Closely allied to this subject is the existence of “comet families,” demonstrated by Hoek of Utrecht in 1865, and mentioned in our chapter on the Outer Planets. These comets are found to be dependent on the planets, Jupiter, Saturn, Uranus, and Neptune, each possessing a comet-group. Various theories have been advanced to account for the existence of these groups. One of these theories is that the comets have been captured by the various planets, who have forced them into their present orbits. A mathematical study by _Jean Pierre Octave Callandrean_ (1852-1904) shows that the large number of comets possessed by the various planets may be explained by the disintegration of large comets into small ones. The capture theory, it must be remembered, is purely hypothetical, and must not be regarded as anything but a theory. All that we really know is the existence of comet-families, and of comets moving in the same orbits. The first photograph of a comet was that of Donati’s, taken in 1858 by Bond. In 1881 Tebbutt’s comet was photographed in England by Huggins, and in America by _Henry Draper_ (1837-1882), while in 1882 Gill secured excellent photographs of the great September comet. The first photographic discovery of a comet was made by Barnard in 1892. Since then photography has been much used in cometary astronomy. No bright comets have appeared since 1882,—if we except the comet of 1901, only seen in the southern hemisphere,—although several have been just visible to the naked eye, among them Swift’s comet of 1892 and Perrine’s in the autumn of 1902. Telescopic comets, however, are very numerous, and a year never passes without one or more being discovered. The ordinary periodic comets, such as Encke’s, Faye’s, and others, are very faint, and are becoming fainter at each return—a clear proof that comets die, as Kepler said three centuries ago. This brings us to the subject of the next chapter, Meteoric Astronomy. CHAPTER VIII. METEORS. There is no more interesting chapter in the history of astronomy than that relating to meteors. A hundred years ago shooting-stars were not considered to be astronomical phenomena. They were supposed to be merely inflammable vapours which caught fire in the upper regions of our atmosphere, although both Halley and the scientist _Ernst Chladni_ (1756-1827) had notions of their celestial origin. For thirty-three years after the beginning of the century, however, nothing was heard of meteoric astronomy, nor was the subject considered as part of the astronomer’s labours.