Page:The story of the comets.djvu/25

I. The physical difference between different comets is a matter very little appreciated or understood by people in general. With such, every thought is concentrated on the comet's tail, if it has one; or if it has not a tail, then the verdict is "no comet". Yet the facts of the case are that the comets with tails are, and always have been, considerably outnumbered by the comets without tails. An explanation of the popular view is to be found in the fact that the tailed comets are very frequently visible to the naked eye, whilst the tailless comets may be said to be never so visible.

An ordinary comet when first discovered by means of a telescope either consists of, or sooner or later developes, three parts. In the latter case the developement takes place somewhat

in the following manner: the telescope reveals a faintly luminous speck; its size increases gradually, and after some little time a nucleus appears. This word indicates that a portion of the comet is more condensed in its light than the rest. Both the size and the brilliancy of the object progressively increase; the cloud-like mass around the nucleus (called the coma ), becomes less symmetrical, and this loss of symmetry, when it occurs, betokens the early developement of a tail. Nucleus and coma taken together are generally spoken of as the head of the comet. When a tail has become manifest it will be found to be brighter near the head than at the tip, and often brighter on one side than on the other.