the heavenly bodies, there was a great desire to find out what comets
are made of. The first opportunity came in 1864, when Donati observed
the spectrum of a comet, and saw three bright bands, thus proving that
it was a gas and at least partly self-luminous. In 1868 Huggins
compared the spectrum of Winnecke's comet with that of a Geissler tube
containing olefiant gas, and found exact agreement. Nearly all comets
have shown the same spectrum.[1] A very few comets have given bright
band spectra differing from the normal type. Also a certain kind of
continuous spectrum, as well as reflected solar light showing
Frauenhofer lines, have been seen.

[Illustration: COPY OF THE DRAWING MADE BY PAUL FABRICIUS. To define
the path of comet 1556. After being lost for 300 years, this drawing
was recovered by the prolonged efforts of Mr. Hind and Professor
Littrow in 1856.]

When Wells's comet, in 1882, approached very close indeed to the sun,
the spectrum changed to a mono-chromatic yellow colour, due to sodium.

For a full account of the wonders of the cometary world the reader is
referred to books on descriptive astronomy, or to monographs on
comets.[2] Nor can the very uncertain speculations about the structure
of comets' tails be given here. A new explanation has been proposed
almost every time that a great discovery has been made in the theory
of light, heat, chemistry, or electricity.

Halley's comet remained the only one of which a prediction of the
return had been confirmed, until the orbit of the small, ill-defined
comet found by Pons in 1819 was computed by Encke, and found to have a
period of 3 1/3 years. It was predicted to return in 1822, and was