PROBLEMS OF MODERN SCIENCE 



fixity with regard to the stars indicated that their 

 distances also were enormous. 



The foundations of Sidereal Astronomy, as we 

 now know it, were laid by the Herschels, who 

 between them explored the whole heavens. Their 

 discovery of binary stars revolving round one 

 another in elliptic orbits demonstrated that the 

 sway of the law of gravitation extended throughout 

 the material universe ; the soundings of the elder 

 Herschel gave the first reliable indications of the 

 extent and shape of that universe, and the dis- 

 covery that the nebulae were to be numbered not 

 by hundreds but by thousands showed that they 

 were not exceptional but normal forms which 

 masses of matter might assume. 



In considering a group of objects we may fix 

 our attention upon the characteristics which they 

 possess in common, or upon those which distinguish 

 one from the other and mark them out as indi- 

 viduals. When the number of objects in the 

 group is small the study of the individual pecu- 

 liarities may be of more importance than that of 

 the qualities common to all. But in dealing with 

 a large population such as that formed by the 

 stars the common characteristics are those which 

 demand chief attention. 



As a rule the characteristics are such that 

 they are possessed by different individuals in 

 different degrees, and one of the first steps in a 

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