﻿Telescopic 
  Vision. 
  799 
  

  

  called 
  its 
  anti-spile. 
  Under 
  some 
  conditions 
  the 
  light 
  seen 
  

   in 
  the 
  spile 
  is 
  the 
  same 
  as 
  the 
  light 
  received 
  by 
  the 
  telescope 
  

   from 
  its 
  anti-spile; 
  but 
  this 
  is 
  by 
  no 
  means 
  always 
  the 
  

   case. 
  

  

  CHAPTER 
  4. 
  

  

  The 
  kind 
  of 
  image 
  that 
  ivill 
  be 
  formed. 
  

  

  38. 
  When 
  some 
  feature, 
  z 
  , 
  upon 
  a 
  planet 
  is 
  to 
  be 
  made 
  

   visible 
  by 
  looking 
  at 
  it 
  through 
  a 
  telescope, 
  then, 
  if 
  what 
  we 
  

   see 
  in 
  the 
  telescope 
  corresponding 
  to 
  that 
  part 
  of 
  the 
  planet, 
  

   is 
  the 
  kind 
  of 
  image 
  which 
  we 
  have 
  called 
  a 
  spile, 
  i. 
  e. 
  if 
  it 
  

   is 
  some 
  one 
  of 
  the 
  elementary 
  parts 
  of 
  which 
  the 
  image 
  of 
  

   the 
  whole 
  planet 
  is 
  seen 
  to 
  consist 
  ; 
  — 
  if, 
  for 
  example, 
  it 
  is 
  one 
  

   of 
  the 
  white 
  polar 
  caps 
  seen 
  upon 
  the 
  image 
  of 
  Mars, 
  or 
  

   one 
  of 
  those 
  extensive 
  regions 
  which 
  are 
  bounded 
  by 
  what- 
  

   ever 
  on 
  the 
  planet 
  corresponds 
  to 
  the 
  faint 
  streaks 
  in 
  its 
  

   image 
  which 
  have 
  been 
  called 
  Canals 
  " 
  upon 
  Mars 
  " 
  — 
  in 
  such 
  

   cases 
  the 
  outline 
  of 
  this 
  spile 
  is 
  often 
  tolerably 
  well 
  seen 
  in 
  

   the 
  image, 
  although 
  the 
  immense 
  amount 
  of 
  detail 
  which 
  

   really 
  exists 
  upon 
  its 
  anti-spile 
  (an 
  extensive 
  region 
  upon 
  

   the 
  planet) 
  is 
  so 
  blurred 
  together 
  that 
  it 
  is 
  unseen 
  or 
  but 
  

   imperfectly 
  seen. 
  Even 
  the 
  image 
  of 
  an 
  entire 
  planet 
  may 
  

   present 
  this 
  appearance. 
  This 
  happens 
  with 
  our 
  best 
  tele- 
  

   scopes 
  when 
  the 
  planet 
  is 
  Uranus 
  or 
  Neptune 
  ; 
  and 
  with 
  

   telescopes 
  of 
  insufficient 
  aperture, 
  however 
  good 
  in 
  other 
  

   respects, 
  all 
  the 
  planets 
  will 
  present 
  this 
  appearance. 
  Under 
  

   such 
  circumstances 
  no 
  detail 
  except 
  the 
  mere 
  outline 
  of 
  the 
  

   planet 
  is 
  well 
  seen. 
  

  

  39. 
  Let 
  us 
  take 
  the 
  case 
  of 
  Neptune, 
  which 
  when 
  in 
  

   opposition 
  subtends 
  at 
  the 
  earth 
  an 
  angle 
  of 
  about 
  1J 
  e, 
  

   where 
  e 
  is 
  the 
  angle 
  2 
  /,# 
  062648 
  which 
  in 
  circular 
  measure 
  

   is 
  0*00001 
  ; 
  or 
  of 
  Uranus 
  which 
  subtends 
  nearly 
  2e. 
  These 
  

   are 
  objects 
  which 
  appear 
  simply 
  as 
  round 
  disks 
  of 
  nearly 
  

   uniform 
  light 
  in 
  the 
  telescope 
  ; 
  that 
  is 
  to 
  say, 
  the 
  image 
  of 
  

   the 
  planet 
  is 
  the 
  kind 
  of 
  plateo-spile 
  or 
  surface-blur 
  which 
  

   we 
  have 
  distinguished 
  by 
  the 
  name 
  of 
  silhouette-blur. 
  Its 
  

   outline 
  only 
  is 
  seen 
  ; 
  and 
  we 
  represent 
  it 
  in 
  our 
  experimental 
  

   apparatus 
  by 
  light 
  coming 
  through 
  a 
  round 
  hole 
  in 
  copper- 
  

   foil 
  at 
  z' 
  . 
  To 
  do 
  this 
  satisfactorily 
  we 
  must 
  adjust 
  the 
  size 
  

   of 
  the 
  round 
  hole 
  at 
  z', 
  which 
  is 
  our 
  pro-planet, 
  and 
  the 
  

   aperture 
  A' 
  of 
  the 
  iris-diaphragm 
  at 
  C, 
  so 
  as 
  correctly 
  to 
  repre- 
  

   sent 
  the 
  size 
  of 
  the 
  planet 
  and 
  the 
  aperture 
  of 
  the 
  astronomical 
  

   telescope 
  whose 
  performance 
  upon 
  it 
  we 
  want 
  to 
  investigate. 
  

   This 
  may 
  be 
  done 
  in 
  any 
  one 
  of 
  the 
  ways 
  described 
  in 
  

  

  