on Al-Kindi's Optics; and, through Alhazen (Ibn al-Haytham), on Ibn Sahl's work on dioptrics. Gunpowder A passage in the and another in the are usually taken as the first European descriptions of a mixture containing the essential ingredients of gunpowder. Partington and others have come to the conclusion that Bacon most likely witnessed at least one demonstration of Chinese firecrackers, possibly obtained by Franciscans—including Bacon's friend William of Rubruck—who visited the Mongol Empire during this period. The most telling passage reads: We have an example of these things (that act on the senses) in [the sound and fire of] that children's toy which is made in many [diverse] parts of the world; i.e. a device no bigger than one's thumb. From the violence of that salt called saltpetre [together with sulphur and willow charcoal, combined into a powder] so horrible a sound is made by the bursting of a thing so small, no more than a bit of parchment [containing it], that we find [the ear assaulted by a noise] exceeding the roar of strong thunder, and a flash brighter than the most brilliant lightning. At the beginning of the 20th century, Henry William Lovett Hime of the Royal Artillery published the theory that Bacon's contained a cryptogram giving a recipe for the gunpowder he witnessed. The theory was criticised by Thorndike in a 1915 letter to Science and several books, a position joined by Muir, Stillman, Steele, and Sarton. Needham et al. concurred with these earlier critics that the additional passage did not originate with Bacon and further showed that the proportions supposedly deciphered (a 7:5:5 ratio of saltpetre to charcoal to sulphur) as not even useful for firecrackers, burning slowly with a great deal of smoke and failing to ignite inside a gun barrel. The ~41% nitrate content is too low to have explosive properties. Secret of Secrets Bacon attributed the Secret of Secrets (), the Islamic "Mirror of Princes" (), to Aristotle, thinking that he had composed it for Alexander the Great. Bacon produced an edition of Philip of Tripoli's Latin translation, complete with his own introduction and notes; and his writings of the 1260s and 1270s cite it far more than his contemporaries did. This led Easton and others, including Robert Steele, to argue that the text spurred Bacon's own transformation into an experimentalist. (Bacon never described such a decisive impact himself.) The dating of Bacon's