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.
In Part IV of the Opus Majus, Bacon proposed a calendrical reform similar to the later system introduced in 1582 under Pope Gregory XIII. Drawing on ancient Greek and medieval Islamic astronomy recently introduced to western Europe via Spain, Bacon continued the work of Robert Grosseteste and criticized the then-current Julian calendar as "intolerable, horrible, and laughable".
Around 1589, Robert Greene adapted the story for the stage as The Honorable Historie of Frier Bacon and Frier Bongay, one of the most successful Elizabethan comedies. As late as the 1640s, Thomas Browne was still complaining that "Every ear is filled with the story of Frier Bacon, that made a brazen head to speak these words, Time is". Greene's Bacon spent seven years creating a brass head that would speak "strange and uncouth aphorisms" to enable him to encircle Britain with a wall of brass that would make it impossible to conquer.
Greene's play prompted a less successful sequel John of Bordeaux and was recast as a children's story for James Baldwin's 1905 Thirty More Famous Stories Retold. "The Brazen Head of Friar Bacon" also appears in Daniel Defoe's 1722 Journal of the Plague Year, Nathaniel Hawthorne's 1843 "The Birth-Mark" and 1844 "The Artist of the Beautiful", william Douglas O'Connor's 1891 "The Brazen Android" (where Bacon devises it to terrify King Henry into accepting Simon de Montfort's demands for greater democracy), John Cowper Powys's 1956 The Brazen Head, and Robertson Davies's 1970 Fifth Business.
To commemorate the 700th anniversary of Bacon's approximate year of birth, Prof. J. Erskine wrote the biographical play A Pageant of the Thirteenth Century, which was performed and published by Columbia University in 1914. A fictionalized account of Bacon's life and times also appears in the second book of James Blish's After Such Knowledge trilogy, the 1964 Doctor Mirabilis. Bacon serves as a mentor to the protagonists of Thomas Costain's 1945 The Black Rose, Umberto Eco's 1980 The Name of the Rose, and David Flusfeder's John the Pupil. He is the protagonist's sidekick in John Bellairs's The Face in the Frost.
At the beginning of the 20th century, Henry william Lovett Hime of the Royal Artillery published the theory that Bacon's Epistola contained a cryptogram giving a recipe for the gunpowder he witnessed. The theory was criticized 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.
As a private scholar, his whereabouts for the next decade are uncertain but he was likely in Oxford c. 1248–51, where he met Adam Marsh, and in Paris in 1251. He seems to have studied most of the known Greek and Arabic works on optics (then known as "perspective", perspectiva). A passage in the Opus Tertium states that at some point he took a two-year break from his studies.
This idea that Bacon was a modern experimental scientist reflected two views of the period: that the principal form of scientific activity is experimentation and that 13th-century Europe still represented the "Dark Ages". This view, which is still reflected in some 21st-century popular science books, portrays Bacon as an advocate of modern experimental science who emerged as a solitary genius in an age hostile to his ideas. Based on Bacon's apocrypha, he is also portrayed as a visionary who predicted the invention of the submarine, aircraft, and automobile.
Research also established that Bacon was not as isolated—and probably not as persecuted—as was once thought. Many medieval sources of and influences on Bacon's scientific activity have been identified. In particular, Bacon often mentioned his debt to the work of Robert Grosseteste: his work on optics and the calendar followed Grosseteste's lead, as did his idea that inductively-derived conclusions should be submitted for verification through experimental testing.
The cryptic Voynich manuscript has been attributed to Bacon by various sources, including by its first recorded owner, but historians of science Lynn Thorndike and George Sarton dismissed these claims as unsupported. and the vellum of the manuscript has since been dated to the 15th century.
As early as the 16th century, natural Philosophers like Bruno, Dee, and Francis Bacon were attempting to rehabilitate Bacon's reputation and to portray him as a scientific pioneer who had avoided the petty bickering of his contemporaries to attempt a rational understanding of nature. By the 19th century, commenters following Whewell considered that "Bacon... was not appreciated in his age because he was so completely in advance of it; he is a 16th or 17th century Philosopher, whose lot has been by some accident cast in the 13th century". His assertions in the Opus Majus that "theories supplied by reason should be verified by sensory data, aided by instruments, and corroborated by trustworthy witnesses" were (and still are) considered "one of the first important formulations of the scientific method on record".