1891. ] VEEDER—ON THE ZODIACAL LIGHT. 139 
hand, should shade off very gradually departing more widely from the 
plane of the ecliptic, and this is precisely what has been found to be 
the case. At this time of year also the northern coronal extension, thus 
opened out to its greatest extent, is in full view on the sunset side of 
the plane of the earth’s orbit. In Autumn, on the other hand, the 
north pole of the sun is directed earthward, and the northern coronal 
extension fails of illumination, and the southern coronal extension is 
opened out and most completely illuminated, and is in full view on the 
sunrise side of the plane of the earth’s orbit. Thus the zodiacal light 
is brightest in Spring and Fall, but in the evening in the one case and in 
morning in the other. In Summer and Winter, on the other hand, the 
earth occupies a position intermediate between these disc-like coronal 
extensions. Coincidently the zodiacal column becomes less clearly 
defined on both edges and diminishes greatly in brightness, less light 
being reflected earthward, and it is seen equally well, though faintly, 
both in the morning and in the evening. 
It is in their relation to magnetic phenomena, however, that the 
peculiarities of the zodiacal light acquire their greatest interest. Indeed 
the present research probably would not have been undertaken had it 
not grown out of the investigation in regard to the aurora and its asso- 
ciated phenomena, some of the results of which were presented before the 
Academy and printed in the Proceedings of Nov. 11, 1889. (See page 
18.) At that time evidence of a periodicity of auroras at intervals of 
about twenty-seven days had been secured. Subsequently, with more 
complete information and longer lists, this period was amended by suc- 
cessive approximations until it became twenty-seven days, six hours, and 
thirty-six minutes. It was then for the first discovered that this result, 
which had been obtained independently from magnetic phenomena 
alone, differed only four minutes from the most generally accepted 
value for a synodic revolution of the sun, as determined from the 
average rate of movement of sun spots. For the sake of uniformity 
this four minutes was added, and tables were constructed, (see plate,) 
showing the numbers of stations reporting auroras each day in all acces- 
sible lists for nearly two hundred years, arranged in periods of twenty- 
seven days, six hours, and forty minutes each. Portions of these tables 
comprising forty-six years were compared with the coincident records of 
magnetic perturbations at Greenwich, and for three years with records 
from the Naval Observatory, Washington, and likewise with the results 
obtained at Point Barrow, in connection with the International Polar 
Expedition in 1882 and 1883. Theconsensus of all this very voluminous 
testimony is to the effect, that there is an unmistakable periodicity at an 
