CHAPTER XIII. 



THE NINETEENTH CENTURY. 



"The greater the sphere ot our knowledge, the larger is the 

 surface of its contact with the infinity of our ignorance." 



272. THE last three chapters have contained some account 

 of progress made in three branches of astronomy which, 

 though they overlap and exercise an important influence on 

 one another, are to a large extent studied by different men 

 and by different methods, and have different aims. The 

 difference is perhaps best realised by thinking of the work 

 of a great master in each department, Bradley, Laplace, 

 and Herschel. So great is the difference that Delambre 

 in his standard history of astronomy all but ignores the 

 work of the great school of mathematical astronomers who 

 were his contemporaries and immediate predecessors, not 

 from any want of appreciation of their importance, but 

 because he regards their work as belonging rather to mathe- 

 matics than to astronomy; while Bessel ( 277), in saying 

 that the function of astronomy is " to assign the places on 

 the sky where sun, moon, planets, comets, and stars have 

 been, are, and will be," excludes from its scope nearly 

 everything towards which Herschel's energies were directed. 



Current modern practice is, however, more liberal in its 

 use of language than either Delambre or Bessel, and finds it 

 convenient to recognise all three of the subjects or groups 

 of subjects referred to as integral parts of one science. 



The mutual relation of gravitational astronomy and what 

 has been for convenience called observational astronomy 

 has been already referred to (chapter x., 196). It should, 

 however, be noticed that the latter term has in this book 

 hitherto been used chiefly fqr only one part of the astrono- 



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