*
[footnote] *Biot, 'Trait?? d'Astron. Physique', 3??me ??d., 1841, t. i., p.
171, 238 and 312.
The phenomena of light depend upon conditions still less understood, and
their variability at twilight, as well as in the zodiacal light, excite our
astonishment.
We have hitherto considered that which belongs to our solare system -- that
world of material forms governed by the Sun -- which includes the primary
and secondary planets, comets of short and long periods of revolution,
meteoric asteroids, which move thronged together in streams, either
sporadically or in closed rings, and finally a luminous nebulous ring, that
revolves round the Sun in the vicinity of the Earth, and for which, owing to
its position, we may retain the name of zodiacal light. Every where the law
of periodicity governs the motions of these bodies, however different may be
the amount of tangential velocity, or the quantity of their agglomerated
material parts; the meteoric asteroids which enter our atmosphere from the
external regions of universal space are alone arrested in the course of
their planetary revolution, and retained within the sphere of a larger
planet. In the solar system, whose boundaries determine the attractive
force of the central body, comets are made to revolve in their elliptical
p 145
orbits at a distance 44 times greater than that of Uranus; may, in those
comets whose nucleus appears to us, from its inconsiderable mass, like a
mere passing cosmical cloud, the Sun exercises its attractive force on the
outermost parts of the emanations radiating from the tail over a space of
many millions of miles.
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