22 LIGHT SCIENCE FOR LEISURE HOURS. 



and then suddenly appears with, full splendor; a phe- 

 nomenon which is utterly inexplicable if the received 

 theory be accepted. The whole account of the phe- 

 nomenon, as given by Baron Humboldt, is so interest- 

 ing, and for my present purpose so significant, that I 

 give it at full length : 



"In the tropical climate of South America," he 

 remarks, " the variable strength of the light of the 

 zodiacal gleam struck me at times with utter amaze- 

 ment. 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 some- 

 times happened that but a few mintues afterward it 

 became notably weakened, and then it suddenly 

 gleamed up again with its former brilliancy. In par- 

 ticular instances, I believed that I remarked not any 

 tiling of a ruddy tinge, or an interior arched obscura- 

 tion, 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 spectroscope of sufficient delicacy to analyze the faint 



