(This section is taken mainly from Heath, The Thirteen Books of Euclid's Elements, pp. 46-47.)
Discovered in 1808 by F. Peyrard, a Frenchman, in the library of the Vatican. At the time, Napoleon was collecting manuscripts from Italy and sending them back to Paris. Peyrard took advantage of the situation and had Vat. ms. gk. 190 sent to Paris for his own use. (Heath, ibid, 103)
This manuscript, a copy of Euclid carefully written by hand in the 10th Century A.D., contains all thirteen books of the Elements as well as some commentary and some other mathematical works formerly attributed to Euclid.
Peyrard noticed that there were differences between the accepted text of Euclid and Vat. 190. The first was that most manuscripts that had been discovered had a little subtitle on the first page, either "from the edition of Theon," or "from the lectures of Theon," which 190 did not have. But the biggest difference appeared at the end of theorem 33 in Book VI: the second part of the theorem was missing.
Theon of Alexandria lived in Euclid's native city in the 4th Century A.D. He was a scholar who wrote a commentary on a book of astronomy as well as a commentary on Euclid. In the astronomical commentary he states, "but that sectors in equal circles are to one another as the angles on which they stand has been proved by me in my edition of the Elements at the end of the sixth book."
This little proof that Theon mentions is exactly what is missing from Vat. 190. Studying Euclid's theorem, Theon found it a bit lacking and simply wrote in what he thought was needed to complete the idea. From that time on copyists included Theon's addition as though it were written by Euclid himself as part of the text. This is called an interpolation, and it has happened many times in the history of Latin and Greek texts. Scholars are always trying to find ways of picking out and eliminating interpolations. Peyrard was lucky to find such definitive proof. He published his own edition of The Elements in 1814-1818, eliminating the interpolation in VI.33 and using manuscript 190 to make other changes in the text. (Heath, ibid, 103)