Advertisement I. P ART of the en fuing Discourse a bout Light was written at the De fire of some Gentlemen of the Royal-Society, in the Year 1675, and then sent to their Secretary, and read at their Meetings, and the rest was added about twelve Years after to complete the Theory; except the third Book, and the last Propofition of the Second, which were since put together out of scatter'd Papers. To avoid being engaged in Disputes about A 2 thefe |