ZODIACAL LIGHT. 131 



and Mars, as the material cause of the zodiacal light. As yet 

 we certainly know nothing definite regarding its actual ma- 

 terial dimensions ; its augmentation* by emanations from 

 the tails of myriads of comets that come within the Sun's 

 vicinity ; the singular changes affecting its expansion, since it 

 sometimes does not appear to extend beyond our Earth's orbit ; 

 r, lastly, regarding its conjectural intimate connection with 

 ihe more condensed cosmical vapour in the vicinity of the Sun. 

 The nebulous particles composing this ring, and revolving 

 round the Sun in accordance with planetary laws, may either 

 be self-luminous or receive light from that luminary. Even in 

 the case of a terrestrial mist (and this fact is very remarkable), 

 which occurred at the time of the new moon at midnight, in 

 1743, the phosphorescence was so intense that objects could 

 be distinctly recognized at a distance of more than 600 feet. 



I have occasionally been astonished, in the tropical climates 

 of South America, to observe the variable intensity of the 

 zodiacal light. As I passed the nights, during many months, 

 in the open air, on the shores of rivers and on llanos, I enjoyed 

 ample opportunities of carefully examining this phenomenon. 

 When the zodiacal light had been most intense, I have ob- 

 served that it would be perceptibly weakened for a few 

 minutes, until it again suddenly shone forth in full bril- 

 liancy. In some few instances I have thought that I could 

 perceive not exactly a reddish coloration, nor the lower 

 portion darkened in an arc-like form, nor even a scintillation, 

 as Mairan affirms he has observed but a kind of flickering 



of Mercury and Venus were visible (throughout their whole, extent), 

 we should invariably observe them with the same figure and in the 

 same position with regard to the Sun, and at the same time of the year, 

 with the zodiacal light." (Mtm. de I'Acad., t. viii. 1730, p. 218, and 

 Biot, in the Comptes Rendus, 1836, t. iii. p. 666.) Cassini believed that 

 the nebulous ring of zodiacal light consisted of innumerable small planet- 

 ary bodies, revolving round the Sun. He even went so far as to believe 

 that the fall of fire balls might be connected with the passage of the Earth 

 through the zodiacal nebulous ring. Olmsted, and especially Biot (op. 

 cit., p. 673), have attempted to establish itb connexion with the Novem- 

 ber phenomenon a connexion which Olbers doubts. (Schum. Jahrb., 

 1837, s. 281.) Regarding the question whether the place of the zodiacal 

 light perfectly coincides with that of the Bun's equator, see Houzeau ia 

 Schum. Astr. Nachr., 1843, Nr. 492, s. 190. 

 * Sir John Herschel, Astron., 487. 



