﻿Telescopic 
  Vision. 
  805 
  

  

  to 
  make 
  special 
  arrangements 
  to 
  enable 
  appendage 
  spectra 
  

   (corresponding 
  to 
  the 
  coloured 
  rings 
  round 
  the 
  image 
  o£ 
  a 
  

   star) 
  to 
  be 
  at 
  all 
  visible. 
  Moreover 
  as 
  the 
  object 
  becomes 
  

   small 
  the 
  phenomenon 
  which 
  corresponds 
  to 
  the 
  dilatation 
  o£ 
  

   the 
  central 
  boss 
  of 
  light 
  in 
  the 
  image 
  of 
  a 
  bright 
  object 
  will 
  

   be 
  an 
  enlargement 
  of 
  the 
  size 
  and 
  lessening 
  of 
  the 
  darkness 
  

   of 
  the 
  image 
  of 
  the 
  dark 
  object. 
  This 
  lessening 
  of 
  its 
  dark- 
  

   ness 
  will 
  prevail 
  all 
  over 
  the 
  enlarged 
  image, 
  but 
  will 
  tell 
  

   with 
  most 
  effect 
  on 
  its 
  marginal 
  portions, 
  so 
  that 
  what 
  is 
  

   usually 
  perceived 
  is 
  that 
  the 
  surrounding 
  brightness 
  en- 
  

   croaches 
  upon 
  the 
  image 
  and 
  causes 
  the 
  central 
  portions 
  of 
  

   the 
  dark 
  image 
  to 
  be 
  alone 
  visible, 
  and 
  that 
  these 
  central 
  

   portions, 
  especially 
  in 
  the 
  later 
  stages, 
  become 
  fainter 
  and 
  

   ultimately 
  cease 
  to 
  be 
  seen. 
  The 
  dark 
  spot 
  upon 
  the 
  planet 
  

   has 
  now 
  become 
  so 
  dim 
  that 
  its 
  image 
  is 
  no 
  longer 
  visible 
  

   to 
  us, 
  being, 
  as 
  it 
  were, 
  swallowed 
  up 
  in 
  the 
  surrounding 
  

   brightness. 
  

  

  45. 
  It 
  need 
  hardly 
  be 
  pointed 
  out 
  that 
  while 
  this 
  necessary 
  

   succession 
  of 
  events 
  is 
  in 
  accordance 
  with 
  the 
  recorded 
  

   appearance 
  of 
  most 
  of 
  the 
  dusky 
  patches 
  entered 
  upon 
  maps 
  

   of 
  Mars, 
  it 
  is 
  inconsistent 
  with 
  the 
  appearance 
  recorded 
  of 
  

   those 
  smallest 
  and 
  darkest 
  specks 
  upon 
  the 
  image 
  which 
  have 
  

   been 
  supposed 
  to 
  represent 
  oases 
  upon 
  the 
  planet. 
  This 
  

   raises 
  the 
  presumption 
  that 
  these 
  nearly 
  black 
  specks 
  are 
  

   almost 
  certainly 
  not 
  aletho-spiles, 
  that 
  is 
  are 
  not 
  markings 
  

   on 
  the 
  image 
  which 
  have 
  been 
  caused 
  by 
  anything 
  like 
  what 
  

   they 
  suggest 
  to 
  the 
  beholder 
  as 
  existing 
  upon 
  the 
  planet, 
  but 
  

   on 
  the 
  contrary 
  are 
  probably 
  phenako-spiles 
  * 
  — 
  misleading 
  

  

  * 
  It 
  may 
  make 
  this 
  clearer 
  to 
  give 
  a 
  possible 
  instance. 
  In 
  June 
  1903 
  

   (see 
  Annals 
  of 
  the 
  Lowell 
  Observatory, 
  vol. 
  iii. 
  p. 
  156) 
  Mars 
  was 
  

   receding 
  from 
  the 
  earth. 
  On 
  the 
  28th 
  of 
  that 
  month 
  the 
  double 
  streaks 
  

   in 
  the 
  image 
  called 
  Euphrates 
  and 
  Phison 
  (longitude 
  333° 
  on 
  maps 
  u 
  of 
  

   Mars") 
  were 
  seen 
  with 
  dark 
  appendages, 
  called 
  carets 
  by 
  Professor 
  

   Lowell, 
  at 
  their 
  southern 
  ends. 
  These 
  had 
  not 
  been 
  seen 
  when 
  Mars 
  

   was 
  less 
  remote 
  ; 
  which 
  Professor 
  Lowell 
  attributes 
  to 
  a 
  seasonal 
  change 
  

   having 
  taken 
  place. 
  But 
  may 
  not 
  the 
  intrusion 
  of 
  these 
  triangular 
  

   specks 
  into 
  the 
  image 
  have 
  been 
  simply 
  because 
  the 
  planet 
  had 
  by 
  that 
  

   time 
  got 
  to 
  be 
  sufficiently 
  far 
  from 
  the 
  telescope 
  ? 
  Experiment 
  shows 
  

   that 
  the 
  appearance 
  is 
  one 
  which 
  will 
  necessarily 
  present 
  itself 
  so 
  soon 
  

   as 
  the 
  inefficiency 
  of 
  the 
  telescope 
  reaches 
  a 
  certain 
  point, 
  and 
  the 
  inefficiency 
  

   of 
  the 
  telescope 
  went 
  on 
  gradually 
  increasing 
  according 
  as 
  the 
  planet 
  

   receded. 
  How 
  considerable 
  it 
  had 
  become 
  at 
  the 
  time 
  of 
  the 
  observation 
  

   will 
  appear 
  from 
  the 
  following 
  considerations. 
  

  

  On 
  that 
  night 
  the 
  distance 
  of 
  Mars 
  from 
  the 
  observer 
  was 
  437 
  times 
  

   the 
  average 
  distance 
  of 
  the 
  Moon 
  when 
  full, 
  from 
  which 
  we 
  learn 
  that 
  

   Professor 
  Lowell's 
  2-1-inch 
  objective 
  was 
  — 
  even 
  if 
  objective 
  and 
  atmo- 
  

   sphere 
  were 
  optically 
  perfect 
  — 
  only 
  as 
  efficient 
  as 
  an 
  aperture 
  of 
  1'4 
  mm. 
  

   employed 
  upon 
  the 
  full 
  Moon 
  ; 
  and 
  that 
  the 
  12-inch 
  objective 
  was 
  no 
  

   more 
  efficient 
  than 
  0*7 
  mm. 
  employed 
  upon 
  the 
  Moon. 
  These 
  apertures 
  

  

  