99 



that these variations are not all of them appear- 

 ances dependant on certain modifications in the 

 state of our atmosphere. Sometimes, during 

 nights equally clear, I sought in vain for the 

 zodiacal light, when the day before it had ap- 

 peared with the greatest brilliancy *. Must we 

 admit, that emanations, which reflect white 

 light, and seem to have some analogy with the 

 tail of comets, are less abundant at certain 

 periods ? The researches on the zodiacal light 

 have acquired a new degree of interest, since 

 geometricians have taught us, that we are igno- 

 rant of the real causes of this phenomenon. 

 The illustrious author of La Mecanique celeste 

 has shown, that the solar atmosphere cannot 

 reach even the orb of Mercury; and that it 

 could not in any case display the lenticular 

 form -f, which observations have attributed to 

 the zodiacal light. We may also entertain the 



* Mairan had been struck with the same phenomenon in 

 our climates. " I cannot pass unnoticed," he says, " that 

 twice, in the month of April, I have not been able to discover 

 toward the West the least trace of the zodiacal light, during 

 weather which seemed most favorable for it's appearance^ 

 and at the hour and season, when this light is most visible. 

 And what is also worthy of benig remarked, it appeared on 

 the morrow of each of these days, very brilliant and very 

 extensive." Mtm de VAcadtmie, 1783, p. 483 ; and Mairan \ 

 Traits de VAurore hot., (2d ed.) p. 263. 



t Syst. du Monde, (4th ed.) p. 270. 



H 2 



