Questions. And to shew that I do not take Gravity for an essential Property of Bodies, I have added one Question concerning its Cause, chusing to propose it by way of a Question, because I am not yet satisfied about it for want of Experiments._ _I. N._ July 16, 1717. Advertisement to this Fourth Edition _This new Edition of Sir_ Isaac Newton's Opticks _is carefully printed from the Third Edition, as it was corrected by the Author's own Hand, and left before his Death with the Bookseller. Since Sir_ Isaac's...