﻿Telescopic 
  Vision. 
  969 
  

  

  83. 
  Accordingly, 
  when 
  z' 
  is 
  a 
  hole 
  1 
  mm. 
  in 
  diameter, 
  the 
  

   radius 
  of 
  the 
  whole 
  of 
  the 
  efficient 
  part 
  of 
  its 
  concentration 
  

   image 
  upon 
  plane 
  Y 
  x 
  will 
  be 
  

  

  T 
  = 
  4*2 
  mm. 
  

  

  and 
  the 
  radius 
  of 
  its 
  central 
  boss 
  of 
  very 
  i 
  

   bright 
  light 
  will 
  be 
  

  

  t 
  x 
  = 
  0*854 
  mm. 
  

  

  From 
  these 
  values 
  we 
  can 
  compute 
  what 
  they 
  would 
  become 
  

   if 
  the 
  diameter 
  of 
  the 
  opening 
  at 
  z 
  were 
  made 
  successively 
  

   1J 
  mm. 
  and 
  11*655 
  mm. 
  so 
  as 
  to 
  represent 
  planets 
  Neptune 
  

   and 
  Mars. 
  We 
  thus 
  find 
  that 
  a 
  round 
  hole 
  at 
  z' 
  which 
  would 
  

   represent 
  Neptune 
  in 
  opposition, 
  would 
  make 
  

  

  4T' 
  = 
  12*6 
  mm. 
  1 
  

  

  4^'= 
  2*562 
  mm. 
  J' 
  ^ 
  

  

  and 
  that 
  the 
  hole 
  at 
  z* 
  which 
  would 
  represent 
  Mars 
  when 
  

   nearest 
  to 
  us 
  in 
  1909, 
  i. 
  e. 
  on 
  the 
  18th 
  of 
  September, 
  

   would 
  make 
  

  

  4T" 
  = 
  1*44 
  mm. 
  ~i 
  

  

  4^" 
  = 
  0*293 
  mm. 
  J 
  ' 
  

  

  In 
  writing 
  these 
  we 
  have 
  introduced 
  a 
  single 
  dash 
  when 
  the 
  

   equations 
  refer 
  to 
  Neptune, 
  and 
  a 
  double 
  dash 
  when 
  they 
  

   refer 
  to 
  Mars 
  ; 
  and 
  the 
  radii 
  have 
  been 
  multiplied 
  by 
  4 
  

   because 
  the 
  annexed 
  diagrams 
  need 
  to 
  be 
  on 
  a 
  larger 
  scale 
  

   than 
  the 
  -scale 
  on 
  which 
  the 
  originals 
  present 
  themselves 
  on 
  

   plane 
  Y'. 
  In 
  these 
  diagrams 
  each 
  inch 
  upon 
  the 
  objective 
  

   of 
  the 
  astronomical 
  telescope 
  is 
  represented 
  by 
  a 
  millimetre. 
  

  

  Fig. 
  1. 
  

  

  Concentration 
  images 
  of 
  Neptune 
  and 
  Mars. 
  

  

  In 
  fig. 
  1 
  a 
  1 
  is 
  four 
  times 
  the 
  size 
  of 
  the 
  concentration 
  image 
  

   upon 
  plane 
  Y' 
  when 
  z' 
  represents 
  Neptune, 
  and 
  a" 
  is 
  four 
  

  

  