﻿Telescopic 
  Vision, 
  

  

  973 
  

  

  is 
  chiefly 
  characterized 
  by 
  this 
  great 
  preponderance 
  of 
  

   optimum 
  partials. 
  The 
  state 
  o£ 
  things 
  which 
  now 
  prevails 
  

   will 
  furnish 
  an 
  image 
  of 
  the 
  edge 
  of 
  the 
  planet 
  which 
  any 
  

   astronomer 
  would 
  pronounce 
  to 
  be 
  an 
  exceedingly 
  sharp 
  * 
  

   image. 
  Of 
  course 
  this 
  will 
  only 
  happen, 
  if 
  the 
  observation 
  

   is 
  made 
  on 
  one 
  of 
  those 
  rare 
  occasions 
  when 
  ' 
  seeing 
  ' 
  is 
  

   very 
  good. 
  

  

  86. 
  We 
  shall 
  next 
  consider 
  how 
  these 
  same 
  objects 
  would 
  

   appear 
  in 
  telescopes 
  of 
  less 
  aperture, 
  such 
  as 
  those 
  with 
  

   which 
  the 
  most 
  successful 
  observations 
  have 
  hitherto 
  been 
  

   made 
  upon 
  the 
  images 
  of 
  Mars 
  in 
  telescopes. 
  The 
  apertures 
  

   we 
  will 
  consider 
  are 
  apertures 
  of 
  24 
  inches 
  and 
  of 
  12 
  inches. 
  

   Proceeding 
  as 
  before, 
  we 
  find 
  that 
  the 
  following 
  figures 
  

   indicate 
  the 
  proportions 
  in 
  which 
  the 
  four 
  classes 
  of 
  partials 
  

   will 
  be 
  present 
  in 
  these 
  images 
  when 
  the 
  objects 
  under 
  

   •examination 
  are 
  the 
  contours 
  of 
  Neptune 
  and 
  Mars. 
  

  

  Fi-. 
  3. 
  

  

  n 
  

  

  i 
  j 
  

  

  Efficiency 
  of 
  a 
  24-incli 
  telescope 
  upon 
  the 
  outline 
  — 
  

   of 
  Neptune. 
  of 
  Mars. 
  

  

  From 
  fig. 
  3, 
  c', 
  we 
  learn 
  that 
  when 
  Neptune 
  is 
  examined 
  

   with 
  an 
  aperture 
  of 
  24 
  inches, 
  the 
  image 
  will 
  contain 
  no 
  

   partials 
  of 
  class 
  k, 
  i.e. 
  none 
  of 
  the 
  best 
  kind 
  ; 
  but 
  that 
  there 
  

   will 
  be 
  present 
  a 
  considerable 
  contingent 
  of 
  the 
  partials 
  of 
  

   class 
  I 
  (without 
  however 
  the 
  best 
  of 
  that 
  class). 
  Those 
  of 
  

   class 
  I 
  tbat 
  are 
  present 
  would 
  produce 
  a 
  fair 
  image, 
  were 
  it 
  

   not 
  for 
  the 
  large 
  admixture 
  of 
  partials 
  of 
  the 
  other 
  two 
  

  

  * 
  This 
  does 
  not 
  in 
  the 
  least 
  imply 
  that 
  such 
  vastly 
  smaller 
  details 
  as 
  

   the 
  profiles 
  of 
  mountains 
  upon 
  the 
  edge 
  of 
  the 
  planet 
  could 
  approach 
  to 
  

   being 
  made 
  visible 
  by 
  a 
  40-inch 
  telescope. 
  

  

  