﻿$02 
  Dr. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  round. 
  If 
  this 
  process 
  is 
  carried 
  far 
  enough 
  the 
  planet 
  will 
  

   be 
  seen 
  to 
  behave 
  more 
  and 
  more 
  like 
  a 
  star, 
  until 
  it 
  at 
  

   length 
  furnishes 
  in 
  great 
  perfection 
  the 
  kind 
  of 
  image 
  pro- 
  

   duced 
  by 
  a 
  star 
  and 
  which 
  we 
  have 
  called 
  a 
  star-burst, 
  

   consisting 
  of 
  a 
  central 
  boss 
  of 
  light 
  (which 
  by 
  that 
  time 
  will 
  

   have 
  become 
  much 
  larger 
  than 
  the 
  geometrical 
  image 
  of 
  the 
  

   planet) 
  surrounded 
  by 
  coloured 
  appendage 
  rings. 
  These 
  

   rings, 
  as 
  seen 
  in 
  the 
  pro-telescope, 
  are 
  apt 
  to 
  be 
  irregularly 
  

   broken 
  up 
  if 
  we 
  have 
  limited 
  the 
  aperture 
  of 
  the 
  pro- 
  

   telescope 
  by 
  an 
  iris-diaphragm, 
  owing 
  to 
  the 
  circumstance 
  

   that 
  the 
  opening 
  in 
  an 
  iris-diaphragm 
  is 
  not 
  bounded 
  by 
  a 
  

   circle 
  but 
  by 
  a 
  polygon. 
  This, 
  however, 
  is 
  a 
  defect 
  which 
  

   can 
  be 
  satisfactorily 
  gotten 
  rid 
  of 
  by 
  dispensing 
  with 
  the 
  

   iris-diaphragm 
  and 
  limiting 
  the 
  aperture 
  of 
  the 
  pro-telescope 
  

   by 
  a 
  clean 
  round 
  hole 
  in 
  copper-foil. 
  With 
  a 
  hole 
  of 
  half 
  a 
  

   millimetre 
  in 
  diameter, 
  which 
  corresponded 
  to 
  using 
  a 
  1J- 
  

   inch 
  telescope 
  upon 
  Neptune, 
  the 
  phenomenon 
  was 
  found 
  to 
  

   be 
  splendidly 
  seen 
  with 
  its 
  central 
  boss 
  of 
  light 
  much 
  larger 
  

   than 
  the 
  geometrical 
  image 
  of 
  the 
  planet, 
  and 
  with 
  five 
  of 
  

   the 
  appendage-rings 
  visible 
  ; 
  but 
  then, 
  all 
  the 
  dark 
  glasses 
  

   at 
  E 
  had 
  been 
  removed 
  and 
  the 
  brightness 
  of 
  the 
  pro-planet 
  

   was 
  much 
  greater 
  than 
  that 
  of 
  the 
  real 
  planet. 
  However, 
  it 
  

   is 
  only 
  increased 
  brightness 
  which 
  the 
  planet 
  Neptune 
  would 
  

   need, 
  to 
  enable 
  it 
  to 
  exhibit 
  the 
  same 
  appearance 
  ; 
  it 
  would, 
  

   however, 
  be 
  seen 
  in 
  the 
  astronomical 
  telescope 
  on 
  a 
  smaller 
  

   scale, 
  since 
  we 
  can 
  with 
  advantage 
  use 
  upon 
  the 
  pro-telescope 
  

   the 
  power 
  12, 
  which 
  corresponds 
  to 
  a 
  power 
  of 
  1200 
  upon 
  

   the 
  astronomical 
  telescope, 
  and 
  this 
  it 
  would 
  be 
  impracticable 
  

   to 
  use 
  on 
  a 
  lj-inch 
  telescope. 
  

  

  It 
  is 
  noteworthy 
  that 
  the 
  star-burst 
  which 
  the 
  image 
  of 
  a 
  

   small 
  bright 
  object 
  can 
  be 
  made 
  to 
  present, 
  may 
  be 
  obtained 
  

   either 
  by 
  contracting 
  A' 
  (the 
  aperture 
  of 
  the 
  pro-telescope) 
  

   or 
  by 
  diminishing 
  the 
  size 
  of 
  z' 
  (which 
  represents 
  either 
  a 
  

   planet 
  or 
  some 
  spot 
  upon 
  a 
  planet). 
  It 
  is 
  also 
  noteworthy 
  

   that 
  in 
  whichever 
  of 
  these 
  ways 
  the 
  appearance 
  is 
  produced, 
  

   it 
  comes 
  on 
  gradually 
  when 
  we 
  diminish 
  the 
  aperture 
  of 
  the 
  

   pro-telescope. 
  In 
  its 
  earlier 
  stages 
  it 
  manifests 
  itself 
  first 
  

   by 
  a 
  very 
  slight 
  enlargement 
  of 
  the 
  disk 
  in 
  the 
  image 
  

   accompanied 
  by 
  an 
  appearance 
  of 
  clumsiness 
  in 
  its 
  outline, 
  

   and 
  at 
  a 
  somewhat 
  later 
  stage 
  by 
  some 
  luminosity 
  at 
  a 
  little 
  

   distance 
  from 
  the 
  principal 
  light, 
  where 
  at 
  a 
  subsequent 
  

   stage 
  appendage-rings 
  will 
  be 
  formed. 
  This 
  intermediate 
  

   stage 
  is 
  interesting, 
  because 
  of 
  its 
  presenting 
  an 
  appearance 
  

   noticed 
  by 
  some 
  exceptionally 
  skilful 
  observers 
  of 
  the 
  images 
  

   made 
  by 
  the 
  polar 
  caps 
  of 
  Mars. 
  Between 
  the 
  principal 
  

   light 
  and 
  the 
  faint 
  luminosity, 
  there 
  appears 
  at 
  this 
  stage 
  a 
  

  

  