﻿804 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  colour 
  which 
  surrounds 
  the 
  image 
  of 
  every 
  bright 
  object 
  

   in 
  an 
  achromatic 
  telescope. 
  I 
  ought 
  here 
  to 
  mention 
  that 
  

   this 
  explanation 
  of 
  the 
  blue 
  colour 
  by 
  the 
  familiar 
  semblance 
  

   of 
  a 
  blue 
  atmosphere 
  which 
  surrounds 
  every 
  bright 
  image 
  in 
  

   an 
  achromatic 
  telescope, 
  was 
  suggested 
  to 
  me 
  in 
  conversation 
  

   by 
  Professor 
  Simon 
  Newcomb. 
  Presumably, 
  the 
  blue 
  color- 
  

   ation 
  would 
  disappear 
  if 
  the 
  object 
  were 
  examined 
  with 
  a 
  

   reflecting 
  telescope, 
  and 
  if 
  the 
  observer 
  takes 
  care 
  not 
  to 
  

   employ 
  an 
  eyepiece 
  which 
  can 
  produce 
  it. 
  

  

  43. 
  A 
  bright 
  object 
  seen 
  in 
  a 
  telescope 
  projected 
  upon 
  a 
  

   darker 
  background 
  will 
  present 
  the 
  succession 
  of 
  appearances 
  

   described 
  above 
  in 
  §§40 
  and 
  41, 
  if 
  we 
  progressively 
  make 
  

   the 
  object 
  smaller 
  without 
  changing 
  its 
  shape. 
  We 
  shall 
  

   here 
  give 
  a 
  synopsis 
  of 
  this 
  succession 
  of 
  appearances. 
  While 
  

   the 
  object 
  is 
  large 
  enough, 
  its 
  shape 
  and 
  size 
  will 
  be- 
  

   correctly 
  seen 
  and 
  its 
  outline 
  will 
  be 
  sharply 
  defined 
  ; 
  when 
  

   somewhat 
  smaller, 
  its 
  image 
  becomes 
  slightly 
  too 
  large 
  for 
  

   the 
  size 
  of 
  the 
  object, 
  and 
  its 
  outline 
  gets 
  a 
  kind 
  of 
  rounded 
  

   appearance, 
  as 
  if 
  the 
  boundary 
  were 
  the 
  kind 
  of 
  spile 
  called 
  

   a 
  rhabdo-spile 
  (i. 
  e. 
  a 
  rod-like 
  marking 
  in 
  the 
  image). 
  At 
  

   the 
  next 
  stage 
  the 
  central 
  boss 
  of 
  light 
  will 
  have 
  become 
  a 
  

   little 
  larger 
  compared 
  with 
  the 
  size 
  of 
  the 
  object, 
  and 
  will 
  be 
  

   surrounded 
  at 
  a 
  little 
  distance 
  by 
  a 
  field 
  of 
  vague 
  luminosity, 
  

   which 
  is 
  separated 
  from 
  the 
  brighter 
  light 
  by 
  a 
  narrow 
  and 
  

   conspicuous 
  dark 
  interval. 
  At 
  the 
  more 
  advanced 
  stages, 
  if 
  

   the 
  bright 
  object 
  is 
  made 
  progressively 
  smaller, 
  the 
  vague- 
  

   luminosity 
  will 
  gradually 
  develop 
  into 
  the 
  coloured 
  spectra 
  

   of 
  an 
  image 
  of 
  the 
  same 
  kind 
  as 
  that 
  which 
  a 
  star 
  produces 
  

   in 
  a 
  telescope. 
  Such 
  is 
  an 
  account 
  of 
  the 
  successive 
  ap- 
  

   pearances 
  which 
  a 
  sufficiently 
  bright 
  object 
  would 
  have 
  if 
  its 
  

   size 
  could 
  be 
  made 
  to 
  dwindle, 
  or 
  if 
  viewed 
  from 
  different 
  

   distances. 
  

  

  44. 
  A 
  dark 
  object 
  seen 
  upon 
  a 
  bright 
  background 
  will, 
  if 
  

   made 
  progressively 
  smaller, 
  present 
  a 
  series 
  of 
  stages 
  which 
  

   can 
  be 
  shown 
  to 
  be 
  closely 
  related 
  to 
  the 
  above, 
  but 
  which 
  

   would 
  appear 
  to 
  our 
  eyes 
  materially 
  different. 
  This 
  is 
  because 
  

   faint 
  additions 
  or 
  subtractions 
  of 
  light 
  at 
  situations 
  upon 
  the 
  

   surrounding 
  brighter 
  background 
  may 
  exist 
  unseen 
  by 
  us, 
  

   under 
  conditions 
  such 
  that 
  similar 
  additions 
  of 
  light 
  upon 
  a 
  

   surrounding 
  dusky 
  background 
  would 
  be 
  conspicuous. 
  A 
  

   familiar 
  and 
  in 
  some 
  degree 
  analogous 
  phenomenon, 
  is 
  the 
  less 
  

   visibility 
  of 
  the 
  Newton's 
  rings 
  seen 
  by 
  refracted 
  light, 
  than 
  

   of 
  those 
  seen 
  by 
  reflected 
  light 
  ; 
  where 
  a 
  part 
  of 
  the 
  faintness 
  

   of 
  their 
  appearance 
  is 
  due 
  to 
  their 
  being 
  superposed 
  upon 
  a 
  

   bright 
  field. 
  When 
  the 
  image 
  of 
  a 
  dark 
  object 
  in 
  a 
  telescope 
  

   is 
  surrounded 
  by 
  a 
  bright 
  background, 
  we 
  have 
  usually 
  

  

  