very important for the chemical reduction potential of the developer solution (not the standard reduction potential of the developing agent) to be somewhere higher than the Fermi energy level of small metallic silver clusters (that is, the latent image) but well below the conduction band of unexposed silver halide crystals. Generally, weakly exposed crystals have smaller silver clusters. Silver clusters of smaller sizes have a higher Fermi level, and therefore more crystals are developed as the developer's reduction potential is increased. However, again, the developer potential must be well below the conduction band of silver halide crystal. Thus there is a limit in increasing the photographic speed of the system by boosting the developer potential; if the solution's reduction potential is set high enough to exploit smaller silver cluster, at some point the solution begins to reduce silver halide crystals regardless of exposure. This is called fog, which is metallic silver made from non-imagewise (exposure-nonspecific) reduction of silver halide crystals. It was also found that, when developer solution is optimally formulated, the maximum photographic speed is rather insensitive to the choice of developing agent (James 1945), and there exists a limit for the size of silver cluster that can be developed. One way to improve this problem is the use of the gold sensitization technique of Koslowski. A small metallic gold cluster whose Fermi level is high enough to prevent development of the crystal is used to decrease the threshold size of metallic silver cluster that can render the crystal developable. For further discussion, refer to Tani 1995 and Hamilton 1988. Stability of latent image Under normal conditions the latent image, which may be as small as a few atoms of metallic silver on each halide grain, is stable for many months. Subsequent development can then reveal a visible metallic image. A famous instance of latent-image stability are the pictures taken by Nils Strindberg, the photographer in S. A. Andrée's ill-fated arctic balloon expedition of 1897. The pictures of the expedition and of the balloon stranded on the ice were not discovered and developed until some 33 years later. See also Film base References Coe, Brian, 1976, The Birth of Photography, Ash & Grant. Mitchell, J.W., 1957, Photographic Sensitivity, Rep. Prog. Phys., vol. 20, pp. 433–515. Tani, T., 1995, Photographic Sensitivity, Oxford University Press., pp. 31–32, 84-85, 89-91. Mitchell, J. W., 1999, Evolution of the concepts of photographic sensitivity, J.