206 A CENTURY'S PROGRESS IN ASTRONOMY. 



system is not of the prevailing type of stellar 

 systems." 



The study of triple and multiple stars is of 

 deep interest, but the orbits of these objects 

 cannot be said to be fully investigated by any 

 means. The first application of the problem of 

 three bodies to stellar astronomy was made by 

 Seeliger in 1889. His researches, relating to 

 the famous star, Cancri, disclosed the exist- 

 ence of three stars revolving round a dark body, 

 apparently the most massive in the system. 

 The system of Cancri, at least, seems to be 

 modelled on the Ptolemaic design. 



In the study of star-clusters and nebulae, as 

 in the investigation of double stars, Herschel's 

 successor was his son. His observations, both in 

 England and at the Cape of Good Hope, resulted 

 in a large number of new discoveries, and the 

 results of his studies in this direction were 

 published in 1864 in his catalogue of all known 

 clusters and nebulae, amounting to 5079. This 

 catalogue was enlarged and revised in 1888 by 

 John Louis Emil Dreyer (born 1852), a Danish 

 astronomer, but director of the Observatory at 

 Armagh, in Ireland ; and the same observer 

 published from 1888 to 1894 a supplementary 

 list, bringing the number of known clusters and 

 nebulae to about 10,000. 



