﻿Telescopic 
  Vision. 
  953 
  

  

  Geometrical 
  Optics, 
  which 
  is 
  one 
  of 
  our 
  most 
  useful 
  sciences, 
  

   is 
  built 
  upon 
  the 
  supposition 
  that 
  light 
  consists 
  of 
  rays 
  — 
  a 
  

   supposition 
  which 
  it 
  would 
  be 
  an 
  error 
  to 
  mistake 
  for 
  a 
  

   theory 
  of 
  light, 
  that 
  is 
  for 
  an 
  attempt 
  to 
  define 
  what 
  light 
  

   really 
  is. 
  That 
  which 
  makes 
  the 
  supposition 
  deserving 
  of 
  

   our 
  acceptance 
  is 
  that 
  it 
  is 
  eminently 
  useful 
  and 
  therefore 
  

   legitimate 
  as 
  hypothesis. 
  In 
  Geometrical 
  Optics 
  what 
  we 
  

   investigate 
  is 
  the 
  succession 
  of 
  events, 
  not 
  in 
  nature, 
  but 
  in 
  

   a 
  model 
  of 
  nature. 
  We 
  substitute 
  in 
  this 
  model 
  machinery 
  

   more 
  easily 
  handled 
  than 
  any 
  which 
  operates 
  in 
  nature, 
  

   every 
  step 
  in 
  the 
  progress 
  of 
  which 
  can 
  be 
  foretold 
  by 
  the 
  

   application 
  of 
  singularly 
  easy 
  mathematical 
  analysis, 
  can 
  

   be 
  represented 
  by 
  easily 
  understood 
  diagrams, 
  and 
  can 
  be 
  

   imagined 
  and 
  followed 
  without 
  difficulty 
  by 
  students 
  who 
  

   possess 
  but 
  little 
  skill. 
  The 
  justification 
  of 
  this 
  hypothesis 
  

   is 
  that 
  it 
  is 
  so 
  easily 
  handled, 
  and 
  that 
  it 
  yields 
  results 
  

   which 
  are 
  true 
  within 
  ascertainable 
  limits. 
  It 
  can 
  be 
  shown 
  

   that 
  the 
  hypothesis 
  that 
  light 
  consists 
  of 
  rays 
  furnishes 
  

   correctly 
  positions 
  which 
  are 
  in 
  close 
  relation 
  with 
  the 
  

   situations 
  really 
  occupied 
  by 
  the 
  several 
  parts 
  of 
  an 
  optical 
  

   image, 
  although 
  what 
  it 
  offers 
  as 
  the 
  image 
  differs 
  in 
  

   material 
  respects 
  from 
  any 
  real 
  image. 
  Thus 
  'the 
  Geo- 
  

   metrical 
  Image/ 
  as 
  it 
  is 
  called, 
  presents 
  us 
  with 
  an 
  unlimited 
  

   amount 
  of 
  detail, 
  most 
  of 
  which 
  must 
  be 
  regarded 
  as 
  false 
  

   because 
  it 
  is 
  detail 
  which 
  does 
  not 
  exist 
  in 
  the 
  images 
  

   produced 
  by 
  nature. 
  The 
  hypothesis 
  is 
  very 
  useful 
  within 
  

   definable 
  limits, 
  but 
  may 
  seriously 
  mislead 
  if 
  it 
  is 
  misapplied. 
  

   Accordingly, 
  when 
  we 
  seek 
  to 
  ascertain 
  what 
  the 
  image 
  

   really 
  is, 
  we 
  must 
  abandon 
  Geometrical 
  Optics 
  and 
  have 
  

   recourse 
  to 
  an 
  investigation 
  based 
  on 
  what 
  we 
  are 
  entitled 
  to 
  

   accept 
  as 
  theory. 
  

  

  58. 
  If 
  we 
  place 
  at 
  z 
  1 
  in 
  our 
  experimental 
  apparatus 
  (see 
  

   Plate 
  XII.) 
  a 
  small 
  object 
  representing 
  some 
  feature 
  or 
  

   features 
  upon 
  the 
  planet, 
  and 
  if 
  we 
  illuminate 
  it 
  by 
  diffused 
  

   light, 
  we 
  shall 
  see 
  in 
  the 
  pro-telescope 
  at 
  x' 
  an 
  image 
  of 
  

   that 
  object. 
  

  

  The 
  diffused 
  light 
  which 
  is 
  to 
  be 
  incident 
  upon 
  z' 
  may 
  be 
  

   light 
  which 
  has 
  issued 
  from 
  an 
  image 
  of 
  the 
  sun 
  as 
  in 
  § 
  39 
  

   (see 
  p. 
  801 
  of 
  last 
  month's 
  Phil. 
  Mag.), 
  or, 
  when 
  we 
  do 
  not 
  

   need 
  it 
  to 
  be 
  of 
  great 
  intensity, 
  it 
  is 
  often 
  more 
  convenient 
  

   to 
  provide 
  it 
  by 
  a 
  suitably 
  placed 
  piece 
  of 
  illuminated 
  white 
  

   paper 
  or 
  by 
  light 
  reflected 
  from 
  a 
  cloud. 
  Whatever 
  is 
  the 
  

   source 
  of 
  the 
  light 
  it 
  will 
  be 
  convenient 
  to 
  call 
  it 
  a 
  ; 
  and 
  

   what 
  we 
  have 
  now 
  to 
  ascertain 
  is 
  the 
  succession 
  of 
  events 
  by 
  

   ^which 
  a 
  part 
  of 
  the 
  light 
  emitted 
  by 
  a 
  succeeds 
  in 
  becoming 
  

   •an 
  image 
  of 
  z' 
  at 
  s' 
  in 
  the 
  pro-telescope. 
  

  

  