582 THE PRINCIPLES OF SCIENCE. [CHAP. 



chemistry and electricity, there is hardly a progressive 

 branch of physical and mathematical science, which has 

 not been developed from the germs of true scientific pro- 

 cedure which he disclosed in the Principia or the Opticks. 

 Overcome by the success of his theory of universal gravi- 

 tation, we are apt to forget that in his theory of sound he 

 originated the mathematical investigation of waves and 

 the mutual action of particles ; that in his corpuscular 

 theory of light, however mistaken, he first ventured to 

 apply mathematical calculation to molecular attractions 

 and repulsions ; that in his prismatic experiments he 

 showed how far experimental verification could be pushed; 

 that in his examination of the coloured rings named after 

 him, he accomplished the most remarkable instance of 

 minute measurement yet known, a mere practical appli- 

 cation of which by Fizeau was recently deemed worthy 

 of a medal by the Royal Society. We only learn by degrees 

 how complete was his scientific insight ; a few words in his 

 third law of motion display his acquaintance with the 

 fundamental principles of modern thermodynamics and 

 the conservation of energy, while manuscripts long over- 

 looked prove that in his inquiries concerning atmospheric 

 refraction he had overcome the main difficulties of ap- 

 plying theory to one of the most complex of physical 

 problems. 



After all, it is only by examining the way in which he 

 effected discoveries, that we can rightly appreciate his 

 greatness. The Principia treats not of gravity so much 

 as of forces in general, and the methods of reasoning 

 about them. He investigates not one hypothesis only, 

 but mechanical hypotheses in general. Nothing so much 

 strikes the reader of the work as the exhaustiveness of his 

 treatment, and the unbounded power of his insight. If he 

 treats of central forces, it is not one law of force which he 

 discusses, but many, or almost all imaginable laws, the 

 results of each of which he sketches out in a few pregnant 

 words. If his subject is a resisting medium, it is not air 

 or water alone, but resisting media in general. We have 

 a good example of his method in the scholium to the 

 twenty-second proposition of the second book, in which he 

 runs rapidly over many suppositions as to the laws of the 

 compressing forces which might conceivably act in an 



