rface within little more than five million miles on
June 10, 1882; and the vicinity had the effect of developing a novel
feature in its incandescence.

During the first half of April its spectrum was of the normal type,
though the carbon bands were unusually weak; but with approach to the
sun they died out, and the entire light seemed to become concentrated
into a narrow, unbroken, brilliant streak, hardly to be distinguished
from the spectrum of a star. This unusual behaviour excited attention,
and a strict watch was kept. It was rewarded at the Dunecht Observatory,
May 27, by the discernment of what had never before been seen in a
comet--the yellow ray of sodium.[1308] By June 1, this had kindled into
a blaze overpowering all other emissions. The light of the comet was
practically monochromatic; and the image of the entire head, with the
root of the tail, could be observed, like a solar prominence, depicted,
in its new saffron vesture of vivid illumination, within the jaws of an
open slit.

At Potsdam, the bright yellow line was perceived with astonishment by
Vogel on May 31, and was next evening identified with Fraunhofer's "D."
Its character led him to infer a very considerable density in the
glowing vapour emitting it.[1309] Hasselberg founded an additional
argument in favour of the electrical origin of cometary light on the
changes in the spectrum of comet Wells.[1310] For they were closely
paralleled by some earlier experiments of Wiedemann, in which the
gaseous spectra of vacuum tubes were at once effaced on the introduction
of metallic vapours. It seemed as if the metal had no sooner been
rendered volatile by heat, than it usurped the entire office of carrying
the discharge, the resulting light being thus exclusively of its
production. Had simple incandescence by heat been in question, the
effect would have been different; the two spectra would have been
superposed without prejudice to either. Similarly, the replacement of
the hydro-carbon bands in the spectrum of the comet by the sodium line
proved electricity to be the exciting agent. For the increasing thermal
power of the sun might, indeed, have ignited the sodium, but it could
not have extinguished the hydro-carbons.

Sir William Huggins succeeded in photographing the spectrum of comet
Wells by an exposure of one hour and a quarter.[1311] The result was to
confirm the novelty of its character. None of the ultra-violet carbon
groups were apparent; but ce