_In this Second Edition of these Opticks I have omitted the Mathematical Tracts publish'd at the End of the former Edition, as not belonging to the Subject. And at the End of the Third Book I have added some 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 Lectiones Opticae, _which he publickly read in the University of_ Cambridge _in the Years 1669, 1670, and 1671, are lately printed, it has been thought proper to make at the bottom of the Pages several Citations from thence, where may be found the Demonstrations, which the Author omitted in these_ Opticks. * * * * * Transcriber's Note: There are several greek letters used in the descriptions of the illustrations. They are signified by [Greek: letter]. Square roots are noted by the letters sqrt before the equation. * * * * * THE FIRST BOOK OF OPTICKS _PART I._ My Design in this Book is not to explain the Properties of Light by Hypotheses, but to propose and prove them by Reason and Experiments: In order to which I shall premise the following Definitions and Axioms. _DEFINITIONS_ DEFIN. I. _By the Rays of Light I understand its least Parts, and those as well Successive in the same Lines, as Contemporary in several Lines._ For it is manifest that Light consists of Parts, both Successive and Contemporary; because in the same place you may stop that which comes one moment, and let pass that which comes presently after; and in the same time you may stop it in any one place, and let it pass in any other. For that part of Light which is stopp'd cannot be the same with that which is let pass. The least Light or part of Light, which may be stopp'd alone without the rest of the Light, or propagated alone, or do or suffer any thing alone, which the rest of the Light doth not or suffers not, I call a Ray of Light. DEFIN. II.