THE AURORA. 15 



recorded by Baron Humboldt that the zodiacal light is 

 often invisible when night first sets in, and then sud- 

 denly appears with full splendour; a phenomenon 

 which is utterly inexplicable if the received theory be 

 accepted. The whole account of the phenomenon, as 

 given by Baron Humboldt, is so interesting, and for 

 my present purpose so significant, that I give it at full 

 length : 



( In the tropical climate of South America,' he re- 

 marks, ' the variable strength of the light of the zodiacal 

 gleam struck me at times with utter amazement. As 

 I there passed the beautiful nights, in the open air, on 

 the banks of rivers, and in the grassy plains for several 

 months together, I had opportunities of observing the 

 phenomenon with attention. When the zodiacal light 

 was at its very brightest, it sometimes happened that 

 but a few minutes afterwards it became notably weak- 

 ened, and then it suddenly gleamed up again with its 

 former brilliancy. In particular instances, I believed 

 that I remarked not anything of a ruddy tinge, or an 

 interior arched obscuration, or an emission of sparks, 

 such as Mairan describes, but a kind of unsteadiness 

 and flickering of the light/ 



Despite these and similar observations, very little 

 doubt had been felt by astronomers that the zodiacal 

 light really indicates the presence of minute bodies 

 travelling in more or less eccentric paths round the sun. 

 And it was confidently expected that whenever a spec- 

 troscope of sufficient delicacy to analyse the faint light 

 of the zodiacal gleam was applied to that purpose, the 



