152 NEWTON S PKINCIPIA. 



great man s discoveries, that nothing should ever be added 

 to them but by the use of means which he had himself 

 furnished, it was only by applying a form of experiment 

 which Sir Isaac Newton had used in examining the 

 colours of thick and thin plates, that this important fact 

 was ascertained, he not having subjected the phenomenon 

 first observed by Grimaldi to that mode of investiga 

 tion.* 



The Fourteenth Section concludes with an elegant so 

 lution of a local problem in Descartes s Geometry, for 

 finding that form of refracting glasses which will make 

 the rays converge to a given focus, a problem, the de 

 monstration of which Descartes had not given. The 

 brilliant discoveries made by Sir Isaac Newton upon the 

 refrangibility and colours of light, not belonging to dy 

 namics, he pursues the subject no further in this place, 

 having reserved the history of those inquiries for his 

 other great work, the Opticsf, perhaps the only monument 

 of human genius that merits a place by the side of the 

 Principle, 



The truths which we have been contemplating respect 

 ing the attractions of bodies are fruitful in important 

 consequences respecting the constitution of the universe. 

 We have seen that the law of attraction which makes it de 

 crease as the squares of the distances increase, and the law 

 which makes it increase as the distances decrease, are the 



* The Undulatory Theory of light, towards which philosophers have of 

 late years appeared to lean, is no exception to this remark; for the princi 

 ples of that Theory may be found in the Eighth Section of the Second 

 Book of the Principia, and the Scholium which concludes that Section 

 seems to anticipate the application of its principles to Optical Science. 



j- An abstract of these discoveries had been given in the Lectiones 

 Opticao at Cambridge seventeen years before the publication of the Prin 

 cipia in 1687. The Optics only appeared in 1704. 



