﻿1*7-1- 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  classes, 
  m 
  and 
  n, 
  which 
  are 
  the 
  kinds 
  of 
  partials 
  which 
  most 
  

   tend 
  to 
  spoil 
  an 
  image. 
  Owing 
  to 
  their 
  presence 
  the 
  image 
  

   of 
  the 
  edge 
  of 
  planet 
  Neptune 
  as 
  seen 
  in 
  a 
  24-inch 
  telescope 
  

   cannot 
  be 
  assigned 
  a 
  higher 
  place 
  than 
  that 
  of 
  a 
  barely 
  fair 
  

   image 
  of 
  that 
  feature 
  of 
  the 
  planet. 
  

  

  8-7. 
  The 
  state 
  <^' 
  things 
  will 
  oi' 
  couv^\ 
  become 
  worse, 
  if 
  the 
  

   aperture 
  of 
  the 
  telescope 
  is 
  cut 
  down 
  to 
  12 
  inches. 
  What 
  

   will 
  then 
  happen 
  is 
  depicted 
  in 
  fig. 
  4 
  </'. 
  From 
  this 
  diagram 
  

  

  Kg. 
  4. 
  

  

  n 
  

  

  d" 
  

  

  Efficiency 
  of 
  a 
  12-inch 
  telescope 
  upon 
  the 
  outline 
  — 
  

   Neptune. 
  of 
  Mars. 
  

  

  e 
  that 
  the 
  image 
  of 
  Neptune's 
  outline 
  is 
  made 
  up 
  chiefly 
  

   of 
  partials 
  of 
  the 
  two 
  bad 
  classes 
  m 
  and 
  n 
  9 
  superposed 
  upon 
  

   only 
  a 
  few 
  of 
  those 
  of 
  class 
  /, 
  and 
  they 
  only 
  the 
  less 
  efficient 
  

   of 
  this 
  class. 
  The 
  resulting 
  * 
  complete 
  image 
  ' 
  will 
  there- 
  

   fore 
  poorly 
  represent 
  the 
  real 
  margin 
  of 
  the 
  planet. 
  

  

  B8. 
  As 
  before, 
  we 
  shall 
  find 
  a 
  great 
  contrast 
  to 
  the 
  state 
  

   of 
  things 
  described 
  in 
  the 
  last 
  few 
  paragraphs 
  when 
  we 
  turn 
  

   the 
  same 
  telescopes 
  upon 
  planet 
  Mars. 
  The 
  visions 
  which 
  

   they 
  will 
  afford 
  of 
  the 
  outline 
  of 
  Mars 
  are 
  expressed 
  diagram- 
  

   matically 
  in 
  fig. 
  3 
  c" 
  and 
  fig. 
  4 
  d" 
  . 
  From 
  these 
  we 
  learn 
  

   that 
  even 
  with 
  the 
  12-inch 
  telescope, 
  the 
  image 
  of 
  the 
  con- 
  

   tour 
  of 
  Mars 
  when 
  nearest 
  the 
  earth 
  in 
  1909, 
  will 
  be 
  decidedly 
  

   well 
  seen 
  on 
  a 
  good 
  night. 
  The 
  image 
  will 
  include 
  a 
  

   considerable 
  number 
  of 
  the 
  h 
  partials 
  — 
  the 
  partials 
  of 
  the 
  

   best 
  kind 
  — 
  with 
  not 
  a 
  very 
  large 
  admixture 
  of 
  partials 
  of 
  the 
  

   three 
  inferior 
  kinds 
  I 
  m 
  and 
  n. 
  With 
  the 
  24-inch 
  telescope 
  

   this 
  satisfactory 
  state 
  of 
  things 
  will 
  be 
  improved 
  on. 
  The 
  

   proportion 
  in 
  which 
  the 
  k 
  partials 
  will 
  now 
  stand 
  to 
  the 
  three 
  

   other 
  classes 
  will 
  have 
  become 
  a 
  much 
  greater 
  proportion, 
  

   with 
  a 
  corresponding 
  improvement 
  in 
  the 
  resulting 
  '• 
  complete 
  

  

  