  Newton was disappointed in the critical response to the first edition. The response in England was adulatory, but the failure to note loose ends must have led Newton to doubt how much anyone had mastered technical details. The leading scientific figure on the Continent, Christiaan Huygens, offered a mixed response to the book in his Discourse on the Cause of Gravity (1690). On the one hand, he was convinced by Newton's argument that inverse-square terrestrial gravity not only extends to the Moon, but is one in kind with the centripetal force holding the planets in orbit; on the other hand,