IN this Second Edition of thefe Opticks I have o-Tracts publifb'd at the End of the Former Edition, as not be-longing to the Subject. And at the End of the Tbird Book I bave added fome 2ueftions. And to Fbew that I do not Take Gravity for an Effential Property of Bodies, I have added one 2ueftion concerning its Caufe, cbufing to propofe it by way of a 2ueftion, becaufe I am not yet fatisfied about it for want of Experiments. July 16. 1717. I. N. ERRATA. Pap. 342. Lin. 6. For feven, read feven and 4 half. Ib. Lin. 8. For 14, read 15, Ib. Lin. 10, for 21, 28, or 35, read 22.50 , 30 or 38. Ib .lin. 21. For 70 , 140, 210 . Read 76, 152, 228. Pag. 366. I. 22. For beigbi read diftavce. THE FIRST BOOK OF OPTICKS. P A R T I M Y Defign in this Book is not to Explain the Pro