﻿Telescopic 
  Vision. 
  811 
  

  

  was 
  in 
  that 
  year 
  so 
  very 
  close 
  a 
  double 
  star. 
  The 
  lower 
  of 
  

   these 
  powers 
  was 
  the 
  usual 
  working 
  power 
  used 
  upon 
  the 
  

   Great 
  Telescope, 
  as 
  it 
  is 
  not 
  far 
  from 
  the 
  lowest 
  power 
  

   which 
  enables 
  the 
  whole 
  pencil 
  of 
  light 
  to 
  enter 
  the 
  pupil 
  of 
  

   the 
  observer's 
  eye. 
  

  

  53. 
  Such 
  being 
  the 
  state 
  of 
  things, 
  I 
  have 
  little 
  doubt 
  

   that 
  on 
  that 
  night, 
  although 
  it 
  was 
  an 
  exceptionally 
  good 
  

   night, 
  the 
  small 
  telescopes 
  used 
  by 
  Professor 
  Lowell, 
  if 
  

   equally 
  good 
  as 
  optical 
  instruments, 
  would 
  have 
  shown 
  Mars 
  

   better 
  than 
  Lord 
  Rosse's 
  enormous 
  telescope 
  with 
  its 
  ex- 
  

   quisite 
  defining 
  power. 
  So 
  that 
  I 
  have 
  not 
  much 
  hope 
  of 
  

   good 
  results 
  from 
  the 
  use 
  upon 
  Mars 
  of 
  any 
  such 
  immense 
  

   aperture. 
  No 
  doubt 
  the 
  instantaneous 
  vision 
  obtained 
  now 
  

   and 
  then 
  would 
  do 
  something, 
  but 
  I 
  fear 
  not 
  much. 
  We 
  

   are 
  therefore 
  confronted 
  by 
  two 
  limitations 
  in 
  examining 
  an 
  

   object 
  such 
  as 
  Mars. 
  We 
  cannot 
  use 
  really 
  large 
  apertures 
  * 
  

   on 
  account 
  of 
  the 
  imperfections 
  of 
  our 
  atmosphere, 
  and 
  with 
  

   such 
  apertures 
  as 
  we 
  can 
  avail 
  ourselves 
  of, 
  the 
  image 
  of 
  

   the 
  planet 
  is 
  remote 
  from 
  representing 
  what 
  is 
  really 
  upon 
  

   the 
  planet. 
  It 
  behoves 
  us 
  then 
  to 
  avoid 
  confident 
  assertions 
  

   as 
  to 
  what 
  exists 
  or 
  does 
  not 
  exist 
  upon 
  Mars, 
  and 
  to 
  refrain 
  

   from 
  guesses 
  as 
  to 
  what 
  is 
  going 
  on 
  there. 
  The 
  necessity 
  

   for 
  this 
  caution 
  in 
  the 
  interests 
  of 
  correct 
  thinking, 
  in 
  no 
  

   degree 
  detracts 
  from 
  the 
  merit 
  of 
  the 
  great 
  achievements 
  

   of 
  those 
  astronomers 
  who 
  have 
  secured 
  for 
  science 
  a 
  know- 
  

   ledge 
  of 
  important 
  details, 
  before 
  unknown, 
  but 
  which 
  they 
  

   have 
  shown 
  the 
  insufficient 
  telescopes 
  that 
  man 
  can 
  employ 
  

   to 
  be 
  competent 
  to 
  reach, 
  and 
  which 
  they 
  have 
  ascertained 
  

   did 
  truly 
  exist 
  upon 
  the 
  eidolons 
  or 
  substitides 
  for 
  Mars 
  

   which 
  those 
  telescopes 
  exhibited, 
  when 
  Mars 
  was 
  at 
  the 
  

   various 
  distances 
  from 
  us 
  at 
  which 
  it 
  was 
  on 
  the 
  nights 
  upon 
  

   which 
  the 
  observations 
  were 
  made. 
  

  

  In 
  the 
  present 
  chapter 
  we 
  have 
  aimed 
  only 
  at 
  ascertaining 
  

   facts. 
  We 
  have 
  sought 
  to 
  discover 
  what 
  that 
  appearance 
  is 
  

   into 
  ichich 
  cm 
  insufficient 
  telescope 
  ivill 
  seem 
  to 
  transform 
  

   an 
  ohject 
  containing 
  certain 
  definite 
  details. 
  But 
  this 
  is 
  not 
  

   enough. 
  We 
  have 
  still 
  to 
  learn 
  the 
  precise 
  process 
  by 
  which 
  

   nature 
  produces 
  this 
  imperfect 
  image, 
  which 
  will 
  lead 
  to 
  our 
  

   discovering 
  its 
  true 
  nature 
  and 
  the 
  cause 
  of 
  its 
  imperfections. 
  

   This, 
  the 
  most 
  important 
  branch 
  of 
  our 
  inquiry, 
  will 
  occupy 
  

   our 
  attention 
  in 
  the 
  next 
  and 
  final 
  part 
  of 
  this 
  memoir. 
  

  

  * 
  Lord 
  Rosse's 
  marvellous 
  instrument 
  if 
  used 
  upon 
  Mars 
  when 
  nearest 
  

   to 
  us 
  would, 
  even 
  if 
  the 
  imperfections 
  of 
  our 
  atmosphere 
  could 
  be 
  

   removed, 
  only 
  have 
  shown 
  Mars 
  as 
  well 
  as 
  a 
  half-inch 
  objective 
  will 
  

   show 
  the 
  Moon 
  when 
  full. 
  

  

  [To 
  be 
  continued.] 
  

  

  