﻿Telescopic 
  Vision. 
  961 
  

  

  the 
  component 
  travelled, 
  which 
  has 
  been 
  concentrated 
  into 
  

   that 
  speck 
  *. 
  

  

  In 
  the 
  next 
  place, 
  the 
  intensity 
  at 
  the 
  various 
  situations 
  of 
  

   the 
  concentration 
  image 
  tells 
  the 
  observer 
  the 
  relative 
  inten- 
  

   sities 
  of 
  the 
  several 
  components 
  of 
  the 
  beam 
  which 
  are 
  con- 
  

   centrated 
  into 
  the 
  corresponding 
  specks. 
  

  

  And, 
  thirdly, 
  the 
  colours 
  of 
  the 
  concentration 
  image 
  at 
  

   those 
  situations 
  indicate 
  in 
  some 
  degree 
  the 
  various 
  pro- 
  

   portions 
  in 
  which 
  wave-lengths 
  are 
  mixed 
  in 
  the 
  com- 
  

   ponents, 
  whether 
  they 
  be 
  punctades 
  or 
  stellades. 
  This 
  

   very 
  considerable 
  amount 
  of 
  information 
  is 
  increased 
  when 
  

   the 
  observer 
  adds 
  to 
  it 
  what 
  he 
  may 
  infer 
  from 
  the 
  cir- 
  

   cumstance 
  that 
  all 
  this 
  light 
  has 
  been 
  furnished 
  by 
  the 
  

   single 
  stellade 
  emitted 
  from 
  pro-star 
  s', 
  which 
  is 
  the 
  light 
  

   that 
  illuminates 
  object 
  z' 
  . 
  This 
  light 
  reached 
  z' 
  in 
  the 
  same 
  

   phase 
  and 
  in 
  the 
  same 
  state 
  of 
  polarization 
  throughout 
  each 
  

   wave-front. 
  Accordingly, 
  all 
  the 
  light 
  which 
  advanced 
  

   beyond 
  screen 
  z\ 
  and 
  with 
  which 
  alone 
  we 
  are 
  concerned, 
  

   started 
  from 
  z' 
  in 
  the 
  same 
  phase 
  and 
  in 
  the 
  same 
  state 
  of 
  

   polarization, 
  and 
  accordingly 
  if 
  at 
  any 
  subsequent 
  stage 
  the 
  

   different 
  parts 
  of 
  this 
  light 
  are 
  brought 
  together 
  again 
  after 
  

   having 
  spent 
  equal 
  times 
  in 
  their 
  several 
  journeys 
  from 
  z\ 
  

   they 
  will 
  arrive 
  at 
  that 
  destination 
  (as, 
  for 
  instance, 
  at 
  w 
  f 
  

   in 
  the 
  pro-telescope) 
  under 
  such 
  conditions 
  that 
  they 
  can 
  

   produce 
  there 
  the 
  kind 
  of 
  persistent 
  interference 
  effects 
  of 
  

   which 
  human 
  eyes 
  can 
  take 
  cognizance. 
  This 
  we 
  shall 
  find 
  

   a 
  very 
  important 
  circumstance 
  : 
  upon 
  it 
  depends 
  the 
  com- 
  

   petence 
  of 
  this 
  light 
  to 
  form 
  at 
  x' 
  the 
  image 
  which 
  it 
  displays 
  

   there. 
  

  

  73. 
  An 
  object 
  upon 
  the 
  planet 
  which 
  is 
  small 
  enough 
  to 
  

   subtend 
  not 
  more 
  than 
  a 
  few 
  seconds 
  of 
  angle 
  at 
  the 
  distance 
  

   of 
  the 
  earth 
  may 
  have 
  its 
  outline 
  correctly 
  represented 
  by 
  

   the 
  boundary 
  of 
  a 
  mere 
  hole 
  of 
  the 
  proper 
  shape 
  and 
  size 
  

   at 
  z' 
  . 
  And 
  the 
  experiments 
  that 
  can 
  then 
  be 
  made 
  with 
  it 
  will 
  

  

  * 
  The 
  direction 
  in 
  which 
  that 
  component 
  of 
  the 
  "beam 
  travelled 
  across 
  

   whatever 
  free 
  space 
  was 
  left 
  for 
  it 
  to 
  the 
  left 
  of 
  screen 
  Z', 
  is 
  made 
  known 
  

   to 
  the 
  observer 
  by 
  the 
  position 
  upon 
  plane 
  Y' 
  of 
  the 
  speck 
  of 
  light 
  into 
  

   which 
  the 
  light 
  of 
  that 
  component 
  is 
  concentrated. 
  He 
  has 
  only 
  to 
  . 
  

   picture 
  to 
  himself 
  the 
  ray 
  (of 
  Geometrical 
  Optics) 
  from 
  that 
  speck 
  to 
  the 
  

   optical 
  centre 
  of 
  lens 
  L', 
  with 
  its 
  continuation 
  to 
  the 
  right 
  of 
  lens 
  L'. 
  

   The 
  portions 
  of 
  this 
  ray 
  which 
  are 
  in 
  air 
  are 
  parallel 
  to 
  one 
  another, 
  and 
  

   their 
  direction 
  when 
  reversed 
  indicates 
  the 
  direction 
  in 
  which 
  the 
  com- 
  

   ponent 
  in 
  question 
  was 
  travelling 
  until 
  it 
  encountered 
  lens 
  L' 
  ; 
  that 
  is 
  to 
  

   say, 
  this 
  is 
  the 
  direction 
  in 
  which 
  the 
  stellade 
  advanced, 
  if 
  the 
  resolu- 
  

   tion 
  of 
  the 
  beam 
  is 
  regarded 
  as 
  being 
  into 
  stellades, 
  and 
  it 
  is 
  the 
  

   direction 
  of 
  the 
  central 
  radius 
  of 
  the 
  punctade, 
  if 
  the 
  resolution 
  is 
  into 
  

   punctades. 
  Either 
  of 
  these 
  resolutions 
  is 
  legitimate. 
  

  

  Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  16. 
  No. 
  96. 
  Dec. 
  1908. 
  3 
  E 
  

  

  