﻿PROFESSOR 
  PIAZZI 
  SMYTH 
  ON 
  COMETARY 
  PHYSICS. 
  135 
  

  

  merely 
  so 
  much 
  the 
  more 
  difficult 
  to 
  distinguish, 
  by 
  reason 
  of 
  the 
  well-known 
  optical 
  or 
  physiological 
  

   law, 
  that 
  a 
  small 
  difference 
  in 
  the 
  brightness 
  of 
  two 
  objects 
  is 
  more 
  difficult 
  to 
  perceive 
  in 
  proportion 
  

   to 
  their 
  absolute 
  brightness. 
  If 
  our 
  sensation 
  and 
  means 
  of 
  measurement 
  are 
  not 
  sufficiently 
  accurate 
  

   in 
  this 
  case 
  of 
  the 
  thin 
  cloud, 
  we 
  have 
  only 
  to 
  turn 
  to 
  a 
  thicker 
  cloud 
  (of 
  the 
  same 
  species, 
  and 
  in 
  a 
  

   similar 
  part 
  of 
  the 
  sky 
  with 
  regard 
  to 
  the 
  sun), 
  and 
  there 
  we 
  shall 
  see 
  the 
  same 
  law 
  which 
  must 
  

   obtain 
  in 
  the 
  former 
  case 
  now 
  visibly 
  developed 
  ; 
  and 
  tben 
  we 
  come 
  to 
  the 
  necessary 
  conclusion, 
  that 
  

   the 
  illuminated 
  side 
  of 
  every 
  cloud 
  must 
  be 
  brighter 
  than 
  the 
  other, 
  i. 
  e., 
  that 
  it 
  must^shew 
  some 
  

   phase. 
  

  

  The 
  comets 
  are 
  undoubtedly 
  far 
  rarer 
  than 
  any 
  description 
  of 
  cloud 
  floating 
  in 
  our 
  atmosphere, 
  

   but 
  they 
  are 
  seen 
  under 
  far 
  more 
  favourable 
  circumstances 
  for 
  exhibiting 
  a 
  phase 
  ; 
  for, 
  they 
  are 
  

   illuminated 
  by 
  the 
  sun 
  from 
  one 
  end, 
  so 
  that 
  there 
  must 
  be 
  a 
  much 
  greater 
  difference 
  of 
  intensity 
  of 
  

   light 
  at 
  the 
  two 
  ends 
  than 
  there 
  would 
  be 
  at 
  the 
  two 
  sides, 
  if 
  transversely 
  lighted, 
  as 
  with 
  the 
  long 
  

   thin 
  films 
  of 
  bright 
  cloud 
  alluded 
  to 
  ; 
  and, 
  further, 
  the 
  comet 
  being 
  of 
  the 
  last 
  degree 
  of 
  faintness, 
  the 
  

   eye 
  is 
  much 
  better 
  able 
  to 
  detect 
  small 
  differences 
  of 
  luminosity. 
  Then 
  again, 
  comets, 
  though 
  they 
  

   be 
  exceedingly 
  rare, 
  are 
  very 
  voluminous, 
  so 
  that 
  the 
  rays 
  of 
  light 
  bave 
  to 
  traverse 
  a 
  great 
  space 
  of 
  

   matter 
  in 
  passing 
  through 
  them 
  ; 
  and 
  if 
  some 
  is 
  reflected 
  in 
  the 
  anterior 
  parts 
  of 
  the 
  body, 
  as 
  we 
  

   see 
  is 
  the 
  case 
  by 
  the 
  fact 
  that 
  the 
  body 
  is 
  rendered 
  visible 
  to 
  us, 
  there 
  cannot 
  possibly 
  be 
  so 
  

   strong 
  an 
  illumination 
  on 
  the 
  posterior 
  parts 
  ; 
  therefore, 
  we 
  shall 
  either 
  see 
  them 
  fainter 
  than 
  the 
  

   others 
  ; 
  or 
  not 
  at 
  all, 
  if 
  the 
  anterior 
  portions 
  themselves 
  are 
  but 
  just 
  visible. 
  

  

  With 
  these 
  preliminaries 
  then, 
  we 
  may 
  ask, 
  what 
  comet 
  has 
  ever 
  been 
  seen 
  without 
  some 
  phase 
  ? 
  

   for 
  in 
  every 
  single 
  instance, 
  the 
  anterior 
  part 
  of 
  the 
  head, 
  or 
  the 
  denser 
  portion 
  of 
  the 
  envelope, 
  has 
  

   been 
  brighter 
  than 
  the 
  posterior, 
  exhibiting 
  sometimes 
  the 
  appearance 
  of 
  a 
  luminous 
  sector 
  in 
  front 
  ; 
  

   and 
  the 
  anterior 
  half 
  only, 
  of 
  the 
  body, 
  has 
  been 
  seen, 
  the 
  comet 
  presenting 
  as 
  a 
  general 
  rule 
  two 
  

   diverging 
  and 
  slightly 
  curved 
  tails. 
  This 
  has 
  been 
  generally 
  held 
  to 
  be 
  merely 
  the 
  effect 
  of 
  looking 
  

   transversely 
  through 
  a 
  conical 
  envelope 
  of 
  luminous 
  matter, 
  when 
  the 
  ray 
  of 
  light 
  passing 
  through 
  the 
  

   central 
  portions 
  would 
  meet 
  with 
  less 
  substance, 
  and 
  that 
  part 
  would 
  therefore 
  appear 
  darker 
  than 
  

   the 
  limbs. 
  This, 
  doubtless, 
  prevails 
  to 
  a 
  great 
  extent, 
  but 
  then 
  we 
  must 
  further 
  remember, 
  that 
  the 
  

   exterior 
  coats 
  of 
  the 
  envelope 
  will 
  be 
  more 
  strongly 
  illuminated 
  than 
  the 
  interior 
  ; 
  and 
  the 
  dark 
  axis 
  

   of 
  the 
  comet's 
  tail 
  becomes 
  therefore 
  a 
  particular 
  character 
  of 
  phase. 
  Further, 
  as 
  we 
  proceed 
  to 
  the 
  

   posterior 
  portions 
  of 
  even 
  the 
  outer 
  coats 
  of 
  the 
  envelope, 
  they 
  will 
  be 
  illuminated 
  by 
  a 
  weaker 
  light 
  

   from 
  the 
  sun, 
  by 
  reason 
  of 
  their 
  greater 
  distance 
  ; 
  and 
  if 
  any 
  convergence 
  of 
  them 
  towards 
  the 
  axis 
  

   should 
  occur, 
  as 
  has 
  actually 
  been 
  observed 
  in 
  some 
  cases, 
  their 
  illuminating 
  rays 
  being 
  then 
  still 
  

   further 
  diminished 
  in 
  intensity 
  by 
  absorption 
  and 
  reflection, 
  they 
  will 
  hardly 
  be 
  enabled 
  to 
  make 
  them- 
  

   selves 
  visible 
  to 
  us. 
  Thus, 
  the 
  diverging 
  limbs 
  of 
  the 
  tail, 
  and 
  its 
  forked 
  or 
  many-pointed 
  termination, 
  

   becomes 
  an 
  effect 
  of 
  phase 
  on 
  a 
  body 
  which 
  may 
  be 
  of 
  a 
  symmetrical 
  and 
  rounded, 
  and 
  complete 
  character. 
  

   This 
  point, 
  it 
  is 
  of 
  the 
  greatest 
  importance 
  to 
  determine, 
  for 
  if 
  the 
  actual 
  forms 
  of 
  comets 
  be 
  as 
  

   we 
  see 
  them, 
  they 
  are 
  altogether 
  anomalous 
  in 
  the 
  heavenly 
  regions 
  ; 
  and 
  merely 
  on 
  the 
  score 
  of 
  the 
  

   form 
  of 
  these 
  supposed 
  conical 
  envelopes 
  and 
  diverging 
  streamers, 
  equally 
  anomalous 
  forces 
  have 
  been 
  

   introduced 
  to 
  explain 
  the 
  phenomena 
  ; 
  electricity 
  and 
  polarity, 
  which 
  have 
  no 
  place 
  in 
  any 
  other 
  

   department 
  of 
  astronomy, 
  being 
  allowed 
  precedence 
  here. 
  

  

  Granting, 
  that 
  a 
  comet 
  is 
  always 
  a 
  prolate 
  spheroidal 
  mass 
  of 
  vapour 
  of 
  different 
  degrees 
  of 
  

   prolateness, 
  and 
  of 
  actual 
  length 
  in 
  various 
  cases, 
  but 
  always 
  illumined 
  from 
  one 
  end, 
  then 
  we 
  may 
  

   expect 
  in 
  the 
  larger 
  and 
  denser 
  comets 
  to 
  see 
  but 
  the 
  anterior 
  half 
  of 
  the 
  body 
  ; 
  the 
  posterior 
  half 
  

   being 
  so 
  much 
  further 
  off 
  from 
  the 
  sun, 
  and 
  the 
  rays 
  of 
  light 
  which 
  reach 
  it, 
  being 
  further 
  so 
  much 
  

   weakened 
  by 
  having 
  passed 
  through 
  the 
  first 
  half; 
  consequently, 
  in 
  this 
  description 
  of 
  comet, 
  we 
  

   might 
  expect, 
  and 
  we 
  absolutely 
  do 
  find, 
  the 
  phenomenon 
  of 
  the 
  forked 
  tail 
  most 
  marked. 
  In 
  the 
  

   smaller 
  and 
  fainter 
  comets, 
  on 
  the 
  other 
  hand, 
  the 
  rays 
  of 
  light 
  which 
  reach 
  the 
  posterior 
  half 
  of 
  the 
  

   body 
  are 
  not 
  much 
  dimmed 
  either 
  from 
  having 
  passed 
  through 
  the 
  excessively 
  tenuous 
  anterior 
  por- 
  

   tion, 
  or 
  from 
  having 
  travelled 
  through 
  any 
  notably 
  greater 
  distance 
  from 
  the 
  centre 
  of 
  radiation 
  ; 
  in 
  

   such 
  cases 
  we 
  may 
  expect 
  to 
  see 
  more 
  completely 
  the 
  whole 
  form 
  of 
  the 
  comet 
  ; 
  and 
  in 
  them 
  we 
  do 
  

   actually 
  find 
  nearly, 
  and 
  sometimes 
  quite 
  oval 
  forms, 
  and 
  all 
  gradations 
  from 
  these, 
  through 
  truncated 
  

   ovals 
  to 
  the 
  foi-ked 
  tails. 
  

  

  These 
  facts 
  induce 
  us 
  to 
  admit 
  the 
  possibility 
  of 
  the 
  bodies 
  of 
  comets 
  being 
  of 
  a 
  far 
  more 
  regular 
  

   geometric 
  form 
  than 
  has 
  hitherto 
  been 
  suspected, 
  if 
  we 
  allow 
  that 
  conclusions 
  derived 
  from 
  small 
  

   comets 
  may 
  be 
  safely 
  so 
  extended, 
  mutatis 
  mutandis, 
  to 
  large 
  ones 
  ; 
  but 
  this 
  view 
  is 
  further 
  confirmed 
  

   by 
  a 
  notable 
  observation 
  of 
  one 
  of 
  the 
  largest 
  and 
  most 
  excentric 
  of 
  comets. 
  

  

  As 
  already 
  observed, 
  comets 
  decrease 
  in 
  size 
  and 
  increase 
  in 
  density 
  on 
  approaching 
  the 
  peri- 
  

   helion, 
  and 
  the 
  reverse 
  on 
  receding 
  therefrom 
  ; 
  hence 
  the 
  phase 
  ought 
  to 
  be 
  most 
  evident, 
  or 
  the 
  tail 
  

  

  VOL. 
  XX. 
  PART 
  I. 
  2 
  O 
  

  

  