﻿960 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  g 
  l5 
  and 
  of 
  which 
  the 
  middle 
  point 
  Avill 
  bo 
  the 
  point 
  upon 
  thai 
  

  

  plane 
  which 
  is 
  conjugate 
  to 
  point 
  jOj 
  upon 
  surface 
  W. 
  How 
  

   small 
  the 
  patch 
  is 
  which 
  will 
  be 
  occupied 
  by 
  this 
  image, 
  may 
  

   be 
  seen 
  from 
  the 
  following 
  considerations. 
  The 
  image 
  of 
  a 
  

   point 
  formed 
  by 
  a 
  lens 
  is 
  o\' 
  the 
  same 
  kind 
  as 
  the 
  image 
  which 
  

   we 
  have 
  called 
  a 
  star-bnrst. 
  This 
  image 
  was 
  investigated 
  by 
  

   Airy 
  and 
  consists 
  of 
  a 
  central 
  boss 
  of 
  bright 
  light 
  (sometimes 
  

   called 
  the 
  spurious 
  disk 
  of 
  the 
  star) 
  surrounded 
  by 
  relatively 
  

   faint 
  coloured 
  rings, 
  of 
  which, 
  asually, 
  only 
  a 
  few 
  of 
  the 
  

   inner 
  ones 
  can 
  he 
  seem 
  A 
  very 
  large 
  percentage 
  of 
  the 
  

   light 
  is 
  concentrated 
  into 
  the 
  central 
  bright 
  boss 
  and 
  nearly 
  

   the 
  whole 
  into 
  it 
  and 
  the 
  two 
  inner 
  appendage 
  rings. 
  Now,. 
  

   From 
  Airy'a 
  formulae 
  we 
  find 
  that 
  in 
  the 
  author's 
  apparatus 
  

   the 
  patch 
  occupied 
  by 
  the 
  central 
  boss 
  and 
  its 
  two 
  inner 
  

   appendages 
  is 
  less 
  than 
  a 
  tenth 
  of 
  a 
  millimetre 
  in 
  diameter,, 
  

   and 
  that 
  the 
  central 
  boss 
  in 
  which 
  most 
  of 
  the 
  light 
  is 
  con^ 
  

   centrated 
  is 
  only 
  a 
  low 
  hundredths 
  of 
  a 
  mm. 
  in 
  diameter*. 
  

   We 
  are 
  therefore 
  justified 
  when 
  making 
  experiments 
  in 
  

   regarding 
  the 
  light 
  of 
  each 
  of 
  the 
  u 
  s 
  W's 
  incident 
  upon 
  lens 
  

   [/, 
  as 
  being 
  concentrated 
  into 
  an 
  image 
  small 
  enough 
  to 
  be 
  

   regarded 
  a- 
  a 
  -peck 
  of 
  light 
  upon 
  plane 
  Y'. 
  In 
  the 
  author's 
  

   apparatus 
  there 
  are 
  about 
  inn 
  of 
  these 
  -pecks 
  in 
  every 
  square 
  

   mm. 
  of 
  plane 
  Y 
  : 
  ami, 
  of 
  course, 
  the 
  size 
  of 
  these 
  specks 
  

   can 
  be 
  made 
  a- 
  much 
  smaller 
  as 
  the 
  observer 
  chooses. 
  

   by 
  substituting 
  for 
  1/ 
  a 
  lens 
  of 
  correspondingly 
  larger 
  

   aperture. 
  

  

  72. 
  Accordingly, 
  what 
  the 
  observer 
  will 
  see 
  on 
  looking 
  

   through 
  his 
  Steinlieil 
  lens 
  at 
  B 
  will 
  be 
  the 
  image 
  which 
  we 
  

   have 
  called 
  the 
  concentration 
  image 
  upon 
  plane 
  \ 
  w 
  . 
  This 
  he 
  

   may 
  regard 
  as 
  consisting 
  of 
  the 
  -peck- 
  of 
  light 
  which 
  occupy 
  

   the 
  patches 
  qi, 
  '/._.. 
  <\x. 
  into 
  which 
  plane 
  Y' 
  has 
  been 
  divided. 
  

   lie 
  should 
  then 
  reflect 
  that 
  each 
  one 
  of 
  these 
  specks, 
  as 
  we 
  

   may 
  call 
  them, 
  is 
  the 
  concentrated 
  light 
  of 
  one 
  of 
  a 
  definite 
  

   sel 
  of 
  punctades 
  of 
  light 
  into 
  which 
  the 
  beam 
  of 
  light 
  which 
  

   has 
  advanced 
  to 
  the 
  left 
  of 
  screen 
  7J 
  has 
  been 
  able 
  to 
  resolve 
  

   itself 
  ; 
  and 
  that 
  what 
  he 
  sees 
  on 
  plane 
  Y' 
  supplies 
  the 
  following 
  

   information 
  about 
  that 
  resolution. 
  In 
  the 
  first 
  place 
  the 
  

   position 
  of 
  the 
  speck 
  upon 
  plane 
  Y' 
  — 
  let 
  us 
  suppose 
  that 
  it 
  is 
  

   q 
  l 
  — 
  can 
  be 
  made 
  to 
  indicate 
  to 
  him 
  the 
  direction 
  in 
  which 
  

  

  * 
  The 
  image 
  in 
  the 
  author's 
  apparatus 
  is 
  in 
  fact 
  larger, 
  because 
  

   of 
  the 
  large 
  spherical 
  aberration 
  of 
  telescope 
  objective 
  L' 
  when 
  

   employed 
  to 
  form 
  an 
  image 
  of 
  a 
  near 
  object. 
  This 
  might 
  have 
  been 
  

   corrected 
  by 
  using 
  as 
  lens 
  L', 
  two 
  telescope 
  objectives 
  put 
  back 
  to 
  back, 
  

   but 
  this 
  more 
  elaborate 
  arrangement 
  was 
  not 
  adopted, 
  as 
  the 
  simpler 
  

   arrangement 
  was 
  found 
  to 
  work 
  well 
  in 
  practice. 
  

  

  