he only plausible hypothesis as to the mode of
their production is that of an opposite state of electrification in the
particles composing the ordinary and extraordinary appendages.

The spectrum of the great comet of 1882 was, in part, a repetition of
that of its immediate predecessor, thus confirming the inference that
the previously unexampled sodium-blaze was in both a direct result of
the intense solar action to which they were exposed. But the D line was,
this time, not seen alone. At Dunecht, on the morning of September 18,
Drs. Copeland and J. G. Lohse succeeded in identifying six brilliant
rays in the green and yellow with as many prominent iron-lines;[1337] a
very significant addition to our knowledge of cometary constitution, and
one which lent countenance to Bredikhine's assumption of various kinds
of matter issuing from the nucleus with velocities inversely as their
atomic weights. All the lines equally showed a slight displacement,
indicating a recession from the earth of the radiating body at the rate
of 37 to 46 miles a second. A similar observation, made by M. Thollon at
Nice on the same day, gave emphatic sanction to the spectroscopic method
of estimating movement in the line of sight. Before anything was as yet
known of the comet's path or velocity, he announced, from the position
of the double sodium-line alone, that at 3 p.m. on September 18 it was
increasing its distance from our planet by from 61 to 76 kilometres per
second.[1338] M. Bigourdan's subsequent calculations showed that its
actual swiftness of recession was at that moment 73 kilometres.

Changes in the inverse order to those seen in the spectrum of comet
Wells soon became apparent. In the earlier body, carbon bands had died
out with _approach_ to perihelion, and had been replaced by sodium
emissions; in its successor, sodium emissions became weakened and
disappeared with _retreat_ from perihelion, and found their substitute
in carbon bands. Professor Ricco was, in fact, able to infer, from the
sequence of prismatic phenomena, that the comet had already passed the
sun; thus establishing a novel criterion for determining the position of
a comet in its orbit by the varying quality of its radiations.

Recapitulating what was learnt from the five conspicuous comets of
1880-82, we find that the leading facts acquired to science were these
three. First, that comets may be met with pursuing each other, after
intervals of many years, in the sam