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  XX 
  Y. 
  Telescopic 
  Vision. 
  

   By 
  G. 
  Johnstone 
  Stoney, 
  M.A., 
  Sc.D., 
  F.R.S* 
  

  

  Part 
  I. 
  — 
  The 
  necessary 
  Apparatus. 
  

   [Plate 
  XII.] 
  

  

  CHAPTER 
  1. 
  

  

  A 
  cosmical 
  apparatus 
  wJtich, 
  if 
  it 
  could 
  be 
  realised, 
  woidd 
  

  

  give 
  the 
  information 
  we 
  seek. 
  

   In 
  this 
  paper 
  : 
  — 
  

  

  1. 
  A 
  wavelet 
  will 
  mean 
  a 
  luminous 
  wave 
  of 
  infinitesimal 
  intensity. 
  

  

  2. 
  An 
  undulation 
  will 
  usually 
  mean 
  a 
  simple 
  undulation, 
  that 
  is 
  a 
  

  

  complete 
  train 
  of 
  similar 
  waves 
  or 
  wavelets. 
  

  

  3. 
  ufw 
  is 
  employed 
  as 
  a 
  short 
  way 
  of 
  writing 
  undulation 
  of 
  flat 
  

  

  wavelets 
  of 
  light, 
  of 
  infinitesimal 
  intensity. 
  

  

  4. 
  Similarly, 
  uf 
  W 
  will 
  mean 
  an 
  undulation 
  of 
  flat 
  waves 
  of 
  light, 
  

  

  of 
  finite 
  intensity, 
  

   o. 
  A 
  sheaf 
  of 
  undulations 
  will 
  mean 
  all 
  the 
  undulations 
  that 
  fulfil 
  

   some 
  condition 
  which 
  will 
  he 
  stated. 
  The 
  term 
  is 
  often 
  used 
  of 
  

   u 
  f 
  w's 
  travelling 
  nearly 
  in 
  the 
  same 
  direction 
  — 
  or,more 
  precisely, 
  

   whose 
  guide 
  lines 
  (lines 
  perpendicular 
  to 
  the 
  wavelets) 
  are 
  

   parallel 
  to 
  lines 
  drawn 
  within 
  a 
  very 
  acute 
  cone, 
  from 
  its 
  vertex. 
  

  

  1. 
  npHE 
  opposition 
  of 
  Mars 
  in 
  September 
  1909 
  will 
  occur 
  

   JL 
  when 
  Mars 
  is 
  near 
  perihelion 
  and 
  the 
  earth 
  near 
  

   aphelion: 
  when, 
  also, 
  the 
  planet 
  will 
  be 
  sufficiently 
  high 
  

   in 
  the 
  sky 
  for 
  convenient 
  observation 
  from 
  all 
  the 
  obser- 
  

   vatories 
  of 
  the 
  southern 
  hemisphere, 
  and 
  from 
  most 
  of 
  those 
  

   in 
  the 
  northern. 
  Accordingly 
  not 
  far 
  short 
  of 
  the 
  best 
  

   conditions 
  for 
  observing 
  the 
  surface 
  of 
  the 
  planet 
  will 
  then 
  

   present 
  themselves, 
  and 
  an 
  even 
  nearer 
  approach 
  to 
  the 
  best 
  

   possible 
  conditions 
  for 
  observing 
  the 
  terminator 
  of 
  the 
  planet 
  

   when 
  slightly 
  gibbous 
  will 
  occur 
  in 
  August. 
  Mars 
  will 
  on 
  

   the 
  13th 
  of 
  that 
  month 
  reach 
  perihelion 
  and 
  exhibit 
  the 
  

   degree 
  of 
  gibbosity 
  which 
  will 
  then 
  exist, 
  with 
  the 
  advantage 
  

   that 
  tne 
  planet 
  will 
  be 
  almost 
  at 
  the 
  shortest 
  distance 
  from 
  

   the 
  observer 
  which 
  is 
  compatible 
  with 
  its 
  being 
  in 
  that 
  degree 
  

   gibbous. 
  Moreover, 
  the 
  position 
  of 
  the 
  planet 
  in 
  September 
  

   will 
  give 
  an 
  opportunity 
  of 
  scrutinising 
  its 
  southern 
  circum- 
  

   polar 
  regions 
  under 
  exceptionally 
  favourable 
  conditions. 
  

  

  These 
  considerations, 
  and 
  a 
  desire 
  that 
  there 
  shall 
  be 
  time 
  

   for 
  adequate 
  preparation 
  before 
  the 
  summer 
  of 
  next 
  year, 
  

   have 
  induced 
  the 
  writer 
  of 
  this 
  paper 
  to 
  publish 
  somewhat 
  

   prematurely 
  one 
  part 
  of 
  an 
  inquiry 
  respecting 
  the 
  process 
  by 
  

   which 
  nature 
  forms 
  those 
  images 
  which 
  we 
  find 
  in 
  optical 
  

   instruments 
  or 
  on 
  the 
  retina, 
  although 
  (being 
  the 
  part 
  of 
  the 
  

   inquiry 
  which 
  concerns 
  the 
  telescope) 
  it 
  would 
  more 
  naturally 
  

   have 
  come 
  after 
  another 
  unpublished 
  part 
  of 
  the 
  investigation 
  

   * 
  Communicated 
  by 
  the 
  Author. 
  

  

  