Sir Isaac Newton (1642-1727) is said to have made his greatest contributions to original thought in science in 1665-6 while at his parents' home in Lincolnshire escaping the Great Plague (which had closed the universities), a period of which he wrote: 'I was in the prime of my age for invention'. It was at this fruitful time that he formulated calculus, hit upon the idea of gravity and did experiments which showed that white light was made up different coloured rays. He returned to Trinity College Cambridge, where in 1699 he became Professor of Mathematics, but at first had to circulate his work privately among other scientists because no one would publish it. He was also interested in the movements of the planets and designed his own telescope; and in later life became MP for Cambridge University, Master of the Mint, and President of the Royal Society. It was not until 1704 that his researches on calculus were published. He was the author of Principia, one of the most important books in the whole history of science, in which he proved the 'laws of motion'. In keeping with his age, Newton blurred the borders between science and speculation: he was as passionate about astrology as about astronomy, dabbled in alchemy, and used the Bible to work out that the date of the earth's creation was 3500 BC.