for the wet
collodion-plates hitherto in use; and this improvement alone reduced the
necessary time of exposure to two hours. It was brought down to half an
hour by Janssen's employment of a reflector specially adapted to give an
image illuminated eight or ten times as strongly as that produced in the
focus of an ordinary telescope.[1298]

The photographic feebleness of cometary rays was not the only obstacle
in the way of success. The proper motion of these bodies is so rapid as
to render the usual devices for keeping a heavenly body steadily in view
quite inapplicable. The machinery by which the diurnal movement of the
sphere is followed, must be especially modified to suit each eccentric
career. This, too, was done, and on June 30, 1881, Janssen secured a
perfect photograph of the brilliant object then visible, showing the
structure of the tail with beautiful distinctness to a distance of
2-1/2 deg. from the head. An impression to nearly 10 deg. was obtained
about the same time by Dr. Henry Draper at New York, with an exposure
of 162 minutes.[1299]

Tebbutt's (or comet 1881 iii.) was also the first comet of which the
spectrum was so much as attempted to be chemically recorded. Both
Huggins and Draper were successful in this respect, but Huggins was more
completely so.[1300] The importance of the feat consisted in its
throwing open to investigation a part of the spectrum invisible to the
eye, and so affording an additional test of cometary constitution. The
result was fully to confirm the origin from carbon-compounds assigned to
the visible rays, by disclosing additional bands belonging to the same
series in the ultra-violet; as well as to establish unmistakably the
presence of a not inconsiderable proportion of reflected solar light by
the clear impression of some of the principal Fraunhofer lines. Thus the
polariscope was found to have told the truth, though not the whole
truth.

The photograph so satisfactorily communicative was taken by Sir William
Huggins on the night of June 24; and on the 29th, at Greenwich, the
tell-tale Fraunhofer lines were perceived to interrupt the visible range
of the spectrum. This was at first so vividly continuous, that the
characteristic cometary bands could scarcely be detached from their
bright background. But as the nucleus faded towards the end of June,
they came out strongly, and were more and more clearly seen, both at
Greenwich and at Princeton, to agree, not with the spect