THE satisfactory recording of a scene by photographic means first became possible in 1839, when Daguerre published details of the process which bears his name. The ‘base’ consisted of a silver or silvered metal plate iodized on the surface, and the exposure amounted to about ten minutes in bright sunlight. ‘Development’ took place in mercury vapour, which acted preferentially upon the exposed portions, yielding a direct positive. A serious disadvantage of the process was the fact that only one picture was obtained, and there was no known process of copying originals. Fox-Talbot in 1840 carried the art a stage further by obtaining negatives on paper, using silver iodide as sensitive material, developing in gallic acid, waxing the paper to make it transparent and making prints by exposing in front of a piece of paper sensitized with silver chloride.