﻿800 
  Dr. 
  Gr. 
  Johnstone 
  Stoney 
  on 
  

  

  the 
  footnote 
  * 
  ; 
  and 
  having 
  got 
  his 
  adjustments 
  made 
  the 
  

   observer 
  should 
  note 
  the 
  position 
  at 
  C 
  (see 
  Plate 
  XII.) 
  of 
  

   the 
  handle 
  of 
  the 
  iris-diaphragm, 
  and 
  the 
  direction 
  in 
  which 
  

   he 
  moved 
  it 
  into 
  position, 
  in 
  order 
  that 
  he 
  may 
  be 
  able 
  to 
  

   restore 
  it 
  to 
  that 
  position, 
  if 
  he 
  should 
  have 
  occasion 
  to 
  

   increase 
  or 
  diminish 
  the 
  opening. 
  He 
  may 
  then 
  freely 
  vary 
  

   that 
  opening, 
  and 
  thus 
  see 
  how 
  the 
  image 
  of 
  the 
  planet 
  will 
  

   be 
  affected 
  by 
  differences 
  in 
  the 
  aperture 
  of 
  his 
  astronomical 
  

  

  * 
  When 
  making 
  experiments 
  the 
  observer 
  often 
  finds 
  it 
  convenient 
  to 
  

   have 
  more 
  than 
  one 
  way 
  of 
  bringing 
  his 
  apparatus 
  into 
  accordance 
  with 
  

   the 
  conditions 
  that 
  have 
  to 
  be 
  complied 
  with. 
  And 
  in 
  the 
  present 
  

   instance, 
  where 
  we 
  want 
  to 
  reproduce 
  in 
  our 
  pro-telescope 
  the 
  appear- 
  

   ance 
  which 
  Neptune 
  will 
  have 
  in 
  the 
  astronomical 
  telescope, 
  we 
  have 
  

   the 
  choice 
  of 
  several 
  ways 
  of 
  doing 
  so. 
  The 
  simplest 
  in 
  principle 
  is 
  to 
  

   give 
  to 
  the 
  hole 
  at 
  ~', 
  which 
  is 
  to 
  represent 
  the 
  plauet, 
  a 
  diameter 
  of 
  

   I,} 
  mm., 
  and 
  to 
  contract 
  the 
  aperture 
  of 
  the 
  pro-telescope 
  by 
  the 
  iris- 
  

   diaphragm 
  at 
  C 
  until 
  it 
  becomes 
  one-hundredth 
  part 
  of 
  the 
  aperture 
  of 
  

   the 
  astronomical 
  telescope. 
  But 
  we 
  may 
  proceed 
  otherwise 
  : 
  we 
  may 
  

   assign 
  any 
  other 
  size, 
  if 
  more 
  convenient, 
  to 
  the 
  opening 
  of 
  z', 
  provided 
  

   that 
  we 
  increase 
  or 
  diminish 
  the 
  aperture 
  of 
  the 
  pro-telescope 
  inversely 
  

   a- 
  we 
  have 
  altered 
  the 
  size 
  of 
  the 
  opening 
  at 
  z'. 
  

  

  A 
  third, 
  and 
  often 
  the 
  most 
  convenient 
  method 
  of 
  making 
  the 
  adjust- 
  

   ment, 
  depends 
  upon 
  the 
  mode 
  of 
  observing 
  which 
  we 
  shall 
  have 
  to 
  adopt 
  

   in 
  Part 
  III. 
  of 
  this 
  Memoir. 
  We 
  shall 
  there 
  find 
  that 
  if 
  we 
  make 
  a 
  

   round 
  opening 
  at 
  z, 
  and 
  illuminate 
  it 
  by 
  light 
  from 
  the 
  pro-star 
  s, 
  it 
  

   will 
  furnish 
  a 
  very 
  remarkable 
  concentration 
  image 
  upon 
  plane 
  Y' 
  ; 
  

   which 
  the 
  observer 
  will 
  see 
  by 
  removing 
  the 
  pro-telescope 
  and 
  looking 
  

   through 
  the 
  Steinheil 
  lens 
  when 
  placed 
  at 
  B. 
  This 
  concentration 
  image 
  

   will 
  consist 
  of 
  a 
  central 
  boss 
  of 
  light 
  surrounded 
  by 
  coloured 
  rings, 
  

   which 
  the 
  observ 
  er 
  can 
  make 
  to 
  present 
  itself 
  centrally 
  in 
  the 
  field 
  of 
  

   view, 
  by 
  adjusting 
  the 
  B 
  end 
  of 
  the 
  board 
  BE, 
  which 
  he 
  has 
  the 
  means 
  

   of 
  doing. 
  Now, 
  it 
  can 
  be 
  proved 
  that 
  the 
  condition 
  to 
  be 
  fulfilled 
  in 
  

   order 
  that 
  the 
  aperture 
  of 
  the 
  pro-telescope 
  shall 
  correspond 
  to 
  that 
  of 
  

   the 
  astronomical 
  telescope 
  is 
  that 
  the 
  iris-diaphragm 
  at 
  shall 
  be 
  closed 
  

   until 
  a 
  definite 
  amount 
  of 
  this 
  concentration 
  image 
  shall 
  remain 
  visible, 
  

   which 
  amount 
  can 
  be 
  calculated 
  by 
  a 
  known 
  formula. 
  Thus, 
  if 
  we 
  

   want 
  to 
  investigate 
  the 
  performance 
  of 
  a 
  12-inch 
  telescope 
  used 
  in 
  

   examining 
  a 
  planet 
  subtending 
  the 
  angle 
  11 
  e, 
  it 
  is 
  found 
  that 
  the 
  iris- 
  

   diaphragm 
  has 
  to 
  be 
  closed 
  until 
  only 
  the 
  central 
  boss 
  of 
  the 
  concen- 
  

   tration 
  image, 
  the 
  whole 
  of 
  the 
  first 
  appendage 
  ring, 
  and 
  nearly 
  but 
  not 
  

   quite 
  the 
  whole 
  of 
  the 
  second 
  appendage 
  ring, 
  remain 
  visible. 
  Accor- 
  

   dingly, 
  as 
  it 
  is 
  the 
  visibility 
  of 
  this 
  amount 
  of 
  the 
  concentration 
  image 
  

   which 
  is 
  the 
  condition 
  to 
  be 
  secured, 
  the 
  observer 
  is 
  at 
  liberty 
  to 
  make 
  

   the 
  round 
  hole 
  at 
  z' 
  of 
  any 
  size 
  that 
  he 
  finds 
  convenient, 
  provided 
  that 
  

   he 
  then 
  adjusts 
  the 
  iris-diaphragm 
  at 
  C 
  to 
  allow 
  him 
  to 
  see 
  this 
  portion 
  

   of 
  the 
  concentration 
  image. 
  He 
  has 
  then 
  only 
  to 
  illuminate 
  z' 
  by 
  diffused 
  

   light, 
  instead 
  of 
  by 
  light 
  from 
  the 
  pro-star, 
  in 
  order 
  to 
  see 
  it 
  through 
  his 
  

   pro-telescope 
  under 
  the 
  same 
  optical 
  conditions 
  as 
  those 
  under 
  which 
  a 
  

   12-inch 
  astronomical 
  telescope 
  would 
  show 
  the 
  disk 
  of 
  the 
  planet 
  

   Neptune 
  when 
  in 
  opposition. 
  

  

  This 
  last 
  method 
  of 
  adjusting 
  has 
  the 
  practical 
  advantage 
  that 
  it 
  

   dispenses 
  with 
  the 
  necessity 
  of 
  accurately 
  making 
  the 
  round 
  hole 
  at 
  z' 
  of 
  

   a 
  prescribed 
  size, 
  and 
  that 
  what 
  the 
  observer 
  has 
  to 
  do 
  in 
  making 
  the 
  

   . 
  adjustment 
  is 
  easily 
  performed. 
  

  

  