. E. Barnard)]

[Illustration: FIG. 2.--COMET C, 1908, NOV. 16d. 13h. 10m.
By permission of Yerkes Observatory (E. E. Barnard).]

PLATE II.

[Illustration: FIG. 3.--HALLEY'S COMET, 1910, APRIL 27.
By permission of Helwan Observatory, Egypt.]

[Illustration: FIG. 4.--HALLEY'S COMET, 1910, MAY 4.
By permission of Yerkes Observatory (E. E. Barnard).]

In the telescope the nucleus of a bright comet appears as an opaque
mass, one or more seconds in diameter, the absolute dimensions comparing
with those of the satellites of the planets, sometimes, indeed, equal to
our moon. But the actual results of micrometric measures are found to
differ very widely. In the case of Donati's comet of 1858 the nucleus
seemed to grow smaller as perihelion was approached. This is evidently
due to the fact that the coma immediately around the nucleus was so
bright as apparently to form a part of it at considerable distances from
the sun. G. P. Bond estimated the diameter of the actual nucleus at 500
m. That the nucleus is a body of appreciable mass seems to be made
probable by the fact that, except for the central attraction of such a
body, a comet would speedily be dissipated by the different attractions
of the sun on different parts of the mass, which would result in each
particle pursuing an orbit of its own. It follows that there must be a
mass sufficient to hold the parts of the comet, if not absolutely
together, at least in each other's immediate neighbourhood. How great a
central mass may be required for this is a subject not yet investigated.
It might be supposed that the amount of matter must be sufficient to
make the nucleus quite opaque. But two considerations based on
observations militate against this view. One is that an opaque body,
reflecting much sunlight, would show a brighter continuous spectrum than
has yet been found in any comet. Another and yet more remarkable
observation is on record which goes far to prove not only the tenuity,
but the transparency of a cometary nucleus. The great comet of 1882 made
a transit over the sun on the 17th of September, an occurrence unique in
the history of astronomy. But the fact of the transit escaped attention
except at the observatory of the Cape of Good Hope. Here the comet was
watched by W. H. Finlay and by W. L. Elkin as it approached the sun, and
was kept in sight until it came almost or quite in contact with the
sun's disk, when it disappeared. It should, if op