In observational astronomy one is continually faced with the problem of interpreting faint images which are close to the limit of recognizability. The problem of threshold recognition of images at low light levels may be subdivided into two cases: (1) the limit of detectability of an image against a black background, and (2) the minimum perceptible change of illumination which can be detected at small but finite light levels. Until such time as the much-discussed space platform is launched into its orbit around the Earth, the astronomer must be content to view astronomical objects through a thick layer of relatively luminous air. Therefore, the second type of problem, namely that of a low-contrast image, is the main consideration.