﻿324 
  Dr. 
  G. 
  Johnstone 
  Stone 
  y 
  on 
  

  

  were 
  both 
  introduced 
  they 
  would 
  simply 
  cancel 
  one 
  another. 
  

   Therefore 
  we 
  are 
  justified 
  in 
  regarding 
  the 
  telescopic 
  image 
  

   as 
  formed 
  by 
  the 
  light 
  A 
  + 
  E— 
  E, 
  since 
  the 
  addition 
  of 
  both 
  

   4-E 
  and 
  — 
  E 
  leaves 
  matters 
  unchanged. 
  But 
  it 
  is 
  also 
  per- 
  

   missible 
  to 
  view 
  the 
  matter 
  otherwise, 
  and 
  this 
  we 
  shall 
  now 
  

   do. 
  The 
  light 
  A 
  + 
  E, 
  if 
  it 
  could 
  all 
  be 
  made 
  use 
  of, 
  would 
  

   form 
  what 
  we 
  have 
  called 
  the 
  optimum 
  image, 
  meaning 
  that 
  

   it 
  is 
  the 
  best 
  image 
  of 
  the 
  planet 
  which 
  the 
  light 
  emitted 
  by 
  

   the 
  planet 
  is 
  capable 
  of 
  yielding. 
  Hence 
  what 
  we 
  see 
  in 
  

   the 
  telescope 
  is 
  this 
  optimum 
  image 
  with 
  light 
  — 
  E 
  super- 
  

   imposed 
  upon 
  it. 
  What 
  the 
  full 
  effect 
  of 
  this 
  superposition 
  

   will 
  be 
  we 
  do 
  not 
  know, 
  but 
  we 
  can 
  at 
  all 
  events 
  see 
  that 
  

   light 
  — 
  E, 
  consisting 
  as 
  it 
  does 
  of 
  ufw's 
  more 
  inclined 
  to 
  one 
  

   another 
  than 
  any 
  of 
  those 
  of 
  light 
  A, 
  may 
  introduce 
  additions 
  

   and 
  defects 
  into 
  the 
  optimum 
  image, 
  some 
  of 
  which 
  may 
  be 
  

   smaller 
  than 
  any 
  of 
  those 
  parts 
  of 
  the 
  telescopic 
  image 
  which 
  

   can 
  represent 
  detail 
  really 
  existing 
  upon 
  the 
  planet. 
  Thus 
  — 
  as 
  

   we 
  shall 
  find 
  farther 
  on 
  — 
  the 
  minute 
  dark 
  specks 
  which 
  have 
  

   been 
  detected 
  upon 
  the 
  image 
  of 
  Mars 
  and 
  to 
  which 
  has 
  been 
  

   given 
  the 
  name 
  of 
  oases 
  under 
  the 
  impression 
  that 
  they 
  

   represent 
  something 
  on 
  the 
  planet, 
  are 
  probably 
  optical 
  

   markings 
  arising 
  in 
  the 
  way 
  described 
  above. 
  Misleading 
  

   appearances 
  of 
  this 
  kind 
  are 
  familiar 
  to 
  microscopists, 
  who 
  

   sometimes 
  speak 
  of 
  them 
  as 
  ' 
  Intercostal 
  Markings.' 
  They 
  

   may 
  perhaps 
  be 
  more 
  appropriately 
  termed 
  Phenako-spiles, 
  

   or 
  misleading 
  markings 
  upon 
  the 
  image 
  presented 
  by 
  the 
  

   telescope, 
  as 
  they 
  are 
  a 
  semblance 
  of 
  detail 
  which 
  has 
  no 
  

   existence 
  upon 
  the 
  object. 
  

  

  It 
  thus 
  appears 
  that 
  what 
  is 
  seen 
  by 
  the 
  Astronomer 
  in 
  his 
  

   telescope 
  represents 
  the 
  celestial 
  object 
  imperfectly 
  ; 
  and 
  

   what 
  we 
  have 
  to 
  aim 
  at 
  in 
  this 
  inquiry 
  is 
  to 
  study 
  the 
  

   excesses 
  and 
  defects 
  in 
  the 
  image, 
  with 
  a 
  view 
  to 
  learning- 
  

   how 
  to 
  minimise 
  them 
  and 
  how 
  to 
  detect 
  and 
  measure 
  and 
  

   make 
  allowance 
  for 
  those 
  that 
  continue 
  outstanding. 
  

  

  12. 
  We 
  may 
  call 
  the 
  image 
  directly 
  formed 
  by 
  the 
  

   objective 
  the 
  primary 
  telescopic 
  image. 
  It 
  is 
  usually 
  on 
  too 
  

   small 
  a 
  scale 
  for 
  the 
  eye 
  to 
  see 
  in 
  it 
  the 
  whole 
  of 
  the 
  detail 
  

   which 
  the 
  objective, 
  if 
  good, 
  can 
  exhibit. 
  It 
  has 
  therefore 
  to 
  

   be 
  magnified 
  by 
  an 
  eyepiece, 
  and 
  we 
  shall 
  use 
  the 
  term 
  eikon 
  

   to 
  signify 
  the 
  resulting 
  larger 
  image 
  which 
  is 
  what 
  is 
  actually 
  

   seen 
  by 
  the 
  observer. 
  The 
  eikon 
  will 
  then 
  be, 
  not 
  the 
  

   primary 
  telescopic 
  image, 
  but 
  an 
  enlargement 
  of 
  it. 
  Again, 
  

   let 
  us 
  use 
  the 
  term 
  eidolon 
  to 
  signify 
  the 
  object 
  which 
  seems 
  

   to 
  the 
  observer 
  to 
  be 
  what 
  he 
  is 
  looking 
  at 
  — 
  in 
  other 
  words, 
  

   the 
  eidolon 
  is 
  a 
  model 
  of 
  the 
  planet, 
  of 
  such 
  a 
  size 
  and 
  with 
  

   such 
  additions 
  imperfections 
  and 
  blurring 
  together 
  of 
  details, 
  

  

  