284 EIGHTEENTH CENTURY. PT. TIL 



other, but are actual pairs of stars moving round and round 

 each other, as if they were connected by a rod suspended by 

 its centre, and then set revolving ! 



To understand how great a discovery this was, it is 

 necessary to bear in mind that Newton had only been able 

 to prove that gravitation acts between the sun and the 

 planets ; but here was a reason for believing that even in 

 the far-off stars, millions of miles away from our system, the 

 same force is holding distant suns together, and keeping 

 them in their orbits. This great discovery has been still 

 more clearly proved by later investigations, and groups of 

 two, three, and even more stars are now known, in which 

 these bodies revolve round a common centre, held together 

 by the force of gravitation. 



Herschel studies Star-clusters and Nebulae, 1786. 

 The next discovery which Herschel made was quite as 

 remarkable as that of the binary stars. As long ago as the 

 time of Ptolemy (138 A.D.) five curious stars had been 

 observed, which he called ' cloudy stars,' because they 

 looked as if they were covered by a mist ; and the number 

 of these cloudy masses had been increased by different 

 astronomers as time went on. When Herschel turned his 

 attention to them he discovered so many that, in 1786, 

 he published a catalogue of no less than a thousand, and 

 added fifteen hundred more a few years later ! Some of 

 these bodies, such as the bright spot called the ' beehive,' in 

 the constellation Cancer, were simply clusters of stars which 

 could be seen distinctly through a telescope. In others the 

 separate stars could not be seen even with the strongest 

 magnifying power, but the group looked so much more 

 distinct through a powerful telescope than through a feeble 

 one, that it seemed most likely the stars were there, if only 

 they could be distinguished But a third set of cloudy 



