﻿334 
  Br. 
  G. 
  Johnstone 
  Stoney 
  on 
  

  

  image 
  when 
  focussed 
  on 
  the 
  copper 
  -foil 
  S', 
  which 
  it 
  is 
  difficult 
  

   to 
  keep 
  on 
  the 
  small 
  hole. 
  Nevertheless 
  these 
  artificial 
  

   illuminants 
  have 
  sometimes 
  been 
  used, 
  when 
  sunshine 
  was 
  

   not 
  available. 
  

  

  25. 
  In 
  making 
  our 
  observations 
  two 
  images 
  have 
  to 
  be 
  

   studied 
  — 
  the 
  telescopic 
  image 
  of 
  z 
  as 
  presented 
  by 
  the 
  pro- 
  

   telescope 
  T', 
  and 
  the 
  concentration 
  image 
  produced 
  by 
  the 
  same 
  

   light 
  at 
  y\ 
  which 
  is 
  to 
  be 
  examined 
  through 
  the 
  Steinheil 
  lens. 
  

   When 
  the 
  sun 
  is 
  the 
  source 
  of 
  light, 
  the 
  concentration 
  image 
  

   is 
  usually 
  seen 
  in 
  great 
  perfection, 
  but 
  the 
  telescopic 
  image 
  

   is 
  apt 
  to 
  be 
  too 
  bright 
  for 
  the 
  eye. 
  When 
  this 
  is 
  the 
  case, 
  a 
  

   dark 
  glass 
  may 
  be 
  put 
  either 
  before 
  the 
  eye, 
  or 
  close 
  beyond 
  

   z', 
  or 
  near 
  s' 
  ; 
  in 
  any 
  of 
  which 
  positions 
  moderate 
  imperfection 
  

   in 
  the 
  dark 
  glass 
  will 
  not 
  spoil 
  the 
  image. 
  The 
  author 
  

   usually 
  hangs 
  the 
  dark 
  glass 
  just 
  beyond 
  s' 
  ; 
  where 
  also 
  

   another 
  dark 
  glass 
  is 
  mounted 
  through 
  which 
  to 
  view 
  the 
  

   inconveniently 
  bright 
  image 
  of 
  the 
  sun 
  formed 
  by 
  lens 
  G 
  

   when 
  adjusting 
  the 
  heliostat 
  to 
  throw 
  the 
  sun's 
  image 
  where 
  

   the 
  hole 
  s 
  1 
  is. 
  

  

  When 
  setting 
  up 
  the 
  apparatus 
  the 
  two 
  following 
  disposi- 
  

   tions 
  have 
  to 
  be 
  attended 
  to. 
  D' 
  is 
  to 
  be 
  one 
  metre 
  ; 
  and 
  

   the 
  object 
  z' 
  has 
  to 
  be 
  pushed 
  into 
  its 
  proper 
  position. 
  It 
  

   has 
  already 
  been 
  explained 
  how 
  to 
  secure 
  that 
  D' 
  shall 
  be 
  a 
  

   metre. 
  To 
  get 
  z 
  into 
  its 
  place 
  the 
  procedure 
  described 
  

   below 
  was 
  found 
  convenient. 
  

  

  26. 
  The 
  objects 
  at 
  z' 
  are 
  usually 
  openings 
  of 
  various 
  

   shapes 
  and 
  sizes 
  made 
  in 
  small 
  pieces 
  of 
  copper-foil. 
  The 
  

   copper-foil 
  is 
  about 
  ^ 
  mm. 
  thick, 
  and 
  has 
  been 
  softened 
  

   by 
  heating 
  it 
  red-hot 
  in 
  a 
  JBunsen's 
  burner. 
  Round 
  and 
  

   triangular 
  holes, 
  to 
  represent 
  objects 
  upon 
  the 
  planet 
  of 
  those 
  

   shapes, 
  can 
  be 
  conveniently 
  made 
  by 
  ordinary 
  and 
  glovers' 
  

   needles. 
  For 
  rectangular 
  and 
  other 
  polygonal 
  openings 
  the 
  

   author 
  crosses 
  pieces 
  of 
  the 
  softened 
  foil 
  out 
  of 
  which 
  strips 
  

   have 
  been 
  cut 
  with 
  a 
  small 
  pair 
  of 
  scissors. 
  The 
  objects 
  z 
  f 
  , 
  

   whether 
  of 
  this 
  kind 
  or 
  any 
  other, 
  are 
  brought 
  into 
  position 
  

   by 
  the 
  following 
  contrivance. 
  A 
  small 
  piece 
  of 
  board 
  is 
  

   fastened 
  to 
  the 
  steel 
  blade 
  of 
  an 
  ordinary 
  carpenter's 
  square, 
  

   and 
  two 
  pieces 
  of 
  the 
  copper-foil 
  about 
  2\ 
  inches 
  square 
  are 
  

   screwed 
  to 
  one 
  edge 
  of 
  the 
  wood 
  so 
  that 
  when 
  the 
  carpenter's 
  

   square 
  is 
  made 
  to 
  slide 
  along 
  the 
  board 
  on 
  which 
  the 
  apparatus 
  

   is 
  mounted, 
  it 
  will 
  push 
  the 
  squares 
  of 
  foil 
  up 
  against 
  lens 
  I/. 
  

   A 
  round 
  opening 
  about 
  half 
  an 
  inch 
  across 
  is 
  provided 
  in 
  

   both 
  the 
  squares 
  of 
  copper, 
  opposite 
  to 
  the 
  middle 
  of 
  lens 
  I/. 
  

   The 
  strip 
  of 
  copper 
  or 
  glass 
  holding 
  whatever 
  is 
  to 
  be 
  viewed 
  

   at 
  z 
  is 
  to 
  be 
  slipped 
  between 
  the 
  squares 
  of 
  foil, 
  and 
  the 
  

   object 
  itself 
  is 
  to 
  be 
  brought 
  to 
  the 
  middle 
  of 
  the 
  half 
  -inch 
  

  

  