to Humboldt, 'we may with great
probability attribute the zodiacal light to the existence of an
extremely oblate ring of nebulous matter, revolving freely in space
between the orbits of Venus and Mars.' On several occasions he
witnessed its fluctuations, night after night, from the plains of
South America, shewing itself at times greatly collapsed or condensed,
with intermittences of vividness and faintness, in the course of a few
minutes, as is observed of the aurora. The light of the stars, of even
the fifth or sixth magnitudes, can be seen through it: the same has
been remarked of comets; and it is known also that the tails of comets
undergo frequent flashings or pulsations, so that the two phenomena
may be analogous in character. It is necessary, however, to
distinguish the fluctuations from such effects as may be produced by
movements in the lower strata of the atmosphere.

Mairan, who wrote in 1731, was of opinion that the zodiacal light
consisted of particles thrown off from the sun by its rapid rotation,
or a species of atmosphere peculiar to the central orb. Others have
supposed the luminosity to be composed of 'revolving planetary
particles,' shining by a direct or reflected light. But, according to
Professor Olmsted, of Yale College, Massachusetts, it is something
which has a motion of its own around the sun, notwithstanding that the
general steadiness of its movements had warranted the notion that it
was in some way attached to the body of the sun itself. Olmsted's
conclusions are drawn from a diligent observation of the light during
a period of six years, and are on this account, as well as from his
scientific reputation, entitled to respect. He states the light to be,
in constitution, colour, and density, similar to that of the tail of a
comet, the portion nearest the sun being brightest, and both admitting
of stars being seen through them. We may, therefore, infer it to be a
nebulous ring surrounding the sun, in the same way that the
magnificent rings of Saturn surround that planet. Of such nebulae as
this there are from 2000 to 3000 visible in the regions of space,
compared with which the dimension of ours is insignificant: at the
same distance, and sought for with the same instruments, it would be
invisible.

In one point, Professor Olmsted's views are particularly interesting,
as, out of one mysterious phenomenon, he endeavours to explain
another, and inquires: 'Whether or not the zodiacal light is t