500 PROFESSOR PIAZZI SMYTH ON THE 
one of peculiar beauty, when a small fleecy cloud is projected against the zodiacal 
light, and detaches itself picturesquely from the illuminated back-ground. A 
passage in my journal during a voyage from Lima to the West Coast of Mexico, 
notices such a picture. ‘For the last three or four nights (between 10° and 14° of 
north latitude), the zodiacal light has appeared with a magnificence which I have 
never before seen. Judging also from the brightness of the stars and nebulze, the 
transparency of the atmosphere in this part of the Pacific must be extremely great. 
From the 14th to the 19th of March, during a very regular interval of three 
quarters of an hour after the disc of the sun had sunk below the horizon, no trace 
of the zodiacal light could be seen, although the night was perfectly dark; but 
an hour after sunset, it became suddenly visible, extending in great brightness 
and beauty, between Aldebaran and the Pleiades, and, on the 18th of March, at- 
taining an altitude of 39° 5’. Long narrow clouds, scattered over the lovely azure 
of the sky, appeared low down on the horizon, as if in front of a golden curtain, 
while bright varied tints played from time to time on the higher clouds: it seemed 
a second sunset. Towards that side of the heavens the light diffused appeared 
almost equal to that of the moon in her first quarter. Towards ten o’clock, in 
this part of the Pacific, the zodiacal light usually becomes very faint, and at mid- 
night I could only see a trace of it remaining. On the 16th of March, when its 
brightness was greatest, a mild reflected glow was visible in the east.’ ” 
He describes also several anomalous features, as that sometimes it did not 
appear for three-quarters of an hour after sunset, though the twilight had been 
for some time ended ; that then it appeared suddenly, and continued long of very 
great brightness; that at other times it would continue to shorten and lengthen 
many degrees in a few minutes, and have an undulatory sort of motion. But 
these peculiarities, when not accounted for by the atmospheric circumstances of 
which he himself takes notice, seem rather to be produced in the eye of the ob- 
server by reason of the extreme faintness of the object to be observed; by the 
length of time that a retina,—which has been initiated by watching the setting 
sun, or even when acted on by ordinary daylight,—requires to recover its full 
degree of sensitivenes; as well as by the deceptive phantasmagoric effect produced 
on the nerves when strained to a greater extent than they can well bear. 
Taking all the above facts into consideration, we are perhaps entitled to con- 
clude, on pretty good foundation, that the zodiacal light is an extremely oblate, 
lenticular, revolving body, nearly in the plane of the sun’s equator, rather excen- 
trically situated, of so vast a size as nearly to fill the whole orbit of the earth, 
and sometimes actually to reach it. But whether it does actually at the present 
time correspond exactly with the sun’s equator, and if it has always done so, and 
always will; whether the manifest changes in the intrinsic brightness, and the 
form and size of the light that have been observed, be due merely to a rotation of 
the excentric or oval body, or to a real periodical increase of the intensity of its 

