﻿Telescopic 
  Vision. 
  967 
  

  

  80. 
  The 
  image 
  formed 
  from 
  light 
  that 
  originally 
  came 
  

   from 
  a 
  star 
  or 
  pro-star 
  we 
  shall 
  call 
  a 
  partial 
  image, 
  or 
  

   partial. 
  The 
  partial 
  image 
  cannot 
  be 
  the 
  same 
  as 
  the 
  image 
  

   of 
  the 
  corresponding 
  feature 
  on 
  the 
  planet 
  which 
  would 
  be 
  

   presented 
  to 
  the 
  observer 
  in 
  his 
  astronomical 
  telescope. 
  This 
  

   is 
  because 
  the 
  planet 
  is 
  an 
  opaque 
  body, 
  the 
  superficial 
  

   molecules 
  of 
  which 
  have 
  been 
  rendered 
  luminous 
  by 
  light 
  

   which 
  has 
  reached 
  them 
  from 
  in 
  front, 
  and 
  which 
  has 
  set 
  up 
  

   such 
  activities 
  in 
  their 
  superficial 
  molecules 
  as 
  scatter 
  light 
  in 
  

   all 
  forward 
  directions. 
  To 
  represent 
  this 
  correctly 
  in 
  our 
  

   experimental 
  apparatus 
  the 
  light 
  reaching 
  z' 
  from 
  behind 
  

   must 
  be 
  light 
  that 
  advances 
  in 
  all 
  forward 
  directions, 
  or 
  in 
  

   other 
  words 
  it 
  must 
  be 
  diffused 
  light. 
  And 
  the 
  image 
  of 
  z' 
  

   which 
  is 
  then 
  formed, 
  and 
  which 
  is 
  identical 
  with 
  that 
  formed 
  

   by 
  the 
  corresponding 
  feature 
  on 
  the 
  planet, 
  is 
  entitled 
  to 
  be 
  

   spoken 
  of 
  as 
  the 
  complete 
  image 
  of 
  z' 
  . 
  To 
  form 
  this 
  com- 
  

   plete 
  image 
  in 
  our 
  apparatus 
  it 
  would 
  suffice 
  to 
  illuminate 
  z' 
  

   from 
  behind 
  by 
  light 
  coming 
  from 
  a 
  sheet 
  of 
  pro-stars 
  instead 
  

   of 
  from 
  a 
  single 
  one. 
  This 
  makes 
  it 
  obvious 
  that 
  the 
  com- 
  

   plete 
  image 
  is 
  that 
  image 
  which 
  is 
  formed 
  when 
  all 
  the 
  

   partials 
  formed 
  by 
  the 
  individual 
  pro-stars 
  are 
  simultaneously 
  

   present 
  on 
  disk 
  x\ 
  These 
  partials, 
  however, 
  we 
  can 
  in 
  the 
  

   experimental 
  apparatus 
  study 
  one 
  by 
  one. 
  As 
  they 
  arise 
  

   from 
  light 
  coming 
  from 
  different 
  pro-stars, 
  viz. 
  from 
  da 
  l9 
  

   da 
  2 
  , 
  &c, 
  they 
  come 
  from 
  independent 
  sources 
  of 
  light, 
  and 
  

   therefore 
  when 
  they 
  are 
  all 
  thrown 
  down 
  simultaneously 
  

   upon 
  disk 
  x\ 
  the 
  portions 
  of 
  light 
  which 
  they 
  individually 
  

   contribute 
  and 
  which 
  form 
  images 
  at 
  x 
  l 
  are 
  unable 
  to 
  pro- 
  

   duce 
  further 
  interference 
  effects 
  than 
  those 
  which 
  they 
  

   separately 
  produce. 
  Hence 
  we 
  arrive 
  at 
  the 
  important 
  con- 
  

   clusion 
  that 
  when 
  we 
  have 
  studied 
  what 
  the 
  partials 
  are 
  when 
  

   examined 
  one 
  by 
  one_, 
  we 
  can 
  arrive 
  at 
  the 
  brightness 
  at 
  any 
  

   point 
  of 
  the 
  ' 
  complete 
  ' 
  image, 
  by 
  simply 
  adding 
  together 
  

   the 
  illuminations 
  which 
  will 
  be 
  produced 
  at 
  that 
  point 
  by 
  the 
  

   partials 
  when 
  they 
  are 
  made 
  to 
  present 
  themselves 
  in 
  

   succession. 
  

  

  81. 
  In 
  order 
  to 
  see 
  the 
  general 
  outcome 
  of 
  this 
  state 
  of 
  

   things, 
  we 
  shall 
  examine 
  how 
  the 
  principles 
  that 
  have 
  been 
  

   laid 
  down 
  will 
  work 
  out 
  when 
  applied 
  to 
  a 
  special 
  instance. 
  

   The 
  feature 
  to 
  be 
  seen 
  we 
  shall 
  suppose 
  to 
  be 
  the 
  simple 
  

   boundary 
  of 
  the 
  whole 
  planet, 
  when 
  the 
  planet 
  is 
  1°, 
  Neptune 
  

   in 
  opposition, 
  and 
  2°, 
  Mars 
  when 
  nearest 
  to 
  the 
  earth 
  in 
  1909. 
  

   In 
  other 
  words 
  the 
  objects 
  are 
  nearly 
  circular 
  disks, 
  and 
  

   these 
  we 
  shall 
  represent 
  by 
  circular 
  openings 
  of 
  the 
  proper 
  

   sizes, 
  at 
  z' 
  , 
  respecting 
  which 
  we 
  are 
  to 
  discover 
  of 
  what 
  

   partials 
  their 
  images 
  in 
  the 
  pro-telescope 
  will 
  be 
  built 
  up 
  

  

  