CHAPTER VIII. 



FROM GALILEI TO NEWTON. 



".Andgr.owjthc kfty telescope, the scale 

 By which they venture heaven itself t'assail, 

 Was raised, and planted full against the moon." 



Hudibras 



152. BETWEEN the publication of Galilei's Two New 

 Sciences (1638) and that of Newton's Principia (1687) a 

 period of not quite half a century elapsed ; during this 

 interval no astronomical discovery of first-rate importance 

 was published, but steady progress was made on lines 

 already laid down. 



On the one hand, while the impetus given to exact observa- 

 tion by Tycho Brahe had not yet spent itself, the invention 

 of the telescope and its gradual improvement opened out an 

 almost indefinite field for possible discovery of new celestial 

 objects of interest. On the other hand, the remarkable 

 character of the three laws in which Kepler had summed 

 up the leading characteristics of the planetary motions 

 could hardly fail to suggest to any intelligent astronomer 

 the question why these particular laws should hold, or, in 

 other words, to stimulate the inquiry into the possibility of 

 shewing them to be necessary consequences of some 

 simpler and more fundamental law or laws, while Galilei's 

 researches into the laws of motion suggested the possibility 

 of establishing some connection between the causes under- 

 lying these celestial motions and those of ordinary terrestrial 

 objects. 



153. It has been already mentioned how closely Galilei 

 was followed by other astronomers (if not in some cases 

 actually anticipated) in most of his telescopic discoveries. 



198 



