<P> The oldest surviving photograph of the image formed in a camera was created by Niépce in 1826 or 1827 . It was made on a polished sheet of pewter and the light - sensitive substance was a thin coating of bitumen, a naturally occurring petroleum tar, which was dissolved in lavender oil, applied to the surface of the pewter and allowed to dry before use . After a very long exposure in the camera (traditionally said to be eight hours, but now believed to be several days), the bitumen was sufficiently hardened in proportion to its exposure to light that the unhardened part could be removed with a solvent, leaving a positive image with the light areas represented by hardened bitumen and the dark areas by bare pewter . To see the image plainly, the plate had to be lit and viewed in such a way that the bare metal appeared dark and the bitumen relatively light . </P> <P> In partnership, Niépce in Chalon - sur - Saône and Louis Daguerre in Paris refined the bitumen process, substituting a more sensitive resin and a very different post-exposure treatment that yielded higher - quality and more easily viewed images . Exposure times in the camera, although substantially reduced, were still measured in hours . </P> <P> Niépce died suddenly in 1833, leaving his notes to Daguerre . More interested in silver - based processes than Niépce had been, Daguerre experimented with photographing camera images directly onto a mirror - like silver - surfaced plate that had been fumed with iodine vapor, which reacted with the silver to form a coating of silver iodide . As with the bitumen process, the result appeared as a positive when it was suitably lit and viewed . Exposure times were still impractically long until Daguerre made the pivotal discovery that an invisibly slight or "latent" image produced on such a plate by a much shorter exposure could be "developed" to full visibility by mercury fumes . This brought the required exposure time down to a few minutes under optimum conditions . A strong hot solution of common salt served to stabilize or fix the image by removing the remaining silver iodide . On 7 January 1839, this first complete practical photographic process was announced at a meeting of the French Academy of Sciences, and the news quickly spread . At first, all details of the process were withheld and specimens were shown only at Daguerre's studio, under his close supervision, to Academy members and other distinguished guests . Arrangements were made for the French government to buy the rights in exchange for pensions for Niépce's son and Daguerre and present the invention to the world (with the exception of Great Britain, where an agent for Daguerre patented it) as a free gift . Complete instructions were made public on 19 August 1839 . Known as the Daguerreotype process, it was the most common commercial process until the late 1850s . It was superseded by the collodion process . </P> <P> After reading early reports of Daguerre's invention, Henry Fox Talbot, who had succeeded in creating stabilized photographic negatives on paper in 1835, worked on perfecting his own process . In early 1839, he acquired a key improvement, an effective fixer, from his friend John Herschel, a polymath scientist who had previously shown that hyposulfite of soda (commonly called "hypo" and now known formally as sodium thiosulfate) would dissolve silver salts . News of this solvent also benefited Daguerre, who soon adopted it as a more efficient alternative to his original hot salt water method . </P>

When was the first picture taken of a person