I turned on my computer and called Bryn Mawr college's library through my modem to search its catalogue for Simson's translation and edition of The Elements, which was of historical value to this project. Simson claims, on his title page, that "The Errors, by which Theon, or others, have long ago vitiated these Books, are corrected, and some of Euclid's Demonstrations are restored."
It is true, but difficult to prove, that Theon of Alexandria heavily edited and rewrote much of The Elements, and that all later Greek manuscripts and printed editions depended on Theon's "recension." (In the 1800's, a Frenchman named Peyrard finally found a manuscript in the Vatican library which proved to be free of some of Theon's interpolations.)
Simson's aim was accuracy, so he "corrected" and "restored" the text to what he thought was the original Euclid, cutting out what he thought Theon had added and adding what he thought Theon had cut out. I'm sure Theon thought he was improving the state of Euclidean scholarship as much as Simson did. They just had different ideas of what an "improvement" entailed.
I expected to get a reprinted edition or a reproduction of Simson's edition, since it was originally published in the l700's. What arrived at Bryn Mawr, to the delight of both the librarian and me, was an original, crumbling, 1781 edition of Simson held together by book tape. It is beyond repair and has no particular value as a rare book in its current condition, so Swarthmore will simply keep it in circulation until it returns to dust.
It had once been rebound, and in the process an elegantly handwritten note had been sewn into the text itself. The note has the title, "proposition suplimentary to VI Book." (Note the mispelling or, perhaps, obsolete spelling, "suplimentary.") The right margin of the note had been cleanly sliced off, removing the writing at the ends of the lines, and the bottom of the note had likewise been lost.
The note describes the last proposition of Book 6, which Theon admits he wrote and added to The Elements, and which Simson removed because he believed that Theon, not Euclid, had written it.
Suppose that, a thousand years from now, archaeologists uncover this book, with its mysterious note bound inside, perfectly preserved in the ashes of the Swarthmore library (?shelled by radical Episcopalians in the American Civil Wars between the Fundamentalist Muslims and the Fundamentalist Christians). And suppose "Classical" scholars, having only this copy of Simson, some fragments of Heiberg's edition, and a few Latin and Arabic translations, study the text to try to distinguish the original Euclid from the additions of later generations. Imagine their difficulties.