This was the first attempt to deal with the dynamics of a system.

In this work, also, was the true determination of the relation between the length of a pendulum and the time of its oscillation.
In 1681 he returned to Holland, influenced, it is believed, by the attitude that was being taken in France against his religion.

Here he continued his investigations, built his immense telescopes, and, among other things, discovered "polarization," which is recorded in Traite de la Lumiere, published at Leyden in 1690.

Five years later he died, bequeathing his manuscripts to the University of Leyden.

It is interesting to note that he never accepted Newton's theory of gravitation as a universal property of matter.
XI.