﻿Telescopic 
  Vision. 
  957 
  

  

  faint 
  that 
  its 
  presence 
  may 
  be 
  disregarded 
  unless 
  object 
  z' 
  is 
  

   too 
  large. 
  We 
  shall 
  be 
  careful 
  not 
  to 
  make 
  it 
  too 
  large 
  ; 
  and 
  

   may 
  therefore 
  regard 
  the 
  light 
  incident 
  upon 
  z' 
  as 
  consisting 
  

   of 
  n 
  stellades 
  and 
  each 
  stellade 
  of 
  m 
  u 
  f 
  W's, 
  these 
  stellades 
  

   advancing 
  upon 
  z' 
  in 
  the 
  n 
  directions 
  represented 
  by 
  lines 
  

   from 
  the 
  middles 
  of 
  the 
  da's 
  to 
  the 
  middle 
  of 
  £. 
  

  

  65. 
  Let 
  us 
  now 
  select 
  some 
  one 
  of 
  these 
  stellades 
  and 
  

   study 
  what 
  further 
  happens 
  to 
  it. 
  To 
  do 
  this 
  we 
  must 
  be 
  

   able 
  to 
  deal 
  with 
  it 
  separately 
  ; 
  that 
  is 
  we 
  must 
  free 
  it 
  from 
  

   the 
  presence 
  of 
  the 
  other 
  stellades 
  which 
  will 
  accompany 
  it 
  

   so 
  long 
  as 
  a 
  is 
  the 
  source 
  of 
  light. 
  This 
  is 
  easily 
  done 
  by 
  

   removing 
  a 
  from 
  the 
  experimental 
  apparatus 
  and 
  employing 
  

   s\ 
  the 
  pro-star, 
  as 
  the 
  source 
  of 
  light. 
  The 
  pro-star 
  will 
  then 
  

   illuminate 
  z' 
  with 
  a 
  single 
  stellade, 
  and 
  this 
  may 
  legitimately 
  

   take 
  the 
  place 
  of 
  the 
  stellade 
  whose 
  future 
  history 
  we 
  want 
  to 
  

   trace, 
  if 
  we 
  make 
  its 
  light 
  fall 
  upon 
  z 
  in 
  the 
  right 
  direction. 
  

   This 
  we 
  can 
  do 
  by 
  availing 
  ourselves 
  of 
  the 
  adjustments 
  pro- 
  

   vided 
  at 
  the 
  B 
  end 
  of 
  the 
  board 
  BE. 
  See 
  § 
  22, 
  p. 
  333. 
  

  

  66. 
  What 
  we 
  have 
  in 
  the 
  present 
  Chapter 
  to 
  study 
  are 
  the 
  

   events 
  that 
  will 
  then 
  occur 
  between 
  object 
  z' 
  and 
  the 
  plane 
  

   Y' 
  which 
  stands 
  in 
  front 
  of 
  the 
  pro-telescope. 
  To 
  do 
  this 
  

   satisfactorily, 
  we 
  must 
  confine 
  our 
  attention 
  to 
  light 
  trans- 
  

   mitted 
  to 
  the 
  astronomical 
  telescope 
  from 
  some 
  definite 
  

   object 
  upon 
  the 
  planet, 
  and 
  the 
  object 
  we 
  shall 
  select 
  as 
  the 
  

   first 
  to 
  be 
  dealt 
  with 
  is 
  an 
  object 
  like 
  the 
  planet 
  Neptune, 
  and 
  

   in 
  some 
  degree 
  like 
  the 
  Solis 
  Lacus 
  upon 
  Mars, 
  at 
  longitude 
  

   87° 
  and 
  south 
  latitude 
  27° 
  upon 
  the 
  maps 
  of 
  the 
  so-called 
  

   t 
  canals 
  upon 
  Mars.' 
  The 
  Solis 
  Lacus 
  is 
  a 
  small 
  but 
  con- 
  

   spicuous 
  spot 
  upon 
  the 
  image 
  of 
  the 
  planet, 
  of 
  a 
  shape 
  not 
  

   far 
  from 
  round, 
  and 
  of 
  a 
  diameter 
  on 
  the 
  planet 
  which 
  when 
  

   the 
  planet 
  comes 
  nearest 
  to 
  us 
  subtends 
  at 
  the 
  distance 
  of 
  the 
  

   earth 
  an 
  angle 
  not 
  far 
  from 
  our 
  standard 
  angle 
  e. 
  We 
  are 
  

   not 
  at 
  present 
  concerned 
  with 
  the 
  faint 
  traces 
  oi 
  detail 
  which 
  

   are 
  supposed 
  to 
  have 
  been 
  at 
  times 
  detected 
  upon 
  its 
  image, 
  

   since 
  the 
  question 
  to 
  which 
  we 
  are 
  at 
  present 
  seeking 
  an 
  

   answer 
  is, 
  what 
  appearance 
  the 
  outline 
  of 
  an 
  object 
  of 
  some 
  

   such 
  shape 
  and 
  size 
  will 
  have 
  in 
  the 
  astronomical 
  telescope, 
  

   if 
  such 
  an 
  object 
  exists 
  upon 
  the 
  planet 
  ? 
  The 
  object 
  we 
  

   shall 
  suppose 
  will, 
  for 
  simplicity,, 
  be 
  an 
  object 
  exactly 
  

   round, 
  and 
  subtending 
  exactly 
  angle 
  e 
  at 
  the 
  distance 
  of 
  the 
  

   astronomical 
  telescope 
  from 
  the 
  planet. 
  The 
  object 
  which 
  

   we 
  have 
  assumed 
  to 
  exist 
  upon 
  the 
  planet 
  will 
  be 
  adequately 
  

   represented 
  in 
  the 
  experimental 
  apparatus 
  by 
  a 
  round 
  hole 
  in 
  

   copper-foil 
  at 
  z', 
  illuminated 
  by 
  light 
  coming 
  from 
  beyond 
  it. 
  

   This 
  will 
  present 
  in 
  the 
  pro-telescope 
  the 
  appearance 
  of 
  a 
  

   bright 
  object 
  with 
  dark 
  surroundings, 
  whereas 
  the 
  Solis 
  

  

  