coated with silver iodide, and exposing this plate again to mercury vapor. By 1837, he was able to fix the images with a common salt solution. He called this process Daguerreotype, and tried unsuccessfully for a couple of years to commercialize it. Eventually, with help of the scientist and politician François Arago, the French government acquired Daguerre's process for public release. In exchange, pensions were provided to Daguerre as well as Niépce's son, Isidore. In the 1830s, the English scientist William Henry Fox Talbot independently invented a process to capture camera images using silver salts. Although dismayed that Daguerre had beaten him to the announcement of photography, he submitted on 31 January 1839, a pamphlet to the Royal Institution entitled Some Account of the Art of Photogenic Drawing, which was the first published description of photography. Within two years, Talbot developed a two-step process for creating photographs on paper, which he called calotypes. The calotype process was the first to utilize negative printing, which reverses all values in the reproduction process – black shows up as white and vice versa. Negative printing allows, in principle, an unlimited number of positive prints to be made from the original negative. The Calotype process also introduced the ability for a printmaker to alter the resulting image through retouching of the negative.Calotypes were never as popular or widespread as daguerreotypes, owing mainly to the fact that the latter produced sharper details. However, because daguerreotypes only produce a direct positive print, no duplicates can be made. It is the two-step negative/positive process that formed the basis for modern photography. The first photographic camera developed for commercial manufacture was a daguerreotype camera, built by Alphonse Giroux in 1839. Giroux signed a contract with Daguerre and Isidore Niépce to produce the cameras in France, with each device and accessories costing 400 francs. The camera was a double-box design, with a landscape lens fitted to the outer box, and a holder for a ground glass focusing screen and image plate on the inner box. By sliding the inner box, objects at various distances could be brought to as sharp a focus as desired. After a satisfactory image had been focused on the screen, the screen was replaced with a sensitized plate. A knurled wheel controlled a copper flap in front of the lens, which functioned as a shutter. The early daguerreotype cameras required long exposure times, which in