﻿PHENOMENA 
  OF 
  THE 
  ZODIACAL 
  LIGHT. 
  493 
  

  

  In 
  order 
  to 
  give 
  a 
  clearer 
  description 
  of 
  what 
  may 
  be 
  expected 
  to 
  be 
  seen, 
  

   than 
  can 
  be 
  conveyed 
  in 
  words 
  alone, 
  I 
  have 
  subjoined 
  a 
  number 
  of 
  drawings,* 
  

   both 
  of 
  what 
  the 
  zodiacal 
  light 
  is, 
  and 
  what 
  it 
  is 
  not, 
  the 
  latter 
  being 
  the 
  great 
  

   comet 
  of 
  1668 
  and 
  1843, 
  mistaken 
  on 
  both 
  occasions 
  for 
  the 
  more 
  permanent 
  

   members 
  of 
  the 
  system. 
  The 
  object 
  in 
  the 
  construction 
  of 
  the 
  drawings 
  has 
  been, 
  

   in 
  so 
  far 
  as 
  it 
  was 
  possible 
  to 
  be 
  compassed 
  by 
  the 
  small 
  skill 
  of 
  the 
  author, 
  to 
  

   give 
  so 
  complete 
  a 
  reproduction 
  of 
  all 
  the 
  attendant 
  phenomena, 
  and 
  circumstances 
  

   of 
  climate 
  or 
  country, 
  as 
  to 
  enable 
  any 
  one 
  who 
  looks 
  at 
  them, 
  to 
  form 
  a 
  tolerable 
  

   idea, 
  whether 
  any 
  of 
  the 
  accompanying 
  conditions 
  under 
  which 
  the 
  original 
  view 
  

   was 
  obtained, 
  were 
  likely 
  to 
  produce 
  an 
  erroneous 
  judgment 
  in 
  the 
  spectator, 
  of 
  

   the 
  exact 
  form 
  and 
  appearance 
  of 
  the 
  zodiacal 
  light. 
  A 
  larger 
  portion 
  of 
  the 
  land- 
  

   scape 
  has 
  therefore 
  been 
  introduced, 
  than 
  would 
  otherwise 
  have 
  been 
  altogether 
  

   appropriate 
  in 
  astronomical 
  drawings. 
  

  

  A 
  more 
  important 
  addition 
  is, 
  however, 
  that 
  which 
  I 
  was 
  advised 
  to 
  make 
  by 
  

   my 
  friend 
  Mr 
  W. 
  A. 
  Cadell, 
  viz., 
  the 
  insertion 
  in 
  the 
  margin 
  of 
  the 
  circles 
  of 
  

   Right 
  Ascension 
  and 
  Declination, 
  which 
  shew 
  what 
  particular 
  projection 
  has 
  

   been 
  employed, 
  and 
  serve 
  to 
  identify 
  the 
  stars, 
  fix 
  the 
  latitude 
  in 
  which 
  the 
  ob- 
  

   servations 
  were 
  made, 
  the 
  time 
  at 
  the 
  instant, 
  and 
  to 
  give 
  an 
  idea 
  of 
  the 
  dimen- 
  

   sions 
  of 
  the 
  body 
  under 
  discussion, 
  and 
  the 
  region 
  of 
  the 
  sky 
  in 
  which 
  it 
  is 
  found. 
  

  

  To 
  represent 
  the 
  eastern 
  or 
  western 
  portion 
  of 
  the 
  sky, 
  in 
  their 
  perfection, 
  or 
  

   as 
  would 
  be 
  thrown 
  on 
  paper 
  by 
  the 
  camera 
  lucida, 
  as 
  Mr 
  Cadell 
  has 
  shewn, 
  

   the 
  horizon 
  should 
  be 
  defined 
  by 
  a 
  straight 
  line 
  in 
  the 
  picture, 
  and 
  the 
  E. 
  or 
  W. 
  

   point 
  must 
  be 
  in 
  the 
  middle 
  of 
  that 
  line 
  ; 
  then 
  the 
  eye 
  of 
  the 
  spectator 
  being 
  

   directed 
  toward 
  it 
  as 
  such, 
  the 
  equator, 
  being 
  a 
  great 
  circle, 
  will 
  be 
  represented 
  

   by 
  a 
  straight 
  line 
  drawn 
  through 
  that 
  point, 
  and 
  rising 
  at 
  an 
  angle 
  to 
  the 
  hori- 
  

   zon, 
  equal 
  to 
  the 
  latitude 
  of 
  the 
  place 
  ; 
  and 
  the 
  meridian 
  lines 
  on 
  the 
  parallels 
  of 
  

   right 
  ascension, 
  being 
  also 
  great 
  circles, 
  must 
  be 
  expressed 
  by 
  straight 
  lines 
  cross- 
  

   ing 
  the 
  equator 
  at 
  right 
  angles 
  ; 
  on 
  the 
  other 
  hand, 
  the 
  parallels 
  of 
  declination 
  

   being 
  small 
  circles, 
  will 
  appear 
  as 
  conoidal 
  curves. 
  A 
  great 
  circle 
  becomes 
  a 
  

   straight 
  line 
  on 
  the 
  picture, 
  since 
  it 
  is 
  a 
  plane 
  passing 
  through 
  the 
  eye, 
  and 
  the 
  

   common 
  section 
  of 
  this 
  plane 
  with 
  the 
  planes 
  of 
  the 
  picture, 
  is 
  a 
  straight 
  line. 
  A 
  

   small 
  circle 
  is 
  a 
  conoidal 
  curve 
  on 
  the 
  picture, 
  because 
  a 
  small 
  circle 
  is 
  seen 
  as 
  

   a 
  cone 
  of 
  which 
  the 
  apex 
  is 
  at 
  the 
  eye, 
  and 
  the 
  common 
  section 
  of 
  this 
  cone 
  with 
  

   the 
  plane 
  of 
  the 
  picture 
  is 
  a 
  conic 
  section. 
  The 
  form 
  of 
  the 
  conic 
  section 
  will 
  vary 
  

   as 
  the 
  inclination 
  of 
  the 
  cone's 
  axis 
  to 
  the 
  plane 
  of 
  the 
  picture 
  varies. 
  

  

  In 
  all 
  the 
  drawings 
  given 
  herewith, 
  the 
  line 
  of 
  sight 
  is 
  seldom 
  directed 
  exactly 
  

   to 
  the 
  E. 
  or 
  W. 
  points, 
  but 
  generally 
  between 
  them 
  and 
  the 
  northern 
  or 
  (point 
  of 
  

   culmination 
  for 
  the 
  southern 
  hemisphere.) 
  Were 
  the 
  spectator 
  to 
  face 
  the 
  

  

  * 
  On 
  account 
  of 
  the 
  expense 
  of 
  first-class 
  engravings, 
  one 
  of 
  the 
  drawings 
  only 
  has 
  heen 
  put 
  

   upon 
  metal. 
  This 
  one, 
  " 
  the 
  appearance 
  of 
  the 
  zodiacal 
  light 
  at 
  the 
  Cape 
  of 
  Good 
  Hope, 
  in 
  July 
  

   1845," 
  will 
  be 
  found 
  amongst 
  the 
  plates 
  at 
  the 
  end 
  of 
  the 
  volume. 
  

  

  VOL. 
  XX. 
  PART 
  III. 
  6 
  R 
  

  

  