Generally speaking, however, a comet consists of three parts: The nucleus--which appears as a more or less clearly defined star-like point, and is the only part of the comet which will bear any magnification to speak of--the coma, and the tail. In many telescopic comets the nucleus is entirely absent, and, in the comets in which it is present, it is of very varied size, and often presents curious irregularities in shape, and even occasionally the appearance of internal motions. It frequently changes very much in size during the period of the comet's visibility. The nucleus is the only part of a comet's structure which has even the most distant claim to solidity; but even so the evidence which has been gradually accumulated all goes to show that while it may be solid in the sense of being composed of particles which have some substance, it is not solid in the sense of being one coherent mass, but rather consists of something like a swarm of small meteoric bodies. Surrounding the nucleus is the coma, from which the comet derives its name. This is a sort of misty cloud through which the nucleus seems to shine like a star in a nebula or a gas-lamp in a fog. Its boundaries are difficult to trace, as it appears to fade away gradually on every side into the background; but generally its appearance is more or less of a globular shape except where the tail streams away from it behind. Sometimes the coma is of enormous extent--the Great Comet of 1811 showed a nucleus of 428 miles diameter, enclosed within a nebulous globe 127,000 miles across, which in its turn was wrapped in a luminous atmosphere of four times greater diameter, with an outside envelope covering all, and extending backwards to form the tail. But it is also of the most extraordinary tenuity, the light of the very faintest stars having been frequently observed to shine undimmed through several millions of miles of coma. Finally, there is the tail, which may be so short as to be barely distinguishable; or may extend, as in the case of Comet 1811 (ii.), to 130,000,000 miles; or, as in that of Comet 1843 (i.), to 200,000,000. The most tenuous substances with which we are acquainted seem to be solidity itself compared with the material of a comet's tail. It is 'such stuff as dreams are made of.'