sell for much higher prices than ordinary portraits. At the time of its introduction, the process required exposures lasting ten minutes or more for brightly sunlit subjects, so portraiture was an impractical ordeal. Samuel Morse was astonished to learn that daguerreotypes of the streets of Paris did not show any people, horses or vehicles, until he realized that due to the long exposure times all moving objects became invisible. Within a few years, exposures had been reduced to as little as a few seconds by the use of additional sensitizing chemicals and "faster" lenses such as Petzval's portrait lens, the first mathematically calculated lens. The daguerreotype was the Polaroid film of its day: it produced a unique image which could only be duplicated by using a camera to photograph the original. Despite this drawback, millions of daguerreotypes were produced. The paper-based calotype process, introduced by Henry Fox Talbot in 1841, allowed the production of an unlimited number of copies by simple contact printing, but it had its own shortcomings—the grain of the paper was obtrusively visible in the image, and the extremely fine detail of which the daguerreotype was capable was not possible. The introduction of the wet collodion process in the early 1850s provided the basis for a negative-positive print-making process not subject to these limitations, although it, like the daguerreotype, was initially used to produce one-of-a-kind images—ambrotypes on glass and tintypes on black-lacquered iron sheets—rather than prints on paper. These new types of images were much less expensive than daguerreotypes, and they were easier to view. By 1860 few photographers were still using Daguerre's process. The same small ornate cases commonly used to house daguerreotypes were also used for images produced by the later and very different ambrotype and tintype processes, and the images originally in them were sometimes later discarded so that they could be used to display photographic paper prints. It is now a very common error for any image in such a case to be described as "a daguerreotype". A true daguerreotype is always an image on a highly polished silver surface, usually under protective glass. If it is viewed while a brightly lit sheet of white paper is held so as to be seen reflected in its mirror-like metal surface, the daguerreotype image will appear as a relatively faint negative—its dark and light areas reversed—instead of a normal positive. Other types of photographic images