﻿Telescopic 
  Vision. 
  797 
  

  

  which 
  o£ 
  necessity 
  * 
  exist 
  in 
  the 
  images 
  which 
  his 
  astro- 
  

   nomical 
  telescopes 
  are 
  competent 
  to 
  furnish 
  ; 
  which 
  will 
  tell 
  

   him 
  how 
  to 
  mitigate, 
  these 
  imperfections 
  and 
  how 
  to 
  guard 
  

   against 
  being 
  misled 
  by 
  them 
  ; 
  and, 
  above 
  all, 
  which 
  will 
  

   reveal 
  to 
  him 
  the 
  causes 
  of 
  all 
  these 
  imperfections. 
  

  

  In 
  this 
  second 
  part 
  of 
  the 
  Memoir, 
  we 
  propose 
  to 
  investi- 
  

   gate 
  by 
  its 
  aid, 
  of 
  what 
  kind 
  the 
  images 
  will 
  be 
  ; 
  reserving 
  

   for 
  the 
  third 
  part 
  which 
  is 
  to 
  follow 
  the 
  most 
  important 
  part 
  

   of 
  our 
  inquiry 
  by 
  which 
  the 
  causes 
  of 
  the 
  observed 
  defects 
  

   will 
  be 
  made 
  fully 
  known 
  to 
  us. 
  

  

  33. 
  An 
  image, 
  whether 
  formed 
  by 
  an 
  optical 
  instrument 
  

   or 
  upon 
  the 
  retina, 
  is 
  found 
  to 
  consist 
  of 
  parts 
  distinguishable 
  

   from 
  one 
  another, 
  which 
  are 
  of 
  such 
  a 
  kind 
  that 
  within 
  each 
  

   of 
  these 
  units 
  there 
  is 
  either 
  no 
  detail 
  to 
  be 
  seen 
  or 
  at 
  least 
  

   none 
  that 
  can 
  be 
  made 
  out 
  satisfactorily. 
  Each 
  such 
  

   elementary 
  part 
  of 
  an 
  image 
  may 
  be 
  called 
  a 
  spile 
  or 
  blur 
  

   (cr7rt\o9, 
  a 
  blot 
  or 
  blur), 
  since 
  it 
  is 
  the 
  outcome 
  of 
  light 
  

   emitted 
  from 
  an 
  immense 
  amount 
  of 
  molecular 
  and 
  other 
  

   events 
  going 
  on 
  upon 
  the 
  object, 
  all 
  parts 
  of 
  which 
  with 
  

   all 
  the 
  activities 
  in 
  or 
  between 
  minutise 
  being 
  in 
  the 
  image 
  

   blurred 
  into 
  a 
  single 
  spile, 
  with, 
  in 
  some 
  cases, 
  an 
  admixture 
  

   of 
  light 
  that 
  has 
  strayed 
  on 
  to 
  this 
  spile 
  from 
  other 
  parts 
  of 
  

   the 
  object. 
  

  

  Of 
  spiles 
  there 
  are 
  varieties, 
  and 
  a 
  somewhat 
  full 
  study 
  

   of 
  them 
  is 
  required 
  in 
  order 
  to 
  understand 
  the 
  images 
  

   seen 
  in 
  microscopes; 
  but 
  only 
  two 
  or 
  three 
  kinds 
  present 
  

   themselves 
  in 
  the 
  telescopic 
  images 
  of 
  celestial 
  objects, 
  and 
  

   among 
  these 
  the 
  plateo-spile 
  or 
  surface-blur 
  is 
  that 
  with 
  

   which 
  we 
  have 
  most 
  to 
  do. 
  The 
  images 
  formed 
  on 
  the 
  retina 
  

   by 
  a 
  grape, 
  a 
  cherry, 
  a 
  sheet 
  of 
  white 
  paper, 
  or 
  a 
  white- 
  

   washed 
  ceiling 
  are 
  examples 
  of 
  plateo-spiles. 
  In 
  the 
  astro- 
  

   nomical 
  telescope 
  this 
  description 
  of 
  spile 
  may 
  be 
  the 
  full 
  

   size 
  of 
  the 
  image 
  of 
  the 
  planet, 
  or 
  it 
  may 
  be 
  of 
  any 
  smaller 
  

   size 
  down 
  to 
  the 
  smallest 
  speck 
  which 
  in 
  any 
  degree 
  exhibits 
  

   the 
  shape 
  of 
  the 
  detail 
  upon 
  the 
  planet 
  which 
  has 
  occasioned 
  

   it. 
  Specks 
  smaller 
  than 
  this 
  or 
  more 
  shapeless 
  are 
  spiles 
  of 
  

   another 
  kind. 
  Most, 
  however, 
  of 
  the 
  features 
  visible 
  in 
  the 
  

   image 
  of 
  a 
  planet 
  are 
  plateo-spiles. 
  

  

  34. 
  Plateo-spiles 
  in 
  many 
  cases 
  have 
  a 
  well 
  defined 
  out- 
  

   line, 
  and 
  then 
  become 
  perigrapho-spiles 
  or 
  silhouette 
  blurs. 
  

   The 
  leaves 
  of 
  a 
  tree 
  viewed 
  from 
  a 
  suitable 
  distance 
  are 
  good 
  

   examples 
  of 
  objects 
  of 
  which 
  the 
  images 
  formed 
  on 
  the 
  retina 
  

   are 
  silhouette-blurs. 
  A 
  leaf 
  may 
  retain 
  a 
  markedly 
  distinct 
  

  

  * 
  By 
  the 
  expression 
  "imperfections 
  which 
  of 
  necessity 
  exist" 
  is 
  

   meant, 
  which 
  would 
  continue 
  to 
  exist 
  if 
  his 
  telescope 
  could 
  be 
  made 
  

   perfect 
  as 
  an 
  optical 
  instrument, 
  and 
  if 
  "seeing 
  " 
  through 
  our 
  atmosphere 
  

   were 
  also 
  perfect. 
  

  

  