﻿144 
  PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  

  

  great 
  length 
  and 
  small 
  breadth 
  of 
  the 
  comet 
  itself. 
  The 
  first 
  day 
  that 
  I 
  saw 
  the 
  

   comet, 
  three 
  days 
  after 
  perihelio, 
  it 
  was 
  not 
  visible 
  ; 
  but, 
  clearly 
  seen 
  on 
  every 
  sub- 
  

   sequent 
  day, 
  it 
  existed 
  until 
  the 
  whole 
  was 
  lost 
  in 
  faintness. 
  

  

  The 
  observation 
  of 
  these 
  bodies 
  is 
  not, 
  however, 
  as 
  hinted 
  above, 
  in 
  a 
  suffi- 
  

   ciently 
  satisfactory 
  state 
  for 
  rigidly 
  testing 
  any 
  calculable 
  theory. 
  This 
  depends 
  not 
  

   only 
  on 
  the 
  rarity 
  of 
  the 
  appearance 
  of 
  comets 
  (a 
  matter 
  beyond 
  our 
  control) 
  ; 
  but 
  

   also 
  on 
  the 
  insufficient 
  means 
  with 
  which, 
  and 
  the 
  untoward 
  circumstances 
  under 
  

   which 
  they 
  are 
  generally 
  observed. 
  The 
  telescopes 
  usually 
  turned 
  upon 
  them, 
  

   have 
  been 
  of 
  so 
  small 
  an 
  optical 
  power, 
  that 
  they 
  would 
  have 
  been 
  considered 
  

   utterly 
  incompetent 
  for 
  ascertaining 
  the 
  nature 
  of 
  nebulae 
  high 
  up 
  in 
  the 
  sky 
  ; 
  how 
  

   much 
  more 
  so, 
  when 
  employed 
  on 
  nebulous 
  objects 
  close 
  to 
  the 
  horizon, 
  as 
  the 
  

   comets 
  usually 
  are, 
  flickering 
  and 
  faint 
  in 
  vapour 
  and 
  smoke, 
  and 
  almost 
  over- 
  

   powered 
  by 
  the 
  strength 
  of 
  the 
  twilight. 
  

  

  But 
  a 
  sufficiently 
  powerful 
  telescope 
  need 
  not 
  any 
  longer 
  be 
  a 
  difficulty, 
  since 
  

   the 
  publication 
  of 
  the 
  inventions 
  of 
  the 
  Earl 
  of 
  Rosse, 
  and 
  Messrs 
  Lassel 
  and 
  

   Nasmyth 
  ; 
  and 
  the 
  effect 
  of 
  the 
  vapour 
  of 
  the 
  horizon, 
  and 
  the 
  glare 
  of 
  twilight, 
  

   might 
  be 
  successfully 
  overcome, 
  by 
  establishing 
  an 
  observatory 
  on 
  high 
  land 
  

   within 
  the 
  tropics, 
  where 
  the 
  geographical 
  position 
  renders 
  the 
  twilight 
  short 
  

   even 
  to 
  the 
  plains 
  ; 
  and 
  the 
  rarity 
  of 
  mountain 
  air 
  would 
  still 
  further 
  reduce 
  the 
  

   reflective 
  power 
  of 
  the 
  atmosphere. 
  Micrometrical 
  measures, 
  with 
  such 
  instru- 
  

   ments, 
  and 
  under 
  such 
  circumstances, 
  should 
  be 
  combined 
  with 
  photometrical 
  deter- 
  

   minations 
  of 
  the 
  brightness 
  of 
  the 
  various 
  parts 
  of 
  the 
  comet, 
  and 
  of 
  the 
  background 
  

   of 
  the 
  sky. 
  The 
  former 
  observations 
  are 
  easy 
  and 
  straight 
  forward 
  enough, 
  but 
  

   the 
  latter 
  are 
  difficult 
  and 
  new 
  ; 
  the 
  zero 
  must 
  inevitably 
  be 
  taken 
  from 
  a 
  stellar 
  

   scale, 
  but 
  none 
  such 
  exists 
  at 
  present 
  ; 
  for 
  the 
  telescope 
  measure 
  has 
  invariably 
  

   failed 
  whenever 
  employed 
  for 
  the 
  purpose, 
  and 
  the 
  eye 
  is 
  still 
  thought 
  the 
  best 
  

   available 
  mean. 
  Hence, 
  none 
  but 
  the 
  brightest 
  stars 
  have 
  had 
  their 
  magni- 
  

   tudes 
  determined, 
  and 
  that 
  but 
  coarsely, 
  while 
  the 
  great 
  question 
  still 
  remains 
  

   in 
  much 
  the 
  same 
  state 
  as 
  that 
  in 
  which 
  the 
  application 
  of 
  the 
  telescope 
  to 
  divided 
  

   instruments 
  was 
  in, 
  before 
  men 
  had 
  learnt 
  how 
  to 
  determine 
  the 
  error 
  of 
  colli- 
  

   mation. 
  They 
  knew 
  that 
  there 
  were 
  vast 
  powers 
  of 
  accuracy 
  in 
  the 
  optic 
  tubes, 
  

   but 
  were 
  afraid 
  of 
  great 
  and 
  mysterious 
  errors, 
  which 
  they 
  neither 
  exactly 
  

   understood, 
  nor 
  saw 
  how 
  to 
  correct. 
  Similarly 
  in 
  photometr} 
  7 
  ", 
  a 
  telescope 
  of 
  

   large 
  aperture, 
  is 
  confessed 
  to 
  have 
  a 
  larger 
  scale 
  and 
  range 
  than 
  the 
  unassisted 
  

   eye, 
  but 
  is 
  suspected 
  of 
  misleading 
  to 
  a 
  greater 
  extent. 
  

  

  This 
  is 
  hardly 
  the 
  place 
  for 
  entering 
  into 
  such 
  an 
  experimental 
  branch 
  of 
  

   practical 
  astronomy 
  ; 
  for 
  pointing 
  out 
  what 
  appears 
  to 
  be 
  the 
  error 
  of 
  the 
  methods 
  

   adopted 
  by 
  others 
  ; 
  and 
  for 
  shewing 
  the 
  correctness 
  and 
  efficacy, 
  as 
  I 
  believe, 
  of 
  

   another 
  plan, 
  which 
  might 
  be 
  adapted 
  to 
  telescopes 
  of 
  any 
  size. 
  But 
  there 
  can 
  

  

  