﻿322 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  inquiry, 
  and 
  which 
  is 
  based 
  on 
  the 
  circumstance 
  that 
  it 
  is 
  

   legitimate 
  to 
  conceive 
  any 
  change 
  to 
  be 
  made 
  at 
  the 
  Mars 
  

   end 
  of 
  our 
  series 
  of 
  events 
  which 
  will 
  preserve 
  the 
  image 
  in 
  

   our 
  telescope 
  without 
  sensible 
  alteration. 
  One 
  such 
  change 
  

   would 
  be 
  if 
  Mars 
  were 
  removed 
  and 
  a 
  pro-Mars 
  substituted 
  

   for 
  it, 
  this 
  pro-Mars 
  being 
  a 
  flat 
  transparent 
  disk 
  set 
  up 
  close 
  

   to 
  Mars 
  and 
  perpendicular 
  to 
  the 
  line 
  of 
  sight 
  from 
  us 
  to 
  

   Mars. 
  We 
  may 
  conceive 
  it 
  to 
  be 
  of 
  the 
  same 
  size 
  as 
  the 
  

   great 
  circle 
  upon 
  Mars 
  which 
  bounds 
  our 
  vision 
  of 
  the 
  

   planet. 
  Upon 
  this 
  disk 
  are 
  to 
  be 
  delineated 
  in 
  transparent 
  

   paints 
  — 
  i. 
  e. 
  in 
  paints 
  visible 
  by 
  transmitted 
  light 
  — 
  all 
  the 
  

   details 
  upon 
  the 
  planet 
  which 
  an 
  artist 
  placed 
  upon 
  the 
  disk 
  

   would 
  see 
  perpendicularly 
  under 
  him 
  if 
  he 
  moved 
  over 
  the 
  

   disk 
  from 
  place 
  to 
  place. 
  We 
  may 
  now 
  conceive 
  Mars 
  to 
  be 
  

   removed 
  and 
  this 
  picture 
  illuminated 
  from 
  behind 
  to 
  be 
  sub- 
  

   stituted, 
  since 
  the 
  painting 
  would 
  then 
  present 
  to 
  the 
  terrestrial 
  

   observer 
  precisely 
  the 
  same 
  appearance 
  as 
  Mars 
  does. 
  Both 
  

   the 
  planet 
  itself 
  and 
  this 
  pro-Mars 
  would 
  obviously 
  contain 
  

   an 
  enormous 
  quantity 
  of 
  more 
  minute 
  detail 
  than 
  any 
  that 
  

   can 
  be 
  seen 
  from 
  the 
  great 
  distance 
  of 
  our 
  earth. 
  

  

  9. 
  The 
  light 
  from 
  behind 
  which 
  we 
  are 
  to 
  suppose 
  illumi- 
  

   nating 
  the 
  pro-Mars, 
  like 
  every 
  other 
  transmission 
  of 
  light 
  

   through 
  a 
  medium, 
  is 
  susceptible 
  of 
  being 
  resolved 
  into 
  

   undulations 
  of 
  flat 
  wavelets 
  ; 
  and 
  it 
  is 
  possible 
  to 
  determine 
  

   the 
  effect 
  which 
  any 
  one 
  of 
  these 
  would 
  have 
  if 
  it 
  acted 
  

   alone, 
  by 
  picturing 
  to 
  ourselves 
  the 
  pro- 
  Mars 
  as 
  illuminated 
  

   from 
  behind 
  by 
  a 
  single 
  bright 
  star 
  properly 
  situated, 
  and 
  by- 
  

   considering 
  what 
  the 
  optical 
  effect 
  would 
  then 
  be. 
  If 
  this 
  

   state 
  of 
  things 
  could 
  be 
  realised, 
  the 
  light 
  incident 
  upon 
  the 
  

   pro-Mars 
  from 
  behind 
  would 
  behave 
  as 
  a 
  single 
  undulation 
  

   of 
  flat 
  waves 
  of 
  light 
  ; 
  and 
  this 
  in 
  passing 
  through 
  the 
  

   pro-Mars 
  would 
  — 
  because 
  of 
  the 
  details 
  upon 
  that 
  object- 
  

   develop 
  into 
  a 
  sheaf 
  of 
  innumerable 
  ufw's 
  travelling- 
  

   forwards 
  in 
  definite 
  directions, 
  one 
  of 
  which 
  may 
  be 
  called 
  

   the 
  direct 
  light 
  and 
  the 
  others 
  the 
  diverted 
  lights 
  associated 
  

   with 
  it. 
  If 
  by 
  any 
  contrivance 
  the 
  proper 
  parts 
  of 
  all 
  

   these 
  undulations 
  could 
  be 
  combined, 
  by 
  being 
  made 
  to 
  fall 
  

   upon 
  a 
  single 
  disk 
  after 
  having 
  occupied 
  identical 
  times 
  in 
  

   their 
  several 
  journeys, 
  they 
  would 
  exhibit 
  on 
  that 
  disk 
  by 
  

   their 
  mutual 
  interferences 
  an 
  optimum 
  image 
  of 
  the 
  pro-Mars, 
  

   i. 
  e. 
  the 
  best 
  image 
  which 
  the 
  light 
  emitted 
  by 
  the 
  pro-Mars 
  is 
  

   competent 
  to 
  produce 
  at 
  that 
  distance. 
  

  

  10. 
  In 
  order 
  to 
  make 
  the 
  transmission 
  forwards 
  of 
  these 
  

   undulations 
  compatible 
  with 
  the 
  formation 
  of 
  the 
  Concen- 
  

   tration 
  Image 
  upon 
  plane 
  Y 
  which 
  we 
  shall 
  need 
  to 
  assist 
  us 
  

   in 
  our 
  further 
  study, 
  we 
  may 
  conceive 
  a 
  lens 
  L 
  of 
  immensely 
  

  

  

  