﻿Telescopic 
  Vision. 
  975 
  

  

  image 
  ' 
  o£ 
  the 
  outline 
  of 
  the 
  planet, 
  which 
  will 
  be 
  o£ 
  the 
  

   kind 
  that 
  an 
  astronomer 
  would 
  pronounce 
  to 
  be 
  excellent. 
  

  

  89. 
  In 
  order 
  to 
  show 
  the 
  efficiency 
  o£ 
  the 
  proposed 
  method 
  

   o£ 
  investigating 
  telescopic 
  vision, 
  and 
  of 
  discovering 
  the 
  

   reason 
  why 
  it 
  is 
  encumbered 
  with 
  the 
  imperfections 
  which 
  

   become 
  troublesome 
  when 
  we 
  endeavour 
  to 
  see 
  minute 
  

   objects, 
  it 
  has 
  been 
  thought 
  that 
  the 
  best 
  course 
  was 
  to 
  

   describe 
  how 
  to 
  apply 
  the 
  new 
  method 
  to 
  some 
  one 
  class 
  of 
  

   objects 
  ; 
  and 
  the 
  class 
  chosen 
  has 
  been 
  round 
  or 
  roundish 
  

   objects 
  such 
  as 
  are 
  some 
  of 
  the 
  spots 
  on 
  the 
  image 
  of 
  Mars, 
  

   including 
  the 
  polar 
  caps 
  at 
  the 
  times 
  when 
  these 
  appear 
  to 
  

   be 
  roundish 
  or 
  oval 
  objects. 
  The 
  telescope 
  is 
  more 
  successful 
  

   in 
  dealing 
  with 
  objects 
  of 
  this 
  kind 
  than 
  with 
  objects 
  of 
  

   angular 
  or 
  other 
  irregular 
  form. 
  This 
  is 
  because 
  in 
  the 
  

   latter 
  case, 
  as 
  experiments 
  with 
  the 
  OE 
  apparatus 
  will 
  show, 
  

   the 
  concentration 
  images 
  which 
  they 
  develop 
  upon 
  plane 
  Y' 
  

   do 
  not, 
  as 
  with 
  round 
  objects, 
  consist 
  of 
  a 
  boss 
  of 
  white 
  light 
  

   surrounded 
  by 
  annular 
  spectra 
  alike 
  in 
  all 
  directions 
  round 
  the 
  

   boss 
  and 
  which 
  after 
  the 
  first 
  four 
  or 
  five 
  become 
  excessively 
  

   faint, 
  but 
  consist 
  of 
  a 
  central 
  boss 
  of 
  some 
  other 
  form 
  

   surrounded 
  by 
  spectra 
  occupying 
  detached 
  patches 
  upon 
  

   plane 
  Y', 
  of 
  very 
  unequal 
  intensity 
  in 
  the 
  various 
  directions 
  

   round 
  the 
  boss, 
  and 
  in 
  some 
  directions 
  retaining 
  sufficient 
  

   brightness 
  to 
  need 
  to 
  be 
  taken 
  into 
  account, 
  when 
  consider- 
  

   ably 
  farther 
  from 
  their 
  central 
  boss, 
  than 
  the 
  fifth 
  annular 
  

   spectrum 
  which 
  is 
  the 
  most 
  distant 
  we 
  have 
  thought 
  it 
  

   necessary 
  to 
  retain 
  when 
  inquiring 
  as 
  to 
  the 
  vision 
  of 
  round 
  

   objects. 
  Long 
  rows 
  of 
  exceptionally 
  bright 
  spectra 
  are 
  in 
  

   such 
  cases 
  apt 
  to 
  radiate 
  out 
  in 
  some 
  directions 
  from 
  the 
  

   central 
  boss, 
  and 
  the 
  more 
  distant 
  of 
  these 
  will 
  be 
  excluded 
  

   if 
  the 
  aperture 
  of 
  the 
  telescope 
  is 
  insufficient. 
  This 
  must 
  

   have 
  often 
  happened 
  when 
  the 
  24-inch 
  telescope 
  and 
  in 
  a 
  

   more 
  intense 
  degree 
  when 
  the 
  12-inch 
  telescope 
  have 
  been 
  

   employed 
  to 
  examine 
  polygonal 
  patches 
  subtending 
  at 
  the 
  

   telescope 
  so 
  small 
  an 
  angle, 
  as 
  do 
  those 
  brigher 
  regions 
  

   upon 
  Mars 
  which 
  are 
  enclosed 
  by 
  whatever 
  on 
  Mars 
  corre- 
  

   spond 
  to 
  the 
  ' 
  canals 
  ' 
  upon 
  the 
  image, 
  or 
  by 
  the 
  darker 
  

   6 
  seas/ 
  When 
  these 
  outlying 
  spectra 
  are 
  excluded 
  from 
  the 
  

   telescope, 
  one 
  effect 
  will 
  be 
  that 
  the 
  angles 
  of 
  the 
  polygon 
  

   w 
  T 
  ill 
  be 
  rounded 
  off, 
  and 
  such 
  appearances 
  as 
  ' 
  oases 
  ' 
  and 
  

   ' 
  carets 
  ' 
  will 
  present 
  themselves. 
  

  

  90- 
  Tn 
  all 
  such 
  cases 
  the 
  astronomer 
  can 
  make 
  at 
  z' 
  a 
  hole 
  

   which 
  will 
  represent 
  both 
  in 
  size 
  and 
  shape 
  any 
  supposed 
  

   polygonal 
  feature 
  upon 
  Mars. 
  This 
  is 
  to 
  be 
  illuminated 
  by 
  

   the 
  pro-star 
  /, 
  and 
  he 
  can 
  then 
  see 
  through 
  the 
  Steinheil 
  

   lens 
  the 
  concentration 
  image 
  it 
  produces 
  on 
  plane 
  Y', 
  and 
  

  

  