﻿Telescopic 
  Vision. 
  965 
  

  

  produces 
  the 
  concentration 
  image, 
  or 
  some 
  part 
  of 
  this 
  light, 
  

   which 
  afterwards 
  develops 
  the 
  image 
  of 
  object 
  z' 
  which 
  is 
  

   seen 
  at 
  x 
  l 
  in 
  the 
  pro-telescope 
  ; 
  and 
  if 
  z' 
  is 
  of 
  an 
  angular 
  

   shape, 
  the 
  light 
  of 
  the 
  more 
  distant 
  spectra 
  must 
  be 
  ad- 
  

   mitted 
  to 
  the 
  pro-telescope 
  in 
  order 
  that 
  the 
  vertices 
  in 
  the 
  

   angular 
  image 
  may 
  be 
  seen 
  sharply 
  defined. 
  If 
  the 
  aperture 
  

   of 
  the 
  pro-telescope 
  is 
  unable 
  to 
  admit 
  the 
  light 
  of 
  these 
  

   distant 
  spectra, 
  the 
  angles 
  in 
  the 
  image 
  of 
  z' 
  will 
  appear 
  

   blunt 
  and 
  not 
  reaching 
  out 
  as 
  far 
  as 
  they 
  should. 
  This 
  

   seems 
  to 
  be 
  the 
  case 
  with 
  most 
  if 
  not 
  all 
  of 
  the 
  polygonal 
  

   spiles 
  in 
  the 
  image 
  of 
  Mars 
  which 
  are 
  bounded 
  by 
  the 
  dark 
  

   streaks 
  in 
  the 
  image 
  which 
  have 
  been 
  called 
  canals. 
  And 
  

   this 
  is 
  a 
  state 
  of 
  things 
  which 
  must 
  apparently 
  cause 
  dark 
  

   specks 
  to 
  present 
  themselves 
  at 
  the 
  angles, 
  and 
  may 
  be 
  the 
  

   true 
  account 
  of 
  the 
  dark 
  specks 
  upon 
  Mars 
  which 
  have 
  been 
  

   interpreted 
  as 
  representing 
  oases 
  upon 
  the 
  planet. 
  

  

  76. 
  Before 
  closing 
  this 
  chapter 
  it 
  may 
  be 
  well 
  to 
  refer 
  to 
  

   a 
  property 
  common 
  to 
  all 
  concentration 
  images, 
  which 
  the 
  

   reader 
  is 
  requested 
  to 
  keep 
  constantly 
  in 
  view 
  when 
  inves- 
  

   tigating 
  the 
  process 
  by 
  which 
  the 
  light 
  emitted 
  by 
  objects 
  at 
  

   z' 
  forms 
  images 
  at 
  x 
  1 
  in 
  the 
  pro-telescope. 
  This 
  property 
  is 
  

   that 
  objects 
  at 
  z 
  f 
  which 
  are 
  similar 
  but 
  of 
  different 
  sizes 
  will 
  

   give 
  rise 
  to 
  concentration 
  images 
  on 
  plane 
  Y' 
  which 
  will 
  

   also 
  be 
  similar, 
  but 
  of 
  linear 
  dimensions 
  which 
  will 
  be 
  

   inversely 
  as 
  the 
  linear 
  dimensions 
  of 
  the 
  objects. 
  See 
  fig. 
  1, 
  

   § 
  83 
  (p. 
  969). 
  

  

  The 
  concentration 
  image 
  has 
  a 
  double 
  meaning. 
  It 
  as 
  it 
  

   were 
  looks 
  backwards 
  and 
  analyses 
  for 
  us 
  the 
  beam 
  of 
  light 
  

   which 
  has 
  passed 
  through 
  the 
  opening 
  at 
  z 
  1 
  ; 
  and 
  it 
  looks 
  

   forward 
  and 
  furnishes 
  the 
  information 
  about 
  the 
  image 
  in 
  

   the 
  telescope 
  which 
  will 
  occupy 
  our 
  attention 
  in 
  the 
  next 
  

   chapter. 
  

  

  CHAPTER 
  8. 
  

  

  How 
  a 
  Telescopic 
  Image 
  is 
  formed. 
  

  

  78. 
  By 
  combining 
  what 
  we 
  have 
  learned 
  in 
  the 
  last 
  two 
  

   chapters, 
  we 
  obtain 
  a 
  complete 
  reply 
  to 
  the 
  second 
  of 
  the 
  

   three 
  questions 
  propounded 
  in 
  § 
  59, 
  in 
  which 
  we 
  are 
  asked 
  

   how 
  a 
  beam 
  of 
  light 
  is 
  to 
  be 
  analysed. 
  If, 
  for 
  example, 
  the 
  

   beam 
  has 
  come 
  from 
  the 
  sun 
  through 
  a 
  hole 
  in 
  a 
  shutter, 
  

   which 
  opening 
  may 
  be 
  of 
  any 
  form, 
  we 
  are 
  first 
  to 
  conceive 
  

   that 
  a 
  sheet 
  of 
  star-like 
  bodies, 
  sufficiently 
  close 
  together 
  

   and 
  sufficiently 
  bright, 
  take 
  the 
  place 
  of 
  the 
  sun 
  as 
  the 
  source 
  

   of 
  light. 
  In 
  Chapter 
  6 
  we 
  found 
  that 
  the 
  light 
  reaching 
  an 
  

   opening 
  of 
  limited 
  size 
  in 
  the 
  shutter 
  will 
  be 
  for 
  practical 
  

  

  