﻿Telescopic 
  Vision. 
  

  

  807 
  

  

  Mars, 
  if 
  he 
  will 
  with 
  sufficient 
  diligence 
  make, 
  in 
  connexion 
  

   with 
  his 
  observations 
  upon 
  the 
  planet, 
  control 
  observations 
  

   upon 
  the 
  Moon 
  when 
  nearly 
  full, 
  by 
  examining 
  it 
  as 
  often 
  as 
  

   weather 
  permits 
  on 
  the 
  day 
  it 
  is 
  full 
  and 
  the 
  day 
  before 
  and 
  

   the 
  day 
  after: 
  and 
  under 
  optical 
  conditions 
  the 
  same 
  as 
  those 
  

   to 
  which 
  he 
  is 
  obliged 
  to 
  confine 
  himself 
  in 
  his 
  observations 
  

   upon 
  Mars 
  while 
  at 
  the 
  distance 
  from 
  the 
  Earth 
  at 
  which 
  it 
  

   happens 
  to 
  be 
  at 
  that 
  time. 
  To 
  facilitate 
  his 
  own 
  observa- 
  

   tions 
  of 
  this 
  kind, 
  the 
  author 
  found 
  a 
  table 
  such 
  as 
  the 
  

   following 
  useful, 
  in 
  which 
  D 
  is 
  the 
  distance 
  of 
  Mars 
  from 
  

   the 
  Earth, 
  L 
  is 
  the 
  estimated 
  mean 
  distance 
  of 
  Full-Moon 
  

   from 
  the 
  observer, 
  and 
  e 
  is 
  our 
  standard 
  angle 
  which 
  is 
  

   2"'062648, 
  or 
  in 
  circular 
  measure 
  0*00001. 
  The 
  table 
  

   furnishes 
  the 
  values 
  of 
  a, 
  the 
  aperture 
  of 
  the 
  telescope 
  to 
  be 
  

   used 
  upon 
  the 
  Moon, 
  and 
  of 
  /x 
  the 
  maximum 
  magnifying 
  

   power 
  to 
  be 
  applied 
  to 
  it 
  — 
  this 
  latter 
  being 
  the 
  power 
  which 
  

   produces 
  the 
  same 
  effect 
  upon 
  the 
  Moon 
  as 
  400 
  does 
  when 
  

   employed 
  upon 
  Mars. 
  

  

  Diameter 
  

   of 
  Mars. 
  

  

  Semi- 
  

   Diameter 
  

   of 
  Mars. 
  

  

  D. 
  

  

  Distance 
  

   of 
  Mars. 
  

  

  a. 
  

   Aperture 
  

   to 
  be 
  used 
  

   on 
  Moon. 
  

  

  Maximum 
  

  

  magnifying 
  

  

  power 
  to 
  

  

  be 
  used 
  

  

  on 
  Moon. 
  

  

  12 
  6 
  

  

  12"-38 
  

  

  150 
  L 
  

  

  2 
  mm. 
  

  

  267 
  

  

  10| 
  e 
  

  

  10"-82 
  

  

  171 
  L 
  

  

  If 
  mm. 
  

  

  2-34 
  

  

  9 
  6 
  

  

  9"-28 
  

  

  200 
  L 
  

  

  1^ 
  mm. 
  

  

  2 
  

  

  He 
  

  

  7"-73 
  

  

  240 
  L 
  

  

  1^ 
  mm. 
  

  

  1-40 
  

  

  6 
  6 
  

  

  6"-19 
  

  

  300 
  L 
  

  

  1 
  mm. 
  

  

  1-23 
  

  

  4±6 
  

  

  4"-64 
  

  

  400 
  L 
  

  

  | 
  mm. 
  

  

  1 
  

  

  3 
  e 
  

  

  3"-09 
  

  

  600 
  L 
  

  

  i 
  mm. 
  

  

  0-67 
  

  

  2 
  e 
  

  

  i 
  

  

  2"06 
  

  

  900 
  L 
  

  

  ^ 
  mm. 
  

  

  0-44 
  

  

  This 
  table 
  is 
  somewhat 
  more 
  convenient 
  when 
  thrown 
  into 
  

   the 
  form 
  of 
  a 
  diagram, 
  to 
  be 
  hung 
  up 
  in 
  the 
  Observatory. 
  

   If 
  we 
  use 
  the 
  numbers 
  of 
  the 
  first 
  column 
  as 
  abscissas 
  and 
  

   the 
  distances 
  of 
  Mars 
  as 
  ordinates, 
  the 
  latter 
  will 
  terminate 
  

   in 
  a 
  hyperbola. 
  Again 
  keeping 
  the 
  same 
  abscissas, 
  and 
  

   using 
  the 
  values 
  of 
  a 
  as 
  ordinates, 
  they 
  will 
  terminate 
  in 
  a 
  

   straight 
  line. 
  And, 
  finally, 
  with 
  the 
  same 
  abscissas 
  and 
  the 
  

   values 
  of 
  yrx 
  as 
  ordinates, 
  they 
  also 
  terminate 
  in 
  a 
  straight 
  line. 
  

   Of 
  course 
  the 
  two 
  straight 
  lines 
  and 
  the 
  hyperbola 
  can 
  be 
  

  

  