﻿320 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  the 
  actual 
  progress 
  of 
  events 
  and 
  can, 
  with 
  least 
  difficulty, 
  

   succeed 
  by 
  deductive 
  reasoning 
  in 
  foreseeing 
  the 
  optical 
  

   effects 
  that 
  will 
  arise. 
  

  

  4. 
  However, 
  in 
  making 
  observations 
  and 
  experiments, 
  it 
  is 
  

   somewhat 
  different 
  : 
  and 
  we 
  shall 
  find 
  it 
  often 
  convenient 
  to 
  

   resolve 
  the 
  light 
  in 
  some 
  other 
  way 
  that 
  is 
  legitimate 
  ; 
  e. 
  g., 
  

   (when 
  dealing 
  with 
  singly 
  refracting 
  media) 
  into 
  undulations 
  

   of 
  spherical 
  wavelets 
  which 
  within 
  a 
  limited 
  volume 
  situated 
  

   at 
  a 
  sufficient 
  distance 
  from 
  the 
  centres 
  of 
  the 
  spheres, 
  will, 
  

   within 
  that 
  space, 
  be 
  undulations 
  of 
  nearly 
  though 
  not 
  quite 
  

   fiat 
  wavelets. 
  Among 
  events 
  of 
  this 
  kind 
  light 
  which 
  reaches 
  

   the 
  earth 
  from 
  a 
  star 
  is 
  by 
  far 
  the 
  nearest 
  approach 
  to 
  light 
  

   consisting 
  of 
  flat 
  waves 
  that 
  is 
  available 
  when 
  we 
  want 
  to 
  

   make 
  actual 
  experiments 
  with 
  it. 
  

  

  5. 
  In 
  treating 
  of 
  vision 
  with 
  the 
  telescope 
  it 
  is 
  found 
  most 
  

   convenient 
  to 
  resolve 
  the 
  light 
  before 
  it 
  enters 
  the 
  telescope 
  

   into 
  a 
  very 
  special 
  system 
  of 
  undulations 
  of 
  spherical 
  

   wavelets, 
  viz. 
  into 
  spherical 
  undulations 
  the 
  centres 
  of 
  which 
  

   shall 
  be 
  the 
  several 
  points 
  of 
  a 
  plane 
  perpendicular 
  to 
  the 
  

   optic 
  axis, 
  situated 
  just 
  in 
  front 
  of 
  the 
  objective 
  of 
  our 
  

   telescope. 
  This 
  plane 
  we 
  shall 
  call 
  plane 
  Y, 
  and 
  the 
  circular 
  

   disk 
  of 
  it 
  in 
  front 
  of 
  the 
  objective 
  we 
  may 
  call 
  disk 
  y, 
  the 
  

   diameter 
  of 
  which 
  will 
  be 
  A, 
  the 
  "Aperture 
  " 
  of 
  the 
  telescope. 
  

   The 
  centres 
  of 
  the 
  undulations, 
  being 
  the 
  points 
  of 
  plane 
  Y, 
  

   are 
  so 
  remote 
  from 
  the 
  object 
  — 
  suppose 
  Mars 
  — 
  that 
  the 
  light 
  

   waves 
  as 
  they 
  leave 
  the 
  planet 
  to 
  converge 
  towards 
  those 
  

   centres 
  are 
  very 
  nearly 
  flat 
  waves 
  in 
  the 
  vicinity 
  of 
  the 
  

   planet 
  ; 
  and, 
  inasmuch 
  as 
  the 
  centres 
  towards 
  which 
  they 
  

   converge 
  are 
  upon 
  a 
  plane 
  close 
  to 
  the 
  objective, 
  they 
  indicate 
  

   at 
  once 
  which 
  of 
  these 
  spherical 
  undulations 
  can 
  and 
  which 
  

   cannot 
  enter 
  the 
  objective. 
  Obviously 
  those 
  converging 
  

   towards 
  points 
  lying 
  within 
  disk 
  y 
  enter 
  the 
  telescope, 
  while 
  

   all 
  the 
  others 
  whose 
  centres 
  are 
  at 
  other 
  points 
  of 
  plane 
  Y 
  

   cannot 
  enter 
  it. 
  Let 
  us 
  fix 
  our 
  attention 
  upon 
  that 
  one 
  of 
  

  

  Disk 
  y. 
  

  

  the 
  spherical 
  undulations 
  which 
  has 
  its 
  centre 
  at 
  the 
  point 
  p 
  

   of 
  disk 
  y. 
  Between 
  Mars 
  and 
  p 
  9 
  the 
  part 
  of 
  this 
  undulation 
  

  

  