﻿970 
  

  

  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  times 
  what 
  its 
  size 
  becomes 
  when 
  :' 
  represents 
  Mars. 
  Fig. 
  2 
  

   will 
  illustrate 
  what 
  kind 
  of 
  vision 
  can 
  be 
  obtained 
  of 
  the 
  

   outline 
  of 
  these 
  planets 
  it' 
  examined 
  with 
  a 
  telescopeof 
  which 
  

  

  Pkr. 
  2. 
  

  

  V 
  

  

  n 
  

  

  y 
  

  

  Efficiency 
  of 
  a 
  40-inch 
  telescope 
  upon 
  the 
  outline 
  — 
  

   of 
  Neptune. 
  

  

  of 
  Mars. 
  

  

  the 
  aperture 
  i- 
  a 
  metre 
  (i. 
  e. 
  40 
  metric 
  inches), 
  and 
  if 
  the 
  

   optical 
  .stuff 
  of 
  our 
  atmosphere 
  is 
  sufficiently 
  good, 
  which 
  it 
  of 
  

   course 
  very 
  rarely 
  is. 
  In 
  this 
  diagram 
  the 
  continuous 
  line 
  is 
  

   the 
  circumference 
  <>i 
  the 
  objective, 
  and 
  the 
  dotted 
  circles 
  

   separate 
  it 
  and 
  the 
  neighbouring 
  space 
  into 
  four 
  regions 
  

   k 
  I 
  m 
  and 
  n. 
  The 
  dotted 
  lines 
  nearest 
  to 
  the 
  continuous 
  

   line 
  are 
  at 
  distances 
  from 
  it 
  =At 
  Y 
  ' 
  in 
  the 
  left-hand 
  figure, 
  

   and 
  = 
  4^" 
  in 
  the 
  right-hand 
  figure; 
  while 
  the 
  outermost 
  and 
  

   the 
  innermost 
  circles 
  are 
  distant 
  from 
  the 
  outline 
  of 
  the 
  

   objective, 
  at 
  distances 
  from 
  it 
  = 
  4T' 
  in 
  the 
  left-hand 
  figure, 
  

   and 
  = 
  4T" 
  in 
  the 
  right-hand 
  figure*. 
  

  

  * 
  In 
  the 
  right-hand 
  diagrams 
  of 
  figs. 
  2, 
  3, 
  and 
  4, 
  the 
  distances 
  of 
  the 
  

   four 
  dotted 
  circles 
  from 
  the 
  continuous 
  circle 
  should 
  be 
  the 
  lengths 
  given 
  

   for 
  4T" 
  and 
  4t" 
  in 
  equation 
  (7). 
  Of 
  these 
  only 
  the 
  more 
  distant 
  dotted 
  

   circles, 
  at 
  the 
  distance 
  4T" 
  from 
  the 
  edge 
  of 
  the 
  objective, 
  can 
  be 
  repre- 
  

   sented 
  in 
  the 
  diagrams. 
  The 
  other 
  two, 
  between 
  which 
  lies 
  region 
  m, 
  can 
  

   be 
  imagined 
  though 
  too 
  close 
  to 
  the 
  continuous 
  circle 
  to 
  be 
  drawn. 
  

  

  