﻿Telescopic 
  Vision. 
  

  

  325 
  

  

  as 
  would 
  cause 
  it 
  to 
  have, 
  it' 
  viewed 
  by 
  the 
  naked 
  eye 
  from 
  

   the 
  distance 
  of 
  most 
  distinct 
  vision, 
  exactly 
  the 
  same 
  appear- 
  

   ance 
  as 
  the 
  image 
  seen 
  in 
  the 
  telescope. 
  We 
  shall 
  assume, 
  

   in 
  accordance 
  with 
  the 
  usual 
  convention, 
  that 
  the 
  average 
  

   eye 
  finds 
  it 
  most 
  convenient 
  to 
  scrutinise 
  an 
  object 
  critically 
  

   when 
  placed 
  in 
  front 
  of 
  it 
  at 
  a 
  distance 
  of 
  10 
  metric 
  inches, 
  

   which 
  is 
  the 
  same 
  as 
  a 
  quarter 
  of 
  a 
  metre. 
  This 
  then 
  is 
  the 
  

   distance 
  from 
  which 
  we 
  are 
  to 
  conceive 
  the 
  eidolon 
  to 
  be 
  

   viewed 
  and 
  criticised 
  by 
  the 
  unassisted 
  eye 
  of 
  a 
  person 
  with 
  

   good 
  sight. 
  The 
  astronomer 
  should 
  never 
  lose 
  sight 
  of 
  the 
  

   fact 
  that 
  what 
  he 
  sees 
  in 
  his 
  telescope 
  is 
  this 
  eidolon 
  and 
  not 
  

   the 
  distant 
  object 
  ; 
  and 
  it 
  behoves 
  him 
  very 
  carefully 
  to 
  

   discriminate 
  between 
  those 
  features 
  on 
  the 
  eidolon 
  which 
  he 
  

   may 
  rely 
  on 
  as 
  representing 
  somewhat 
  similar 
  details 
  upon 
  

   the 
  planet, 
  and 
  those 
  others 
  which 
  are 
  due 
  to 
  a 
  very 
  different 
  

   cause. 
  

  

  13. 
  It 
  is 
  advantageous 
  to 
  the 
  observer 
  to 
  be 
  readily 
  able 
  to 
  

   form 
  a 
  correct 
  estimate 
  of 
  the 
  actual 
  size 
  of 
  the 
  eidolon, 
  or 
  

   object 
  which 
  appears 
  to 
  be 
  what 
  is 
  seen 
  in 
  his 
  telescope. 
  Its 
  

   size 
  can 
  be 
  easily 
  computed 
  from 
  a 
  number 
  which 
  is 
  recorded 
  

   in 
  the 
  Nautical 
  Almanac 
  for 
  each 
  day, 
  viz. 
  : 
  the 
  number 
  

   of 
  seconds 
  of 
  angle 
  in 
  the 
  apparent 
  semidiameter 
  of 
  the 
  planet 
  

   on 
  that 
  day. 
  Let 
  p" 
  be 
  the 
  number 
  of 
  seconds 
  in 
  the 
  semi- 
  

   diameter 
  of 
  any 
  planet, 
  and 
  M 
  the 
  magnifying 
  power 
  we 
  are 
  

   using 
  upon 
  our 
  telescope. 
  Then 
  the 
  angular 
  diameter 
  of 
  the 
  

   eikon, 
  the 
  image 
  of 
  the 
  planet 
  we 
  see 
  in 
  the 
  telescope, 
  will 
  

   be 
  2M/j 
  ,; 
  ; 
  and 
  the 
  diameter 
  of 
  the 
  eidolon, 
  the 
  object 
  which 
  

   will 
  then 
  seem 
  to 
  be 
  presented 
  to 
  us, 
  will 
  be 
  

  

  ,, 
  . 
  48481.36811 
  

  

  m 
  P 
  dx 
  . 
  

  

  where 
  

  

  48481,36811 
  

  

  "10 
  15 
  

  

  is 
  the 
  value 
  of 
  1" 
  in 
  circular 
  measure, 
  

  

  and 
  d 
  stands 
  for 
  the 
  average 
  distance 
  of 
  most 
  distinct 
  vision. 
  

   We 
  may 
  adopt 
  the 
  usual 
  convention 
  and 
  assume 
  d 
  to 
  be 
  

   250 
  millimetres. 
  Introducing 
  this 
  value 
  we 
  find 
  

  

  M 
  

  

  Diameter 
  of 
  eidolon 
  = 
  ^ 
  p 
  millimetres. 
  

  

  Hence 
  if 
  we 
  arm 
  our 
  telescope 
  with 
  an 
  eyepiece 
  which 
  

   will 
  make 
  M, 
  the 
  magnifying 
  power 
  of 
  the 
  telescope, 
  

   = 
  412*53, 
  we 
  shall 
  have 
  the 
  extremely 
  convenient 
  relation 
  

   that 
  the 
  resulting 
  eidolon 
  of 
  the 
  planet 
  will 
  have 
  the 
  number 
  

   of 
  millimetres 
  in 
  its 
  diameter 
  the 
  same 
  as 
  the 
  number 
  entered 
  

   in 
  the 
  Nautical 
  Almanac 
  as 
  the 
  number 
  of 
  seconds 
  in 
  the 
  

   apparent 
  semidiameter 
  of 
  the 
  planet 
  for 
  the 
  day 
  on 
  which 
  

  

  