io8 GREAT MEN OF SCIENCE 



gradually developed further. His communications on these 

 matters to his university teachers^ resulted in his being made 

 the successor of one of these, and thus professor of mathema- 

 tics at Cambridge in 1669 at the age of twenty-seven. 

 He held this position actively until 1695, and all the achieve- 

 ments which we have described above fall in this period, 

 which lasted to his fifty-second year. 



Right at the beginning he also gave lectures concerning 

 his optical discoveries. As a result of these, Newton also 

 came upon the idea at that time, that the production of 

 colours by refraction stood in the way of improving tele- 

 scopes in which lenses were used, and this led him, in 1668, 

 to make the first reflecting telescope. This excited general 

 interest, so that after it had been tested by the Royal Society 

 and a description sent to Huygens, it was also shown to the 

 King, Charles H.^ 



The fame of the telescope soon caused him to be made a 

 member of the Royal Society in 1672, and he remained 

 closely connected with it during the whole of his life. New- 

 ton's consideration of the motions of the planets, which 

 already began in 1666, led him, on the basis of Kepler's 

 laws, to assume a power proceeding from the sun and in- 

 versely proportional to the square of the distance. An 

 identical force must also proceed from the earth and keep 

 the moon in its path, and this force might - if everything 

 were as simple as possible and nothing else came into play - 

 even be the same as the ordinary force of gravity of the earth, 

 reduced in proportion to the square of the distance. One of 

 the most important ways of testing this idea of universal 



^ The beginnings of the calculus of fluxions were already communi- 

 cated to wider circles in 1669, and later also in letters by Newton himself. 

 (Thus in 1679 to Leibniz - see the preface to the Opticks); the com- 

 plete publication of the calculus of fluxions only occurred in 1736, after 

 Newton's death. 



2 This telescope, or a second apparently likewise built by Newton 

 himself, is still carefully preserved in the library of the Royal Society; it 

 bears the date 1671. 



