136 A CENTURY'S PROGRESS IN ASTRONOMY. 



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 ex- 

 plained by the disintegration of large comets into 

 small ones. The capture theory, it must be re- 

 membered, 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 Teb- 

 butt'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, 



