﻿Telescopic 
  Vision. 
  803 
  

  

  •conspicuously 
  dark 
  streak, 
  like 
  a 
  dark 
  border 
  to 
  the 
  principal 
  

  

  light, 
  

  

  The 
  round 
  form 
  of 
  the 
  object 
  z 
  f 
  is 
  not 
  necessary 
  for 
  this 
  

   phenomenon. 
  It 
  presents 
  itself 
  when 
  that 
  object 
  has 
  any 
  

   other 
  shape, 
  provided 
  that 
  it 
  is 
  sufficiently 
  narrow 
  and 
  

   sufficiently 
  bright. 
  It 
  is 
  beautifully 
  seen 
  by 
  using 
  as 
  object 
  

   a 
  slit 
  at 
  c', 
  or 
  a 
  row 
  of 
  holes 
  (which 
  in 
  the 
  image 
  will 
  be 
  seen 
  

   as 
  a 
  rhabdo-spile, 
  i. 
  <?., 
  run 
  together 
  into 
  the 
  appearance 
  of 
  a 
  

   rod); 
  and 
  it 
  maybe 
  exhibited 
  conveniently 
  by 
  placing 
  at 
  z 
  

   the 
  adjustable 
  slit 
  of 
  a 
  spectroscope, 
  which 
  can 
  be 
  gradually 
  

   narrowed, 
  whereupon 
  all 
  the 
  phases 
  of 
  the 
  phenomenon 
  will 
  

   successively 
  present 
  themselves. 
  And, 
  of 
  course, 
  the 
  ob- 
  

   server 
  can 
  induce 
  the 
  same 
  succession 
  of 
  appearances 
  without 
  

   changing 
  the 
  slit, 
  by 
  altering 
  the 
  aperture 
  of 
  his 
  pro- 
  

   telescope. 
  The 
  astronomer 
  would 
  do 
  well 
  to 
  familiarize 
  

   himself 
  with 
  several 
  other 
  details 
  of 
  this 
  phenomenon, 
  which 
  

   cannot 
  be 
  included 
  in 
  so 
  brief 
  an 
  account 
  of 
  it 
  as 
  that 
  to 
  

   which 
  we 
  must 
  confine 
  ourselves 
  here. 
  Our 
  aim 
  in 
  the 
  

   present 
  Memoir 
  is 
  to 
  go 
  far 
  enough 
  into 
  detail 
  to 
  indicate 
  

   the 
  value 
  of 
  the 
  work 
  which 
  can 
  be 
  done 
  with 
  an 
  Observatory- 
  

   experiments 
  apparatus, 
  rather 
  than 
  to 
  attempt 
  to 
  describe 
  

   what 
  has 
  been 
  accomplished 
  by 
  it— 
  an 
  adequate 
  account 
  of 
  

   which 
  would 
  be 
  too 
  voluminous. 
  

  

  42. 
  What 
  has 
  been 
  already 
  described 
  has 
  an 
  obvious 
  

   bearing 
  upon 
  the 
  interpretation 
  to 
  be 
  put 
  on 
  some 
  o£ 
  

   the 
  appearances 
  detected 
  at 
  Flagstaff, 
  when 
  we 
  bear 
  in 
  

   mind 
  that 
  the 
  12- 
  and 
  the 
  24-inch 
  telescopes 
  employed 
  in 
  

   Professor 
  Lowell's 
  observatory 
  (and 
  we 
  might 
  indeed 
  add, 
  

   all 
  existing 
  astronomical 
  telescopes) 
  belong 
  essentially 
  to 
  the 
  

   class 
  of 
  instruments 
  whose 
  apertures 
  are 
  vastly 
  too 
  small 
  for 
  

   dealing 
  adequately 
  with 
  the 
  details 
  upon 
  Mars. 
  The 
  ex- 
  

   periments 
  made 
  with 
  the 
  OE 
  apparatus 
  suggest 
  that 
  the 
  

   dusky 
  band 
  filled 
  with 
  blue 
  light, 
  which 
  has 
  sometimes 
  been 
  

   seen 
  to 
  border 
  the 
  image 
  of 
  a 
  polar 
  cap 
  upon 
  Mars, 
  may 
  

   prove 
  to 
  be 
  produced 
  in 
  the 
  way 
  described 
  in 
  the 
  last 
  two 
  

   paragraphs. 
  Any 
  object 
  of 
  the 
  kind 
  seen 
  with 
  a 
  telescope 
  

   of 
  insufficient 
  aperture 
  will 
  if 
  bright 
  enough 
  exhibit 
  an 
  image 
  

   consisting 
  of 
  a 
  principal 
  patch 
  of 
  light 
  near 
  to 
  which, 
  

   though 
  separated 
  from 
  it 
  by 
  a 
  conspicuous 
  dark 
  band, 
  will 
  

   be 
  a 
  detached 
  field 
  of 
  milder 
  luminosity. 
  If, 
  therefore, 
  the 
  

   polar 
  cap 
  upon 
  Mars 
  is 
  bright 
  enough 
  and 
  if 
  the 
  albedo 
  

   of 
  the 
  neighbouring 
  regions 
  is 
  sufficiently 
  low, 
  this 
  dark 
  

   interval 
  will 
  appear 
  very 
  dark, 
  and 
  may 
  sometimes 
  be 
  so 
  

   dark 
  that 
  its 
  faint 
  luminosity 
  (which 
  will 
  be 
  whatever 
  is 
  the 
  

   intrinsic 
  luminosity 
  of 
  that 
  part 
  of 
  Mars) 
  shall 
  not 
  suffice 
  to 
  

   prevent 
  colour 
  being 
  there 
  seen 
  due 
  to 
  the 
  blue 
  and 
  violet 
  

  

  3G2 
  

  

  