﻿$62 
  Dr. 
  Gr. 
  Johnstone 
  Stonoy 
  on 
  

  

  show 
  what 
  approach 
  the 
  astronomical 
  telescope 
  can 
  make 
  to 
  

   reproducing 
  this 
  outline 
  in 
  its 
  image 
  of 
  the 
  planet. 
  If 
  the 
  

   astronomical 
  telescope 
  shows 
  further 
  detail 
  upon 
  the 
  image 
  

   of 
  the 
  small 
  object, 
  the 
  conditions 
  under 
  which 
  this 
  further 
  

   detail 
  becomes 
  visible 
  must 
  be 
  made 
  the 
  subject 
  of 
  a 
  separate 
  

   investigation. 
  What 
  a 
  simple 
  hole 
  at 
  z 
  suffices 
  to 
  do 
  is 
  to 
  

   investigate 
  the 
  vision 
  which 
  can 
  be 
  obtained 
  of 
  the 
  outline, 
  

   and 
  this 
  is 
  usually 
  what 
  is 
  wanted 
  in 
  practice. 
  In 
  the 
  few 
  

   cases 
  when 
  more 
  is 
  required 
  we 
  must 
  substitute 
  something 
  

   other 
  than 
  a 
  mere 
  hole 
  at 
  : 
  '. 
  Ilowevt 
  r. 
  for 
  the 
  present 
  we 
  

   confine 
  our 
  attention 
  t<> 
  mere 
  holes, 
  and 
  to 
  those 
  of 
  elementary 
  

   Bhapes 
  such 
  as 
  round, 
  rectangular, 
  or 
  triangular. 
  For 
  forms 
  

   less 
  Bimple 
  much 
  the 
  best 
  course 
  is 
  to 
  inspect, 
  and 
  if 
  neces- 
  

   sary 
  measure, 
  the 
  concentration 
  image 
  in 
  the 
  OE 
  apparatus, 
  

   which 
  will 
  then 
  give 
  the 
  astronomer 
  the 
  information 
  he 
  

   requires. 
  

  

  7 
  1. 
  A 
  circular 
  opening 
  is 
  the 
  iirst 
  with 
  which 
  we 
  shall 
  

   deal, 
  inasmuch 
  as 
  with 
  a 
  small 
  hole 
  of 
  this 
  shape 
  in 
  screen 
  Z' 
  

   i: 
  is 
  specially 
  easy 
  to 
  Eoresee 
  what 
  the 
  optical 
  effect 
  on 
  

   plane 
  Y' 
  will 
  be, 
  when 
  light 
  from 
  the 
  pro-star 
  s' 
  is 
  admitted 
  

   through 
  that 
  opening. 
  For, 
  in 
  the 
  first 
  place, 
  since 
  object 
  

   : 
  is 
  small 
  the 
  light 
  which 
  reaches 
  it 
  from 
  the 
  pro-star 
  is 
  not 
  

   appreciably 
  distinguishable 
  from 
  the 
  light 
  which 
  would 
  reach 
  

   it 
  from 
  a 
  Luminous 
  point 
  on 
  surface 
  W, 
  and 
  accordingly 
  we 
  

   may 
  regard 
  the 
  light 
  which 
  reaches 
  z' 
  as 
  light 
  that 
  has 
  

   emanated 
  from 
  that 
  luminous 
  point, 
  which 
  we 
  may 
  call 
  w. 
  

   Again, 
  objective 
  1/ 
  and 
  the 
  Steinheil 
  lens 
  at 
  B 
  may 
  be 
  

   regarded 
  as 
  the 
  objective 
  and 
  eyepiece 
  of 
  a 
  telescope 
  with 
  

   which 
  the 
  observer 
  is 
  looking 
  at 
  luminous 
  point 
  w. 
  The 
  

   aperture 
  of 
  his 
  tele-cope 
  is 
  limited 
  hy 
  the 
  size; 
  of 
  the 
  round 
  

   hole 
  ;', 
  which 
  we 
  -hall 
  suppose 
  cuts 
  it 
  down 
  to 
  being 
  one 
  

   millimetre. 
  It 
  thus 
  appeals 
  that 
  what 
  the 
  observer 
  sees 
  

   through 
  his 
  Steinheil 
  lens 
  is 
  the 
  image 
  of 
  luminous 
  point 
  w 
  

   which 
  the 
  tele-cope 
  just 
  described 
  has 
  been 
  able 
  to 
  form 
  

   when 
  its 
  aperture 
  is 
  only 
  1 
  mm. 
  Now 
  the 
  image 
  of 
  a 
  

   luminous 
  point 
  in 
  a 
  telescope 
  is 
  of 
  the 
  same 
  kind 
  as 
  the 
  

   image 
  of 
  a 
  star. 
  It 
  is 
  the 
  kind 
  of 
  image 
  which 
  we 
  have 
  

   called 
  a 
  star-burst, 
  all 
  the 
  principal 
  details 
  of 
  which 
  are 
  

   known 
  to 
  us 
  from 
  Airy's 
  investigation, 
  and 
  the 
  dimensions 
  

   of 
  which 
  can 
  he 
  computed 
  by 
  his 
  formulas. 
  The 
  image 
  

   consists 
  of 
  a 
  very 
  bright 
  central 
  boss 
  of 
  nearly 
  white 
  light, 
  

   often 
  spoken 
  of 
  as 
  the 
  spurious 
  disk 
  of 
  the 
  star, 
  surrounded 
  

   by 
  coloured 
  rings 
  which 
  are 
  so 
  much 
  fainter 
  and 
  so 
  rapidly 
  

   deteriorate 
  in 
  brightness, 
  that 
  Airy's 
  formulas 
  only 
  give 
  full 
  

   particulars 
  for 
  the 
  central 
  boss 
  and 
  the 
  two 
  inner 
  rings 
  which 
  

   are 
  nearly 
  all 
  that 
  can 
  he 
  seen 
  of 
  the 
  phenomenon 
  in 
  astro- 
  

  

  