﻿PHENOMENA 
  OF 
  THE 
  ZODIACAL 
  LIGHT. 
  497 
  

  

  of 
  the 
  two 
  objects, 
  however, 
  will 
  probably 
  convince 
  every 
  one, 
  that 
  Chardin's 
  

   Persian 
  expression 
  " 
  niazouk," 
  or 
  in 
  French 
  " 
  petit 
  lance," 
  which 
  was 
  applied 
  by 
  

   the 
  Persians 
  to 
  the 
  phenomenon 
  they 
  saw, 
  conld 
  only 
  be 
  considered 
  as 
  at 
  all 
  

   suitable 
  in 
  the 
  case 
  of 
  the 
  comet's 
  tail. 
  

  

  In 
  1683, 
  the 
  subject 
  was 
  taken 
  up 
  by 
  Dominic 
  Cassini, 
  and 
  to 
  him 
  belongs 
  the 
  

   merit 
  of 
  first 
  scientifically 
  investigating 
  the 
  laws 
  of 
  the 
  phenomenon, 
  determining 
  

   its 
  cosmical 
  nature, 
  and 
  giving 
  it 
  the 
  appropriate 
  name 
  of 
  the 
  zodiacal 
  light. 
  His 
  

   series 
  of 
  observations, 
  extending 
  over 
  nearly 
  six 
  years, 
  is 
  still 
  unrivalled 
  ; 
  and 
  if 
  

   he 
  is 
  not 
  correct 
  in 
  all 
  his 
  conclusions, 
  it 
  is 
  chiefly 
  because 
  his 
  observations 
  were 
  

   almost 
  entirely 
  confined 
  to 
  his 
  own 
  high 
  northern 
  latitude 
  ; 
  and 
  were 
  therefore 
  

   affected 
  to 
  a 
  great 
  and 
  unknown 
  extent 
  by 
  circumstances 
  of 
  climate 
  and 
  geogra- 
  

   phical 
  position. 
  He 
  had 
  much 
  wished 
  to 
  eliminate 
  these 
  effects 
  by 
  means 
  of 
  ob- 
  

   servations 
  made 
  in 
  the 
  southern 
  hemisphere, 
  but 
  unfortunately 
  was 
  not 
  able 
  to 
  

   obtain 
  any 
  ; 
  and 
  indeed 
  those 
  which 
  have 
  been 
  made 
  by 
  the 
  author, 
  and 
  recorded 
  

   in 
  this 
  paper, 
  are 
  perhaps 
  the 
  first 
  which 
  have 
  been 
  published 
  and 
  brought 
  to 
  

   bear 
  on 
  the 
  theory 
  of 
  the 
  subject. 
  

  

  Cassini' 
  s 
  conclusions 
  were, 
  that 
  the 
  zodiacal 
  light 
  is 
  a 
  flat 
  luminous 
  ring 
  en- 
  

   circling 
  the 
  sun, 
  nearly 
  in 
  the 
  plane 
  of 
  his 
  equator, 
  and 
  is 
  therefore 
  seen 
  always 
  

   more 
  or 
  less 
  in 
  profile, 
  and 
  perfectly 
  so 
  at 
  two 
  periods 
  of 
  the 
  year, 
  April 
  and 
  August, 
  

   when 
  like 
  Saturn's 
  ring, 
  and 
  for 
  similar 
  reasons, 
  he 
  supposed 
  it 
  to 
  vanish 
  to 
  our 
  

   sight 
  ; 
  while 
  the 
  nonvisibility 
  at 
  any 
  period 
  between 
  these 
  two 
  months, 
  he 
  con- 
  

   sidered 
  to 
  be 
  produced 
  mainly 
  by 
  the 
  overpowering 
  effect 
  of 
  the 
  lengthened 
  sum- 
  

   mer 
  twilight. 
  But 
  these 
  ideas, 
  on 
  being 
  tested 
  by 
  the 
  Cape 
  observations, 
  com- 
  

   pletely 
  fall 
  to 
  the 
  ground 
  ; 
  for 
  during 
  the 
  whole 
  period 
  of 
  invisibility 
  to 
  Cassini 
  

   (caused 
  in 
  reality 
  by 
  the 
  lengthened 
  twilight 
  of 
  summer 
  in 
  his 
  northern 
  hemi- 
  

   sphere), 
  the 
  phenomenon 
  was 
  most 
  visible 
  at 
  the 
  Cape, 
  as 
  winter 
  then 
  prevails 
  in 
  

   the 
  southern 
  hemisphere 
  ; 
  and, 
  indeed, 
  the 
  very 
  reverse 
  effect 
  from 
  that 
  expected 
  

   by 
  Cassini 
  should 
  follow, 
  when 
  a 
  transparent 
  and 
  oblate 
  luminous 
  ring 
  is 
  viewed 
  

   in 
  profile, 
  for 
  it 
  will 
  then 
  be 
  seen 
  at 
  its 
  brightest, 
  on 
  account 
  of 
  all 
  the 
  infinitely 
  

   small 
  light-giving 
  particles 
  being 
  brought 
  closer 
  together 
  ; 
  so 
  small 
  are 
  they, 
  that 
  

   they 
  can 
  by 
  no 
  means 
  be 
  distinguished 
  separately, 
  or 
  when 
  thinly 
  scattered 
  over 
  

   the 
  sky, 
  but 
  only 
  make 
  themselves 
  sensible 
  to 
  the 
  eye, 
  and 
  the 
  telescope, 
  when 
  

   they 
  are 
  crowded 
  together 
  in 
  a 
  smaller 
  space. 
  The 
  idea, 
  moreover, 
  of 
  the 
  zodiacal 
  

   light 
  being 
  in 
  the 
  form 
  of 
  a 
  ring 
  at 
  all, 
  is 
  discountenanced 
  by 
  the 
  observed 
  appear- 
  

   ances, 
  they 
  being 
  all 
  conformable 
  to 
  the 
  phenomena 
  which 
  would 
  be 
  afforded 
  by 
  

   a 
  thin 
  lenticular 
  body, 
  excentrically 
  situated 
  and 
  revolving 
  about 
  the 
  sun. 
  

  

  Cassini's 
  friend, 
  M. 
  Fatio, 
  made 
  observations 
  of 
  the 
  zodiacal 
  light 
  about 
  the 
  

   same 
  time, 
  as 
  did 
  also 
  M. 
  Kirch, 
  and 
  Eimmart, 
  and 
  Mr 
  Derham. 
  

  

  But 
  the 
  subject 
  was 
  not 
  carried 
  further, 
  until 
  taken 
  up 
  by 
  Mairan, 
  in 
  1731, 
  

   He 
  was 
  rather 
  wild 
  in 
  his 
  notion 
  of 
  the 
  manner 
  in 
  which 
  the 
  body 
  was 
  formed, 
  

   vol. 
  xx. 
  part 
  in. 
  6 
  s 
  

  

  