Note in [Fig. 102] the great variety of aspect presented by some of the more famous comets, which are here represented upon a very small scale.
[Fig. 103] is from a photograph of one of the faint comets of the year 1893, which appears here as a rather feeble streak of light amid the stars which are scattered over the background of the picture. An apparently detached portion of this comet is shown at the extreme left of the picture, looking almost like another independent comet. The clean, straight line running diagonally across the picture is the flash of a bright meteor that chanced to pass within the range of the camera while the comet was being photographed.
A more striking representation of a moderately bright telescopic comet is contained in Figs. [104] and [105], which present two different views of the same comet, showing a considerable change in its appearance. A striking feature of [Fig. 105] is the star images, which are here drawn out into short lines all parallel with each other. During the exposure of 2h. 20m. required to imprint this picture upon the photographic plate, the comet was continually changing its position among the stars on account of its orbital motion, and the plate was therefore moved from time to time, so as to follow the comet and make its image always fall at the same place. Hence the plate was continually shifted relative to the stars whose images, drawn out into lines, show the direction in which the plate was moved—i. e., the direction in which the comet was moving across the sky. The same effect is shown in the other photographs, but less conspicuously than here on account of their shorter exposure times.
These pictures all show that one end of the comet is brighter and apparently more dense than the other, and it is customary to call this bright part the head of the comet, while the brushlike appendage that streams away from it is called the comet's tail.
160. The parts of a comet.—It is not every comet that has a tail, though all the large ones do, and in [Fig. 103] the detached piece of cometary matter at the left of the picture represents very well the appearance of a tailless comet, a rather large but not very bright star of a fuzzy or hairy appearance. The word comet means long-haired or hairy star. Something of this vagueness of outline is found in all comets, whose exact boundaries are hard to define, instead of being sharp and clean-cut like those of a planet or satellite. Often, however, there is found in the head of a comet a much more solid appearing part, like the round white ball at the center of [Fig. 106], which is called the nucleus of the comet, and appears to be in some sort the center from which its activities radiate. As shown in Figs. [106] and [107], the nucleus is sometimes surrounded by what are called envelopes, which have the appearance of successive wrappings or halos placed about it, and odd, spurlike projections, called jets, are sometimes found in connection with the envelopes or in place of them. These figures also show what is quite a common characteristic of large comets, a dark streak running down the axis of the tail, showing that the tail is hollow, a mere shell surrounding empty space.
