  The years between the publication of the Principia and Newton's permanent move to London in 1696 were marked by his increasing disenchantment with his situation in Cambridge. In January 1689, following the Glorious Revolution at the end of 1688, he was elected to represent Cambridge University in the Convention Parliament, which he did until January 1690. During this time he formed friendships with John Locke and Nicolas Fatio de Duillier, and in the summer of 1689 he finally met Christiaan Huygens face to face for two extended discussions. Perhaps because of disappointment with Huygens not being convinced by the argument for universal gravity, in the early 1690s Newton initiated a radical rewriting of the Principia. During these same years he wrote (but withheld) his principal treatise in alchemy, Praxis; he corresponded with Richard Bentley on religion and allowed Locke to read some of his writings on the subject; he once again entered into an effort to put his work on the calculus in a form suitable for publication; and he carried out experiments on diffraction with the intent of completing his Opticks, only to withhold the manuscript from publication because of dissatisfaction with its treatment of diffraction. The radical revision of the Principia became abandoned by 1693, during the middle of which Newton suffered, by his own testimony, what in more recent times would be called a nervous breakdown. In the two years following his recovery that autumn, he continued his experiments in chymistry and he put substantial effort into trying to refine and extend the gravity-based theory of the lunar orbit in the Principia, but with less success than he had hoped.