170 DISCOVERY OH. 



Halley returned to London with the knowledge that 

 the problem had been solved, but without the demon- 

 stration, which, however, reached him in November, 

 and early in the following year was communicated to 

 the Royal Society. Halley was thus the means of 

 bringing Newton's immortal discovery to light ; and 

 he used justly to boast that he had been " the Ulysses 

 who produced this Achilles." 



Twenty years before this, in 1665, Newton's mind 

 had been directed to the subject of gravity. He had 

 left Cambridge on account of the plague, and was in 

 the garden of his home at Woolsthorpe, Lincolnshire, 

 speculating upon the power of gravity, when an apple 

 fell from a tree and is said to have started a train of 

 thought which led to the discovery of the universal law 

 "of gravitation. There is little foundation for this 

 anecdote ; and in Newton's case the fall of an apple 

 was certainly not needed to originate a speculation to 

 which several natural philosophers of his day had given 

 attention. 



An isolated fact can be observed by all eyes ; by those of the 

 ordinary person as well as of the wise. But it is the true physicist 

 alone who may see the bond which unites several facts among 

 which the relationship is important, though obscure. The 

 story of Newton's apple is probably not true, but it is symbolical ; 

 so let us think of it as true. Well, we must believe that many 

 before Newton had seen apples fall, but they made no deduction. 

 Facts are sterile until there are minds capable of choosing between 

 them and discerning those which conceal something and recog- 

 nising that which is concealed ; minds which under the bare 

 fact see the soul of the fact. Henri Poincare. 



The free fall of an object on the earth made a con- 

 venient point of departure for Newton's mental excursion 

 into space ; and the line of reasoning was : a body falls, 

 or is pulled toward the earth's centre by a force which is 



