It is supposed that it was at Woolsthorpe in the summer of 1666 that Newton's thoughts were directed to the subject of gravity. They are said to have been inspired by Newton's seeing an apple fall from a tree on his mother's farm, a version for which there is reasonable historical evidence. In one version of the story, the apple is supposed to have fallen on Newton's head; this version appears to have been invented by Isaac D'Israeli. Voltaire is the authority for the former version of the story. He had his information from Newton's favourite niece Catherine Barton, who married John Conduitt, a fellow of the Royal Society, and one of Newton's intimate friends. How much truth there is in what is a plausible and a favourite story can never be known, but it is certain that tradition marked a tree as that from which the apple fell, until 1866, when, owing to decay, the tree was cut down and its wood carefully preserved.