ZODIACAL LIGHT. 131 



and Mars. We can, indeed, at present, form no certain 

 judgment concerning the true dimensions of the supposed 

 ring ; its possible augmentation ( 97 ) by emanations from the 

 tails of many millions of comets when at their perihelia ; the 

 singular variability of its extension (which seems sometimes 

 not to exceed that of our own orbit) ; or concerning its not 

 improbable intimate connection with the more condensed 

 cosmical vapour in the vicinity of the Sun. The nebulous 

 particles of which the ring consists, and which revolve 

 around the Sun according to the same laws as the planets, 

 may either be self-luminous, or may reflect the light of the 

 Sun. The first supposition is not inadmissible; even a 

 terrestrial fog (and it is a very remarkable fact), shewed 

 itself in 1743, at the time of the new moon and in the 

 middle of the night, so phosphorescent, that objects could 

 be distinctly recognised at a distance of above 600 feet( 98 ). 

 In the tropical regions of South America, I have some- 

 times observed with astonishment the variations in in- 

 tensity of the Zodiacal light. Having passed the nights 

 during several months in the open air and under a 

 serene sky, on the banks of the great rivers or in the midst 

 of the wide grassy plains or llanos, I had frequent oppor- 

 tunities of carefully observing the phenomenon. Some- 

 times in a few minutes after the Zodiacal light had been at 

 the strongest, it would become sensibly weakened, then 

 suddenly reappear in full brilliancy. In a few instances I 

 thought that I perceived, not indeed a tinge of red colour, 

 or a dark arch beneath, or, as Mairan describes, a jet 

 of sparks, but an undulatory motion of the light. Are 

 there, then, processes going on in the nebulous ring 

 itself? or is it not more probable that, though near the 



