﻿806 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  details 
  such 
  as 
  under 
  some 
  circumstances 
  intrude 
  into 
  the 
  

   image 
  and 
  seem 
  to 
  the 
  observer 
  to 
  betoken, 
  although 
  not 
  

   really 
  indicating, 
  that 
  there 
  is 
  something 
  more 
  or 
  less 
  like 
  

   themselves 
  upon 
  the 
  object. 
  We 
  shall 
  have 
  occasion 
  to 
  

   return 
  to 
  this 
  subject 
  farther 
  on. 
  

  

  46. 
  The 
  ordinary 
  experience 
  of 
  every 
  man 
  all 
  through 
  his 
  

   life 
  is 
  almost 
  exclusively 
  taken 
  up 
  in 
  dealing 
  with 
  objects 
  of 
  

   such 
  a 
  kind 
  and 
  at 
  such 
  distances, 
  that 
  he 
  can 
  see 
  their 
  size 
  

   and 
  their 
  outline 
  ; 
  which 
  means 
  that 
  they 
  are 
  such 
  that 
  an 
  

   optical 
  apparatus 
  with 
  an 
  aperture 
  so 
  moderate 
  as 
  the 
  pupil 
  

   of 
  the 
  eye* 
  can 
  form 
  an 
  adequate 
  image 
  of 
  them. 
  It 
  is 
  by 
  

   this 
  abundant 
  and 
  unfortunately 
  one-sided 
  experience 
  that 
  

   our 
  judgments 
  as 
  to 
  how 
  we 
  are 
  to 
  interpret 
  visual 
  objects 
  

   has 
  been 
  trained. 
  We 
  must 
  therefore 
  be 
  prepared 
  to 
  find 
  

   ourselves 
  tilled 
  by 
  it 
  with 
  prepossessions, 
  which 
  become 
  

   embarrassing 
  whenever 
  we 
  have 
  occasion, 
  like 
  the 
  astro- 
  

   nomer, 
  to 
  deal 
  with 
  an 
  entirely 
  new 
  field 
  of 
  exploration 
  

   which 
  lies 
  outside 
  that 
  ordinary 
  experience. 
  The 
  astronomer 
  

   will 
  therefore 
  do 
  well 
  to 
  lose 
  no 
  opportunity 
  of 
  acquiring 
  

   effectual 
  experience 
  within 
  this 
  new 
  field, 
  so 
  as 
  thus 
  to 
  do 
  

   what 
  he 
  can 
  to 
  counteract 
  the 
  misapprehensions 
  in 
  the 
  inter- 
  

   pretation 
  of 
  astronomical 
  images 
  into 
  which 
  his 
  (common 
  

   sense), 
  which 
  here 
  means 
  the 
  misapplication 
  of 
  ordinary 
  ex- 
  

   perience, 
  tends 
  to 
  lead 
  him 
  with 
  an 
  insistence 
  that 
  is 
  too 
  

   often 
  successful. 
  

  

  47. 
  A 
  valuable 
  opportunity 
  of 
  acquiring 
  experience 
  which 
  

   will 
  help 
  to 
  correct 
  these 
  errors 
  is 
  open 
  to 
  the 
  student 
  of 
  

  

  may 
  be 
  taken 
  to 
  be 
  about 
  half 
  and 
  a 
  quarter 
  of 
  what 
  -we 
  employ 
  when 
  

   we 
  look 
  at 
  the 
  Moon 
  with 
  the 
  naked 
  eye, 
  so 
  that 
  the 
  vision 
  of 
  Mars 
  

   which 
  Professor 
  Lowell 
  had 
  at 
  his 
  disposal 
  was 
  immensely 
  inferior 
  to 
  

   that 
  which 
  we 
  obtain 
  when 
  we 
  look 
  at 
  the 
  full 
  Moon 
  with 
  one 
  eye. 
  

  

  Any 
  trained 
  astronomer 
  can 
  easily 
  take 
  steps 
  which 
  will 
  enable 
  him 
  

   to 
  appreciate 
  with 
  some 
  exactness 
  what 
  Professor 
  Lowell's 
  telescope 
  

   could 
  succeed 
  in 
  substituting 
  for 
  Mars 
  on 
  that 
  night, 
  if 
  the 
  state 
  of 
  the 
  

   atmosphere 
  was 
  at 
  its 
  best. 
  He 
  has 
  only 
  to 
  make 
  holes 
  07 
  mm. 
  and 
  

   1*4 
  mm. 
  in 
  diameter 
  with 
  sufficiently 
  fine 
  needles, 
  in 
  a 
  piece 
  of 
  card, 
  and 
  

   then 
  to 
  see 
  for 
  himself 
  the 
  deterioration 
  which 
  the 
  image 
  of 
  the 
  full 
  

   Moon 
  undergoes 
  when 
  examined 
  with 
  the 
  naked 
  eye 
  restricted 
  to 
  looking 
  

   through 
  one 
  or 
  other 
  of 
  these 
  holes. 
  

  

  It 
  seems 
  wonderful 
  that 
  Professor 
  Lowell 
  could 
  succeed 
  in 
  securing 
  

   the 
  remarkable 
  observations 
  he 
  made 
  under 
  such 
  difficult 
  circumstances 
  ; 
  

   and 
  the 
  question 
  arises, 
  What 
  can 
  these 
  " 
  carets 
  " 
  mean? 
  It 
  is 
  to 
  this 
  

   inquiry 
  that 
  experiments 
  give 
  an 
  answer. 
  The 
  control 
  observations 
  

   only 
  go 
  so 
  far 
  as 
  to 
  show 
  that 
  their 
  presence 
  in 
  the 
  image 
  does 
  not 
  

   betoken 
  that 
  anything 
  like 
  them 
  exists 
  upon 
  the 
  planet. 
  

  

  * 
  The 
  reader 
  will 
  find 
  an 
  inquiry 
  as 
  to 
  the 
  ways 
  in 
  which 
  vision 
  witk 
  

   the 
  naked 
  eve 
  is 
  limited 
  in 
  Section 
  1 
  of 
  a 
  paper 
  on 
  Insect 
  Vision 
  in 
  the 
  

   Phil. 
  Mag. 
  for 
  March 
  1894, 
  p. 
  317. 
  

  

  

  