30 A CENTURY'S PROGRESS IN ASTRONOMY. 



y Leonis 1200 years ; 8 Serpentis 375 years ; 

 and e Bootis 1681 years. Herschel did not 

 compute the orbits mathematically. This was 

 not done for nearly thirty years, when the cal- 

 culation of binary star -objects was commenced 

 by Savary, Sir John Herschel, and Encke. 



In 1782 the French astronomer, Charles 

 Messier (1730-1817), published a list of 103 

 nebulae. In the following year Herschel com- 

 menced his famous sweeps of the heavens with 

 his large reflectors, and during these he made 

 many remarkable discoveries. In 1786 he pub- 

 lished in the ' Philosophical Transactions ' of the 

 Royal Society a catalogue of a thousand new 

 nebulae and star-clusters, in which he gave the 

 position of each object with a short description 

 of its appearance, written by Caroline Herschel 

 while her brother actually had the object before 

 his eyes. In 1786 Herschel published a cata- 

 logue of another thousand clusters and nebulae, 

 followed in 1802 by a list of 500 ; making a 

 total of 2500 clusters and nebulae discovered by 

 the great astronomer. This alone would have 

 gained a great name for William Herschel in 

 this branch of astronomy. In the space of only 

 twenty years 2500 nebulae and clusters had been 

 discovered. The various nebulae and clusters 

 were divided into eight classes, as follows : the 



