﻿798 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  outline 
  even 
  when 
  the 
  spectator 
  has 
  retreated 
  to 
  such 
  a 
  

   distance 
  that 
  all 
  appearance 
  of 
  detail 
  upon 
  the 
  surface 
  of 
  the 
  

   leaf 
  ceases 
  to 
  be 
  seen. 
  It 
  indeed 
  seems 
  marvellous, 
  when 
  

   we 
  come 
  to 
  think 
  of 
  it, 
  that 
  the 
  boundary 
  of 
  each 
  leaf 
  can 
  

   continue 
  to 
  be 
  so 
  distinctly 
  seen, 
  when 
  perhaps 
  detail 
  upon 
  

   the 
  leaf 
  has 
  become 
  unseen 
  or 
  so 
  confusedly 
  seen 
  as 
  to 
  

   indicate 
  that 
  light 
  from 
  one 
  part 
  of 
  the 
  leaf 
  is 
  able 
  to 
  stray 
  

   to 
  other 
  parts 
  on 
  the 
  image, 
  while, 
  at 
  the 
  same 
  time, 
  no 
  

   light 
  from 
  any 
  part 
  of 
  the 
  leaf 
  is 
  allowed 
  outside 
  the 
  

   boundary 
  of 
  the 
  image 
  of 
  the 
  whole 
  leaf, 
  nor 
  is 
  any 
  of 
  the 
  

   light 
  from 
  the 
  background 
  against 
  which 
  the 
  leaf 
  is 
  seen 
  

   allowed 
  to 
  trespass 
  within 
  that 
  barrier. 
  The 
  green 
  of 
  the 
  

   leaf 
  seems 
  to 
  stop 
  abruptly 
  at 
  the 
  boundary 
  of 
  the 
  image 
  ; 
  

   and 
  the 
  light 
  from 
  the 
  background 
  stops 
  there 
  as 
  abruptly. 
  

   We 
  shall 
  be 
  able 
  to 
  discover 
  by 
  our 
  experimental 
  apparatus, 
  

   the 
  conditions 
  which 
  have 
  to 
  be 
  fulfilled 
  in 
  the 
  process 
  by 
  which 
  

   the 
  image 
  is 
  formed 
  in 
  order 
  to 
  secure 
  these 
  results. 
  

  

  35. 
  An 
  object 
  may 
  of 
  course 
  furnish 
  very 
  different 
  images 
  

   when 
  viewed 
  under 
  different 
  conditions. 
  Thus 
  a 
  ruling 
  upon 
  

   a 
  sheet 
  of 
  paper 
  of 
  equidistant 
  parallel 
  lines 
  at 
  intervals 
  of 
  a 
  

   millimetre, 
  when 
  well 
  illuminated, 
  and 
  when 
  looked 
  at 
  by 
  a 
  

   spectator 
  whose 
  eyesight 
  is 
  good 
  from 
  a 
  distance 
  exceeding 
  

   12 
  feet, 
  will 
  present 
  a 
  mere 
  uniform 
  appearance, 
  i. 
  e. 
  the 
  

   image 
  upon 
  the 
  retina 
  will 
  be 
  a 
  simple 
  plateo-spile 
  ; 
  while 
  

   from 
  distances 
  less 
  than 
  11 
  feet 
  the 
  image 
  will 
  be 
  quite 
  

   different, 
  since 
  the 
  ruling 
  itself 
  will 
  then 
  come 
  into 
  view. 
  

   The 
  limit 
  between 
  the 
  two 
  appearances 
  will 
  usually 
  be 
  found 
  

   when 
  the 
  eve 
  is 
  somewhere 
  between 
  11 
  and 
  12 
  feet 
  from 
  the 
  

   ruling 
  (see 
  Phil. 
  Mag. 
  for 
  March 
  1894, 
  p. 
  317). 
  

  

  36. 
  Two 
  quite 
  similar 
  objects 
  upon 
  a 
  planet, 
  but 
  of 
  different 
  

   sizes, 
  may 
  be 
  such 
  that 
  the 
  larger 
  of 
  them 
  will 
  furnish 
  an 
  

   image 
  w 
  r 
  hich 
  is 
  a 
  silhouette-blur, 
  retaining 
  the 
  shape 
  of 
  the 
  

   object 
  with 
  its 
  outline 
  well 
  defined 
  ; 
  while 
  the 
  image 
  of 
  the 
  

   smaller 
  one 
  may 
  be 
  hazy 
  in 
  outline 
  and 
  distorted 
  in 
  shape, 
  

   representing 
  the 
  object 
  as 
  too 
  large 
  in 
  some 
  places 
  and 
  too 
  

   small 
  in 
  others, 
  and 
  sometimes 
  introducing 
  features 
  that 
  do 
  

   not 
  exist 
  upon 
  the 
  object. 
  

  

  So, 
  again, 
  things 
  that 
  appear 
  to 
  be 
  streaks 
  in 
  the 
  image 
  

   may 
  in 
  some 
  cases 
  be 
  the 
  kind 
  of 
  spile 
  which 
  may 
  be 
  

   called 
  rhabdo-spiles, 
  i. 
  e. 
  rod-like 
  blurs 
  occasioned 
  by 
  features 
  

   upon 
  the 
  planet 
  which 
  are 
  not 
  streaks. 
  

  

  The 
  causes 
  of 
  all 
  these 
  and 
  of 
  many 
  other 
  effects 
  will 
  be 
  

   satisfactorily 
  exposed 
  to 
  our 
  view 
  by 
  the 
  experiments 
  of 
  

   which 
  we 
  shall 
  be 
  able 
  to 
  give 
  examples. 
  

  

  37. 
  The 
  part 
  of 
  the 
  planet 
  or 
  other 
  object 
  which 
  corresponds 
  

   in 
  position, 
  shape 
  and 
  size 
  to 
  a 
  spile 
  in 
  the 
  images, 
  may 
  be 
  

  

  