14 LIGHT SCIENCE FOR LEISURE HOURS. 



the auroral electric discharge takes place, or of the 

 substance between which the spark travels, is altogether 

 precluded. 



But interesting as the discovery undoubtedly is, we 

 have now to deal with one of a yet more interesting 

 character. 



Most of my readers have doubtless heard of the 

 zodiacal light, and many of them have perhaps seen 

 that mysterious radiance, pointing obliquely upwards 

 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, astronomers have associ- 

 ated it with the planetary orbits, and have come to 

 look on it as formed by the light reflected from a multi- 

 tude of minute bodies travelling around the sun within 

 the orbit of our earth. 



Yet it had long been recognised that there are diffi- 

 culties 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 pecu- 

 liarity. Lastly, there is the strange circumstance 



