set. Its lustre--setting aside the light derived
from the tail--was, at that date, 6,300 times what it had been on June
15, though _theoretically_--taking into account, that is, only the
differences of distance from sun and earth--it should have been only
1/33 of that amount. Here, it might be thought, was convincing evidence
of the comet itself becoming ignited under the growing intensity of the
solar radiations. Yet experiments with the polariscope were interpreted
in an adverse sense, and Bond's conclusion that the comet sent us
virtually unmixed reflected sunshine was generally acquiesced in. It
was, nevertheless, negatived by the first application of the
spectroscope to these bodies.

Very few comets have been so well or so long observed as Donati's. It
was visible to the naked eye during 112 days; it was telescopically
discernible for 275, the last observation having been made by Mr.
William Mann at the Cape of Good Hope, March 4, 1859. Its course through
the heavens combined singularly with the orbital place of the earth to
favour curious inspection. The tail, when near its greatest development,
lost next to nothing by the effects of perspective, and at the same time
lay in a plane sufficiently inclined to the line of sight to enable it
to display its exquisite curves to the greatest advantage. Even the
weather was, on both sides of the Atlantic, propitious during the period
of greatest interest, and the moon as little troublesome as possible.
The volume compiled by the younger Bond is a monument to the care and
skill with which these advantages were turned to account. Yet this
stately apparition marked no turning-point in the history of cometary
science. By its study knowledge was indeed materially advanced, but
along the old lines. No quick and vivid illumination broke upon its
path. Quite insignificant objects--as we have already partly seen--have
often proved more vitally instructive.

Donati's comet has been identified with no other. Its path is an
immensely elongated ellipse, lying in a plane far apart from that of the
planetary movements, carrying it at perihelion considerably within the
orbit of Venus, and at aphelion out into space to 5-1/2 times the
distance from the sun of Neptune. The entire circuit occupies over 2,000
years, and is performed in a retrograde direction, or against the order
of the Signs. Before its next return, about the year 4000 A.D., the
enigma of its presence and its purpose may h