74 GREAT MEN OF SCIENCE 



the microscope, and introduced dark field illumination,^ 

 which, when improved by modern means, gave us the ultra- 

 microscope. 



These discoveries, together with the invention of the 

 pendulum clock, resulted in Huygens being called to the 

 newly-founded Academy in Paris, where he occupied an 

 influential position with a substantial salary and free quarters, 

 from 1666 to 1 68 1. Thereafter, Louis XIV went to war with 

 Holland; the minister Holbert, who had a high appreciation 

 of Huygens, lost his influence, and the banishment of the 

 Protestants (Revocation of the Edict of Nantes, 1685) 

 threatened. Huygens therefore gave up for lost all that 

 France had offered him, and returned to Holland to his 

 paternal estate Hofwijck near the Hague, where he died at 

 the age of sixty-seven, after fourteen years of further 

 uninterrupted activity. He did not found a family, and 

 otherwise remained a lonely man. Only a journey to 

 England occurred in this latter period of his life, where he 

 made the personal acquaintance of Newton, his fourteen- 

 year younger contemporary. Newton had a high opinion of 

 him, and would have liked to keep him at the University of 

 Cambridge, but nothing came of this. 



Huygens' consideration of the movement of the pen- 

 dulum first led him to discover the present well-known 

 formula for calculating the time of oscillation from the 

 length, which formula summarises Galileo's laws of the 

 pendulum, and further gives the exact connection with the 

 acceleration of gravity which is operative in free falling. ^ 

 He also saw that constancy of the period of oscillation with 

 varying arc or amplitude cannot hold good exactly for the 

 ordinary pendulum, but only for the cycloidal pendulum, in 

 which the bob moves along a cycloid, instead of along the ordi- 

 nary circular path. He arrived at this view by recognising 



1 See Oeuvres complies, vol. 13, 2, p. 696 (1692). 



2 In Horologium Oscillatoriiim, second part, paragraph 25. 



