INTRODUCTION. VII 



Cassini had requested travellers in southern latitudes to give attention to the subject; and in 

 1684, Noel, a Jesuit priest in the East Indies, when near the equator, saw this Light so bril- 

 liantly exhibited, that he thought of giving it the name of the Second Crepuscule. 



In 1687, M. de la Loubere, French envoy to Siam, saw it repeatedly; and in 1690, it was 

 noticed at Pondichery ; but the accounts we have from all these observers are little more than 

 a simple notice that it was seen. 



In 1*731, Mairan, in his important work on the Aurora Borealis, took notice of the Zodiacal 

 Light, to which he had given considerable attention ; but his work affords us very little of a 

 tangible and reliable character. He is remarkable for a theory that the Zodiacal Light is 

 reflected from the sun's atmosphere, stretched out into a flattened spheroid, or lenticular-shaped 

 body, revolving -with the sun ; an idea which La Place has forever set at rest, by demonstrating 

 that the sun's atmosphere "can extend no further than to the orbit of a planet, whose periodi- 

 cal revolution is performed in the same time as the sun's rotary motion about its axis, or in 

 twenty-five days and a half; that is, only as far as -fy of Mercury's distance from the sun." 



From the time of Mairan little seems to have been done on this subject, and we are scarcely 

 furnished with a notice of it, until in 1796, the publication of the Systeme du Monde, in note 

 7 appended to it, gave the world La Place's nebular theory, and with it also a theory 

 respecting the substance producing the Zodiacal Light. This great writer says: "If in the 

 zones abandoned by the atmosphere of the sun, there are any molecules too volatile to be united 

 to each other, or to the planets, they ought, in continuing to circulate around this star, to 

 offer all the appearances of the Zodiacal Light, without opposing any sensible resistance to the 

 different bodies of the planetary system, either ou account of their extreme rarity, or because 

 their motion is nearly the same as that of the planets with which they come in contact. "- 

 Exposition du Systeme du Monde, note 7, p. 471. 



It will be observed that all the theories and all the reasonings on the Zodiacal Light, which 

 we have in these various authors, seem to have been founded on Cassini' s conclusion, that the 

 axis of this light has a relation to the sun's equator, rising and sinking with it; which con- 

 clusion was drawn after only ten observations the first detailed observations ever made. It is 

 to be regretted that, in the one hundred and seventy years since his time, we have been fur- 

 nished with so small an amount of facts instead of theories. My own observations, I believe, 

 are the first, of any detailed character, ever carried into latitudes south of the equator, or even 

 into regions about the equatorial line. 



When the French corvette La JBonite, was about to start on her voyage of circumnavigation, 

 in 1836, a special committee was appointed by the French Academy of Sciences to draw up 

 instructions for scientific observations on board; and Arago was deputed by them to select 

 matters connected with the physique of the globe. In his paper he enjoins particular attention 

 to the Zodiacal Light ; but even these very instructions themselves show how blindly the scientific 

 world were then groping their way in a matter where facts only could give reliable evidence. 

 He ends with saying, "Our young compatriots can, therefore, give themselves up zealously to 

 the observations which we here designate. The question is important, and no one can yet 

 flatter himself that he has given it a definite solution." 



milieu de 1'e'paisseur qu'ellcs enferment est marque'e par une surface pareilletueiit courlie et ondoyante, qui passe par les 

 orbites de toutes les planetes et determine le milieu de la lumiere. Les particules qui la renvoyent sont comprises dans 

 1'orbe annuel au temps qu'elle parait. II leur donne un mouvement par lequel elles vont ou sont porte'es autour du soleil 

 par des cercles eiitiers, avec la meme force que les planetos mmes." Mem. de VAcad. Ray. da Sci., tome viii. p. 158. 



