396 A Short History of Astronomy [Cn. xiu. 



certain systems of comets, the members of which revolve 

 in the same orbit but separated by considerable intervals 

 of time, have also been discovered. Tebbutt's comet of 

 1 88 1 moves in practically the same path as one seen in 

 1807, and the great comet of 1880, the great comet of 1882 

 (shewn in fig. 99), and a third which appeared in 1887, 

 all move in paths closely resembling that of the comet of 

 1843, while that of 1668 is more doubtfully connected 

 with the same system. And it is difficult to avoid regarding 

 the members of a system as fragments of an earlier comet, 

 which has passed through the stages in which we have 

 actually seen the comets of Biela and Brooks. 



Evidence of such different kinds points to an intimate 

 connection between comets and meteors, though it is 

 perhaps still premature to state confidently that meteors 

 are fragments of decayed comets, or that conversely comets 

 are swarms of meteors. 



306. Each of the great problems of sidereal astronomy 

 which Herschel formulated and attempted to solve has 

 been elaborately studied by the astronomers of the i9th 

 century. The multiplication of observatories, improve- 

 ments in telescopes, and the introduction of photography to 

 mention only three obvious factors of progress have added 

 enormously to the extent and accuracy of our knowledge of 

 the stars, while the invention of spectrum analysis has thrown 

 an entirely new light on several important problems. 



William Herschel's most direct successor was his son 

 John Frederick William (1792-1871), who was not only an 

 astronomer, but also made contributions of importance to 

 pure mathematics, to physics, to the nascent art of photo- 

 graphy, and to the philosophy of scientific discovery. He 

 began his astronomical career about 1816 by re-measuring, 

 first alone, then in conjunction with fames South (1785- 

 1867), a number of his father's double stars. The first 

 result of this work was a catalogue, with detailed measure- 

 ments, of some hundred double and multiple stars (published 

 in 1824), which formed a valuable third term of comparison 

 with his father's observations of 1781-82 and 1802-03, and 

 confirmed in several cases the slow motions of revolution 

 the beginnings of which had been observed before. A 

 great survey of nebulae followed, resulting in a catalogue 



