rum of
hydro-carbons glowing in a vacuum tube, but with that of the same
substances burning in a Bunsen flame.[1301] It need not, however, be
inferred that cometary materials are really in a state of combustion.
This, from all that we know, may be called an impossibility. The
additional clue furnished was rather to the manner of their electrical
illumination.[1302]

The spectrum of the tail was, in this comet, found to be not essentially
different from that of the head. Professor Wright of Yale College
ascertained a large percentage of its light to be polarized in a plane
passing through the sun, and hence to be reflected sunlight.[1303] A
faint continuous spectrum corresponded to this portion of its radiance;
but gaseous emissions were also present. At Potsdam, on June 30, the
hydro-carbon bands were indeed traced by Vogel to the very end of the
tail;[1304] and they were kept in sight by Young at a greater distance
from the nucleus than the more equably dispersed light. There seems
little doubt that, as in the solar corona, the relative strength of the
two orders of spectra is subject to fluctuations.

The comet of 1881 iii. was thus of signal service to science. It
afforded, when compared with the comet of 1807, the first undeniable
example of two such bodies travelling so nearly in the same orbit as to
leave absolutely no doubt of the existence of a genetic tie between
them. Cometary photography came to its earliest fruition with it; and
cometary spectroscopy made a notable advance by means of it. Before it
was yet out of sight, it was provided with a successor.

At Ann Arbor Observatory, Michigan, on July 14, a comet was discovered
by Dr. Schaeberle, which, as his claim to priority is undisputed, is
often allowed to bear his name, although designated, in strict
scientific parlance, comet 1881 iv. It was observed in Europe after
three days, became just discernible by the naked eye at the end of July,
and brightened consistently up to its perihelion passage, August 22,
when it was still about fifty million miles from the sun. During many
days of that month, the uncommon spectacle was presented of two bright
comets circling together, though at widely different distances, round
the North pole of the heavens. The newcomer, however, never approached
the pristine lustre of its predecessor. Its nucleus, when brightest, was
comparable to the star Cor Caroli, a narrow, perfectly straight ray
proceeding from it to a distanc