﻿TOTAL 
  ECLIPSE 
  OF 
  THE 
  SUN, 
  JULY 
  28, 
  1851. 
  337 
  

  

  seen 
  on 
  the 
  moon's 
  limb 
  at 
  the 
  eclipse 
  of 
  1842, 
  it 
  seemed 
  very 
  desirable 
  to 
  have 
  

   some 
  means 
  of 
  noting 
  with 
  accuracy 
  the 
  positions 
  of 
  any 
  objects 
  of 
  a 
  similar 
  nature 
  

   that 
  might 
  appear 
  at 
  the 
  approaching 
  eclipse; 
  and 
  in 
  a 
  letter 
  in 
  the 
  Athenaeum 
  

   of 
  12th 
  July 
  1851, 
  I 
  suggested 
  a 
  species 
  of 
  position 
  micrometer 
  suitable 
  for 
  that 
  

   purpose. 
  The 
  instrument 
  there 
  described, 
  with 
  a 
  slight 
  addition, 
  was 
  constructed 
  

   for 
  me 
  by 
  Mr 
  John 
  Adie, 
  and 
  its 
  performance 
  proved 
  very 
  satisfactory. 
  It 
  consists 
  

   of 
  a 
  circular 
  plate 
  of 
  metal, 
  AB, 
  fig. 
  1, 
  (Plate 
  XI.) 
  8 
  inches 
  in 
  diameter, 
  attached 
  

   to 
  the 
  sliding 
  tube 
  of 
  the 
  telescope 
  by 
  a 
  split 
  collar 
  with 
  a 
  tightening 
  screw, 
  

   not 
  seen 
  in 
  the 
  figure, 
  so 
  as 
  to 
  prevent 
  it 
  from 
  turning 
  round. 
  The 
  face 
  of 
  this 
  

   plate, 
  next 
  the 
  eye-end 
  of 
  the 
  telescope, 
  was 
  covered 
  with 
  a 
  disc 
  of 
  card, 
  attached 
  to 
  

   it 
  by 
  four 
  screws, 
  eeee. 
  Inside 
  the 
  tube 
  carrying 
  the 
  plate, 
  another 
  tube 
  carrying 
  

   the 
  eye-piece 
  slid 
  smoothly, 
  so 
  as 
  to 
  admit 
  of 
  being 
  freely 
  turned 
  round. 
  To 
  this 
  

   were 
  attached, 
  by 
  another 
  split 
  collar 
  and 
  clamping 
  screw, 
  two 
  springy 
  arms, 
  FC, 
  

   FD, 
  bearing 
  steel 
  points, 
  by 
  which 
  holes 
  could 
  be 
  pricked 
  in 
  the 
  card 
  disc, 
  and 
  a 
  

   small 
  level, 
  G, 
  was 
  fixed 
  at 
  right 
  angles 
  to 
  one 
  of 
  the 
  arms. 
  In 
  the 
  eye-piece 
  were 
  

   three 
  equidistant 
  parallel 
  spider-lines, 
  ah, 
  cd, 
  ef, 
  fig. 
  2 
  ; 
  the 
  two 
  outer, 
  ab, 
  cd, 
  being 
  

   placed 
  at 
  an 
  interval 
  equal 
  to 
  the 
  apparent 
  diameter 
  of 
  the 
  moon, 
  calcu- 
  

   lated 
  for 
  the 
  time 
  of 
  the 
  total 
  phase 
  of 
  the 
  eclipse 
  ; 
  so 
  that 
  when 
  they 
  were 
  made 
  

   to 
  embrace 
  the 
  moon's 
  disc, 
  gh, 
  the 
  middle 
  wire 
  would 
  pass 
  through 
  its 
  centre, 
  o. 
  

   The 
  instrument 
  was 
  adjusted 
  for 
  observation 
  by 
  making 
  the 
  middle 
  wire 
  coincide 
  

   with 
  a 
  plumb 
  line, 
  seen 
  at 
  a 
  distance 
  of 
  about 
  150 
  yards, 
  while 
  at 
  the 
  same 
  time 
  

   the 
  bubble 
  of 
  the 
  level 
  was 
  brought 
  to 
  the 
  middle 
  of 
  its 
  tube 
  ; 
  and 
  the 
  arms 
  with 
  

   the 
  level 
  were 
  then 
  clamped 
  to 
  the 
  tube 
  carrying 
  the 
  eye-piece. 
  

  

  When 
  this 
  adjustment 
  was 
  completed, 
  it 
  is 
  obvious 
  that 
  the 
  wires 
  in 
  the 
  eye- 
  

   piece 
  would 
  point 
  vertically 
  whenever 
  the 
  bubble 
  of 
  the 
  level 
  was 
  again 
  brought 
  to 
  

   the 
  middle 
  of 
  the 
  tube. 
  If 
  now 
  the 
  bubble 
  were 
  brought 
  to 
  the 
  middle 
  of 
  the 
  

   tube, 
  while 
  the 
  outer 
  wires 
  were 
  made 
  to 
  embrace 
  the 
  moon's 
  disc, 
  the 
  middle 
  

   wire 
  would 
  pass 
  through 
  its 
  vertex, 
  g 
  ; 
  and 
  two 
  holes 
  being 
  pricked 
  in 
  the 
  card, 
  

   the 
  line 
  joining 
  them 
  would 
  represent 
  the 
  moon's 
  (or, 
  with 
  sufficient 
  accuracy, 
  

   the 
  sun's) 
  vertical 
  diameter 
  at 
  the 
  moment 
  of 
  observation. 
  If 
  next, 
  while 
  the 
  

   moon 
  was 
  still 
  kept 
  between 
  the 
  outer 
  wires, 
  the 
  middle 
  wire 
  were 
  made 
  to 
  bisect 
  

   any 
  object, 
  A, 
  near 
  its 
  limb, 
  the 
  wires 
  now 
  having 
  the 
  positions 
  a'b', 
  cfd', 
  e 
  1 
  /', 
  and 
  

   holes 
  were 
  again 
  made 
  in 
  the 
  card, 
  the 
  angle 
  between 
  the 
  lines 
  joining 
  the 
  

   respective 
  pairs 
  of 
  holes 
  would 
  measure, 
  goh, 
  the 
  angular 
  distance 
  of 
  the 
  object 
  

   from 
  the 
  sun's 
  vertex. 
  It 
  is 
  easy 
  to 
  see 
  how, 
  in 
  this 
  manner, 
  the 
  positions 
  of 
  

   the 
  red 
  prominences 
  seen 
  during 
  a 
  total 
  eclipse, 
  could 
  be 
  rapidly 
  registered 
  on 
  

   the 
  card 
  without 
  ever 
  removing 
  the 
  eye 
  from 
  the 
  telescope. 
  In 
  order 
  to 
  repeat 
  

   the 
  observations, 
  the 
  steel 
  points 
  admitted 
  of 
  being 
  moved 
  in 
  longitudinal 
  slits 
  

   in 
  the 
  arms, 
  so 
  as 
  to 
  describe 
  circles 
  of 
  different 
  radii 
  on 
  the 
  card; 
  and 
  the 
  

   reading 
  point 
  was 
  distinguished 
  from 
  the 
  other 
  by 
  being 
  placed 
  a 
  little 
  farther 
  

   from 
  the 
  centre. 
  

  

  