
  

   that 
  little 
  patch, 
  wdiich 
  we 
  may 
  call 
  patch 
  a, 
  the 
  part 
  of 
  the 
  

   spherical 
  undulation 
  which 
  has 
  travelled 
  within 
  the 
  cone 
  falls, 
  

   and 
  would 
  there 
  produce 
  a 
  uniform 
  illumination 
  over 
  the 
  

   whole 
  of 
  that 
  patch 
  if 
  it 
  w 
  r 
  ere 
  the 
  only 
  light 
  admitted 
  to 
  the 
  

   telescope. 
  Similarly 
  the 
  portions 
  that 
  happen 
  to 
  lie 
  within 
  

   the 
  corresponding 
  cones 
  of 
  the 
  undulations 
  which 
  converge 
  

   upon 
  the 
  other 
  points 
  p\ 
  p" 
  9 
  &c. 
  of 
  disk 
  y, 
  will 
  after 
  entering 
  

   the 
  telescope 
  be 
  made 
  to 
  fall 
  upon 
  the 
  same 
  tiny 
  patch 
  x, 
  and 
  

   would 
  each 
  of 
  them 
  simply 
  illuminate 
  it 
  uniformly, 
  if 
  they 
  

   could 
  be 
  isolated 
  from 
  one 
  another. 
  But 
  as 
  they 
  are 
  all 
  

   present 
  together 
  upon 
  patch 
  a, 
  they 
  interfere 
  with 
  one 
  

   another, 
  and 
  it 
  is 
  by 
  their 
  interferences 
  that 
  the 
  complex 
  image 
  

   of 
  Mars 
  is 
  formed. 
  

  

  G. 
  It 
  is 
  of 
  importance 
  to 
  keep 
  in 
  mind 
  that 
  in 
  looking 
  at 
  

   such 
  an 
  object 
  as 
  Mars, 
  the 
  differences 
  of 
  distance 
  from 
  the 
  

   observer 
  of 
  the 
  features 
  seen 
  upon 
  different 
  parts 
  of 
  the 
  

   surface 
  of 
  the 
  planet, 
  are 
  of 
  no 
  optical 
  account. 
  Screen 
  X 
  

   may 
  therefore 
  be 
  practically 
  treated 
  as 
  a 
  simple 
  plane 
  per- 
  

   pendicular 
  to 
  the 
  optic 
  axis, 
  and 
  all 
  parts 
  of 
  the 
  image 
  will 
  

   be 
  simultaneously 
  in 
  focus 
  upon 
  it. 
  This 
  is 
  an 
  advantage 
  

   which 
  the 
  telescope 
  has 
  over 
  the 
  microscope. 
  

  

  7. 
  In 
  the 
  rest 
  of 
  this 
  Chapter 
  we 
  shall 
  request 
  the 
  reader 
  

   to 
  picture 
  to 
  himself 
  apparatus 
  which 
  in 
  fact 
  is 
  unattainable, 
  

   but 
  which 
  if 
  it 
  existed 
  would 
  enable 
  man 
  to 
  make 
  what 
  we 
  

   may 
  call 
  cosmical 
  experiments; 
  and 
  we 
  shall 
  invite 
  him 
  to 
  

   consider 
  what 
  w 
  T 
  ould 
  be 
  the 
  optical 
  effect 
  of 
  bringing 
  that 
  

   apparatus 
  into 
  operation 
  if 
  such 
  a 
  thing 
  were 
  possible. 
  In 
  

   the 
  next 
  following 
  chapter 
  we 
  shall 
  describe 
  how 
  to 
  

   repeat 
  these 
  experiments 
  with 
  apparatus 
  that 
  is 
  under 
  our 
  

   control, 
  and 
  shall 
  show 
  how 
  by 
  employing 
  it 
  we 
  need 
  no 
  

   longer 
  be 
  content 
  with 
  merely 
  conceiving, 
  but 
  can 
  actually 
  

   see 
  the 
  results. 
  

  

  8. 
  We 
  shall 
  begin 
  by 
  asking 
  the 
  reader 
  to 
  permit 
  us 
  to 
  

   effect 
  a 
  simplification 
  which 
  will 
  be 
  a 
  help 
  to 
  us 
  in 
  our 
  

  

  