These variations of form have been most
accurately and admirably described in the comet of 1744, by Hensius, at St.
Petersburg, and in Halley's comet, on its last reappearance in 1835, by
Bessel, at Konigsberg. A more or less well-defined tuft of rays emanated
from that part of the nucleus which was turned toward the Sun; and the rays
being bent backward, formed a part of the tail. The nucleus of Halley's
comet; with its emanations, presented the appearance of a burning rocket,
the end of which was turned sideways by the force of the wind. The rays
issuing from the head were seen by Arago and myself, at the Observatory at
Paris, to assume very different forms on successive nights.*
[footnote] *Arago, 'Des Changements physiques de la Comete de Halley du
15-23 Oct., 1835. 'Annuaire', 1836, p. 218, 221. The ordinary direction of
the emanations was noticed even in Nero's time. "Comae radios solis
effugiunt." -- Seneca, 'Nat. Quaest.', vii., 20.
The great Konigsberg astronomer concluded from many measurements, and from
theoretical considerations, "that the cone of light issuing from the comet
deviated considerably both to the right and the left of the true direction
of the Sun, but that it always returned to that direction, and passed over
to the opposite side, so that both the cone of light and the body of the
comet from whence it emanated experienced a rotatory, or, rather, a
vibratory motion in the plane of the orbit.
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