272 POPULAR SCIENCE MONTHLY. 



the negative charge in the upper atmosphere is high enough, discharges 

 are brought about by the powerful ultra-violet radiation from the sun, 

 and particles are driven o£E radially from the earth on the side turned 

 towards the sun, only to be drifted back with the other streams into 

 the tail. The effect will be as if a sheaf of light projected from her 

 towards the sun. 



Compare with this the description of the Zodiacal Light (Newcomb, 

 'Popular Astronomy/ p. 416) : 



This object consists of a very soft faint column of light, which may be 

 seen rising from the western horizon after twilight on any clear winter or 

 spring evening; it may also be seen rising from the eastern horizon just before 

 daybreak in the summer or autumn. It really extends out on each side of the 

 sun, and lies nearly in the plane of the ecliptic. . . Near the equator, where 

 the ecliptic always rises high above the horizon, the light can be seen about 

 equally well all the year round. . . It is due to a lens-shaped appendage of 

 some sort surrounding the sun, and extending out a little beyond the earth's 

 orbit. 



The nature of the substance from which this light emanates is entirely 

 unknown. . . Professor Wright of Yale College finds its spectrum to be con- 

 tinuous. Accepting this, we should be led to the conclusion that the phe- 

 nomenon in question is due to reflected sunlight, probably from an immense 

 cloud of meteorites, filling up the space between the earth and the sun. 



The difficulty in this view is that the orbits of such swarms of 

 meteorites as are known to us are distributed irregularly with regard to 

 the ecliptic. On the other hand, Arrhenius' streams of particles, when 

 near enough to be visible, necessarily lie in or near the ecliptic, as re- 

 quired by observation. More than this, the particles emitted by the 

 earth herself should be most abundant over those regions which have 

 been exposed for many hours to the sun. Now it has been observed that 

 the zodiacal light is stronger on what Arrhenius calls the 'evening side' 

 of the earth (t. e., that side which is in the act of turning away from the 

 sun, and has the sun in the west) than on the 'morning side.' 



Even at night, when the sun is below the horizon, faint reflections 

 should reach us from the streamers behind the earth, and by an effect 

 of perspective, these should have a maximum in the point opposite to 

 the sun, where they will appear most dense. Let Professor Newcomb 

 describe the Gegenschein : 



Another mysterious phenomenon associated with the zodiacal light is 

 known by its German appellation, the Gegenschein. It is said that in that 

 point of the heavens directly opposite the sun there is an elliptical patch of 

 light, a few degrees in extent, of such extreme faintness that it can be seen 

 only by the most sensitive eyes, under the best conditions, and through the 

 clearest atmosphere. This phenomenon seems so difficult to account for that its 

 existence is sometimes doubted; yet the testimony in its favor is difficult to 

 set aside. 



How is it that the moon does not exhibit such tails ? The moon has 



