" (From a lettr written by Dr. Olbers to myself,
and dated Bremen, Marth 26th, 1833.)
As, however, the separate particles of a comet's tail, measuring millions of
miles,
p 144
are very unequally distant from earth, it is not possible, according to the
laws of the velocity and transmission of light, that we should be able, in
so short a period of time, to perceive any actual changes in a cosmical body
of such vast extent. There considerations in no way exclude the realith of
the changes that have been observed in the emanations from the more
condensed envelopes around the nucleus of a comet, nor that of the sudden
irradiation of the zodiacal light, from internal molecular motion, nor of
the increased or diminished reflection of light in the cosmical vapor of the
luminous ring, but should simply be the means of drawing our attention to
the differences existing between that which appertains to the air of heaven
(the realms of universal space) and that which belongs to the strata of our
terrestrial atmosphere. It is not possible, as well-attested facts prove,
perfectly to explain the operations at work in the much-contested upper
boundaries of our atmosphere. The extraordinary lightness of whole nights
in the year 1831, during which small print might be read at midnight in the
latitudes of Italy and the north of Germany is a fact directly at variance
with all that we know, according to the most recent and acute researches on
the crepuscular theory, and of the height of the atmosphere.
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