n
first seen, it wore the aspect of a nebula; later it put on the
distinctive garb of a comet; it next appeared as a star; finally, it
dilated, first in a spherical, then in a paraboloidal form, until May 5,
1836, when it vanished from Herschel's observation at Feldhausen as if
by melting into adjacent space from the excessive diffusion of its
light. A very uncommon circumstance in its development was that it lost
all trace of tail _previous_ to its arrival at perihelion on the 16th of
November. Nor did it begin to recover its elongated shape for more than
two months afterwards. On the 23rd of January, Boguslawski perceived it
as a star of the sixth magnitude, _without measurable disc_.[284] Only
two nights later, Maclear, director of the Cape Observatory, found the
head to be 131 seconds across.[285] And so rapidly did the augmentation
of size progress, that Sir John Herschel estimated the actual bulk of
this singular object to have increased forty-fold in the ensuing week.
"I can hardly doubt," he remarks, "that the comet was fairly evaporated
in perihelio by the heat, and resolved into transparent vapour, and is
now in process of rapid condensation and re-precipitation on the
nucleus."[286] A plausible, but no longer admissible, interpretation of
this still unexplained phenomenon. The next return of this body, which
will be considerably accelerated by Jupiter's influence, is expected to
take place in 1910.[287]

By means of an instrument devised to test the quality of light, Arago
obtained decisive evidence that some at least of the radiance proceeding
from Halley's comet was derived by reflection from the sun.[288]
Indications of the same kind had been afforded[289] by the comet which
suddenly appeared above the north-western horizon of Paris, July 3,
1819, after having enveloped (as already stated) our terrestrial abode
in its filmy appendages; but the "polariscope" had not then reached the
perfection subsequently given to it, and its testimony was accordingly
far less reliable than in 1835. Such experiments, however, are in
reality more beautiful and ingenious than instructive, since ignited as
well as obscure bodies possess the power of throwing back light incident
upon them, and will consequently transmit to us from the neighbourhood
of the sun rays partly direct, partly reflected, of which a certain
proportion will exhibit the peculiarity known as polarisation.

The most brilliant comets of the century were su