  Save for a few weeks away from Cambridge, from late 1684 until early 1687, Newton concentrated on lines of research that expanded the short ten-proposition tract into the 500 page Principia, with its 192 derived propositions. Initially the work was to have a two book structure, but Newton subsequently shifted to three books, and replaced the original version of the final book with one more mathematically demanding. The manuscript for Book 1 was sent to London in the spring of 1686, and the manuscripts for Books 2 and 3, in March and April 1687, respectively. The roughly three hundred copies of the Principia came off the press in the summer of 1687, thrusting the 44 year old Newton into the forefront of natural philosophy and forever ending his life of comparative isolation.