﻿Telescopic 
  Vision. 
  959 
  

  

  regard 
  these 
  as 
  the 
  centres 
  of 
  u 
  s 
  W's 
  (undulations 
  of 
  spherical 
  

   waves) 
  each 
  of 
  which 
  will 
  take 
  the 
  place 
  of 
  a 
  sheaf 
  of 
  the 
  

   us 
  w's 
  and 
  may 
  be 
  bright 
  enough 
  to 
  be 
  seen. 
  It 
  is 
  accord- 
  

   ingly 
  into 
  these 
  u 
  s 
  W's 
  that, 
  when 
  dealing 
  with 
  experiments, 
  

   we 
  shall 
  resolve 
  the 
  light 
  between 
  object 
  z' 
  and 
  lens 
  L'. 
  

  

  69. 
  Letp 
  1? 
  p 
  2 
  , 
  &c. 
  be 
  the 
  numerous 
  but 
  not 
  innumerable 
  

   points 
  which 
  we 
  have 
  supposed 
  to 
  be 
  distributed 
  over 
  surface 
  

   W, 
  and 
  which 
  are 
  the 
  centres 
  of 
  the 
  spherical 
  undulations 
  

   into 
  which 
  we 
  have 
  resolved 
  the 
  light 
  within 
  the 
  limited 
  

   space 
  between 
  screen 
  7/ 
  and 
  lens 
  L'. 
  As 
  this 
  light 
  contains 
  

   light 
  of 
  all 
  the 
  wave-lengths 
  present 
  in 
  white 
  light, 
  there 
  will, 
  

   in 
  general, 
  be 
  spherical 
  undulations 
  of 
  some 
  or 
  all 
  of 
  these 
  

   wave-lengths 
  emanating 
  from 
  each 
  of 
  the 
  p's. 
  Any 
  complete 
  

   System 
  of 
  undulations 
  of 
  the 
  different 
  wave-lengths 
  with 
  

   which 
  we 
  are 
  concerned, 
  and 
  all 
  of 
  them 
  consisting 
  of 
  

   spherical 
  waves 
  which 
  have 
  one 
  common 
  centre 
  we 
  shall 
  find 
  

   It 
  convenient 
  to 
  call 
  a 
  punctade 
  of 
  light, 
  which 
  will 
  then 
  mean 
  

   light 
  of 
  the 
  same 
  kind 
  as 
  that 
  which 
  issues 
  from 
  a 
  luminous 
  

   punctum 
  or 
  optical 
  point, 
  whether 
  white 
  or 
  coloured. 
  

   Accordingly 
  we 
  may 
  regard 
  the 
  light 
  within 
  the 
  limited 
  

   space 
  between 
  screen 
  Z' 
  and 
  lens 
  1/ 
  as 
  resolved 
  into 
  punctades 
  

   Pi 
  P 
  2 
  , 
  &c, 
  which 
  have 
  their 
  centres 
  at 
  the 
  points 
  pip 
  2 
  , 
  &c; 
  

   and 
  each 
  of 
  these 
  punctades 
  will 
  consist 
  of 
  the 
  u 
  s 
  W's 
  of 
  

   different 
  wave-lengths 
  into 
  which 
  w 
  T 
  e 
  may 
  resolve 
  white 
  or 
  

   coloured 
  light 
  emanating 
  from 
  a 
  point. 
  

  

  70. 
  Having 
  got 
  so 
  far, 
  we 
  shall 
  facilitate 
  the 
  next 
  step 
  of 
  

   our 
  inquiry 
  by 
  substituting 
  instead 
  of 
  the 
  real 
  source 
  of 
  light 
  

   *', 
  and 
  the 
  obstruction 
  it 
  meets 
  with 
  at 
  screen 
  Z' 
  ? 
  the 
  much 
  

   simpler 
  machinery 
  which 
  we 
  can 
  imagine 
  by 
  making 
  the 
  

   hypothesis 
  that 
  this 
  source 
  of 
  light 
  and 
  that 
  obstruction 
  have 
  

   been 
  removed, 
  and 
  that 
  at 
  the 
  same 
  time 
  the 
  puncta 
  p 
  x 
  p 
  2 
  , 
  

   &c. 
  upon 
  surface 
  W 
  have 
  been 
  rendered 
  luminous 
  and 
  are 
  

   now 
  what 
  emit 
  the 
  light 
  which 
  exists 
  between 
  plane 
  Z' 
  and 
  

   lens 
  I/. 
  This 
  is 
  a 
  hypothesis 
  of 
  the 
  same 
  legitimate 
  kind 
  as 
  

   the 
  familiar 
  one 
  which 
  we 
  make 
  when 
  we 
  find 
  it 
  convenient 
  

   to 
  treat 
  light 
  reflected 
  from 
  a 
  plane 
  mirror 
  as 
  having 
  come 
  

   directly 
  from 
  the 
  virtual 
  images 
  of 
  the 
  objects 
  in 
  front 
  of 
  the 
  

   mirror 
  instead 
  of 
  indirectly 
  from 
  those 
  objects 
  themselves. 
  

  

  71. 
  When 
  we 
  view 
  matters 
  in 
  this 
  way 
  we 
  see 
  that 
  these 
  

   punctades 
  are 
  incident 
  upon 
  lens 
  I/, 
  and 
  accordingly 
  that 
  

   lens 
  L' 
  will 
  concentrate 
  the 
  light 
  of 
  any 
  one 
  of 
  them, 
  suppose 
  

   of 
  P 
  l5 
  into 
  an 
  image 
  of 
  the 
  kind 
  which 
  that 
  lens 
  is 
  capable 
  of 
  

   forming 
  of 
  the 
  luminous 
  punctum 
  from 
  which 
  we 
  have 
  asked 
  

   the 
  reader 
  to 
  regard 
  the 
  light 
  of 
  this 
  punctade 
  as 
  having 
  

   issued. 
  This 
  will 
  be 
  into 
  an 
  image 
  which 
  we 
  may 
  call 
  Q 
  1( 
  , 
  

   formed 
  upon 
  a 
  small 
  patch 
  of 
  plane 
  Y', 
  which 
  w 
  T 
  e 
  shall 
  call 
  

  

  