﻿966 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  purposes 
  the 
  same 
  whether 
  it 
  comes 
  from 
  the 
  sun 
  or 
  from 
  

   this 
  sheet 
  of 
  stars. 
  Regarding 
  it 
  as 
  coming 
  from 
  the 
  stars, 
  

   we 
  at 
  once 
  see 
  that 
  the 
  sunbeam 
  may 
  be 
  regarded 
  as 
  an 
  

   aggregate 
  of 
  star-beams, 
  one 
  coming 
  through 
  the 
  opening 
  

   in 
  the 
  shutter 
  from 
  each 
  of 
  the 
  stars 
  in 
  the 
  sheet 
  of 
  stars. 
  

   The 
  light 
  which 
  reaches 
  the 
  shutter 
  Erom 
  each 
  star 
  is 
  the 
  

   kind 
  of 
  light 
  we 
  have 
  called 
  a 
  stellade; 
  and 
  lights 
  from 
  any 
  

   two 
  of 
  these 
  stellades, 
  having 
  come 
  from 
  independent 
  sources, 
  

   are 
  Lights 
  incapable 
  of 
  producing 
  with 
  one 
  another 
  visible 
  

   interference 
  effects. 
  One 
  consequence 
  of 
  this 
  is 
  that 
  the 
  

   intensity 
  of 
  the 
  sunbeam 
  at 
  any 
  situation 
  will 
  be 
  the 
  mere 
  

   Mini 
  of 
  the 
  intensities 
  which 
  the 
  starbeams 
  would 
  separately 
  

   produce 
  at 
  that 
  place. 
  

  

  Knowing 
  that 
  light 
  of 
  any 
  kind 
  and 
  therefore 
  that 
  a 
  star- 
  

   beam 
  i- 
  capable 
  <>r 
  resolving 
  itself 
  into 
  components 
  which 
  

   may 
  I)" 
  cither 
  stellades 
  or 
  punctades, 
  we 
  learn 
  in 
  Chapter 
  7 
  

   much 
  about 
  these 
  components. 
  To 
  gain 
  this 
  knowledge 
  the 
  

   observer 
  inspects 
  that 
  concentration 
  image 
  which 
  by 
  the 
  help 
  

   of 
  lens 
  L 
  will 
  be 
  produced 
  upon 
  plane 
  V 
  whenever 
  light 
  

   from 
  pro-star 
  / 
  passes 
  through 
  an 
  opening 
  at 
  .-'and 
  becomes 
  

   a 
  star-beam. 
  Knowing 
  that 
  each 
  luminous 
  speck 
  of 
  this 
  

   concentration 
  image 
  is 
  the 
  concentrated 
  lio-ht 
  of 
  one 
  of 
  the 
  

   components 
  of 
  the 
  star-beam, 
  he 
  is 
  told 
  by 
  the 
  position 
  of 
  

   the 
  speck 
  upon 
  plane 
  Y' 
  in 
  what 
  direction 
  thai 
  component 
  

   travelled 
  between 
  Bcreen 
  Z' 
  and 
  lens 
  I/; 
  and 
  from 
  the 
  

   intensity 
  and 
  colour 
  of 
  the 
  speck 
  he 
  learns 
  what 
  the 
  intensity 
  

  

  and 
  colour 
  of 
  the 
  Component 
  are. 
  When 
  to 
  this 
  information 
  

  

  we 
  add 
  that 
  all 
  parts 
  of 
  the 
  light 
  of 
  this 
  concentration 
  image 
  

   have 
  started 
  from 
  :' 
  in 
  the 
  same 
  phase 
  and 
  in 
  the 
  sinic 
  state 
  

   of 
  polarization 
  at 
  each 
  instant 
  of 
  time, 
  hut 
  that 
  the 
  light 
  

   of 
  any 
  two 
  of 
  the 
  concentration 
  images 
  may 
  have 
  keen 
  

   in 
  various 
  other 
  phases 
  and 
  in 
  various 
  other 
  states 
  of 
  

   polarization 
  — 
  when 
  these 
  particular- 
  are 
  added 
  we 
  have 
  all 
  

   the 
  information 
  we 
  shall 
  need 
  in 
  the 
  inquiry 
  upon 
  which 
  we 
  

   are 
  now 
  to 
  enter. 
  

  

  79. 
  Let 
  light 
  from 
  the 
  pro-star 
  s 
  be 
  allowed 
  to 
  fall 
  on 
  

   screen 
  Z'. 
  A 
  portion 
  of 
  this 
  light 
  passes 
  through 
  the 
  opening 
  

   at 
  z', 
  and 
  becomes 
  a 
  starbeam 
  between 
  screen 
  Z' 
  and 
  lens 
  L', 
  

   and 
  this 
  starbeam 
  by 
  the 
  intervention 
  of 
  lens 
  L/ 
  produces 
  the 
  

   image 
  on 
  plane 
  Y' 
  which 
  we 
  have 
  called 
  the 
  concentration 
  

   image. 
  After 
  forming 
  this 
  image 
  the 
  light 
  continues 
  its 
  

   advance 
  so 
  that 
  the 
  pro-telescope 
  can 
  be 
  placed 
  in 
  such 
  a 
  

   position 
  that 
  the 
  whole 
  or 
  some 
  part 
  of 
  the 
  advancing 
  light 
  

   will 
  enter 
  the 
  pro-telescope, 
  and 
  will 
  then 
  form 
  at 
  a? 
  the 
  

   image 
  of 
  object 
  z 
  which 
  the 
  portion 
  of 
  the 
  light 
  which 
  gains 
  

   admission 
  to 
  the 
  pro-telescope 
  is 
  able 
  to 
  yield. 
  

  

  