comets' tails, the coeona, and the attrora. 187 



The diHicnlty in this A'iew is that tho ()r])its oi" such swarms of 

 meteorites as ar(^ known to ns are distributed irrey'idarly with regard 

 to the eeliptie. On the other hand, Arrhenius's streams of particles, 

 when near enough to be visible, necessarily lie in or near the ecliptic, 

 as recjuii'ed 1)}^ observation. Moi'e than this, the particles emitted Ijy 

 the earth herself should be most abundant o\er those regions which 

 have been exposed for many hours to the sun. Now it has l)een 

 observed that the zodiacal light is stronger on what Arrhenius calls the 

 "evening side" of the earth (i. c. , 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 persp ctive, these should have a maxinnun in the point opposite to 

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

 describe the Gegenschein; 



"Another mysterioQs 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 (^llip- 

 tical jjatch of light, a few degrees in extent of such extreme faintness 

 that it can be seen only })y the most sensitive eyes, under the best con- 

 ditions, and through the clearest atmosphere. This phenomenon secMus 

 so difficult to account for that its existence is sometimes doubted; yet 

 the testimon}' in its favor is difficult to set aside." 



How is it that the moon does not exhil)it such tails ^ The moon has 

 no atmos-phere, so that the particles which reach her give up their nega- 

 tive charge to her directly, and it spreads equally all over her surface. 

 When in turn she herself discharges the particles, it will be uniforndy 

 in all directions, and she should appear surrounded with a uniform 

 sheath. Possibl}" this sheath of cosmical dust affords the reason that 

 in a lunar eclipse the shadow of the earth can bt> traced a short distance 

 l)e3'ond the limb of the moon on each side. 



THE aurora B(JREALIS. 



Perhaps the most interesting application of Arrhenius's theory is his 

 explanation of the Aurora. In a well-known experiment the streams 

 of negative particles forming cathode rays in a Crookes tu])e are 

 exposed to a magnetic field, when they are seen to describe h(dices 

 round the lines of force. If the field is powerful enough they may 

 thus be bent into a complete cir(de inside a moderately large tube. 



Now the negative particles discharged from the sun arrive most 

 thickly over the equatorial regions of the earth, wdiich ai'(^ most 

 directly exposed to him. Long before they reach any atmosjjhere 

 dense enough to excite kHuinescence, the}' are caught by the lines of 

 force of the earth's magnetic field, which are horizontal over theecpia- 

 tor, and have to follow them, winding round them in helices whose 



