Original proof of Pappus' Hexagon Theorem

It is in book 7 of Pappus's Collection. There is an English translation published by Springer. It's an expensive book, so I'll reproduce the proof & figure here:

It relies on earlier lemmas; there's an explanation that includes them on Wikipedia.

This is available in the Loeb Classics Library, Greek Mathematical Works, Vol II (edited by Ivor Thomas), page 600.The Loeb CL can probably be found in its entirety in many liberal arts college libraries. For your convenience, you can find the book here.

Stumbled by coincidence today over the book "Sir Thomas Heath: A History of Greek Mathematics, II". Chapter XIX (pp. 355-439) is devoted to Pappus and contains a lot of (mathematical) stuff. Perhaps you can find there what you are looking for.

You can find the book at the Internet Archive (PDF link on the left, download may take a while).