e spring months of 1882, Professor Copeland discovered that a
new bright yellow line, coinciding in position with the D-line of
sodium, had suddenly appeared, and it was subsequently, both by him and
by other observers, seen beautifully double. In fact, sodium was so
strongly represented in this comet, that both the head and the tail
could be perfectly well seen in sodium light by merely opening the slit
of the spectroscope very wide, just as a solar prominence may be seen in
hydrogen light. The sodium line attained its greatest brilliance at the
time when the comet was nearest to the sun, while the hydrocarbon bands
were either invisible or very faint. The same connection between the
intensity of the sodium line and the distance from the sun was noticed
in the great September comet of 1882.

The spectrum of the great comet of 1882 was observed by Copeland and
Lohse on the 18th September in daylight, and, in addition to the sodium
line, they saw a number of other bright lines, which seemed to be due to
iron vapour, while the only line of manganese visible at the temperature
of a Bunsen burner was also seen. This very remarkable observation was
made less than a day after the perihelion passage, and illustrates the
wonderful activity in the interior of a comet when very close to the
sun.

[Illustration: PLATE XVII.

THE COMET OF 1882,

AS SEEN FROM STREATHAM, NOV. 4TH, 4 A.M.

FROM A DRAWING BY T.E. KEY.]

In addition to the bright lines comets generally show a faint
continuous spectrum, in which dark Fraunhofer lines can occasionally be
distinguished. Of course, this shows that the continuous spectrum is to
a great extent due to reflected sunlight, but there is no doubt that
part of it is often due to light actually developed in the comets. This
was certainly the case in the first comet of 1884, as a sudden outburst
of light in this body was accompanied by a considerable increase of
brightness of the continuous spectrum. A change in the relative
brightness of the three hydrocarbon bands indicated a considerable rise
of temperature, during the continuance of which the comet emitted white
light.

As comets are much nearer to the earth than the stars, it will
occasionally happen that the comet must arrive at a position directly
between the earth and a star. There is quite a similar phenomenon in the
movement of the moon. A star is frequently occulted in this way, and the
observations of such phenomena are familiar