THE AURORA. 21 



we have now to deal with one of a yet more interest- 

 ing character. 



Most of my readers have doubtless heard of the 

 zodiacal light, and many of them have perhaps seen 

 that mysterious radiance, pointing obliquely upward 

 from the western horizon soon after sunset in the 

 spring months, or in autumn shortly before sunrise, 

 above the eastern horizon. The light, as its name 

 indeed implies, lies upon that region of the heavens 

 along which the planets travel. Accordingly, astron- 

 omers have associated it with the planetary orbits, and 

 have come to look on it as formed by the light reflected 

 from a multitude of minute bodies travelling around 

 the sun within the orbit of our earth. 



Yet it had long been recognized that there are 

 difficulties in the way of this theory. Passing over 

 those which depend On the position of the zodiacal 

 light upon the heavens, there are difficulties connected 

 with the appearance of the object. For example, its 

 light has often been observed to flicker or coruscate in 

 a manner which it seemed difficult to ascribe to the 

 motions of our own atmosphere. Then again there 

 have been seasons when the zodiacal light has shone 

 with unusual intensity for months together, and there 

 is nothing in the received theory which can account 

 for such a peculiarity. Lastly, there is the strange 

 circumstance recorded by Baron Humboldt that the 

 zodiacal light is often invisible when night first sets in, 



