﻿9(U 
  Dr. 
  G. 
  Johnstone 
  S 
  ton 
  oy 
  on 
  

  

  this 
  paper 
  of 
  convenient 
  size. 
  "When 
  white 
  lighl 
  is 
  used 
  

   the 
  centra] 
  boss 
  will 
  occupy 
  the 
  space 
  within 
  the 
  inner 
  

   circle, 
  and 
  the 
  innermost 
  two 
  appendage 
  rings 
  will 
  occupy 
  

   the 
  spaces 
  between 
  the 
  circles, 
  since 
  the 
  overlapping 
  of 
  

   spectra 
  doe- 
  not 
  become 
  conspicuous 
  till 
  farther 
  out. 
  If 
  

   the 
  phenomenon 
  is 
  particularly 
  well 
  seen 
  three 
  or 
  four 
  more 
  

   of 
  the 
  Luminous 
  rings 
  will 
  also 
  lie 
  visible. 
  The 
  space 
  within 
  

   the 
  dotted 
  line 
  of 
  the 
  figure 
  would 
  include 
  five 
  of 
  the 
  ap- 
  

   pendage 
  rings, 
  which 
  are 
  usually 
  all 
  thai 
  we 
  need 
  consider. 
  

  

  If 
  one 
  wishes 
  to 
  Bee 
  what 
  the 
  more 
  definite 
  effect 
  is 
  with 
  

  

  monochromatic 
  Light, 
  the 
  absorbing 
  contrivance 
  known 
  as 
  a 
  

   thallium 
  screen 
  ma} 
  be 
  held 
  before 
  ihe 
  eve. 
  

  

  7."). 
  By 
  these 
  considerations 
  we 
  Learn 
  that 
  when 
  the 
  opening 
  

  

  at 
  zf 
  is 
  round, 
  and 
  when 
  the 
  Lighl 
  incident 
  upon 
  :' 
  has 
  been 
  

  

  a 
  single 
  stellade 
  of 
  Light, 
  ii< 
  concentration 
  image 
  will 
  ho 
  

  

  identical 
  except 
  in 
  -i/«- 
  with 
  the 
  image 
  o( 
  a 
  star 
  as 
  seen 
  in 
  a 
  

  

  tele-cope. 
  It 
  i-what 
  wo 
  have 
  (ailed 
  a 
  star-hurst, 
  and 
  with 
  

  

  ;i 
  round 
  opening 
  is 
  the 
  kind 
  of 
  star-burs( 
  which 
  Airy 
  inves- 
  

   tigated. 
  If 
  the 
  opening 
  at 
  ;' 
  is 
  of 
  some 
  other 
  shape, 
  the 
  

   concentration 
  image 
  on 
  plane 
  V 
  produced 
  when 
  z' 
  is 
  illumi- 
  

   nated 
  by 
  a 
  single 
  stellade 
  will 
  >iill 
  be 
  a 
  star-bursl 
  or 
  image 
  

   of 
  a 
  star, 
  for 
  in 
  Bad 
  it 
  will 
  be 
  the 
  kind 
  of 
  image 
  which 
  a 
  

   telescope 
  would 
  Eurnish 
  if 
  its 
  objective 
  had 
  this 
  new 
  shape. 
  

   Several 
  such 
  images 
  produced 
  by 
  stars 
  were 
  examined 
  by 
  

   sir 
  John 
  Herschel, 
  and 
  some 
  of 
  them 
  are 
  figured 
  in 
  Plates 
  

   IX. 
  and 
  X. 
  of 
  his 
  article 
  on 
  Lighl 
  in 
  the 
  Encyclopaedia 
  

   Metropolitana. 
  But 
  they 
  an- 
  displayed 
  in 
  fuller 
  detail 
  by 
  

   the 
  (JE 
  apparatus, 
  and 
  with 
  great 
  additions 
  to 
  what 
  can 
  

   be 
  made 
  visible 
  by 
  examining 
  the 
  images 
  of 
  stars 
  in 
  tele- 
  

   Bcopes. 
  The 
  astronomer 
  when 
  using 
  the 
  OE 
  apparatus 
  

   should 
  hear 
  in 
  mind 
  that 
  in 
  all 
  cases 
  the 
  concentration 
  intake 
  

   produced 
  by 
  a 
  single 
  stellade 
  of 
  incident 
  light 
  must 
  he 
  

   essentially 
  a 
  star-burst 
  or 
  image 
  of 
  a 
  star 
  whatever 
  the 
  

   shape 
  of 
  the 
  opening 
  at 
  ~' 
  may 
  he. 
  Jt 
  is 
  the 
  image 
  which 
  

   tie- 
  telescope 
  would 
  furnish 
  if 
  its 
  ohjective 
  w^ere 
  of 
  tin; 
  shape 
  

   of 
  the 
  opening 
  or 
  openings 
  which 
  lie 
  has 
  made 
  at 
  z 
  '. 
  And 
  

   he 
  will 
  rind 
  a 
  study 
  of 
  the 
  concentration 
  image 
  which 
  will 
  

   result 
  from 
  different 
  forms 
  of 
  openings 
  or 
  from 
  rows 
  of 
  

   openings 
  most 
  suggestive. 
  When 
  the 
  form 
  of 
  the 
  opening- 
  

   is 
  polygonal 
  the 
  spectra 
  which 
  are 
  associated 
  in 
  its 
  concen- 
  

   tration 
  image 
  with 
  a 
  central 
  hoss 
  of 
  light 
  are 
  no 
  longer 
  

   circular 
  as 
  they 
  are 
  when 
  z' 
  is 
  a 
  round 
  hole. 
  They 
  hecome 
  

   detached 
  patches 
  of 
  coloured 
  light, 
  and 
  some 
  o£ 
  the 
  more 
  

   distant 
  ones 
  are 
  more 
  conspicuous 
  than 
  the 
  corresponding 
  

   spectra 
  when 
  z' 
  is 
  round. 
  This 
  is 
  a 
  very 
  instructive 
  fact. 
  

   It 
  must 
  be 
  remembered 
  that 
  it 
  is 
  the 
  same 
  light 
  which 
  

  

  