s reduced below this limit the ratio of
the repulsive to the attractive force increases, but soon reaches a
maximum, after which it diminishes down to a diameter of 0.00007 mm.,
when the two actions are again balanced. Below this limit the light
speedily ceases to act. It follows that a purely gaseous body, such as
would emit a characteristic bright line spectrum, would not be subject
to the repulsion. We must therefore conclude that both the solid and
gaseous forms of matter are here at play, and this view is consonant
with the fact that the comet leaves behind it particles of meteoric
matter.

Another possible cause is electrical repulsion. The probability of this
cause is suggested by recent discoveries in radioactivity and by the
fact that the sun undoubtedly sends forth electrical emanations which
may ionize the gaseous molecules rising from the nucleus, and lead to
their repulsion from the sun, thus resulting in the phenomena of the
tail. But well-established laws are not yet sufficiently developed to
lead to definite conclusions on this point, and the question whether
both causes are combined, and, if not, to which one the phenomena in
question are mainly due, must be left to the future.

A curious circumstance, which may be explained by a duplex character of
the matter forming a cometary tail, is the great difference between the
visual and photographic aspect of these bodies. The soft, delicate,
feathery-like form which the comet with its tail presents to the eye is
wanting in a photograph, which shows principally a round head with an
irregularly formed tail much like the knotted stalk of a plant. It
follows that the light emitted by the central axis of the tail greatly
exceeds in actinic power the diffuse light around it. A careful
comparison of the form and intensity of the photographic and visual
tails may throw much light on the question of the constitution of these
bodies, but no good opportunity of making the comparison has been
afforded since the art of celestial photography has been brought to its
present state of perfection.

The main conclusion to which the preceding facts and considerations
point is that the matter of a comet is partly solid and partly gaseous.
The gaseous form is shown conclusively by the spectroscope, but in view
of the extreme delicacy of the indications with this instrument no
quantitative estimate of the gas can be made. As there is no central
mass sufficient to hold together