HELTER-SKELTER FLIGHT OF THE STARS 325 



doubt that they were in reality revolving, one around the other. 

 With this clue he set about searching for others : he found 

 half-a-dozen. His observations enabled him to attempt a 

 rough guess at the periods of their revolution ; that of Castor 

 and its twin he put at three hundred years. It has slowed down 

 since ; the present estimate is about a thousand years. 



From this small beginning the number of double stars, or 

 Binary Systems, as they have come to be called, has grown 

 until they are now numbered by the hundreds. Their discovery 

 naturally led to a search for triple and multiple systems. The 

 probable existence of such multiples is no longer a matter of 

 much question. So it was that Herschel's studies opened the 

 way for some insight as to the structure of the universe. It 

 was this latter which eventually became the especial predi- 

 lection of his life. 



But the heavens show something more than stars. Just as 

 here and there -appear little clumps and clusters, so even to 

 the unaided eye there are here and there faint blurs which do 

 not seem to be made up of distinct stars. The old-time observers 

 had given to these the name of nebulae or clouds. The first 

 effect of the introduction of the telescope was to resolve these 

 star clouds in part into star clusters ; naturally it was supposed 

 that telescopes of higher power would resolve them all. Even 

 before Herschel's time, such a view was found to be erroneous. 

 Several astronomers just preceding him had begun to catalogue 

 the nebulae, with the idea of eventually determining their density 

 and the changes that might take place in them. One of these 

 observers was Messier, whose specialty was hunting comets. 

 It was often very difficult to distinguish a comet from a nebula, 

 so he made a record of over a hundred of these puzzling appari- 

 tions. This was where the matter stood when Herschel took 

 it up. 



Before he had done he had catalogued twenty-five hundred 

 disclosed, as well, their extraordinary variety. What was 

 still more of interest was that he came to conceive them as the 

 stuff from which suns and stars are made. He found them of 

 all shapes and sizes, some bright and some dull, some with a 

 seeming nucleus in the centre, some like cloudy stars surrounded 

 by a nebulous atmosphere. As he phrased it, he found that 

 they could be selected " so that an insensible gradation shall 



